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  • 101. De Marchis, M.
    et al.
    Milici, B.
    Sardina, Gaetano
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Napoli, E.
    Interaction between turbulent structures and particles in roughened channel2016Inngår i: International Journal of Multiphase Flow, ISSN 0301-9322, E-ISSN 1879-3533, Vol. 78, s. 117-131Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The distribution of inertial particles in turbulent flows is highly non-uniform and is driven by the local dynamics of the turbulent structures of the underlying carrier flow field. In the specific context of dilute particle-laden wall-bounded flows, deposition and resuspension mechanisms are dominated by the interaction between inertial particles and coherent turbulent structures characteristic of the wall region. The macroscopic behavior of these two-phase systems is influenced by particle inertia, which plays a role at the microscale of a single dispersed element. These turbulent structures, which control the turbulent regeneration cycles, are strongly affected by the wall roughness. The effect of the roughness on turbulent transport in dilute suspension has been still poorly investigated. The issue is discussed here by addressing direct numerical simulation (DNS), at friction Reynolds number Re-tau = 180, of a dilute dispersion of heavy particles in a turbulent channel flow, spanning two orders of magnitude of particle inertia. The irregular wall roughness is obtained through the superimposition of four sinusoidal functions of different wavelengths and random amplitudes. We use DNS combined with Lagrangian particle tracking to characterize the effect of inertia on particle preferential accumulation, looking at the effect of roughness on particle distribution, by comparing the statistics computed for fluid and particles of different size and observing differences in terms of distribution patterns and preferential sampling.

  • 102. Decremer, Damien
    et al.
    Chung, Chul E.
    Ekman, Annica M. L.
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Brandefelt, Jenny
    Which significance test performs the best in climate simulations?2014Inngår i: Tellus. Series A, Dynamic meteorology and oceanography, ISSN 0280-6495, E-ISSN 1600-0870, Vol. 66, s. 23139-Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Climate change simulated with climate models needs a significance testing to establish the robustness of simulated climate change relative to model internal variability. Student's t-test has been the most popular significance testing technique despite more sophisticated techniques developed to address autocorrelation. We apply Student's t-test and four advanced techniques in establishing the significance of the average over 20 continuous-year simulations, and validate the performance of each technique using much longer (375-1000 yr) model simulations. We find that all the techniques tend to perform better in precipitation than in surface air temperature. A sizable performance gain using some of the advanced techniques is realised in the model Ts output portion with strong positive lag-1 yr autocorrelation (> +/- 0.6), but this gain disappears in precipitation. Furthermore, strong positive lag-1 yr autocorrelation is found to be very uncommon in climate model outputs. Thus, there is no reason to replace Student's t-test by the advanced techniques in most cases.

  • 103. Denby, B. R.
    et al.
    Sundvor, I.
    Johansson, Christer
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för tillämpad miljövetenskap (ITM).
    Pirjola, L.
    Ketzel, M.
    Norman, M.
    Kupiainen, K.
    Gustafsson, M.
    Blomqvist, G.
    Kauhaniemi, M.
    Omstedt, G.
    A coupled road dust and surface moisture model to predict non-exhaust road traffic induced particle emissions (NORTRIP). Part 2: Surface moisture and salt impact modelling2013Inngår i: Atmospheric Environment, ISSN 1352-2310, E-ISSN 1873-2844, Vol. 81, s. 485-503Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Non-exhaust traffic induced emissions are a major source of airborne particulate matter in most European countries. This is particularly important in Nordic and Alpine countries where winter time road traction maintenance occurs, e.g. salting and sanding, and where studded tyres are used. Though the total mass generated by wear sources is a key factor in non-exhaust emissions, these emissions are also strongly controlled by surface moisture conditions. In this paper, Part 2, the road surface moisture sub-model of a coupled road dust and surface moisture model (NORTRIP) is described. We present a description of the road surface moisture part of the model and apply the coupled model to seven sites in Stockholm, Oslo, Helsinki and Copenhagen over 18 separate periods, ranging from 3.5 to 24 months. At two sites surface moisture measurements are available and the moisture sub-model is compared directly to these observations. The model predicts the frequency of wet roads well at both sites, with an average fractional bias of −2.6%. The model is found to correctly predict the hourly surface state, wet or dry, 85% of the time. From the 18 periods modelled using the coupled model an average absolute fractional bias of 15% for PM10 concentrations was found. Similarly the model predicts the 90'th daily mean percentiles of PM10 with an average absolute bias of 19% and an average correlation (R2) of 0.49. When surface moisture is not included in the modelling then this average correlation is reduced to 0.16, demonstrating the importance of the surface moisture conditions. Tests have been carried out to assess the sensitivity of the model to model parameters and input data. The model provides a useful tool for air quality management and for improving our understanding of non-exhaust traffic emissions.

