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  • 1. Abdelkader, M.
    et al.
    Metzger, S.
    Mamouri, R. E.
    Astitha, M.
    Barrie, Leonard
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Levin, Z.
    Lelieveld, J.
    Dust-air pollution dynamics over the eastern Mediterranean2015In: Atmospheric Chemistry And Physics, ISSN 1680-7316, E-ISSN 1680-7324, Vol. 15, no 16, p. 9173-9189Article in journal (Refereed)
    Abstract [en]

    Interactions of desert dust and air pollution over the eastern Mediterranean (EM) have been studied, focusing on two distinct dust transport events on 22 and 28 September 2011. The atmospheric chemistry-climate model EMAC has been used at about 50 km grid spacing, applying an on-line dust emission scheme and calcium as a proxy for dust reactivity. EMAC includes a detailed tropospheric chemistry mechanism, aerosol microphysics and thermodynamics schemes to describe dust aging. The model is evaluated using ground-based observations for aerosol concentrations and aerosol optical depth (AOD) as well as satellite observations. Simulation results and back trajectory analysis show that the development of synoptic disturbances over the EM can enhance dust transport from the Sahara and Arabian deserts in frontal systems that also carry air pollution to the EM. The frontal systems are associated with precipitation that controls the dust removal. Our results show the importance of chemical aging of dust, which increases particle size, dust deposition and scavenging efficiency during transport, overall reducing the lifetime relative to non-aged dust particles. The relatively long travel periods of Saharan dust result in more sustained aging compared to Arabian dust. Sensitivity simulations indicate 3 times more dust deposition of aged relative to pristine dust, which significantly decreases the dust lifetime and loading.

  • 2. Abdelkader, Mohamed
    et al.
    Metzger, Swen
    Steil, Benedikt
    Klingmüller, Klaus
    Tost, Holger
    Pozzer, Andrea
    Stenchikov, Georgiy
    Barrie, Leonard
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Lelieveld, Jos
    Sensitivity of transatlantic dust transport to chemical aging and related atmospheric processes2017In: Atmospheric Chemistry And Physics, ISSN 1680-7316, E-ISSN 1680-7324, Vol. 17, no 6, p. 3799-3821Article in journal (Refereed)
    Abstract [en]

    We present a sensitivity study on transatlantic dust transport, a process which has many implications for the atmosphere, the ocean and the climate. We investigate the impact of key processes that control the dust outflow, i.e., the emission flux, convection schemes and the chemical aging of mineral dust, by using the EMAC model following Abdelkader et al. (2015). To characterize the dust outflow over the Atlantic Ocean, we distinguish two geographic zones: (i) dust interactions within the Intertropical Convergence Zone (ITCZ), or the dust-ITCZ interaction zone (DIZ), and (ii) the adjacent dust transport over the Atlantic Ocean (DTA) zone. In the latter zone, the dust loading shows a steep and linear gradient westward over the Atlantic Ocean since particle sedimentation is the dominant removal process, whereas in the DIZ zone aerosol-cloud interactions, wet deposition and scavenging processes determine the extent of the dust outflow. Generally, the EMAC simulated dust compares well with CALIPSO observations; however, our reference model configuration tends to overestimate the dust extinction at a lower elevation and underestimates it at a higher elevation. The aerosol optical depth (AOD) over the Caribbean responds to the dust emission flux only when the emitted dust mass is significantly increased over the source region in Africa by a factor of 10. These findings point to the dominant role of dust removal (especially wet deposition) in transatlantic dust transport. Experiments with different convection schemes have indeed revealed that the transatlantic dust transport is more sensitive to the convection scheme than to the dust emission flux parameterization. To study the impact of dust chemical aging, we focus on a major dust outflow in July 2009. We use the calcium cation as a proxy for the overall chemical reactive dust fraction and consider the uptake of major inorganic acids (i.e., H2SO4, HNO3 and HCl) and their anions, i.e., sulfate (SO42-), bisulfate (HSO4-), nitrate (NO 3) and chloride (Cl), on the surface of mineral particles. The subsequent neutralization reactions with the calcium cation form various salt compounds that cause the uptake of water vapor from the atmosphere, i.e., through the chemical aging of dust particles leading to an increase of 0.15 in the AOD under subsaturated conditions (July 2009 monthly mean). As a result of the radiative feedback on surface winds, dust emissions increased regionally. On the other hand, the aged dust particles, compared to the non-aged particles, are more efficiently removed by both wet and dry deposition due to the increased hygroscopicity and particle size (mainly due to water uptake). The enhanced removal of aged particles decreases the dust burden and lifetime, which indirectly reduces the dust AOD by 0.05 (monthly mean). Both processes can be significant (major dust outflow, July 2009), but the net effect depends on the region and level of dust chemical aging.

  • 3.
    Achtert, Peggy
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Karlsson Andersson, M.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Khosrawi, Farahnaz
    Stockholm University, Faculty of Science, Department of Meteorology .
    Gumbel, Jörg
    Stockholm University, Faculty of Science, Department of Meteorology .
    On the linkage between tropospheric and Polar Stratospheric clouds in the Arctic as observed by space-borne lidar2012In: Atmospheric Chemistry And Physics, ISSN 1680-7316, E-ISSN 1680-7324, Vol. 12, no 8, p. 3791-3798Article in journal (Refereed)
    Abstract [en]

    The type of Polar stratospheric clouds (PSCs) as well as their temporal and spatial extent are important for the occurrence of heterogeneous reactions in the polar stratosphere. The formation of PSCs depends strongly on temperature. However, the mechanisms of the formation of solid PSCs are still poorly understood. Recent satellite studies of Antarctic PSCs have shown that their formation can be associated with deep-tropospheric clouds which have the ability to cool the lower stratosphere radiatively and/or adiabatically. In the present study, lidar measurements aboard the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) satellite were used to investigate whether the formation of Arctic PSCs can be associated with deep-tropospheric clouds as well. Deep-tropospheric cloud systems have a vertical extent of more than 6.5 km with a cloud top height above 7 km altitude. PSCs observed by CALIPSO during the Arctic winter 2007/2008 were classified according to their type (STS, NAT, or ice) and to the kind of underlying tropospheric clouds. Our analysis reveals that 172 out of 211 observed PSCs occurred in connection with tropospheric clouds. 72% of these 172 observed PSCs occurred above deep-tropospheric clouds. We also find that the type of PSC seems to be connected to the characteristics of the underlying tropospheric cloud system. During the Arctic winter 2007/2008 PSCs consisting of ice were mainly observed in connection with deep-tropospheric cloud systems while no ice PSC was detected above cirrus. Furthermore, we find no correlation between the occurrence of PSCs and the top temperature of tropospheric clouds. Thus, our findings suggest that Arctic PSC formation is connected to adiabatice cooling, i.e. dynamic effects rather than radiative cooling.

  • 4. Achtert, Peggy
    et al.
    O'Connor, Ewan J.
    Brooks, Ian M.
    Sotiropoulou, Georgia
    Stockholm University, Faculty of Science, Department of Meteorology .
    Shupe, Matthew D.
    Pospichal, Bernhard
    Brooks, Barbara J.
    Tjernström, Michael
    Stockholm University, Faculty of Science, Department of Meteorology .
    Properties of Arctic liquid and mixed-phase clouds from shipborne Cloudnet observations during ACSE 20142020In: Atmospheric Chemistry And Physics, ISSN 1680-7316, E-ISSN 1680-7324, Vol. 20, no 23, p. 14983-15002Article in journal (Refereed)
    Abstract [en]

    This study presents Cloudnet retrievals of Arctic clouds from measurements conducted during a 3-month research expedition along the Siberian shelf during summer and autumn 2014. During autumn, we find a strong reduction in the occurrence of liquid clouds and an increase for both mixed-phase and ice clouds at low levels compared to summer. About 80 % of all liquid clouds observed during the research cruise show a liquid water path below the infrared black body limit of approximately 50 g m(-2). The majority of mixed-phase and ice clouds had an ice water path below 20 g m(-2). Cloud properties are analysed with respect to cloud-top temperature and boundary layer structure. Changes in these parameters have little effect on the geometric thickness of liquid clouds while mixed-phase clouds during warm-air advection events are generally thinner than when such events were absent. Cloud-top temperatures are very similar for all mixed-phase clouds. However, more cases of lower cloudtop temperature were observed in the absence of warm-air advection. Profiles of liquid and ice water content are normalized with respect to cloud base and height. For liquid water clouds, the liquid water content profile reveals a strong increase with height with a maximum within the upper quarter of the clouds followed by a sharp decrease towards cloud top. Liquid water content is lowest for clouds observed below an inversion during warm-air advection events. Most mixedphase clouds show a liquid water content profile with a very similar shape to that of liquid clouds but with lower maximum values during events with warm air above the planetary boundary layer. The normalized ice water content profiles in mixed-phase clouds look different from those of liquid water content. They show a wider range in maximum values with the lowest ice water content for clouds below an inversion and the highest values for clouds above or extending through an inversion. The ice water content profile generally peaks at a height below the peak in the liquid water content profile - usually in the centre of the cloud, sometimes closer to cloud base, likely due to particle sublimation as the crystals fall through the cloud.

  • 5. Adachi, Kouji
    et al.
    Tobo, Yutaka
    Koike, Makoto
    Pereira Freitas, Gabriel
    Stockholm University, Faculty of Science, Department of Environmental Science.
    Zieger, Paul
    Stockholm University, Faculty of Science, Department of Environmental Science.
    Krejci, Radovan
    Stockholm University, Faculty of Science, Department of Environmental Science.
    Composition and mixing state of Arctic aerosol and cloud residual particles from long-term sinale-particle observations at Zeppelin Observatory, Svalbard2022In: Atmospheric Chemistry And Physics, ISSN 1680-7316, E-ISSN 1680-7324, Vol. 22, no 21, p. 14421-14439Article in journal (Refereed)
    Abstract [en]

    The Arctic region is sensitive to climate change and is warming faster than the global average. Aerosol particles change cloud properties by acting as cloud condensation nuclei and ice-nucleating particles, thus influencing the Arctic climate system. Therefore, understanding the aerosol particle properties in the Arctic is needed to interpret and simulate their influences on climate. In this study, we collected ambient aerosol particles using whole-air and PM10 inlets and residual particles of cloud droplets and ice crystals from Arctic low-level clouds (typically, all-liquid or mixed-phase clouds) using a counterflow virtual impactor inlet at the Zeppelin Observatory near Ny-Ålesund, Svalbard, within a time frame of 4 years. We measured the composition and mixing state of individual fine-mode particles in 239 samples using transmission electron microscopy. On the basis of their composition, the aerosol and cloud residual particles were classified as mineral dust, sea salt, K-bearing, sulfate, and carbonaceous particles. The number fraction of aerosol particles showed seasonal changes, with sulfate dominating in summer and sea salt increasing in winter. There was no measurable difference in the fractions between ambient aerosol and cloud residual particles collected at ambient temperatures above 0 C. On the other hand, cloud residual samples collected at ambient temperatures below 0 C had several times more sea salt and mineral dust particles and fewer sulfates than ambient aerosol samples, suggesting that sea spray and mineral dust particles may influence the formation of cloud particles in Arctic mixed-phase clouds. We also found that 43 % of mineral dust particles from cloud residual samples were mixed with sea salt, whereas only 18 % of mineral dust particles in ambient aerosol samples were mixed with sea salt. This study highlights the variety in aerosol compositions and mixing states that influence or are influenced by aerosol–cloud interactions in Arctic low-level clouds.

  • 6.
    Ahlm, Lars
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology . University of Oslo, Norway.
    Jones, Andy
    Stjern, Camilla W.
    Muri, Helene
    Kravitz, Ben
    Kristjánsson, Jón Egill
    Marine cloud brightening - as effective without clouds2017In: Atmospheric Chemistry And Physics, ISSN 1680-7316, E-ISSN 1680-7324, Vol. 17, no 21, p. 13071-13087Article in journal (Refereed)
    Abstract [en]

    Marine cloud brightening through sea spray injection has been proposed as a climate engineering method for avoiding the most severe consequences of global warming. A limitation of most of the previous modelling studies on marine cloud brightening is that they have either considered individual models or only investigated the effects of a specific increase in the number of cloud droplets. Here we present results from coordinated simulations with three Earth system models (ESMs) participating in the Geoengineering Model Intercomparison Project (GeoMIP) G4sea-salt experiment. Injection rates of accumulation-mode sea spray aerosol particles over ocean between 30 degrees N and 30 degrees S are set in each model to generate a global-mean effective radiative forcing (ERF) of -2.0 W m(-2) at the top of the atmosphere. We find that the injection increases the cloud droplet number concentration in lower layers, reduces the cloud-top effective droplet radius, and increases the cloud optical depth over the injection area. We also find, however, that the global-mean clear-sky ERF by the injected particles is as large as the corresponding total ERF in all three ESMs, indicating a large potential of the aerosol direct effect in regions of low cloudiness. The largest enhancement in ERF due to the presence of clouds occur as expected in the subtropical stratocumulus regions off the west coasts of the American and African continents. However, outside these regions, the ERF is in general equally large in cloudy and clear-sky conditions. These findings suggest a more important role of the aerosol direct effect in sea spray climate engineering than previously thought.

  • 7.
    Ahlm, Lars
    et al.
    Stockholm University, Faculty of Science, Department of Applied Environmental Science (ITM).
    Krejci, Radovan
    Stockholm University, Faculty of Science, Department of Applied Environmental Science (ITM).
    Nilsson, Douglas
    Stockholm University, Faculty of Science, Department of Applied Environmental Science (ITM).
    Mårtensson, Monica
    Stockholm University, Faculty of Science, Department of Applied Environmental Science (ITM).
    Vogt, Matthias
    Stockholm University, Faculty of Science, Department of Applied Environmental Science (ITM).
    Artaxo, Paulo
    Emission and dry deposition of accumulation mode particles in the Amazon Basin2010In: Atmospheric Chemistry And Physics, ISSN 1680-7316, E-ISSN 1680-7324, Vol. 10, no 21, p. 10237-10253Article in journal (Refereed)
    Abstract [en]

    Size-resolved vertical aerosol number fluxes of particles in the diameter range 0.25–2.5 μm were measured with the eddy covariance method from a 53 m high tower over the Amazon rain forest, 60 km NNW of Manaus, Brazil. This study focuses on data measured during the relatively clean wet season, but a shorter measurement period from the more polluted dry season is used as a comparison. Size-resolved net particle fluxes of the five lowest size bins, representing 0.25–0.45 μm in diameter, pointed downward in more or less all wind sectors in the wet season. This is an indication that the source of primary biogenic aerosol particles may be small in this particle size range. In the diameter range 0.5–2.5 μm, vertical particle fluxes were highly dependent on wind direction. In wind sectors where anthropogenic influence was low, net emission fluxes dominated. However, in wind sectors associated with higher anthropogenic influence, net deposition fluxes dominated. The net emission fluxes were interpreted as primary biogenic aerosol emission, but deposition of anthropogenic particles seems to have masked this emission in wind sectors with higher anthropogenic influence. The emission fluxes were at maximum in the afternoon when the mixed layer is well developed, and these emissions were best correlated with horizontal wind speed by the equation log10F=0.47·U+2.26 where F is the emission number flux of 0.5–2.5 μm particles [m−2s−1] and U is the horizontal wind speed [ms−1] at the top of the tower.