  • 104. Denby, B. R.
    et al.
    Sundvor, I.
    Johansson, Christer
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för tillämpad miljövetenskap (ITM).
    Pirjola, L.
    Ketzel, M.
    Norman, M.
    Kupiainen, K.
    Gustafsson, M.
    Blomqvist, G.
    Omstedt, G.
    A coupled road dust and surface moisture model to predict non-exhaust road traffic induced particle emissions (NORTRIP). Part 1: Road dust loading and suspension modelling2013Inngår i: Atmospheric Environment, ISSN 1352-2310, E-ISSN 1873-2844, Vol. 77, s. 283-300Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Non-exhaust traffic induced emissions are a major source of particle mass in most European countries. This is particularly important in Nordic and Alpine countries where winter time road traction maintenance occurs, e.g. salting and sanding, and where studded tyres are used. In this paper, Part 1, the road dust sub-model of a coupled road dust and surface moisture model (NORTRIP) is described. The model provides a generalised process based formulation of the non-exhaust emissions, with emphasis on the contribution of road wear, suspension, surface dust loading and the effect of road surface moisture (retention of wear particles and suspended emissions). The model is intended for use as a tool for air quality managers to help study the impact of mitigation measures and policies. We present a description of the road dust sub-model and apply the model to two sites in Stockholm and Copenhagen where seven years of data with surface moisture measurements are available. For the site in Stockholm, where studded tyres are in use, the model predicts the PM10 concentrations very well with correlations (R-2) in the range of R-2 = 0.76-0.91 for daily mean PM10. The model also reproduces well the impact of a reduction in studded tyres at this site. For the site in Copenhagen the correlation is lower, in the range 0.44-0.51. The addition of salt is described in the model and at both sites this leads to improved correlations due to additional salt emissions. For future use of the model a number of model parameters, e.g. wear factors and suspension rates, still need to be refined. The effect of sanding on PM10 emissions is also presented but more information will be required before this can be confidently applied for management applications.

  • 105.
    Destouni, Georgia
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för naturgeografi och kvartärgeologi (INK).
    Jaramillo, Fernando
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för naturgeografi och kvartärgeologi (INK).
    Prieto, Carmen
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för naturgeografi och kvartärgeologi (INK).
    Hydroclimatic shifts driven by human water use for food and energy production2013Inngår i: Nature Climate Change, ISSN 1758-678X, E-ISSN 1758-6798, Vol. 3, nr 3, s. 213-217Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Hydrological change is a central part of global change(1-3). Its drivers in the past need to be understood and quantified for accurate projection of disruptive future changes(4). Here we analyse past hydro-climatic, agricultural and hydropower changes from twentieth century data for nine major Swedish drainage basins, and synthesize and compare these results with other regional(5-7) and global(2) assessments of hydrological change by irrigation and deforestation. Cross-regional comparison shows similar increases of evapotranspiration by non-irrigated agriculture and hydropower as for irrigated agriculture. In the Swedish basins, non-irrigated agriculture has also increased, whereas hydropower has decreased temporal runoff variability. A global indication of the regional results is a net total increase of evapotranspiration that is larger than a proposed associated planetary boundary(8). This emphasizes the need for climate and Earth system models to account for different human uses of water as anthropogenic drivers of hydro-climatic change. The present study shows how these drivers and their effects can be distinguished and quantified for hydrological basins on different scales and in different world regions. This should encourage further exploration of greater basin variety for better understanding of anthropogenic hydro-climatic change.

  • 106. Devasthale, A.
    et al.
    Sedlar, J.
    Tjernström, Michael
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Characteristics of water-vapour inversions observed over the Arctic by Atmospheric Infrared Sounder (AIRS) and radiosondes2011Inngår i: Atmospheric Chemistry And Physics, ISSN 1680-7316, E-ISSN 1680-7324, Vol. 11, nr 18, s. 9813-9823Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    An accurate characterization of the vertical structure of the Arctic atmosphere is useful in climate change and attribution studies as well as for the climate modelling community to improve projections of future climate over this highly sensitive region. Here, we investigate one of the dominant features of the vertical structure of the Arctic atmosphere, i.e. water-vapour inversions, using eight years of Atmospheric Infrared Sounder data (2002-2010) and radiosounding profiles released from the two Arctic locations (North Slope of Alaska at Barrow and during SHEBA). We quantify the characteristics of clear-sky water vapour inversions in terms of their frequency of occurrence, strength and height covering the entire Arctic for the first time. We found that the frequency of occurrence of water-vapour inversions is highest during winter and lowest during summer. The inversion strength is, however, higher during summer. The observed peaks in the median inversion-layer heights are higher during the winter half of the year, at around 850 hPa over most of the Arctic Ocean, Siberia and the Canadian Archipelago, while being around 925 hPa during most of the summer half of the year over the Arctic Ocean. The radiosounding profiles agree with the frequency, location and strength of water-vapour inversions in the Pacific sector of the Arctic. In addition, the radiosoundings indicate that multiple inversions are the norm with relatively few cases without inversions. The amount of precipitable water within the water-vapour inversion structures is estimated and we find a distinct, two-mode contribution to the total column precipitable water. These results suggest that water-vapour inversions are a significant source to the column thermodynamics, especially during the colder winter and spring seasons. We argue that these inversions are a robust metric to test the reproducibility of thermodynamics within climate models. An accurate statistical representation of water-vapour inversions in models would mean that the large-scale coupling of moisture transport, precipitation, temperature and water-vapour vertical structure and radiation are essentially captured well in such models.

  • 107. Devasthale, A.
    et al.
    Tjernström, Michael
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Caian, M.
    Thomas, M. A.
    Kahn, B. H.
    Fetzer, E. J.
    Influence of the arctic oscillation on the vertical distribution of clouds as observed by the a train constellation of satellites2012Inngår i: Atmospheric Chemistry And Physics, ISSN 1680-7316, E-ISSN 1680-7324, Vol. 12, nr 21, s. 10535-10544Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The main purpose of this study is to investigate the influence of the Arctic Oscillation (AO), the dominant mode of natural variability over the northerly high latitudes, on the spatial (horizontal and vertical) distribution of clouds in the Arctic. To that end, we use a suite of sensors on-board NASA's A-Train satellites that provide accurate observations of the distribution of clouds along with information on atmospheric thermodynamics. Data from three independent sensors are used (AQUA-AIRS, CALIOP-CALIPSO and CPR-CloudSat) covering two time periods (winter half years, November through March, of 2002-2011 and 2006-2011, respectively) along with data from the ERA-Interim reanalysis. We show that the zonal vertical distribution of cloud fraction anomalies averaged over 67-82 degrees N to a first approximation follows a dipole structure (referred to as Greenland cloud dipole anomaly, GCDA), such that during the positive phase of the AO, positive and negative cloud anomalies are observed eastwards and westward of Greenland respectively, while the opposite is true for the negative phase of AO. By investigating the concurrent meteorological conditions (temperature, humidity and winds), we show that differences in the meridional energy and moisture transport during the positive and negative phases of the AO and the associated thermodynamics are responsible for the conditions that are conducive for the formation of this dipole structure. All three satellite sensors broadly observe this large-scale GCDA despite differences in their sensitivities, spatio-temporal and vertical resolutions, and the available lengths of data records, indicating the robustness of the results. The present study also provides a compelling case to carry out process-based evaluation of global and regional climate models.