  • 8.
    Ahlm, Lars
    et al.
    Stockholm University, Faculty of Science, Department of Applied Environmental Science (ITM).
    Nilsson, Douglas
    Stockholm University, Faculty of Science, Department of Applied Environmental Science (ITM).
    Krejci, Radovan
    Stockholm University, Faculty of Science, Department of Applied Environmental Science (ITM).
    Mårtensson, Monica
    Stockholm University, Faculty of Science, Department of Applied Environmental Science (ITM).
    Vogt, Matthias
    Stockholm University, Faculty of Science, Department of Applied Environmental Science (ITM).
    Artaxo, Paulo
    A comparison of dry and wet season aerosol number fluxes over the Amazon rain forest2010In: Atmospheric Chemistry And Physics, ISSN 1680-7316, E-ISSN 1680-7324, Vol. 10, no 6, p. 3063-3079Article in journal (Refereed)
    Abstract [en]

    Vertical number fluxes of aerosol particles and vertical fluxes of CO2 were measured with the eddy covariance method at the top of a 53m high tower in the Amazon rain forest as part of the LBA (The Large Scale Biosphere Atmosphere Experiment in Amazonia) experiment. The observed aerosol number fluxes included particles with sizes down to 10 nm in diameter. The measurements were carried out during the wet and dry season in 2008. In this study focus is on the dry season aerosol fluxes, with significant influence from biomass burning, and these are compared with aerosol fluxes measured during the wet season. Net particle deposition fluxes dominated in daytime in both seasons and the deposition flux was considerably larger in the dry season due to the much higher dry season particle concentration. The particle transfer velocity increased linearly with increasing friction velocity in both seasons. The difference in transfer velocity between the two seasons was small, indicating that the seasonal change in aerosol number size distribution is not enough for causing any significant change in deposition velocity. In general, particle transfer velocities in this study are low compared to studies over boreal forests. The reasons are probably the high percentage of accumulation mode particles and the low percentage of nucleation mode particles in the Amazon boundary layer, both in the dry and wet season, and low wind speeds in the tropics compared to the midlatitudes. In the dry season, nocturnal particle fluxes behaved very similar to the nocturnal CO2 fluxes. Throughout the night, the measured particle flux at the top of the tower was close to zero, but early in the morning there was an upward particle flux peak that is not likely a result of entrainment or local pollution. It is possible that these morning upward particle fluxes are associated with emission of primary biogenic particles from the rain forest. Emitted particles may be stored within the canopy during stable conditions at nighttime, similarly to CO2, and being released from the canopy when conditions become more turbulent in the morning.

  • 9.
    Ahlm, Lars
    et al.
    Stockholm University, Faculty of Science. Stockholm University, Faculty of Science, Department of Applied Environmental Science (ITM).
    Nilsson, Douglas
    Stockholm University, Faculty of Science, Department of Applied Environmental Science (ITM).
    Krejci, Radovan
    Stockholm University, Faculty of Science, Department of Applied Environmental Science (ITM).
    Mårtensson, Monica
    Stockholm University, Faculty of Science, Department of Applied Environmental Science (ITM).
    Vogt, Matthias
    Stockholm University, Faculty of Science, Department of Applied Environmental Science (ITM).
    Artaxo, Paulo
    Aerosol number fluxes over the Amazon rain forest during the wet season2009In: Atmospheric Chemistry And Physics, ISSN 1680-7316, E-ISSN 1680-7324, Vol. 9, no 24, p. 9381-9400Article in journal (Refereed)
    Abstract [en]

    Number fluxes of particles with diameter larger than 10 nm were measured with the eddy covariance method over the Amazon rain forest during the wet season as part of the LBA (The Large Scale Biosphere Atmosphere Experiment in Amazonia) campaign 2008. The primary goal was to investigate whether sources or sinks dominate the aerosol number flux in the tropical rain forest-atmosphere system. During the measurement campaign, from 12 March to 18 May, 60% of the particle fluxes pointed downward, which is a similar fraction to what has been observed over boreal forests. The net deposition flux prevailed even in the absolute cleanest atmospheric conditions during the campaign and therefore cannot be explained only by deposition of anthropogenic particles. The particle transfer velocity vt increased with increasing friction velocity and the relation is described by the equation vt=2.4×10−3×u* where u* is the friction velocity. Upward particle fluxes often appeared in the morning hours and seem to a large extent to be an effect of entrainment fluxes into a growing mixed layer rather than primary aerosol emission. In general, the number source of primary aerosol particles within the footprint area of the measurements was small, possibly because the measured particle number fluxes reflect mostly particles less than approximately 200 nm. This is an indication that the contribution of primary biogenic aerosol particles to the aerosol population in the Amazon boundary layer may be low in terms of number concentrations. However, the possibility of horizontal variations in primary aerosol emission over the Amazon rain forest cannot be ruled out.

  • 10. Alastuey, Andres
    et al.
    Querol, Xavier
    Aas, Wenche
    Lucarelli, Franco
    Perez, Noemi
    Moreno, Teresa
    Cavalli, Fabrizia
    Areskoug, Hans
    Stockholm University, Faculty of Science, Department of Environmental Science and Analytical Chemistry.
    Balan, Violeta
    Catrambone, Maria
    Ceburnis, Darius
    Cerro, Jose C.
    Conil, Sebastien
    Gevorgyan, Lusine
    Hueglin, Christoph
    Imre, Kornelia
    Jaffrezo, Jean-Luc
    Leeson, Sarah R.
    Mihalopoulos, Nikolaos
    Mitosinkova, Marta
    O'Dowd, Colin D.
    Pey, Jorge
    Putaud, Jean-Philippe
    Riffault, Veronique
    Ripoll, Anna
    Sciare, Jean
    Sellegri, Karine
    Spindler, Gerald
    Yttri, Karl Espen
    Geochemistry of PM10 over Europe during the EMEP intensive measurement periods in summer 2012 and winter 20132016In: Atmospheric Chemistry And Physics, ISSN 1680-7316, E-ISSN 1680-7324, Vol. 16, no 10, p. 6107-6129Article in journal (Refereed)
    Abstract [en]

    The third intensive measurement period (IMP) organised by the European Monitoring and Evaluation Programme (EMEP) under the UNECE CLTRAP took place in summer 2012 and winter 2013, with PM10 filter samples concurrently collected at 20 (16 EMEP) regional background sites across Europe for subsequent analysis of their mineral dust content. All samples were analysed by the same or a comparable methodology. Higher PM10 mineral dust loadings were observed at most sites in summer (0.5-10aEuro-A mu gaEuro-m(-3)) compared to winter (0.2-2aEuro-A mu gaEuro-m(-3)), with the most elevated concentrations in the southern- and easternmost countries, accounting for 20-40aEuro-% of PM10. Saharan dust outbreaks were responsible for the high summer dust loadings at western and central European sites, whereas regional or local sources explained the elevated concentrations observed at eastern sites. The eastern Mediterranean sites experienced elevated levels due to African dust outbreaks during both summer and winter. The mineral dust composition varied more in winter than in summer, with a higher relative contribution of anthropogenic dust during the former period. A relatively high contribution of K from non-mineral and non-sea-salt sources, such as biomass burning, was evident in winter at some of the central and eastern European sites. The spatial distribution of some components and metals reveals the influence of specific anthropogenic sources on a regional scale: shipping emissions (V, Ni, and SO42-) in the Mediterranean region, metallurgy (Cr, Ni, and Mn) in central and eastern Europe, high temperature processes (As, Pb, and SO42-) in eastern countries, and traffic (Cu) at sites affected by emissions from nearby cities.

  • 11. Aliaga, Diego
    et al.
    Sinclair, Victoria A.
    Andrade, Marcos
    Artaxo, Paulo
    Carbone, Samara
    Kadantsev, Evgeny
    Laj, Paolo
    Wiedensohler, Alfred
    Krejci, Radovan
    Stockholm University, Faculty of Science, Department of Environmental Science.
    Bianchi, Federico
    Identifying source regions of air masses sampled at the tropical high-altitude site of Chacaltaya using WRF-FLEXPART and cluster analysis2021In: Atmospheric Chemistry And Physics, ISSN 1680-7316, E-ISSN 1680-7324, Vol. 21, no 21, p. 16453-16477Article in journal (Refereed)
    Abstract [en]

    Observations of aerosol and trace gases in the remote troposphere are vital to quantify background concentrations and identify long-term trends in atmospheric composition on large spatial scales. Measurements made at high altitude are often used to study free-tropospheric air; however such high-altitude sites can be influenced by boundary layer air masses. Thus, accurate information on air mass origin and transport pathways to high-altitude sites is required. Here we present a new method, based on the source-receptor relationship (SRR) obtained from backwards WRF-FLEXPART simulations and a k-means clustering approach, to identify source regions of air masses arriving at measurement sites. Our method is tailored to areas of complex terrain and to stations influenced by both local and long-range sources. We have applied this method to the Chacaltaya (CHC) GAW station (5240 m a.s.l.; 16.35 degrees S, 68.13 degrees W) for the 6-month duration of the Southern Hemisphere high-altitude experiment on particle nucleation and growth (SALILNA) to identify where sampled air masses originate and to quantify the influence of the surface and the free troposphere. A key aspect of our method is that it is probabilistic, and for each observation time, more than one air mass (cluster) can influence the station, and the percentage influence of each air mass can be quantified. This is in contrast to binary methods, which label each observation time as influenced by either boundary layer or free-troposphere air masses. Air sampled at CHC is a mix of different provenance. We find that on average 9 % of the air, at any given observation time, has been in contact with the surface within 4 d prior to arriving at CHC. Furthermore, 24 % of the air has been located within the first 1.5 km above ground level (surface included). Consequently, 76 % of the air sampled at CHC originates from the free troposphere. However, pure free-tropospheric influences are rare, and often samples are concurrently influenced by both boundary layer and free-tropospheric air masses. A clear diurnal cycle is present, with very few air masses that have been in contact with the surface being detected at night. The 6-month analysis also shows that the most dominant air mass (cluster) originates in the Amazon and is responsible for 29 % of the sampled air. Furthermore, short-range clusters (origins within 100 km of CHC) have high temporal frequency modulated by local meteorology driven by the diurnal cycle, whereas the mid- and long-range clusters' (> 200 km) variability occurs on timescales governed by synoptic-scale dynamics. To verify the reliability of our method, in situ sulfate observations from CHC are combined with the SRR clusters to correctly identify the (pre-known) source of the sulfate: the Sabancaya volcano located 400 km north-west from the station.

  • 12. Allen, G.
    et al.
    Coe, H.
    Clarke, A.
    Bretherton, C.
    Wood, R.
    Abel, S. J.
    Barrett, P.
    Brown, P.
    George, R.
    Freitag, S.
    McNaughton, C.
    Howell, S.
    Shank, L.
    Kapustin, V.
    Brekhovskikh, V.
    Kleinman, L.
    Lee, Y-N
    Springston, S.
    Toniazzo, T.
    Krejci, Radovan
    Stockholm University, Faculty of Science, Department of Applied Environmental Science (ITM).
    Fochesatto, J.
    Shaw, G.
    Krecl, P.
    Brooks, B.
    McMeeking, G.
    Bower, K. N.
    Williams, P. I.
    Crosier, J.
    Crawford, I.
    Connolly, P.
    Allan, J. D.
    Covert, D.
    Bandy, A. R.
    Russell, L. M.
    Trembath, J.
    Bart, M.
    McQuaid, J. B.
    Wang, J.
    Chand, D.
    South East Pacific atmospheric composition and variability sampled = ong 20 degrees S during VOCALS-REx2011In: Atmospheric Chemistry And Physics, ISSN 1680-7316, E-ISSN 1680-7324, Vol. 11, no 11, p. 5237-5262Article in journal (Refereed)
    Abstract [en]

    The VAMOS Ocean-Cloud-Atmosphere-Land Regional Experiment (VOCALS-REx) was conducted from 15 October to 15 November 2008 in the South East Pacific (SEP) region to investigate interactions between land, sea and atmosphere in this unique tropical eastern ocean environment and to improve the skill of global and regional models in = presenting the region. This study synthesises selected aircraft, ship = d surface site observations from VOCALS-REx to statistically summarise = d characterise the atmospheric composition and variability of the = rine Boundary Layer (MBL) and Free Troposphere (FT) along the 20 = grees S parallel between 70 degrees W and 85 degrees W. Significant = nal gradients in mean MBL sub-micron aerosol particle size and = mposition, carbon monoxide, sulphur dioxide and ozone were seen over = e campaign, with a generally more variable and polluted coastal = vironment and a less variable, more pristine remote maritime regime. = adients in aerosol and trace gas concentrations were observed to be = sociated with strong gradients in cloud droplet number. The FT was = ten more polluted in terms of trace gases than the MBL in the mean; = wever increased variability in the FT composition suggests an episodic = ture to elevated concentrations. This is consistent with a complex = rtical interleaving of airmasses with diverse sources and hence = llutant concentrations as seen by generalised back trajectory = alysis, which suggests contributions from both local and long-range = urces. Furthermore, back trajectory analysis demonstrates that the = served zonal gradients both in the boundary layer and the free = oposphere are characteristic of marked changes in airmass history with = stance offshore - coastal boundary layer airmasses having been in = cent contact with the local land surface and remote maritime airmasses = ving resided over ocean for in excess of ten days. Boundary layer = mposition to the east of 75 degrees W was observed to be dominated by = astal emissions from sources to the west of the Andes, with evidence = r diurnal pumping of the Andean boundary layer above the height of the = rine capping inversion. Analysis of intra-campaign variability in = mospheric composition was not found to be significantly correlated = th observed low-frequency variability in the large scale flow pattern; = mpaign-average interquartile ranges of CO, SO(2) and O(3) = ncentrations at all longitudes were observed to dominate over much = aller differences in median concentrations calculated between periods = different flow regimes. The campaign climatology presented here aims = provide a valuable dataset to inform model simulation and future = ocess studies, particularly in the context of aerosol-cloud = teraction and further evaluation of dynamical processes in the SEP = gion for conditions analogous to those during VOCALS-REx. To this end, = r results are discussed in terms of coastal, transitional and remote = atial regimes in the MBL and FT and a gridded dataset are provided as = resource.

  • 13.
    Andersson, August
    et al.
    Stockholm University, Faculty of Science, Department of Environmental Science.
    Kirillova, Elena N.
    Stockholm University, Faculty of Science, Department of Environmental Science. National Research Council of Italy, Italy.
    Decesari, Stefano
    DeWitt, H. Langley
    Gasore, Jimmy
    Potter, Katherine
    Prinn, Ronald G.
    Rupakheti, Maheswar
    Ndikubwimana, Jean de Dieu
    Nkusi, Julius
    Safari, Bonfils
    Seasonal source variability of carbonaceous aerosols at the Rwanda Climate Observatory2020In: Atmospheric Chemistry And Physics, ISSN 1680-7316, E-ISSN 1680-7324, Vol. 20, no 8, p. 4561-4573Article in journal (Refereed)
    Abstract [en]

    Sub-Saharan Africa (SSA) is a global hot spot for aerosol emissions, which affect the regional climate and air quality. In this paper, we use ground-based observations to address the large uncertainties in the source-resolved emission estimation of carbonaceous aerosols. Ambient fine fraction aerosol was collected on filters at the high-altitude (2590 m a.s.1.) Rwanda Climate Observatory (RCO), a SSA background site, during the dry and wet seasons in 2014 and 2015. The concentrations of both the carbonaceous and inorganic ion components show a strong seasonal cycle, with highly elevated concentrations during the dry season. Source marker ratios, including carbon isotopes, show that the wet and dry seasons have distinct aerosol compositions. The dry season is characterized by elevated amounts of biomass burning products, which approach similar to 95 % for carbonaceous aerosols. An isotopic mass-balance estimate shows that the amount of the carbonaceous aerosol stemming from savanna fires may increase from 0.2 mu g m(-3) in the wet season up to 10 mu g m(-3) during the dry season. Based on these results, we quantitatively show that savanna fire is the key modulator of the seasonal aerosol composition variability at the RCO.

  • 14. Ansmann, A.
    et al.
    Seifert, P.
    Tesche, Matthias
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Wandinger, U.
    Profiling of fine and coarse particle mass: case studies of Saharan dust and Eyjafjallajokull/Grimsvotn volcanic plumes2012In: Atmospheric Chemistry And Physics, ISSN 1680-7316, E-ISSN 1680-7324, Vol. 12, no 20, p. 9399-9415Article in journal (Refereed)
    Abstract [en]

    The polarization lidar photometer networking (POLIPHON) method introduced to separate coarse-mode and fine-mode particle properties of Eyjafjallajokull volcanic aerosols in 2010 is extended to cover Saharan dust events as well. Furthermore, new volcanic dust observations performed after the Grimsvotn volcanic eruptions in 2011 are presented. The retrieval of particle mass concentrations requires mass-specific extinction coefficients. Therefore, a review of recently published mass-specific extinction coefficients for Saharan dust and volcanic dust is given. Case studies of four different scenarios corroborate the applicability of the profiling technique: (a) Saharan dust outbreak to central Europe, (b) Saharan dust plume mixed with biomass-burning smoke over Cape Verde, and volcanic aerosol layers originating from (c) the Eyjafjallajokull eruptions in 2010 and (d) the Grimsvotn eruptions in 2011. Strong differences in the vertical aerosol layering, aerosol mixing, and optical properties are observed for the different volcanic events.

  • 15. Ashworth, Kirsti
    et al.
    Bucci, Silvia
    Gallimore, Peter J.
    Lee, Junghwa
    Nelson, Beth S.
    Sanchez-Marroquín, Alberto
    Schimpf, Marina B.
    Smith, Paul D.
    Drysdale, Will S.
    Hopkins, Jim R.
    Lee, James D.
    Pitt, Joe R.
    Di Carlo, Piero
    Krejci, Radovan
    Stockholm University, Faculty of Science, Department of Environmental Science and Analytical Chemistry.
    McQuaid, James B.
    Megacity and local contributions to regional air pollution: an aircraft case study over London2020In: Atmospheric Chemistry And Physics, ISSN 1680-7316, E-ISSN 1680-7324, Vol. 20, no 12, p. 7193-7216Article in journal (Refereed)
    Abstract [en]

    In July 2017 three research flights circumnavigating the megacity of London were conducted as a part of the STANCO training school for students and early career researchers organised by EUFAR (European Facility for Airborne Research). Measurements were made from the UK's Facility for Airborne Atmospheric Measurements (FAAM) BAe-146-301 atmospheric research aircraft with the aim to sample, characterise and quantify the impact of megacity outflow pollution on air quality in the surrounding region. Conditions were extremely favourable for airborne measurements, and all three flights were able to observe clear pollution events along the flight path. A small change in wind direction provided sufficiently different air mass origins over the 2 d such that a distinct pollution plume from London, attributable marine emissions and a double-peaked dispersed area of pollution resulting from a combination of local and transported emissions were measured. We were able to analyse the effect of London emissions on air quality in the wider region and the extent to which local sources contribute to pollution events. The background air upwind of London was relatively clean during both days; concentrations of CO were 88-95 ppbv, total (measured) volatile organic compounds (VOCs) were 1.6-1.8 ppbv and NOx was 0.7-0.8 ppbv. Downwind of London, we encountered elevations in all species with CO>100 ppbv, VOCs 2.8-3.8 ppbv, CH4> 2080 ppbv and NOx >4 ppbv, and peak concentrations in individual pollution events were higher still. Levels of O-3 were inversely correlated with NOx, during the first flight, with O-3 concentrations of 37 ppbv upwind falling to similar to 26 ppbv in the well-defined London plume. Total pollutant fluxes from London were estimated through a vertical plane downwind of the city. Our calculated CO2 fluxes are within the combined uncertainty of those estimated previously, but there was a greater disparity in our estimates of CH4 and CO. On the second day, winds were lighter and downwind O-3 concentrations were elevated to similar to 39-43 ppbv (from similar to 32 to 35 ppbv upwind), reflecting the contribution of more aged pollution to the regional background. Elevations in pollutant concentrations were dispersed over a wider area than the first day, although we also encountered a number of clear transient enhancements from local sources. This series of flights demonstrated that even in a region of megacity outflow, such as the south-east of the UK, local fresh emissions and more distant UK sources of pollution can all contribute substantially to pollution events. In the highly complex atmosphere around a megacity where a high background level of pollution mixes with a variety of local sources at a range of spatial and temporal scales and atmospheric dynamics are further complicated by the urban heat island, the use of pollutant ratios to track and determine the ageing of air masses may not be valid. The individual sources must therefore all be well-characterised and constrained to understand air quality around megacities such as London. Research aircraft offer that capability through targeted sampling of specific sources and longitudinal studies monitoring trends in emission strength and profiles over time.

  • 16. Asmi, A.
    et al.
    Wiedensohler, A.
    Laj, P.
    Fjaeraa, A. -M
    Sellegri, K.
    Birmili, W.
    Weingartner, E.
    Baltensperger, U.
    Zdimal, V.
    Zikova, N.
    Putaud, J. -P
    Marinoni, A.
    Tunved, Peter
    Stockholm University, Faculty of Science, Department of Applied Environmental Science (ITM).
    Hansson, Hans-Christen
    Stockholm University, Faculty of Science, Department of Applied Environmental Science (ITM).
    Fiebig, M.
    Kivekas, N.
    Lihavainen, H.
    Asmi, E.
    Ulevicius, V.
    Aalto, P. P.
    Swietlicki, E.
    Kristensson, A.
    Mihalopoulos, N.
    Kalivitis, N.
    Kalapov, I.
    Kiss, G.
    de Leeuw, G.
    Henzing, B.
    Harrison, R. M.
    Beddows, D.
    O'Dowd, C.
    Jennings, S. G.
    Flentje, H.
    Weinhold, K.
    Meinhardt, F.
    Ries, L.
    Kulmala, M.
    Number size distributions and seasonality of submicron particles in = rope 2008-20092011In: Atmospheric Chemistry And Physics, ISSN 1680-7316, E-ISSN 1680-7324, Vol. 11, no 11, p. 5505-5538Article in journal (Refereed)
    Abstract [en]

    Two years of harmonized aerosol number size distribution data from 24 = ropean field monitoring sites have been analysed. The results give a = mprehensive overview of the European near surface aerosol particle = mber concentrations and number size distributions between 30 and 500 = of dry particle diameter. Spatial and temporal distribution of = rosols in the particle sizes most important for climate applications = e presented. We also analyse the annual, weekly and diurnal cycles of = e aerosol number concentrations, provide log-normal fitting parameters = r median number size distributions, and give guidance notes for data = ers. Emphasis is placed on the usability of results within the aerosol = delling community.

  • 17. Baars, Holger
    et al.
    Kanitz, Thomas
    Engelmann, Ronny
    Althausen, Dietrich
    Heese, Birgit
    Komppula, Mika
    Preissler, Jana
    Tesche, Matthias
    Stockholm University, Faculty of Science, Department of Environmental Science and Analytical Chemistry. Stockholm University, Faculty of Science, Department of Meteorology .
    Ansmann, Albert
    Wandinger, Ulla
    Lim, Jae-Hyun
    Ahn, Joon Young
    Stachlewska, Iwona S.
    Amiridis, Vassilis
    Marinou, Eleni
    Seifert, Patric
    Hofer, Julian
    Skupin, Annett
    Schneider, Florian
    Bohlmann, Stephanie
    Foth, Andreas
    Bley, Sebastian
    Pfuller, Anne
    Giannakaki, Eleni
    Lihavainen, Heikki
    Viisanen, Yrjo
    Hooda, Rakesh Kumar
    Pereira, Sergio Nepomuceno
    Bortoli, Daniele
    Wagner, Frank
    Mattis, Ina
    Janicka, Lucja
    Markowicz, Krzysztof M.
    Achtert, Peggy
    Stockholm University, Faculty of Science, Department of Environmental Science and Analytical Chemistry. Stockholm University, Faculty of Science, Department of Meteorology .
    Artaxo, Paulo
    Pauliquevis, Theotonio
    Souza, Rodrigo A. F.
    Sharma, Ved Prakesh
    van Zyl, Pieter Gideon
    Beukes, Johan Paul
    Sun, Junying
    Rohwer, Erich G.
    Deng, Ruru
    Mamouri, Rodanthi-Elisavet
    Zamorano, Felix
    An overview of the first decade of Polly(NET): an emerging network of automated Raman-polarization lidars for continuous aerosol profiling2016In: Atmospheric Chemistry And Physics, ISSN 1680-7316, E-ISSN 1680-7324, Vol. 16, no 8, p. 5111-5137Article in journal (Refereed)
    Abstract [en]

    A global vertically resolved aerosol data set covering more than 10 years of observations at more than 20 measurement sites distributed from 63 degrees N to 52 degrees S and 72 degrees W to 124 degrees E has been achieved within the Raman and polarization lidar network Polly(NET). This network consists of portable, remote-controlled multiwavelength-polarization-Raman lidars (Polly) for automated and continuous 24/7 observations of clouds and aerosols. Polly(NET) is an independent, voluntary, and scientific network. All Polly lidars feature a standardized instrument design with different capabilities ranging from single wavelength to multiwavelength systems, and now apply unified calibration, quality control, and data analysis. The observations are processed in near-real time without manual intervention, and are presented online at polly.tropos.de. The paper gives an overview of the observations on four continents and two research vessels obtained with eight Polly systems. The specific aerosol types at these locations (mineral dust, smoke, dust-smoke and other dusty mixtures, urban haze, and volcanic ash) are identified by their Angstrom exponent, lidar ratio, and depolarization ratio. The vertical aerosol distribution at the Polly(NET) locations is discussed on the basis of more than 55 000 automatically retrieved 30 min particle backscatter coefficient profiles at 532 nm as this operating wavelength is available for all Polly lidar systems. A seasonal analysis of measurements at selected sites revealed typical and extraordinary aerosol conditions as well as seasonal differences. These studies show the potential of Polly(NET) to support the establishment of a global aerosol climatology that covers the entire troposphere.

  • 18. Ball, William T.
    et al.
    Alsing, Justin
    Mortlock, Daniel J.
    Stockholm University, Faculty of Science, Department of Astronomy. Imperial College London, UK .
    Rozanov, Eugene V.
    Tummon, Fiona
    Haigh, Joanna D.
    Reconciling differences in stratospheric ozone composites2017In: Atmospheric Chemistry And Physics, ISSN 1680-7316, E-ISSN 1680-7324, Vol. 17, no 20, p. 12269-12302Article in journal (Refereed)
    Abstract [en]

    Observations of stratospheric ozone from multiple instruments now span three decades; combining these into composite datasets allows long-term ozone trends to be estimated. Recently, several ozone composites have been published, but trends disagree by latitude and altitude, even between composites built upon the same instrument data. We confirm that the main causes of differences in decadal trend estimates lie in (i) steps in the composite time series when the instrument source data changes and (ii) artificial sub-decadal trends in the underlying instrument data. These artefacts introduce features that can alias with regressors in multiple linear regression (MLR) analysis; both can lead to inaccurate trend estimates. Here, we aim to remove these artefacts using Bayesian methods to infer the underlying ozone time series from a set of composites by building a joint-likelihood function using a Gaussian-mixture density to model outliers introduced by data artefacts, together with a data-driven prior on ozone variability that incorporates knowledge of problems during instrument operation. We apply this Bayesian self-calibration approach to stratospheric ozone in 10 degrees bands from 60 degrees S to 60 degrees N and from 46 to 1 hPa (similar to 21-48 km) for 1985-2012. There are two main outcomes: (i) we independently identify and confirm many of the data problems previously identified, but which remain unaccounted for in existing composites; (ii) we construct an ozone composite, with uncertainties, that is free from most of these problems - we call this the BAyeSian Integrated and Consolidated (BASIC) composite. To analyse the new BASIC composite, we use dynamical linear modelling (DLM), which provides a more robust estimate of long-term changes through Bayesian inference than MLR. BASIC and DLM, together, provide a step forward in improving estimates of decadal trends. Our results indicate a significant recovery of ozone since 1998 in the upper stratosphere, of both northern and southern midlatitudes, in all four composites analysed, and particularly in the BASIC composite. The BASIC results also show no hemispheric difference in the recovery at midlatitudes, in contrast to an apparent feature that is present, but not consistent, in the four composites. Our overall conclusion is that it is possible to effectively combine different ozone composites and account for artefacts and drifts, and that this leads to a clear and significant result that upper stratospheric ozone levels have increased since 1998, following an earlier decline.

  • 19. Ball, William T.
    et al.
    Alsing, Justin
    Mortlock, Daniel J.
    Stockholm University, Faculty of Science, Department of Astronomy. Imperial College London, UK.
    Staehelin, Johannes
    Haigh, Joanna D.
    Peter, Thomas
    Tummon, Fiona
    Stübi, Rene
    Stenke, Andrea
    Anderson, John
    Bourassa, Adam
    Davis, Sean M.
    Degenstein, Doug
    Frith, Stacey
    Froidevaux, Lucien
    Roth, Chris
    Sofieva, Viktoria
    Wang, Ray
    Wild, Jeannette
    Yu, Pengfei
    Ziemke, Jerald R.
    Rozanov, Eugene V.
    Evidence for a continuous decline in lower stratospheric ozone offsetting ozone layer recovery2018In: Atmospheric Chemistry And Physics, ISSN 1680-7316, E-ISSN 1680-7324, Vol. 18, no 2, p. 1379-1394Article in journal (Refereed)
    Abstract [en]

    Ozone forms in the Earth's atmosphere from the photodissociation of molecular oxygen, primarily in the tropical stratosphere. It is then transported to the extratropics by the Brewer-Dobson circulation (BDC), forming a protective ozone layer around the globe. Human emissions of halogen-containing ozone-depleting substances (hODSs) led to a decline in stratospheric ozone until they were banned by the Montreal Protocol, and since 1998 ozone in the upper stratosphere is rising again, likely the recovery from halogen-induced losses. Total column measurements of ozone between the Earth's surface and the top of the atmosphere indicate that the ozone layer has stopped declining across the globe, but no clear increase has been observed at latitudes between 60 degrees S and 60 degrees N outside the polar regions (60-90 degrees). Here we report evidence from multiple satellite measurements that ozone in the lower stratosphere between 60 degrees S and 60 degrees N has indeed continued to decline since 1998. We find that, even though upper stratospheric ozone is recovering, the continuing downward trend in the lower stratosphere prevails, resulting in a downward trend in stratospheric column ozone between 60 degrees S and 60 degrees N. We find that total column ozone between 60 degrees S and 60 degrees N appears not to have decreased only because of increases in tropospheric column ozone that compensate for the stratospheric decreases. The reasons for the continued reduction of lower stratospheric ozone are not clear; models do not reproduce these trends, and thus the causes now urgently need to be established.

  • 20. Ball, William T.
    et al.
    Alsing, Justin
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC). Imperial College London, UK.
    Staehelin, Johannes
    Davis, Sean M.
    Froidevaux, Lucien
    Peter, Thomas
    Stratospheric ozone trends for 1985-2018: sensitivity to recent large variability2019In: Atmospheric Chemistry And Physics, ISSN 1680-7316, E-ISSN 1680-7324, Vol. 19, no 19, p. 12731-12748Article in journal (Refereed)
    Abstract [en]

    The Montreal Protocol, and its subsequent amendments, has successfully prevented catastrophic losses of stratospheric ozone, and signs of recovery are now evident. Nevertheless, recent work has suggested that ozone in the lower stratosphere (< 24 km) continued to decline over the 1998-2016 period, offsetting recovery at higher altitudes and preventing a statistically significant increase in quasi-global (60 degrees S-60 degrees N) total column ozone. In 2017, a large lower stratospheric ozone resurgence over less than 12 months was estimated (using a chemistry transport model; CTM) to have offset the long-term decline in the quasi-global integrated lower stratospheric ozone column. Here, we extend the analysis of space-based ozone observations to December 2018 using the BASIC(SG) ozone composite. We find that the observed 2017 resurgence was only around half that modelled by the CTM, was of comparable magnitude to other strong interannual changes in the past, and was restricted to Southern Hemisphere (SH) midlatitudes (60-30 degrees S). In the SH midlatitude lower stratosphere, the data suggest that by the end of 2018 ozone is still likely lower than in 1998 (probability similar to 80 %). In contrast, tropical and Northern Hemisphere (NH) ozone continue to display ongoing decreases, exceeding 90 % probability. Robust tropical (> 95 %, 30 degrees S-30 degrees N) decreases dominate the quasi-global integrated decrease (99 % probability); the integrated tropical stratospheric column (1-100 hPa, 30 degrees S-30 degrees N) displays a significant overall ozone decrease, with 95 % probability. These decreases do not reveal an inefficacy of the Montreal Protocol; rather, they suggest that other effects are at work, mainly dynamical variability on long or short timescales, counteracting the positive effects of the Montreal Protocol on stratospheric ozone recovery. We demonstrate that large interannual midlatitude (30-60 degrees) variations, such as the 2017 resurgence, are driven by non-linear quasi-biennial oscillation (QBO) phase-dependent seasonal variability. However, this variability is not represented in current regression analyses. To understand if observed lower stratospheric ozone decreases are a transient or long-term phenomenon, progress needs to be made in accounting for this dynamically driven variability.

  • 21. Ball, William T.
    et al.
    Chiodo, Gabriel
    Abalos, Marta
    Alsing, Justin
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC). Imperial College London, UK.
    Stenke, Andrea
    Inconsistencies between chemistry-climate models and observed lower stratospheric ozone trends since 19982020In: Atmospheric Chemistry And Physics, ISSN 1680-7316, E-ISSN 1680-7324, Vol. 20, no 16, p. 9737-9752Article in journal (Refereed)
    Abstract [en]

    The stratospheric ozone layer shields surface life from harmful ultraviolet radiation. Following the Montreal Protocol ban on long-lived ozone-depleting substances (ODSs), rapid depletion of total column ozone (TCO) ceased in the late 1990s, and ozone above 32 km is now clearly recovering. However, there is still no confirmation of TCO recovery, and evidence has emerged that ongoing quasiglobal (60 degrees S-60 degrees N) lower stratospheric ozone decreases may be responsible, dominated by low latitudes (30 degrees S-30 degrees N). Chemistry-climate models (CCMs) used to project future changes predict that lower stratospheric ozone will decrease in the tropics by 2100 but not at mid-latitudes (30-60 degrees). Here, we show that CCMs display an ozone decline similar to that observed in the tropics over 1998-2016, likely driven by an increase in tropical upwelling. On the other hand, mid-latitude lower stratospheric ozone is observed to decrease, while CCMs that specify real-world historical meteorological fields instead show an increase up to present day. However, these cannot be used to simulate future changes; we demonstrate here that free-running CCMs used for projections also show increases. Despite opposing lower stratospheric ozone changes, which should induce opposite temperature trends, CCMs and observed temperature trends agree; we demonstrate that opposing model- observation stratospheric water vapour (SWV) trends, and their associated radiative effects, explain why temperature changes agree in spite of opposing ozone trends. We provide new evidence that the observed mid-latitude trends can be explained by enhanced mixing between the tropics and extratropics. We further show that the temperature trends are consistent with the observed mid-latitude ozone decrease. Together, our results suggest that large-scale circulation changes expected in the future from increased greenhouse gases (GHGs) may now already be underway but that most CCMs do not simulate mid-latitude ozone layer changes well. However, it is important to emphasise that the periods considered here are short, and internal variability that is both intrinsic to each CCM and different to observed historical variability is not well-characterised and can influence trend estimates. Nevertheless, the reason CCMs do not exhibit the observed changes needs to be identified to allow models to be improved in order to build confidence in future projections of the ozone layer.

  • 22. Baron, Phillippe
    et al.
    Murtagh, Donal
    Urban, J
    Sagawa, S
    Ochiai, H
    Koernich, H
    Khosrawi, Farahnaz
    Stockholm University, Faculty of Science, Department of Meteorology .
    Kikuchi, K
    Mizobuchi, S
    Sagi, K
    Kasai, Y
    Yasui, M
    Observation of horizontal winds in the middle-atmosphere between 30° S and 55° N during the northern winter 2009–20102012In: Atmospheric Chemistry And Physics, ISSN 1680-7316, E-ISSN 1680-7324, Vol. 13, p. 6049-6064Article in journal (Refereed)
    Abstract [en]

    Although the links between stratospheric dynamics, climate and weather have been demonstrated, direct observations of stratospheric winds are lacking. We report observations of winds between 8 and 0.01 hPa (~35–80 km) from October 2009 to April 2010 by the Superconducting Submillimeter-Wave Limb-Emission Sounder (SMILES) on the International Space Station. The altitude range covers the region between 35–60 km where previous space-borne wind instruments show a lack of sensitivity. Both zonal and meridional wind components were obtained, though not simultaneously, in the latitude range from 30° S to 55° N and with a single profile precision of 7–9 m s−1 between 8 and 0.6 hPa and better than 20 m s−1 at altitudes above. The vertical resolution is 5–7 km except in the upper part of the retrieval range (10 km at 0.01 hPa). In the region between 1–0.05 hPa, a mean difference <2 m s−1 is found between SMILES profiles retrieved from different spectroscopic lines and instrumental settings. Good agreement (mean difference of ~2 m s−1) is also found with the European Centre for Medium-Range Weather Forecasts (ECMWF) analysis in most of the stratosphere except for the zonal winds over the equator (mean difference of 5–10 m s−1). In the mesosphere, SMILES and ECMWF zonal winds exhibit large differences (> 20 m s−1), especially in the tropics. We illustrate our results by showing daily and monthly zonal wind variations, namely the semi-annual oscillation in the tropics and reversals of the flow direction between 50° N–55° N during sudden stratospheric warmings in the stratosphere. The daily comparison with ECMWF winds reveals that in the beginning of February, a significantly stronger zonal westward flow is measured in the tropics at 2 hPa compared to the flow computed in the analysis (difference of ~20 m s−1). The results show that the comparison between SMILES and ECMWF winds is not only relevant for the quality assessment of the new SMILES winds but it also provides insights on the quality of the ECMWF winds themselves. Although the instrument was not specifically designed for measuring winds, the results demonstrate that space-borne sub-mm wave radiometers have the potential to provide good quality data for improving the stratospheric winds in atmospheric models.

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  • 23.
    Baró Pérez, Alejandro
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Devasthale, Abhay
    Bender, Frida A.-M.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Ekman, Annica M. L.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Impact of smoke and non-smoke aerosols on radiation and low-level clouds over the southeast Atlantic from co-located satellite observations2021In: Atmospheric Chemistry And Physics, ISSN 1680-7316, E-ISSN 1680-7324, Vol. 21, no 8, p. 6053-6077Article in journal (Refereed)
    Abstract [en]

    Data derived from instruments on board the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) and CloudSat satellites as well as meteorological parameters from reanalysis are used to explore situations when moist aerosol layers overlie stratocumulus clouds over the southeast Atlantic during the biomass burning season (June to October). To separate and quantify the impacts of aerosol loading, aerosol type, and humidity on the radiative fluxes (including cloud top cooling), the data are split into different levels of aerosol and moisture loadings. The aerosol classification available from the CALIPSO products is used to compare and contrast situations with pristine air, with smoke, and with other (non-smoke) types of aerosols. A substantial number of cases with non-smoke aerosols above clouds are found to occur under similar meteorological conditions to the smoke cases. In contrast, the meteorology is substantially different for the pristine situations, making a direct comparison with the aerosol cases ambiguous. The moisture content is enhanced within the aerosol layers, but the relative humidity does not always increase monotonously with increasing optical depth. Shortwave (SW) heating rates within the moist aerosol plumes increase with increasing aerosol loading and are higher in the smoke cases compared to the non-smoke cases. However, there is no clear correlation between moisture changes and SW absorption. Cloud top cooling rates do not show a clear correlation with moisture within the overlying aerosol layers due to the strong variability of the cooling rates caused by other meteorological factors (most notably cloud top temperature). No clear influence of aerosol type or loading on cloud top cooling rates is detected. Further, there is no correlation between aerosol loading and the thermodynamic structure of the atmosphere nor the cloud top height.

  • 24. Beddows, D. C. S.
    et al.
    Dall'Osto, M.
    Harrison, R. M.
    Kulmala, M.
    Asmi, A.
    Wiedensohler, A.
    Laj, P.
    Fjaeraa, A. M.
    Sellegri, K.
    Birmili, W.
    Bukowiecki, N.
    Weingartner, E.
    Baltensperger, U.
    Zdimal, V.
    Zikova, N.
    Putaud, J-P
    Marinoni, A.
    Tunved, Peter
    Stockholm University, Faculty of Science, Department of Applied Environmental Science (ITM).
    Hansson, Hans-Christen
    Stockholm University, Faculty of Science, Department of Applied Environmental Science (ITM).
    Fiebig, M.
    Kivekas, N.
    Swietlicki, E.
    Lihavainen, H.
    Asmi, E.
    Ulevicius, V.
    Aalto, P. P.
    Mihalopoulos, N.
    Kalivitis, N.
    Kalapov, I.
    Kiss, G.
    de Leeuw, G.
    Henzing, B.
    O'Dowd, C.
    Jennings, S. G.
    Flentje, H.
    Meinhardt, F.
    Ries, L.
    van der Gon, H. A. C. Denier
    Visschedijk, A. J. H.
    Variations in tropospheric submicron particle size distributions across the European continent 2008-20092014In: Atmospheric Chemistry And Physics, ISSN 1680-7316, E-ISSN 1680-7324, Vol. 14, no 8, p. 4327-4348Article in journal (Refereed)
    Abstract [en]

    Cluster analysis of particle number size distributions from background sites across Europe is presented. This generated a total of nine clusters of particle size distributions which could be further combined into two main groups, namely: a south-to-north category (four clusters) and a west-to-east category (five clusters). The first group was identified as most frequently being detected inside and around northern Germany and neighbouring countries, showing clear evidence of local afternoon nucleation and growth events that could be linked to movement of air masses from south to north arriving ultimately at the Arctic contributing to Arctic haze. The second group of particle size spectra proved to have narrower size distributions and collectively showed a dependence of modal diameter upon the longitude of the site (west to east) at which they were most frequently detected. These clusters indicated regional nucleation (at the coastal sites) growing to larger modes further inland. The apparent growth rate of the modal diameter was around 0.6-0.9 nm h(-1). Four specific air mass back-trajectories were successively taken as case studies to examine in real time the evolution of aerosol size distributions across Europe. While aerosol growth processes can be observed as aerosol traverses Europe, the processes are often obscured by the addition of aerosol by emissions en route. This study revealed that some of the 24 stations exhibit more complex behaviour than others, especially when impacted by local sources or a variety of different air masses. Overall, the aerosol size distribution clustering analysis greatly simplifies the complex data set and allows a description of aerosol aging processes, which reflects the longer-term average development of particle number size distributions as air masses advect across Europe.

  • 25. Bell, David M.
    et al.
    Wu, Cheng
    Stockholm University, Faculty of Science, Department of Environmental Science.
    Bertrand, Amelie
    Graham, Emelie
    Stockholm University, Faculty of Science, Department of Environmental Science.
    Schoonbaert, Janne
    Giannoukos, Stamatios
    Baltensperger, Urs
    Prevot, Andre S. H.
    Riipinen, Ilona
    Stockholm University, Faculty of Science, Department of Environmental Science.
    El Haddad, Imad
    Mohr, Claudia
    Stockholm University, Faculty of Science, Department of Environmental Science.
    Particle-phase processing of α-pinene NO3 secondary organic aerosol in the dark2022In: Atmospheric Chemistry And Physics, ISSN 1680-7316, E-ISSN 1680-7324, Vol. 22, no 19, p. 13167-13182Article in journal (Refereed)
    Abstract [en]

    The NO3 radical represents a significant night-time oxidant present in or downstream of polluted environments. There are studies that investigated the formation of secondary organic aerosol (SOA) from NO3 radicals focusing on yields, general composition, and hydrolysis of organonitrates. However, there is limited knowledge about how the composition of NO3-derived SOA evolves as a result of particle phase reactions. Here, SOA was formed from the reaction of α-pinene with NO3 radicals generated from N2O5, and the resulting SOA aged in the absence of external stimuli. The initial composition of NO3-derived α-pinene SOA was slightly dependent upon the concentration of N2O5 injected (excess of NO3 or excess of α-pinene), but was largely dominated by dimer dinitrates (C20H32N2O8-13). Oxidation reactions (e.g. C20H32N2O8 → C20H32N2O9 → C20H32N2O10 etc...) accounted for 60–70 % of the particle phase reactions observed. Fragmentation reactions and dimer degradation pathways made up the remainder of the particle-phase processes occurring. The exact oxidant is not known, though suggestions are offered (e.g. N2O5, organic peroxides, or peroxy-nitrates). Hydrolysis of −ONO2 functional groups was not an important loss term during dark aging under the relative humidity conditions of our experiments (58–62 %), and changes in the bulk organonitrate composition were likely driven by evaporation of highly nitrogenated molecules. Overall, 25–30 % of the particle-phase composition changes as a function of particle-phase reactions during dark aging representing an important atmospheric aging pathway.

  • 26. Berchet, Antoine
    et al.
    Pison, Isabelle
    Crill, Patrick
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Thornton, Brett
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Bousquet, Philippe
    Thonat, Thibaud
    Hocking, Thomas
    Thanwerdas, Joël
    Paris, Jean-Daniel
    Saunois, Marielle
    Using ship-borne observations of methane isotopic ratio in the Arctic Ocean to understand methane sources in the Arctic2020In: Atmospheric Chemistry And Physics, ISSN 1680-7316, E-ISSN 1680-7324, Vol. 20, no 6, p. 3987-3998Article in journal (Refereed)
    Abstract [en]

    Characterizing methane sources in the Arctic remains challenging due to the remoteness, heterogeneity and variety of such emissions. In situ campaigns provide valuable datasets to reduce these uncertainties. Here we analyse data from the summer 2014 SWERUS-C3 campaign in the eastern Arctic Ocean, off the shore of Siberia and Alaska. Total concentrations of methane, as well as relative concentrations of (CH4)-C-12 and (CH4)-C-13, were measured continuously during this campaign for 35 d in July and August. Using a chemistry-transport model, we link observed concentrations and isotopic ratios to regional emissions and hemispheric transport structures. A simple inversion system helped constrain source signatures from wetlands in Siberia and Alaska, and oceanic sources, as well as the isotopic composition of lower-stratosphere air masses. The variation in the signature of lower-stratosphere air masses, due to strongly fractionating chemical reactions in the stratosphere, was suggested to explain a large share of the observed variability in isotopic ratios. These results point towards necessary efforts to better simulate large-scale transport and chemistry patterns to make relevant use of isotopic data in remote areas. It is also found that constant and homogeneous source signatures for each type of emission in a given region (mostly wetlands and oil and gas industry in our case at high latitudes) are not compatible with the strong synoptic isotopic signal observed in the Arctic. A regional gradient in source signatures is highlighted between Siberian and Alaskan wetlands, the latter having lighter signatures (more depleted in C-13). Finally, our results suggest that marine emissions of methane from Arctic continental-shelf sources are dominated by thermogenicorigin methane, with a secondary biogenic source as well.

  • 27. Berthet, Gwenaël
    et al.
    Jégou, Fabrice
    Catoire, Valéry
    Krysztofiak, Gisèle
    Renard, Jean-Baptiste
    Bourassa, Adam E.
    Degenstein, Doug A.
    Brogniez, Colette
    Dorf, Marcel
    Kreycy, Sebastian
    Pfeilsticker, Klaus
    Werner, Bodo
    Lefèvre, Franck
    Roberts, Tjarda J.
    Lurton, Thibaut
    Vignelles, Damien
    Bègue, Nelson
    Bourgeois, Quentin
    Stockholm University, Faculty of Science, Department of Meteorology .
    Daugeron, Daniel
    Chartier, Michel
    Robert, Claude
    Gaubicher, Bertrand
    Guimbaud, Christophe
    Impact of a moderate volcanic eruption on chemistry in the lower stratosphere: balloon-borne observations and model calculations2017In: Atmospheric Chemistry And Physics, ISSN 1680-7316, E-ISSN 1680-7324, Vol. 17, no 3, p. 2229-2253Article in journal (Refereed)
    Abstract [en]

    The major volcanic eruption of Mount Pinatubo in 1991 has been shown to have significant effects on stratospheric chemistry and ozone depletion even at midlatitudes. Since then, only moderate but recurrent volcanic eruptions have modulated the stratospheric aerosol loading and are assumed to be one cause for the reported increase in the global aerosol content over the past 15 years. This particularly enhanced aerosol context raises questions about the effects on stratospheric chemistry which depend on the latitude, altitude and season of injection. In this study, we focus on the midlatitude Sarychev volcano eruption in June 2009, which injected 0.9 Tg of sulfur dioxide (about 20 times less than Pinatubo) into a lower stratosphere mainly governed by high-stratospheric temperatures. Together with in situ measurements of aerosol amounts, we analyse high-resolution in situ and/or remote-sensing observations of NO2, HNO3 and BrO from balloon-borne infrared and UV-visible spectrometers launched in Sweden in August-September 2009. It is shown that differences between observations and three-dimensional (3-D) chemistry-transport model (CTM) outputs are not due to transport calculation issues but rather reflect the chemical impact of the volcanic plume below 19 km altitude. Good measurement-model agreement is obtained when the CTM is driven by volcanic aerosol loadings derived from in situ or space-borne data. As a result of enhanced N2O5 hydrolysis in the Sarychev volcanic aerosol conditions, the model calculates reductions of similar to 45% and increases of similar to 11% in NO2 and HNO3 amounts respectively over the August-September 2009 period. The decrease in NOx abundances is limited due to the expected saturation effect for high aerosol loadings. The links between the various chemical catalytic cycles involving chlorine, bromine, nitrogen and HOx compounds in the lower stratosphere are discussed. The increased BrO amounts (similar to 22 %) compare rather well with the balloon-borne observations when volcanic aerosol levels are accounted for in the CTM and appear to be mainly controlled by the coupling with nitrogen chemistry rather than by enhanced BrONO2 hydrolysis. We show that the chlorine partitioning is significantly controlled by enhanced BrONO2 hydrolysis. However, simulated effects of the Sarychev eruption on chlorine activation are very limited in the high-temperature conditions in the stratosphere in the period considered, inhibiting the effect of ClONO2 hydrolysis. As a consequence, the simulated chemical ozone loss due to the Sarychev aerosols is low with a reduction of -22 ppbv (-1.5 %) of the ozone budget around 16 km. This is at least 10 times lower than the maximum ozone depletion from chemical processes (up to -20 %) reported in the Northern Hemisphere lower stratosphere over the first year following the Pinatubo eruption. This study suggests that moderate volcanic eruptions have limited chemical effects when occurring at midlatitudes (restricted residence times) and outside winter periods (high-temperature conditions). However, it would be of interest to investigate longer-lasting tropical volcanic plumes or sulfur injections in the wintertime low-temperature conditions.

  • 28. Birch, C. E.
    et al.
    Brooks, I. M.
    Tjernström, Michael
    Stockholm University, Faculty of Science, Department of Meteorology .
    Shupe, M. D.
    Mauritsen, T.
    Sedlar, J.
    Lock, A. P.
    Earnshaw, P.
    Persson, P. O. G.
    Milton, S. F.
    Leck, Caroline
    Stockholm University, Faculty of Science, Department of Meteorology .
    Modelling atmospheric structure, cloud and their response to CCN in the central Arctic: ASCOS case studies2012In: Atmospheric Chemistry And Physics, ISSN 1680-7316, E-ISSN 1680-7324, Vol. 12, no 7, p. 3419-3435Article in journal (Refereed)
    Abstract [en]

    Observations made during late summer in the central Arctic Ocean, as part of the Arctic Summer Cloud Ocean Study (ASCOS), are used to evaluate cloud and vertical temperature structure in the Met Office Unified Model (MetUM). The observation period can be split into 5 regimes; the first two regimes had a large number of frontal systems, which were associated with deep cloud. During the remainder of the campaign a layer of low-level cloud occurred, typical of central Arctic summer conditions, along with two periods of greatly reduced cloud cover. The short-range operational NWP forecasts could not accurately reproduce the observed variations in near-surface temperature. A major source of this error was found to be the temperature-dependant surface albedo parameterisation scheme. The model reproduced the low-level cloud layer, though it was too thin, too shallow, and in a boundary-layer that was too frequently well-mixed. The model was also unable to reproduce the observed periods of reduced cloud cover, which were associated with very low cloud condensation nuclei (CCN) concentrations (< 1 cm(-3)). As with most global NWP models, the MetUM does not have a prognostic aerosol/cloud scheme but uses a constant CCN concentration of 100 cm(-3) over all marine environments. It is therefore unable to represent the low CCN number concentrations and the rapid variations in concentration frequently observed in the central Arctic during late summer. Experiments with a single-column model configuration of the MetUM show that reducing model CCN number concentrations to observed values reduces the amount of cloud, increases the near-surface stability, and improves the representation of both the surface radiation fluxes and the surface temperature. The model is shown to be sensitive to CCN only when number concentrations are less than 10-20 cm(-3).

  • 29.
    Bolinius, Damien Johann
    et al.
    Stockholm University, Faculty of Science, Department of Environmental Science and Analytical Chemistry.
    Jahnke, Annika
    Stockholm University, Faculty of Science, Department of Environmental Science and Analytical Chemistry. Helmholtz Centre for Environmental Research (UFZ), Germany.
    MacLeod, Matthew
    Stockholm University, Faculty of Science, Department of Environmental Science and Analytical Chemistry.
    Comparison of eddy covariance and modified Bowen ratio methods for measuring gas fluxes and implications for measuring fluxes of persistent organic pollutants2016In: Atmospheric Chemistry And Physics, ISSN 1680-7316, E-ISSN 1680-7324, Vol. 16, no 8, p. 5315-5322Article in journal (Refereed)
    Abstract [en]

    Semi-volatile persistent organic pollutants (POPs) cycle between the atmosphere and terrestrial surfaces; however measuring fluxes of POPs between the atmosphere and other media is challenging. Sampling times of hours to days are required to accurately measure trace concentrations of POPs in the atmosphere, which rules out the use of eddy covariance techniques that are used to measure gas fluxes of major air pollutants. An alternative, the modified Bowen ratio (MBR) method, has been used instead. In this study we used data from FLUXNET for CO2 and water vapor (H2O) to compare fluxes measured by eddy covariance to fluxes measured with the MBR method using vertical concentration gradients in air derived from averaged data that simulate the long sampling times typically required to measure POPs. When concentration gradients are strong and fluxes are unidirectional, the MBR method and the eddy covariance method agree within a factor of 3 for CO2, and within a factor of 10 for H2O. To remain within the range of applicability of the MBR method, field studies should be carried out under conditions such that the direction of net flux does not change during the sampling period. If that condition is met, then the performance of the MBR method is neither strongly affected by the length of sample duration nor the use of a fixed value for the transfer coefficient.

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  • 30.
    Bourgeois, Quentin
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Ekman, Annica M. L.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Renard, Jean-Baptiste
    Krejci, Radovan
    Stockholm University, Faculty of Science, Department of Environmental Science and Analytical Chemistry.
    Devasthale, Abhay
    Bender, Frida A. -M.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Riipinen, Ilona
    Stockholm University, Faculty of Science, Department of Environmental Science and Analytical Chemistry.
    Berthet, Gwenaël
    Tackett, Jason L.
    How much of the global aerosol optical depth is found in the boundary layer and free troposphere?2018In: Atmospheric Chemistry And Physics, ISSN 1680-7316, E-ISSN 1680-7324, Vol. 18, no 10, p. 7709-7720Article in journal (Refereed)
    Abstract [en]

    The global aerosol extinction from the CALIOP space lidar was used to compute aerosol optical depth (AOD) over a 9-year period (2007-2015) and partitioned between the boundary layer (BL) and the free troposphere (FT) using BL heights obtained from the ERA-Interim archive. The results show that the vertical distribution of AOD does not follow the diurnal cycle of the BL but remains similar between day and night highlighting the presence of a residual layer during night. The BL and FT contribute 69 and 31 %, respectively, to the global tropospheric AOD during daytime in line with observations obtained in Aire sur l'Adour (France) using the Light Optical Aerosol Counter (LOAC) instrument. The FT AOD contribution is larger in the tropics than at mid-latitudes which indicates that convective transport largely controls the vertical profile of aerosols. Over oceans, the FT AOD contribution is mainly governed by long-range transport of aerosols from emission sources located within neighboring continents. According to the CALIOP aerosol classification, dust and smoke particles are the main aerosol types transported into the FT. Overall, the study shows that the fraction of AOD in the FT - and thus potentially located above low-level clouds - is substantial and deserves more attention when evaluating the radiative effect of aerosols in climate models. More generally, the results have implications for processes determining the overall budgets, sources, sinks and transport of aerosol particles and their description in atmospheric models.

  • 31. Boy, Michael
    et al.
    Thomson, Erik S.
    Acosta Navarro, Juan-C.
    Arnalds, Olafur
    Batchvarova, Ekaterina
    Back, Jaana
    Berninger, Frank
    Bilde, Merete
    Brasseur, Zoe
    Dagsson-Waldhauserova, Pavla
    Castarede, Dimitri
    Dalirian, Maryam
    Stockholm University, Faculty of Science, Department of Environmental Science and Analytical Chemistry.
    de Leeuw, Gerrit
    Dragosics, Monika
    Duplissy, Ella-Maria
    Duplissy, Jonathan
    Ekman, Annica M. L.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Fang, Keyan
    Gallet, Jean-Charles
    Glasius, Marianne
    Gryning, Sven-Erik
    Grythe, Henrik
    Stockholm University, Faculty of Science, Department of Environmental Science and Analytical Chemistry. NILU–Norwegian Institute for Air Research, Norway.
    Hansson, Hans-Christen
    Stockholm University, Faculty of Science, Department of Environmental Science and Analytical Chemistry.
    Hansson, Margareta
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Isaksson, Elisabeth
    Iversen, Trond
    Jonsdottir, Ingibjorg
    Kasurinen, Ville
    Kirkevag, Alf
    Korhola, Atte
    Krejci, Radovan
    Stockholm University, Faculty of Science, Department of Environmental Science and Analytical Chemistry.
    Kristjansson, Jon Egill
    Lappalainen, Hanna K.
    Lauri, Antti
    Lepparanta, Matti
    Lihavainen, Heikki
    Makkonen, Risto
    Massling, Andreas
    Meinander, Outi
    Nilsson, E. Douglas
    Stockholm University, Faculty of Science, Department of Environmental Science and Analytical Chemistry.
    Olafsson, Haraldur
    Pettersson, Jan B. C.
    Prisle, Nonne L.
    Riipinen, Ilona
    Stockholm University, Faculty of Science, Department of Environmental Science and Analytical Chemistry.
    Roldin, Pontus
    Ruppel, Meri
    Salter, Matthew
    Stockholm University, Faculty of Science, Department of Environmental Science and Analytical Chemistry.
    Sand, Maria
    Seland, Oyvind
    Seppa, Heikki
    Skov, Henrik
    Soares, Joana
    Stohl, Andreas
    Ström, Johan
    Stockholm University, Faculty of Science, Department of Environmental Science and Analytical Chemistry.
    Svensson, Jonas
    Swietlicki, Erik
    Tabakova, Ksenia
    Thorsteinsson, Throstur
    Virkkula, Aki
    Weyhenmeyer, Gesa A.
    Wu, Yusheng
    Zieger, Paul
    Stockholm University, Faculty of Science, Department of Environmental Science and Analytical Chemistry.
    Kulmala, Markku
    Interactions between the atmosphere, cryosphere, and ecosystems at northern high latitudes2019In: Atmospheric Chemistry And Physics, ISSN 1680-7316, E-ISSN 1680-7324, Vol. 19, no 3, p. 2015-2061Article in journal (Refereed)
    Abstract [en]

    The Nordic Centre of Excellence CRAICC (Cryosphere-Atmosphere Interactions in a Changing Arctic Climate), funded by NordForsk in the years 2011-2016, is the largest joint Nordic research and innovation initiative to date, aiming to strengthen research and innovation regarding climate change issues in the Nordic region. CRAICC gathered more than 100 scientists from all Nordic countries in a virtual centre with the objectives of identifying and quantifying the major processes controlling Arctic warming and related feedback mechanisms, outlining strategies to mitigate Arctic warming, and developing Nordic Earth system modelling with a focus on short-lived climate forcers (SLCFs), including natural and anthropogenic aerosols. The outcome of CRAICC is reflected in more than 150 peer-reviewed scientific publications, most of which are in the CRAICC special issue of the journal Atmospheric Chemistry and Physics. This paper presents an overview of the main scientific topics investigated in the centre and provides the reader with a state-of-the-art comprehensive summary of what has been achieved in CRAICC with links to the particular publications for further detail. Faced with a vast amount of scientific discovery, we do not claim to completely summarize the results from CRAICC within this paper, but rather concentrate here on the main results which are related to feedback loops in climate change-cryosphere interactions that affect Arctic amplification.

  • 32. Boyer, Matthew
    et al.
    Aliaga, Diego
    Pernov, Jakob Boyd
    Angot, Hélène
    Quéléver, Lauriane L. J.
    Dada, Lubna
    Heutte, Benjamin
    Dall'Osto, Manuel
    Beddows, David C. S.
    Brasseur, Zoé
    Beck, Ivo
    Bucci, Silvia
    Duetsch, Marina
    Stohl, Andreas
    Laurila, Tiia
    Asmi, Eija
    Massling, Andreas
    Thomas, Daniel Charles
    Klenø Nøjgaard, Jakob
    Chan, Tak
    Sharma, Sangeeta
    Tunved, Peter
    Stockholm University, Faculty of Science, Department of Environmental Science.
    Krejci, Radovan
    Stockholm University, Faculty of Science, Department of Environmental Science.
    Hansson, Hans-Christen
    Stockholm University, Faculty of Science, Department of Environmental Science.
    Bianchi, Federico
    Lehtipalo, Katrianne
    Wiedensohler, Alfred
    Weinhold, Kay
    Kulmala, Markku
    Petäjä, Tuukka
    Sipilä, Mikko
    Schmale, Julia
    Jokinen, Tuija
    A full year of aerosol size distribution data from the central Arctic under an extreme positive Arctic Oscillation: insights from the Multidisciplinarydrifting Observatory for the Study of Arctic Climate (MOSAiC) expedition2023In: Atmospheric Chemistry And Physics, ISSN 1680-7316, E-ISSN 1680-7324, Vol. 23, no 1, p. 389-415Article in journal (Refereed)
    Abstract [en]

    The Arctic environment is rapidly changing due to accelerated warming in the region. The warming trend is driving a decline in sea ice extent, which thereby enhances feedback loops in the surface energy budget in the Arctic. Arctic aerosols play an important role in the radiative balance and hence the climate response in the region, yet direct observations of aerosols over the Arctic Ocean are limited. In this study, we investigate the annual cycle in the aerosol particle number size distribution (PNSD), particle number concentration (PNC), and black carbon (BC) mass concentration in the central Arctic during the Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) expedition. This is the first continuous, year-long data set of aerosol PNSD ever collected over the sea ice in the central Arctic Ocean. We use a k-means cluster analysis, FLEXPART simulations, and inverse modeling to evaluate seasonal patterns and the influence of different source regions on the Arctic aerosol population. Furthermore, we compare the aerosol observations to land-based sites across the Arctic, using both long-term measurements and observations during the year of the MOSAiC expedition (2019–2020), to investigate interannual variability and to give context to the aerosol characteristics from within the central Arctic. Our analysis identifies that, overall, the central Arctic exhibits typical seasonal patterns of aerosols, including anthropogenic influence from Arctic haze in winter and secondary aerosol processes in summer. The seasonal pattern corresponds to the global radiation, surface air temperature, and timing of sea ice melting/freezing, which drive changes in transport patterns and secondary aerosol processes. In winter, the Norilsk region in Russia/Siberia was the dominant source of Arctic haze signals in the PNSD and BC observations, which contributed to higher accumulation-mode PNC and BC mass concentrations in the central Arctic than at land-based observatories. We also show that the wintertime Arctic Oscillation (AO) phenomenon, which was reported to achieve a record-breaking positive phase during January–March 2020, explains the unusual timing and magnitude of Arctic haze across the Arctic region compared to longer-term observations. In summer, the aerosol PNCs of the nucleation and Aitken modes are enhanced; however, concentrations were notably lower in the central Arctic over the ice pack than at land-based sites further south. The analysis presented herein provides a current snapshot of Arctic aerosol processes in an environment that is characterized by rapid changes, which will be crucial for improving climate model predictions, understanding linkages between different environmental processes, and investigating the impacts of climate change in future Arctic aerosol studies.

  • 33. Brean, James
    et al.
    Beddows, David C. S.
    Harrison, Roy M.
    Song, Congbo
    Tunved, Peter
    Stockholm University, Faculty of Science, Department of Environmental Science. Stockholm University, Faculty of Science, The Bolin Centre for Climate Research (together with KTH & SMHI).
    Ström, Johan
    Stockholm University, Faculty of Science, Department of Environmental Science. Stockholm University, Faculty of Science, The Bolin Centre for Climate Research (together with KTH & SMHI).
    Krejci, Radovan
    Stockholm University, Faculty of Science, Department of Environmental Science. Stockholm University, Faculty of Science, The Bolin Centre for Climate Research (together with KTH & SMHI).
    Freud, Eyal
    Stockholm University, Faculty of Science, Department of Environmental Science. Stockholm University, Faculty of Science, The Bolin Centre for Climate Research (together with KTH & SMHI).
    Massling, Andreas
    Skov, Henrik
    Asmi, Eija
    Lupi, Angelo
    Dall'Osto, Manuel
    Collective geographical ecoregions and precursor sources driving Arctic new particle formation2023In: Atmospheric Chemistry And Physics, ISSN 1680-7316, E-ISSN 1680-7324, Vol. 23, no 3, p. 2183-2198Article in journal (Refereed)
    Abstract [en]

    The Arctic is a rapidly changing ecosystem, with complex ice–ocean–atmosphere feedbacks. An important process is new particle formation (NPF), from gas-phase precursors, which provides a climate forcing effect. NPF has been studied comprehensively at different sites in the Arctic, ranging from those in the High Arctic and those at Svalbard to those in the continental Arctic, but no harmonised analysis has been performed on all sites simultaneously, with no calculations of key NPF parameters available for some sites. Here, we analyse the formation and growth of new particles from six long-term ground-based stations in the Arctic (Alert, Villum, Tiksi, Zeppelin Mountain, Gruvebadet, and Utqiaġvik). Our analysis of particle formation and growth rates in addition to back-trajectory analysis shows a summertime maxima in the frequency of NPF and particle formation rate at all sites, although the mean frequency and particle formation rates themselves vary greatly between sites, with the highest at Svalbard and lowest in the High Arctic. The summertime growth rate, condensational sinks, and vapour source rates show a slight bias towards the southernmost sites, with vapour source rates varying by around an order of magnitude between the northernmost and southernmost sites. Air masses back-trajectories during NPF at these northernmost sites are associated with large areas of sea ice and snow, whereas events at Svalbard are associated with more sea ice and ocean regions. Events at the southernmost sites are associated with large areas of land and sea ice. These results emphasise how understanding the geographical variation in surface type across the Arctic is key to understanding secondary aerosol sources and providing a harmonised analysis of NPF across the Arctic.

  • 34.
    Broman, Lina
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Benze, Susanne
    Stockholm University, Faculty of Science, Department of Meteorology .
    Gumbel, Jörg
    Stockholm University, Faculty of Science, Department of Meteorology .
    Christensen, Ole Martin
    Stockholm University, Faculty of Science, Department of Meteorology .
    Randall, Cora E.
    Common volume satellite studies of polar mesospheric clouds with Odin/OSIRIS tomography and AIM/CIPS nadir imaging2019In: Atmospheric Chemistry And Physics, ISSN 1680-7316, E-ISSN 1680-7324, Vol. 19, no 19, p. 12455-12475Article in journal (Refereed)
    Abstract [en]

    Two important approaches for satellite studies of polar mesospheric clouds (PMCs) are nadir measurements adapting phase function analysis and limb measurements adapting spectroscopic analysis. Combining both approaches enables new studies of cloud structures and microphysical processes but is complicated by differences in scattering conditions, observation geometry and sensitivity. In this study, we compare common volume PMC observations from the nadir-viewing Cloud Imaging and Particle Size (CIPS) instrument on the Aeronomy of Ice in the Mesosphere (AIM) satellite and a special set of tomographic limb observations from the Optical Spectrograph and InfraRed Imager System (OSIRIS) on the Odin satellite performed over 18 d for the years 2010 and 2011 and the latitude range 78 to 80 degrees N. While CIPS provides preeminent horizontal resolution, the OSIRIS tomographic analysis provides combined horizontal and vertical PMC information. This first direct comparison is an important step towards co-analysing CIPS and OSIRIS data, aiming at unprecedented insights into horizontal and vertical cloud processes. Important scientific questions on how the PMC life cycle is affected by changes in humidity and temperature due to atmospheric gravity waves, planetary waves and tides can be addressed by combining PMC observations in multiple dimensions. Two- and three-dimensional cloud structures simultaneously observed by CIPS and tomographic OSIRIS provide a useful tool for studies of cloud growth and sublimation Moreover, the combined CIPS/tomographic OSIRIS dataset can be used for studies of even more fundamental character, such as the question of the assumption of the PMC particle size distribution. We perform the first thorough error characterization of OSIRIS tomographic cloud brightness and cloud ice water content (IWC). We establish a consistent method for comparing cloud properties from limb tomography and nadir observations, accounting for differences in scattering conditions, resolution and sensitivity. Based on an extensive common volume and a temporal coincidence criterion of only 5 min, our method enables a detailed comparison of PMC regions of varying brightness and IWC. However, since the dataset is limited to 18 d of observations this study does not include a comparison of cloud frequency. The cloud properties of the OSIRIS tomographic dataset are vertically resolved, while the cloud properties of the CIPS dataset is vertically integrated. To make these different quantities comparable, the OSIRIS tomographic cloud properties cloud scattering coefficient and ice mass density (IMD) have been integrated over the vertical extent of the cloud to form cloud albedo and IWC of the same quantity as CIPS cloud products. We find that the OSIRIS albedo (obtained from the vertical integration of the primary OSIRIS tomography product, cloud scattering coefficient) shows very good agreement with the primary CIPS product, cloud albedo, with a correlation coefficient of 0.96. However, OSIRIS systematically reports brighter clouds than CIPS and the bias between the instruments (OSIRIS -CIPS) is 3.4 x 10(-6) sr(-1) (+/- 2.9x 10(-6) sr(-1)) on average. The OSIRIS tomography IWC (obtained from the vertical integration of IMD) agrees well with the CIPS IWC, with a correlation coefficient of 0.91. However, the IWC reported by OSIRIS is lower than CIPS, and we quantify the bias to -22 g km(-2) (+/- 14 g km(-2)) on average.

  • 35. Browse, J.
    et al.
    Carslaw, K. S.
    Mann, G. W.
    Birch, C. E.
    Arnold, S. R.
    Leck, Caroline
    Stockholm University, Faculty of Science, Department of Meteorology .
    The complex response of Arctic aerosol to sea-ice retreat2014In: Atmospheric Chemistry And Physics, ISSN 1680-7316, E-ISSN 1680-7324, Vol. 14, no 14, p. 7543-7557Article in journal (Refereed)
    Abstract [en]

    Loss of summertime Arctic sea ice will lead to a large increase in the emission of aerosols and precursor gases from the ocean surface. It has been suggested that these enhanced emissions will exert substantial aerosol radiative forcings, dominated by the indirect effect of aerosol on clouds. Here, we investigate the potential for these indirect forcings using a global aerosol microphysics model evaluated against aerosol observations from the Arctic Summer Cloud Ocean Study (ASCOS) campaign to examine the response of Arctic cloud condensation nuclei (CCN) to sea-ice retreat. In response to a complete loss of summer ice, we find that north of 70 degrees N emission fluxes of sea salt, marine primary organic aerosol (OA) and dimethyl sulfide increase by a factor of similar to 10, similar to 4 and similar to 15 respectively. However, the CCN response is weak, with negative changes over the central Arctic Ocean. The weak response is due to the efficient scavenging of aerosol by extensive drizzling stratocumulus clouds. In the scavenging-dominated Arctic environment, the production of condensable vapour from oxidation of dimethyl sulfide grows particles to sizes where they can be scavenged. This loss is not sufficiently compensated by new particle formation, due to the suppression of nucleation by the large condensation sink resulting from sea-salt and primary OA emissions. Thus, our results suggest that increased aerosol emissions will not cause a climate feedback through changes in cloud microphysical and radiative properties.

  • 36. Buchholz, Angela
    et al.
    Lambe, Andrew T.
    Ylisirniö, Arttu
    Li, Zijun
    Tikkanen, Olli-Pekka
    Faiola, Celia
    Kari, Eetu
    Hao, Liqing
    Luoma, Olli
    Huang, Wei
    Mohr, Claudia
    Stockholm University, Faculty of Science, Department of Environmental Science and Analytical Chemistry. Karlsruhe Institute of Technology, Germany.
    Worsnop, Douglas R.
    Nizkorodov, Sergey A.
    Yli-Juuti, Taina
    Schobesberger, Siegfried
    Virtanen, Annele
    Insights into the O: C-dependent mechanisms controlling the evaporation of alpha-pinene secondary organic aerosol particles2019In: Atmospheric Chemistry And Physics, ISSN 1680-7316, E-ISSN 1680-7324, Vol. 19, no 6, p. 4061-4073Article in journal (Refereed)
    Abstract [en]

    The volatility of oxidation products of volatile organic compounds (VOCs) in the atmosphere is a key factor to determine if they partition into the particle phase contributing to secondary organic aerosol (SOA) mass. Thus, linking volatility and measured particle composition will provide insights into SOA formation and its fate in the atmosphere. We produced alpha-pinene SOA with three different oxidation levels (characterized by average oxygen-to-carbon ratio; (O:C) over bar = 0.53, 0.69, and 0.96) in an oxidation flow reactor. We investigated the particle volatility by isothermal evaporation in clean air as a function of relative humidity (RH < 2 %, 40 %, and 80 %) and used a filter-based thermal desorption method to gain volatility and chemical composition information. We observed reduced particle evaporation for particles with increasing <(O:C )over bar> ratio, indicating that particles become more resilient to evaporation with oxidative aging. Particle evaporation was increased in the presence of water vapour and presumably particulate water; at the same time the resistance of the residual particles to thermal desorption was increased as well. For SOA with (O:C ) over bar = 0.96, the unexpectedly large increase in mean thermal desorption temperature and changes in the thermogram shapes under wet conditions (80 % RH) were an indication of aqueous phase chemistry. For the lower (O:C ) over bar cases, some water-induced composition changes were observed. However, the enhanced evaporation under wet conditions could be explained by the reduction in particle viscosity from the semi-solid to liquid-like range, and the observed higher desorption temperature of the residual particles is a direct consequence of the increased removal of high-volatility and the continued presence of low-volatility compounds.

  • 37. Buchholz, Angela
    et al.
    Ylisirniö, Arttu
    Huang, Wei
    Mohr, Claudia
    Stockholm University, Faculty of Science, Department of Environmental Science and Analytical Chemistry.
    Canagaratna, Manjula
    Worsnop, Douglas
    Schobesberger, Siegfried
    Virtanen, Annele
    Deconvolution of FIGAERO-CIMS thermal desorption profiles using positive matrix factorisation to identify chemical and physical processes during particle evaporation2020In: Atmospheric Chemistry And Physics, ISSN 1680-7316, E-ISSN 1680-7324, Vol. 20, no 13, p. 7693-7716Article in journal (Refereed)
    Abstract [en]

    The measurements of aerosol particles with a filter inlet for gases and aerosols (FIGAERO) together with a chemical ionisation mass spectrometer (CIMS) yield the overall chemical composition of the particle phase. In addition, the thermal desorption profiles obtained for each detected ion composition contain information about the volatility of the detected compounds, which is an important property for understanding many physical properties like gas-particle partitioning. We coupled this thermal desorption method with isothermal evaporation prior to the sample collection to investigate the chemical composition changes during isothermal particle evaporation and particulate-water-driven chemical reactions in alpha-pinene secondary organic aerosol (SOA) of three different oxidative states. The thermal desorption profiles of all detected elemental compositions were then analysed with positive matrix factorisation (PMF) to identify the drivers of the chemical composition changes observed during isothermal evaporation. The keys to this analysis were to use the error matrix as a tool to weight the parts of the data carrying most information (i.e. the peak area of each thermogram) and to run PMF on a combined data set of multiple thermograms from different experiments to enable a direct comparison of the individual factors between separate measurements. The PMF was able to identify instrument background factors and separate them from the part of the data containing particle desorption information. Additionally, PMF allowed us to separate the direct desorption of compounds detected at a specific elemental composition from other signals with the same composition that stem from the thermal decomposition of thermally instable compounds with lower volatility. For each SOA type, 7-9 factors were needed to explain the observed thermogram behaviour. The contribution of the factors depended on the prior isothermal evaporation. Decreased contributions from the factors with the lowest desorption temperatures were observed with increasing isothermal evaporation time. Thus, the factors identified by PMF could be interpreted as volatility classes. The composition changes in the particles due to isothermal evaporation could be attributed to the removal of volatile factors with very little change in the desorption profiles of the individual factors (i.e. in the respective temperatures of peak desorption, T-max). When aqueous-phase reactions took place, PMF was able to identify a new factor that directly identified the ions affected by the chemical processes. We conducted a PMF analysis of the FIGAERO-CIMS thermal desorption data for the first time using laboratory-generated SOA particles. But this method can be applied to, for example, ambient FIGAERO-CIMS measurements as well. There, the PMF analysis of the thermal desorption data identifies organic aerosol (OA) sources (such as biomass burning or oxidation of different precursors) and types, e.g. hydrocarbon-like (HOA) or oxygenated organic aerosol (OOA). This information could also be obtained with the traditional approach, namely the PMF analysis of the mass spectra data integrated for each thermogram. But only our method can also obtain the volatility information for each OA source and type. Additionally, we can identify the contribution of thermal decomposition to the overall signal.

  • 38.
    Bulatovic, Ines
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Igel, Adele L.
    Leck, Caroline
    Stockholm University, Faculty of Science, Department of Meteorology .
    Heintzenberg, Jost
    Stockholm University, Faculty of Science, Department of Meteorology . Leibniz Institute for Tropospheric Research, Germany.
    Riipinen, Ilona
    Stockholm University, Faculty of Science, Department of Environmental Science.
    Ekman, Annica M. L.
    Stockholm University, Faculty of Science, Department of Meteorology .
    The importance of Aitken mode aerosol particles for cloud sustenance in the summertime high Arctic - a simulation study supported by observational data2021In: Atmospheric Chemistry And Physics, ISSN 1680-7316, E-ISSN 1680-7324, Vol. 21, no 5, p. 3871-3897Article in journal (Refereed)
    Abstract [en]

    The potential importance of Aitken mode particles (diameters similar to 25-80 nm) for stratiform mixed-phase clouds in the summertime high Arctic (> 80 degrees N) has been investigated using two large-eddy simulation models. We find that, in both models, Aitken mode particles significantly affect the simulated microphysical and radiative properties of the cloud and can help sustain the cloud when accumulation mode concentrations are low (< 10-20 cm(-3)), even when the particles have low hygroscopicity (hygroscopicity parameter - kappa = 0.1). However, the influence of the Aitken mode decreases if the overall liquid water content of the cloud is low, either due to a higher ice fraction or due to low radiative cooling rates. An analysis of the simulated supersaturation (ss) statistics shows that the ss frequently reaches 0.5 % and sometimes even exceeds 1 %, which confirms that Aitken mode particles can be activated. The modelling results are in qualitative agreement with observations of the Hoppel minimum obtained from four different expeditions in the high Arctic. Our findings highlight the importance of better understanding Aitken mode particle formation, chemical properties and emissions, particularly in clean environments such as the high Arctic.

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  • 39.
    Bulatovic, Ines
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Savre, Julien
    Tjernström, Michael
    Stockholm University, Faculty of Science, Department of Meteorology .
    Leck, Caroline
    Stockholm University, Faculty of Science, Department of Meteorology .
    Ekman, Annica M. L.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Large-eddy simulation of a two-layer boundary-layer cloud system from the Arctic Ocean 2018 expedition2023In: Atmospheric Chemistry And Physics, ISSN 1680-7316, E-ISSN 1680-7324, Vol. 23, no 12, p. 7033-7055Article in journal (Refereed)
    Abstract [en]

    Climate change is particularly noticeable in the Arctic. The most common type of cloud at these latitudes is mixed-phase stratocumulus. These clouds occur frequently and persistently during all seasons and play a critical role in the Arctic energy budget. Previous observations in the central (north of 80 N) Arctic have shown a high occurrence of prolonged periods of a shallow, single-layer mixed-phase stratocumulus at the top of the boundary layer (BL; altitudes ∼ 300 to 400 m). However, recent observations from the summer of 2018 instead showed a prevalence of a two-layer boundary-layer cloud system. Here we use large-eddy simulation to examine the maintenance of one of the cloud systems observed in the summer of 2018 and the sensitivity of the cloud layers to different micro- and macro-scale parameters. We find that the model generally reproduces the observed thermodynamic structure well, with two near-neutrally stratified layers in the BL caused by a low cloud (located within the first few hundred meters) capped by a lower-altitude temperature inversion and an upper cloud layer (based around one kilometer or slightly higher) capped by the main temperature inversion of the BL. The simulated cloud structure is persistent unless there are low aerosol number concentrations (≤ 5 cm−3), which cause the upper cloud layer to dissipate, or high large-scale wind speeds (≥ 8.5 m s−1), which erode the lower inversion and the related cloud layer. The changes in cloud structure alter both the short- and longwave cloud radiative effect at the surface. This results in changes in the net radiative effect of the modeled cloud system, which can impact the surface melting or freezing. The findings highlight the importance of better understanding and representing aerosol sources and sinks over the central Arctic Ocean. Furthermore, they underline the significance of meteorological parameters, such as the large-scale wind speed, for maintaining the two-layer boundary-layer cloud structure encountered in the lower atmosphere of the central Arctic.

  • 40.
    Burgos, Maria A.
    et al.
    Stockholm University, Faculty of Science, Department of Environmental Science.
    Andrews, Elisabeth
    Titos, Gloria
    Benedetti, Angela
    Bian, Huisheng
    Buchard, Virginie
    Curci, Gabriele
    Kipling, Zak
    Kirkevag, Alf
    Kokkola, Harri
    Laakso, Anton
    Letertre-Danczak, Julie
    Lund, Marianne T.
    Matsui, Hitoshi
    Myhre, Gunnar
    Randles, Cynthia
    Schulz, Michael
    van Noije, Twan
    Zhang, Kai
    Alados-Arboledas, Lucas
    Baltensperger, Urs
    Jefferson, Anne
    Sherman, James
    Sun, Junying
    Weingartner, Ernest
    Zieger, Paul
    Stockholm University, Faculty of Science, Department of Environmental Science.
    A global model-measurement evaluation of particle light scattering coefficients at elevated relative humidity2020In: Atmospheric Chemistry And Physics, ISSN 1680-7316, E-ISSN 1680-7324, Vol. 20, no 17, p. 10231-10258Article in journal (Refereed)
    Abstract [en]

    The uptake of water by atmospheric aerosols has a pronounced effect on particle light scattering properties, which in turn are strongly dependent on the ambient relative humidity (RH). Earth system models need to account for the aerosol water uptake and its influence on light scattering in order to properly capture the overall radiative effects of aerosols. Here we present a comprehensive model-measurement evaluation of the particle light scattering enhancement factor f (RH), defined as the particle light scattering coefficient at elevated RH (here set to 85 %) divided by its dry value. The comparison uses simulations from 10 Earth system models and a global dataset of surface-based in situ measurements. In general, we find a large diversity in the magnitude of predicted f (RH) amongst the different models, which can not be explained by the site types. Based on our evaluation of sea salt scattering enhancement and simulated organic mass fraction, there is a strong indication that differences in the model parameterizations of hygroscopicity and model chemistry are driving at least some of the observed diversity in simulated f (RH). Additionally, a key point is that defining dry conditions is difficult from an observational point of view and, depending on the aerosol, may influence the measured f (RH). The definition of dry also impacts our model evaluation, because several models exhibit significant water uptake between RH = 0% and 40 %. The multisite average ratio between model outputs and measurements is 1.64 when RH = 0% is assumed as the model dry RH and 1.16 when RH = 40% is the model dry RH value. The overestimation by the models is believed to originate from the hygroscopicity parameterizations at the lower RH range which may not implement all phenomena taking place (i.e., not fully dried particles and hysteresis effects). This will be particularly relevant when a location is dominated by a deliquescent aerosol such as sea salt. Our results emphasize the need to consider the measurement conditions in such comparisons and recognize that measurements referred to as dry may not be dry in model terms. Recommendations for future model-measurement evaluation and model improvements are provided.

  • 41. Cai, Jing
    et al.
    Chu, Biwu
    Yao, Lei
    Yan, Chao
    Heikkinen, Liine M.
    Zheng, Feixue
    Li, Chang
    Fan, Xiaolong
    Zhang, Shaojun
    Yang, Daoyuan
    Wang, Yonghong
    Kokkonen, Tom V.
    Chan, Tommy
    Zhou, Ying
    Dada, Lubna
    Liu, Yongchun
    He, Hong
    Paasonen, Pauli
    Kujansuu, Joni T.
    Petaja, Tuukka
    Mohr, Claudia
    Stockholm University, Faculty of Science, Department of Environmental Science.
    Kangasluoma, Juha
    Bianchi, Federico
    Sun, Yele
    Croteau, Philip L.
    Worsnop, Douglas R.
    Kerminen, Veli-Matti
    Du, Wei
    Kulmala, Markku
    Daellenbach, Kaspar R.
    Size-segregated particle number and mass concentrations from different emission sources in urban Beijing2020In: Atmospheric Chemistry And Physics, ISSN 1680-7316, E-ISSN 1680-7324, Vol. 20, no 21, p. 12721-12740Article in journal (Refereed)
    Abstract [en]

    Although secondary particulate matter is reported to be the main contributor of PM2.5 during haze in Chinese megacities, primary particle emissions also affect particle concentrations. In order to improve estimates of the contribution of primary sources to the particle number and mass concentrations, we performed source apportionment analyses using both chemical fingerprints and particle size distributions measured at the same site in urban Beijing from April to July 2018. Both methods resolved factors related to primary emissions, including vehicular emissions and cooking emissions, which together make up 76% and 24% of total particle number and organic aerosol (OA) mass, respectively. Similar source types, including particles related to vehicular emissions (1.6 +/- 1.1 mu gm(-3); 2.4 +/- 1.8 x 10(3) cm(-3) and 5.5 +/- 2.8 x 10(3) cm(-3) for two traffic-related components), cooking emissions (2.6 +/- 1.9 mu gm(-3) and 5.5 +/- 3.3 x 10(3) cm(-3)) and secondary aerosols (51 +/- 41 mu gm(-3) and 4.2 +/- 3.0 x 10(3) cm(-3)), were resolved by both methods. Converted mass concentrations from particle size distributions components were comparable with those from chemical fingerprints. Size distribution source apportionment separated vehicular emissions into a component with a mode diameter of 20 nm (traffic-ultrafine) and a component with a mode diameter of 100 nm (traffic-fine). Consistent with similar day- and nighttime diesel vehicle PM2.5 emissions estimated for the Beijing area, traffic-fine particles, hydrocarbon-like OA (HOA, traffic-related factor resulting from source apportionment using chemical fingerprints) and black carbon (BC) showed similar diurnal patterns, with higher concentrations during the night and morning than during the afternoon when the boundary layer is higher. Traffic-ultrafine particles showed the highest concentrations during the rush-hour period, suggesting a prominent role of local gasoline vehicle emissions. In the absence of new particle formation, our re-sults show that vehicular-related emissions (14% and 30% for ultrafine and fine particles, respectively) and cooking-activity-related emissions (32 %) dominate the particle number concentration, while secondary particulate matter (over 80 %) governs PM2.5 mass during the non-heating season in Beijing.

  • 42. Cai, Jing
    et al.
    Wu, Cheng
    Stockholm University, Faculty of Science, Department of Environmental Science.
    Wang, Jiandong
    Du, Wei
    Zheng, Feixue
    Hakala, Simo
    Fan, Xiaolong
    Chu, Biwu
    Yao, Lei
    Feng, Zemin
    Liu, Yongchun
    Sun, Yele
    Zheng, Jun
    Yan, Chao
    Bianchi, Federico
    Kulmala, Markku
    Mohr, Claudia
    Stockholm University, Faculty of Science, Department of Environmental Science.
    Daellenbach, Kaspar R.
    Influence of organic aerosol molecular composition on particle absorptive properties in autumn Beijing2022In: Atmospheric Chemistry And Physics, ISSN 1680-7316, E-ISSN 1680-7324, Vol. 22, no 2, p. 1251-1269Article in journal (Refereed)
    Abstract [en]

    Organic aerosol (OA) is a major component of fine particulate matter (PM), affecting air quality, human health, and the climate. The absorptive and reflective behavior of OA components contributes to determining particle optical properties and thus their effects on the radiative budget of the troposphere. There is limited knowledge on the influence of the molecular composition of OA on particle optical properties in the polluted urban environment. In this study, we characterized the molecular composition of oxygenated OA collected on filter samples in the autumn of 2018 in Beijing, China, with a filter inlet for gases and aerosols coupled to a high-resolution time-of-flight chemical ionization mass spectrometer (FIGAERO–CIMS). Three haze episodes occurred during our sampling period with daily maximum concentrations of OA of 50, 30, and 55 µg m−3. We found that the signal intensities of dicarboxylic acids and sulfur-containing compounds increased during the two more intense haze episodes, while the relative contributions of wood-burning markers and other aromatic compounds were enhanced during the cleaner periods. We further assessed the optical properties of oxygenated OA components by combining detailed chemical composition measurements with collocated particle light absorption measurements. We show that light absorption enhancement (Eabs) of black carbon (BC) was mostly related to more oxygenated OA (e.g., dicarboxylic acids), likely formed in aqueous-phase reactions during the intense haze periods with higher relative humidity, and speculate that they might contribute to lensing effects. Aromatics and nitro-aromatics (e.g., nitrocatechol and its derivatives) were mostly related to a high light absorption coefficient (babs) consistent with light-absorbing (brown) carbon (BrC). Our results provide information on oxygenated OA components at the molecular level associated with BrC and BC particle light absorption and can serve as a basis for further studies on the effects of anthropogenic OA on radiative forcing in the urban environment.

  • 43. Chang, R. Y. -W
    et al.
    Leck, Caroline
    Stockholm University, Faculty of Science, Department of Meteorology .
    Graus, M.
    Mueller, M.
    Paatero, J.
    Burkhart, J. F.
    Stohl, A.
    Orr, L. H.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Hayden, K.
    Li, S. -M
    Hansel, A.
    Tjernström, Michael
    Stockholm University, Faculty of Science, Department of Meteorology .
    Leaitch, W. R.
    Abbatt, J. P. D.
    Aerosol composition and sources in the central Arctic Ocean during ASCOS2011In: Atmospheric Chemistry And Physics, ISSN 1680-7316, E-ISSN 1680-7324, Vol. 11, no 20, p. 10619-10636Article in journal (Refereed)
    Abstract [en]

    Measurements of submicron aerosol chemical composition were made over the central Arctic Ocean from 5 August to 8 September 2008 as a part of the Arctic Summer Cloud Ocean Study (ASCOS) using an aerosol mass spectrometer (AMS). The median levels of sulphate and organics for the entire study were 0.051 and 0.055 mu gm(-3), respectively. Positive matrix factorisation was performed on the entire mass spectral time series and this enabled marine biogenic and continental sources of particles to be separated. These factors accounted for 33% and 36% of the sampled ambient aerosol mass, respectively, and they were both predominantly composed of sulphate, with 47% of the sulphate apportioned to marine biogenic sources and 48% to continental sources, by mass. Within the marine biogenic factor, the ratio of methane sulphonate to sulphate was 0.25+/-0.02, consistent with values reported in the literature. The organic component of the continental factor was more oxidised than that of the marine biogenic factor, suggesting that it had a longer photochemical lifetime than the organics in the marine biogenic factor. The remaining ambient aerosol mass was apportioned to an organic-rich factor that could have arisen from a combination of marine and continental sources. In particular, given that the factor does not correlate with common tracers of continental influence, we cannot rule out that the organic factor arises from a primary marine source.

  • 44. Chauvigné, Aurélien
    et al.
    Aliaga, Diego
    Sellegri, Karine
    Montoux, Nadège
    Krejci, Radovan
    Stockholm University, Faculty of Science, Department of Environmental Science and Analytical Chemistry.
    Močnik, Griša
    Moreno, Isabel
    Müller, Thomas
    Pandolfi, Marco
    Velarde, Fernando
    Weinhold, Kay
    Ginot, Patrick
    Wiedensohler, Alfred
    Andrade, Marcos
    Laj, Paolo
    Biomass burning and urban emission impacts in the Andes Cordillera region based on in situ measurements from the Chacaltaya observatory, Bolivia (5240 m a.s.l.)2019In: Atmospheric Chemistry And Physics, ISSN 1680-7316, E-ISSN 1680-7324, Vol. 19, no 23, p. 14805-14824Article in journal (Refereed)
    Abstract [en]

    This study documents and analyses a 4-year continuous record of aerosol optical properties measured at the Global Atmosphere Watch (GAW) station of Chacaltaya (CHC; 5240 m a.s.l.), in Bolivia. Records of particle light scattering and particle light absorption coefficients are used to investigate how the high Andean Cordillera is affected by both long-range transport and by the fast-growing agglomeration of La Paz-El Alto, located approximately 20 km away and 1.5 km below the sampling site. The extended multiyear record allows us to study the properties of aerosol particles for different air mass types, during wet and dry seasons, also covering periods when the site was affected by biomass burning in the Bolivian lowlands and the Amazon Basin. The absorption, scattering, and extinction coefficients (median annual values of 0.74, 12.14, and 12.96 Mm(-1) respectively) show a clear seasonal variation with low values during the wet season (0.57, 7.94, and 8.68 Mm(-1) respectively) and higher values during the dry season (0.80, 11.23, and 14.51 Mm(-1) respectively). The record is driven by variability at both seasonal and diurnal scales. At a diurnal scale, all records of intensive and extensive aerosol properties show a pronounced variation (daytime maximum, night-time minimum), as a result of the dynamic and convective effects. The particle light absorption, scattering, and extinction coefficients are on average 1.94, 1.49, and 1.55 times higher respectively in the turbulent thermally driven conditions than the more stable conditions, due to more efficient transport from the boundary layer. Retrieved intensive optical properties are significantly different from one season to the other, reflecting the changing aerosol emission sources of aerosol at a larger scale. Using the wavelength dependence of aerosol particle optical properties, we discriminated between contributions from natural (mainly mineral dust) and anthropogenic (mainly biomass burning and urban transport or industries) emissions according to seasons and local circulation. The main sources influencing measurements at CHC are from the urban area of La Paz-El Alto in the Altiplano and from regional biomass burning in the Amazon Basin. Results show a 28 % to 80 % increase in the extinction coefficients during the biomass burning season with respect to the dry season, which is observed in both tropospheric dynamic conditions. From this analysis, long-term observations at CHC provide the first direct evidence of the impact of biomass burning emissions of the Amazon Basin and urban emissions from the La Paz area on atmospheric optical properties at a remote site all the way to the free troposphere.

  • 45.
    Christensen, Ole Martin
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Benze, Susanne
    Stockholm University, Faculty of Science, Department of Meteorology .
    Eriksson, Patrick
    Gumbel, Jörg
    Stockholm University, Faculty of Science, Department of Meteorology .
    Megner, Linda
    Stockholm University, Faculty of Science, Department of Meteorology .
    Murtagh, Donal P.
    The relationship between polar mesospheric clouds and their background atmosphere as observed by Odin-SMR and Odin-OSIRIS2016In: Atmospheric Chemistry And Physics, ISSN 1680-7316, E-ISSN 1680-7324, Vol. 16, no 19, p. 12587-12600Article in journal (Refereed)
    Abstract [en]

    In this study the properties of polar mesospheric clouds (PMCs) and the background atmosphere in which they exist are studied using measurements from two instruments, OSIRIS and SMR, on board the Odin satellite. The data comes from a set of tomographic measurements conducted by the satellite during 2010 and 2011. The expected ice mass density and cloud frequency for conditions of thermodynamic equilibrium, calculated using the temperature and water vapour as measured by SMR, are compared to the ice mass density and cloud frequency as measured by OSIRIS. We find that assuming thermodynamic equilibrium reproduces the seasonal, latitudinal and vertical variations in ice mass density and cloud frequency, but with a high bias of a factor of 2 in ice mass density. To investigate this bias, we use a simple ice particle growth model to estimate the time it would take for the observed clouds to sublimate completely and the time it takes for these clouds to reform. We find a difference in the median sublimation time (1.8 h) and the reformation time (3.2 h) at peak cloud altitudes (82-84 km). This difference implies that temperature variations on these timescales have a tendency to reduce the ice content of the clouds, possibly explaining the high bias of the equilibrium model. Finally, we detect and are, for the first time, able to positively identify cloud features with horizontal scales of 100 to 300 km extending far below the region of supersaturation (>2 km). Using the growth model, we conclude these features cannot be explained by sedimentation alone and suggest that these events may be an indication of strong vertical transport.

  • 46. Chung, Chul Eddy
    et al.
    Lewinschal, Anna
    Stockholm University, Faculty of Science, Department of Meteorology .
    Wilcox, Eric
    Relationship between low-cloud presence and the amount of overlying aerosols2016In: Atmospheric Chemistry And Physics, ISSN 1680-7316, E-ISSN 1680-7324, Vol. 16, no 9, p. 5781-5792Article in journal (Refereed)
    Abstract [en]

    Aerosols are often advected above cloud decks, and the amount of aerosols over cloud has been assumed to be similar to that at the same heights in nearby clear sky. In this assumption, cloud and aerosol above cloud-top height are considered randomly located with respect to each other. The Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) data are analyzed here to investigate this assumption on global scales. The CALIPSO data reveal that the aerosol optical depth (AOD) above low cloud tends to be smaller than in nearby clear sky during the daytime, and the opposite is true during the nighttime. In particular, over oceanic regions with wide-spread low cloud, such as the tropical southeastern Atlantic Ocean and northeastern Pacific Ocean, the daytime AOD above low cloud is often 40aEuro-% smaller than in surrounding clear skies.

  • 47. Collaud Coen, Martine
    et al.
    Andrews, Elisabeth
    Alastuey, Andres
    Petkov Arsov, Todor
    Backman, John
    Brem, Benjamin T.
    Bukowiecki, Nicolas
    Couret, Cedric
    Eleftheriadis, Konstantinos
    Flentje, Harald
    Fiebig, Markus
    Gysel-Beer, Martin
    Hand, Jenny L.
    Hoffer, Andras
    Hooda, Rakesh
    Hueglin, Christoph
    Joubert, Warren
    Keywood, Melita
    Kim, Jeong Eun
    Kim, Sang-Woo
    Labuschagne, Casper
    Lin, Neng-Huei
    Lin, Yong
    Lund Myhre, Cathrine
    Luoma, Krista
    Lyamani, Hassan
    Marinoni, Angela
    Mayol-Bracero, Olga L.
    Mihalopoulos, Nikos
    Pandolfi, Marco
    Prats, Natalia
    Prenni, Anthony J.
    Putaud, Jean-Philippe
    Ries, Ludwig
    Reisen, Fabienne
    Sellegri, Karine
    Sharma, Sangeeta
    Sheridan, Patrick
    Sherman, James Patrick
    Sun, Junying
    Titos, Gloria
    Torres, Elvis
    Tuch, Thomas
    Weller, Rolf
    Wiedensohler, Alfred
    Zieger, Paul
    Stockholm University, Faculty of Science, Department of Environmental Science and Analytical Chemistry.
    Laj, Paolo
    Multidecadal trend analysis of in situ aerosol radiative properties around the world2020In: Atmospheric Chemistry And Physics, ISSN 1680-7316, E-ISSN 1680-7324, Vol. 20, no 14, p. 8867-8908Article in journal (Refereed)
    Abstract [en]

    In order to assess the evolution of aerosol parameters affecting climate change, a long-term trend analysis of aerosol optical properties was performed on time series from 52 stations situated across five continents. The time series of measured scattering, backscattering and absorption coefficients as well as the derived single scattering albedo, backscattering fraction, scattering and absorption Angstrom exponents covered at least 10 years and up to 40 years for some stations. The non-parametric seasonal Mann-Kendall (MK) statistical test associated with several pre-whitening methods and with Sen's slope was used as the main trend analysis method. Comparisons with general least mean square associated with autoregressive bootstrap (GLS/ARB) and with standard least mean square analysis (LMS) enabled confirmation of the detected MK statistically significant trends and the assessment of advantages and limitations of each method. Currently, scattering and backscattering coefficient trends are mostly decreasing in Europe and North America and are not statistically significant in Asia, while polar stations exhibit a mix of increasing and decreasing trends. A few increasing trends are also found at some stations in North America and Australia. Absorption coefficient time series also exhibit primarily decreasing trends. For single scattering albedo, 52 % of the sites exhibit statistically significant positive trends, mostly in Asia, eastern/northern Europe and the Arctic, 22 % of sites exhibit statistically significant negative trends, mostly in central Europe and central North America, while the remaining 26 % of sites have trends which are not statistically significant. In addition to evaluating trends for the overall time series, the evolution of the trends in sequential 10-year segments was also analyzed. For scattering and backscattering, statistically significant increasing 10-year trends are primarily found for earlier periods (10-year trends ending in 2010-2015) for polar stations and Mauna Loa. For most of the stations, the present-day statistically significant decreasing 10-year trends of the single scattering albedo were preceded by not statistically significant and statistically significant increasing 10-year trends. The effect of air pollution abatement policies in continental North America is very obvious in the 10-year trends of the scattering coefficient - there is a shift to statistically significant negative trends in 2009-2012 for all stations in the eastern and central USA. This long-term trend analysis of aerosol radiative properties with a broad spatial coverage provides insight into potential aerosol effects on climate changes.

  • 48. Croft, Betty
    et al.
    Martin, Randall V.
    Leaitch, W. Richard
    Tunved, Peter
    Stockholm University, Faculty of Science, Department of Environmental Science and Analytical Chemistry.
    Breider, Thomas J.
    D'Andrea, Stephen D.
    Pierce, Jeffrey R.
    Processes controlling the annual cycle of Arctic aerosol number and size distributions2016In: Atmospheric Chemistry And Physics, ISSN 1680-7316, E-ISSN 1680-7324, Vol. 16, no 6, p. 3665-3682Article in journal (Refereed)
    Abstract [en]

    Measurements at high-Arctic sites (Alert, Nunavut, and Mt. Zeppelin, Svalbard) during the years 2011 to 2013 show a strong and similar annual cycle in aerosol number and size distributions. Each year at both sites, the number of aerosols with diameters larger than 20 nm exhibits a minimum in October and two maxima, one in spring associated with a dominant accumulation mode (particles 100 to 500 nm in diameter) and a second in summer associated with a dominant Aitken mode (particles 20 to 100 nm in diameter). Seasonal-mean aerosol effective diameter from measurements ranges from about 180 in summer to 260 nm in winter. This study interprets these annual cycles with the GEOS-Chem-TOMAS global aerosol microphysics model. Important roles are documented for several processes (new-particle formation, coagulation scavenging in clouds, scavenging by precipitation, and transport) in controlling the annual cycle in Arctic aerosol number and size. Our simulations suggest that coagulation scavenging of interstitial aerosols in clouds by aerosols that have activated to form cloud droplets strongly limits the total number of particles with diameters less than 200 nm throughout the year. We find that the minimum in total particle number in October can be explained by diminishing new-particle formation within the Arctic, limited transport of pollution from lower latitudes, and efficient wet removal. Our simulations indicate that the summertime-dominant Aitken mode is associated with efficient wet removal of accumulation-mode aerosols, which limits the condensation sink for condensable vapours. This in turn promotes new-particle formation and growth. The dominant accumulation mode during spring is associated with build up of transported pollution from outside the Arctic coupled with less-efficient wet-removal processes at colder temperatures. We recommend further attention to the key processes of new-particle formation, interstitial coagulation, and wet removal and their delicate interactions and balance in size-resolved aerosol simulations of the Arctic to reduce uncertainties in estimates of aerosol radiative effects on the Arctic climate.

  • 49.
    Dalirian, Maryam
    et al.
    Stockholm University, Faculty of Science, Department of Environmental Science and Analytical Chemistry.
    Keskinen, H.
    Ahlm, Lars
    Stockholm University, Faculty of Science, Department of Environmental Science and Analytical Chemistry.
    Ylisirniö, A.
    Romakkaniemi, S.
    Laaksonen, A.
    Virtanen, A.
    Riipinen, Ilona
    Stockholm University, Faculty of Science, Department of Environmental Science and Analytical Chemistry.
    CCN activation of fumed silica aerosols mixed with soluble pollutants2015In: Atmospheric Chemistry And Physics, ISSN 1680-7316, E-ISSN 1680-7324, Vol. 15, no 7, p. 3815-3829Article in journal (Refereed)
    Abstract [en]

    Particle-water interactions of completely soluble or insoluble particles are fairly well understood but less is known of aerosols consisting of mixtures of soluble and insoluble components. In this study, laboratory measurements were performed to investigate cloud condensation nuclei (CCN) activity of silica particles mixed with ammonium sulfate (a salt), sucrose (a sugar) and bovine serum albumin known as BSA (a protein). The agglomerated structure of the silica particles was investigated using measurements with a differential mobility analyser (DMA) and an aerosol particle mass analyser (APM). Based on these data, the particles were assumed to be compact agglomerates when studying their CCN activation capabilities. Furthermore, the critical super-saturations of particles consisting of pure and mixed soluble and insoluble compounds were explored using existing theoretical frameworks. These results showed that the CCN activation of single-component particles was in good agreement with Kohler- and adsorption theory based models when the agglomerated structure was accounted for. For mixed particles the CCN activation was governed by the soluble components, and the soluble fraction varied considerably with particle size for our wet-generated aerosols. Our results confirm the hypothesis that knowing the soluble fraction is the key parameter needed for describing the CCN activation of mixed aerosols, and highlight the importance of controlled coating techniques for acquiring a detailed understanding of the CCN activation of atmospheric insoluble particles mixed with soluble pollutants.

  • 50. Dall'Osto, Manuel
    et al.
    Beddows, David C. S.
    Tunved, Peter
    Stockholm University, Faculty of Science, Department of Environmental Science and Analytical Chemistry.
    Harrison, Roy M.
    Lupi, Angelo
    Vitale, Vito
    Becagli, Silvia
    Traversi, Rita
    Park, Ki-Tae
    Yoon, Young Jun
    Massling, Andreas
    Skov, Henrik
    Lange, Robert
    Ström, Johan
    Stockholm University, Faculty of Science, Department of Environmental Science and Analytical Chemistry.
    Krejci, Radovan
    Stockholm University, Faculty of Science, Department of Environmental Science and Analytical Chemistry.
    Simultaneous measurements of aerosol size distributions at three sites in the European high Arctic2019In: Atmospheric Chemistry And Physics, ISSN 1680-7316, E-ISSN 1680-7324, Vol. 19, no 11, p. 7377-7395Article in journal (Refereed)
    Abstract [en]

    Aerosols are an integral part of the Arctic climate system due to their direct interaction with radiation and indirect interaction through cloud formation. Understanding aerosol size distributions and their dynamics is crucial for the ability to predict these climate relevant effects. When of favourable size and composition, both long-rangetransported - and locally formed particles - may serve as cloud condensation nuclei (CCN). Small changes of composition or size may have a large impact on the low CCN concentrations currently characteristic of the Arctic environment. We present a cluster analysis of particle size distributions (PSDs; size range 8-500 nm) simultaneously collected from three high Arctic sites during a 3-year period (20132015). Two sites are located in the Svalbard archipelago: Zeppelin research station (ZEP; 474 m above ground) and the nearby Gruvebadet Observatory (GRU; about 2 km distance from Zeppelin, 67 m above ground). The third site (Villum Research Station at Station Nord, VRS; 30 m above ground) is 600 km west-northwest of Zeppelin, at the tip of northeastern Greenland. The GRU site is included in an inter-site comparison for the first time. K-means cluster analysis pro- vided eight specific aerosol categories, further combined into broad PSD classes with similar characteristics, namely pristine low concentrations (12 %-14 % occurrence), new particle formation (16 %-32 %), Aitken (21 %-35 %) and accumulation (20 %-50 %). Confined for longer time periods by consolidated pack sea ice regions, the Greenland site GRU shows PSDs with lower ultrafine-mode aerosol concentrations during summer but higher accumulation-mode aerosol concentrations during winter, relative to the Svalbard sites. By association with chemical composition and cloud condensation nuclei properties, further conclusions can be derived. Three distinct types of accumulation-mode aerosol are observed during winter months. These are associated with sea spray (largest detectable sizes, > 400 nm), Arctic haze (main mode at 150 nm) and aged accumulation-mode (main mode at 220 nm) aerosols. In contrast, locally produced particles, most likely of marine biogenic origin, exhibit size distributions dominated by the nucleation and Aitken mode during summer months. The obtained data and analysis point towards future studies, including apportioning the relative contribution of primary and secondary aerosol formation processes and elucidating anthropogenic aerosol dynamics and transport and removal processes across the Greenland Sea. In order to address important research questions in the Arctic on scales beyond a singular station or measurement events, it is imperative to continue strengthening international scientific cooperation.

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