  • 108. Devasthale, Abhay
    et al.
    Tjernström, Michael
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Ali, Omar
    The vertical distribution of thin features over the Arctic analysed from CALIPSO observations: Part 2: Aerosols2011Inngår i: Tellus. Series B, Chemical and physical meteorology, ISSN 0280-6509, E-ISSN 1600-0889, Vol. 63, nr 1, s. 86-95Artikkel i tidsskrift (Fagfellevurdert)
  • 109. Devasthale, Abhay
    et al.
    Tjernström, Michael
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Karlsson, Karl-Goran
    Thomas, Manu Anna
    Jones, Colin
    Sedlar, Joseph
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Omar, Ali H.
    The vertical distribution of thin features over the Arctic analysed from CALIPSO observations: Part 1: Optically thin clouds2011Inngår i: Tellus. Series B, Chemical and physical meteorology, ISSN 0280-6509, E-ISSN 1600-0889, Vol. 63, nr 1, s. 77-85Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Clouds play a crucial role in the Arctic climate system. Therefore, it is essential to accurately and reliably quantify and understand cloud properties over the Arctic. It is also important to monitor and attribute changes in Arctic clouds. Here, we exploit the capability of the CALIPSO-CALIOP instrument and provide comprehensive statistics of tropospheric thin clouds, otherwise extremely difficult to monitor from passive satellite sensors. We use 4 yr of data (June 2006-May 2010) over the circumpolar Arctic, here defined as 67-82 degrees N, and characterize probability density functions of cloud base and top heights, geometrical thickness and zonal distribution of such cloud layers, separately for water and ice phases, and discuss seasonal variability of these properties. When computed for the entire study area, probability density functions of cloud base and top heights and geometrical thickness peak at 200-400, 1000-2000 and 400-800 m, respectively, for thin water clouds, while for ice clouds they peak at 6-8, 7-9 and 400-1000 m, respectively. In general, liquid clouds were often identified below 2 km during all seasons, whereas ice clouds were sensed throughout the majority of the upper troposphere and also, but to a smaller extent, below 2 km for all seasons.

  • 110.
    Dey, Dipanjan
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Döös, Kristofer
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    The coupled ocean–atmosphere hydrologic cycle2019Inngår i: Tellus. Series A, Dynamic meteorology and oceanography, ISSN 0280-6495, E-ISSN 1600-0870, Vol. 71, nr 1, s. 1-11Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The freshwater cycle has in the present study been traced as one integrated process in the coupled ocean–atmosphere system for both present and possible future climates simulated with an Earth-System Model. A method based on water-mass conservation was used in order to calculate mass fluxes of water from regions of evaporation to regions of precipitation. These fluxes include not only advection of moisture by the winds but also the vertical water-mass transport due to precipitation forming hence a mass-conserved 3D water-mass transport field. Six atmospheric hydrological cells were revealed, which cross the sea surface, where they join the oceanic overturning circulation. These atmospheric water cells can be interpreted as an extension of the oceanic overturning circulation, since the otherwise open ocean streamlines at the surface continue into the atmosphere due to evaporation and back into the ocean due to precipitation. Although these atmospheric water cells are related to the usual air cells, they are only half part of the coupled water cells and located differently. The future-climate scenario shows that the mid- and high-latitude atmospheric water-mass cells will transport more moisture towards the poles as well as increase of the northward cross-Equatorial atmospheric water-mass transport.

  • 111.
    Dey, Dipanjan
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU). Indian Institute of Technology Bhubaneswar, India.
    Sil, Sourav
    Jana, Sudip
    Pramanik, Saikat
    Pandey, P. C.
    An assessment of TropFlux and NCEP air-sea fluxes on ROMS simulations over the Bay of Bengal region2017Inngår i: Dynamics of atmospheres and oceans (Print), ISSN 0377-0265, E-ISSN 1872-6879, Vol. 80, s. 47-61Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    This study presents an assessment of the TropFlux and the National Centers for Environmental Prediction (NCEP) reanalysis air-sea fluxes in simulating the surface and subsurface oceanic parameters over the Bay of Bengal (BoB) region during 2002-2014 using the Regional Ocean Modelling System (ROMS). The assessment has been made by comparing the simulated fields with in-situ and satellite observations. The simulated surface and subsurface temperatures in the TropFlux forced experiment (TropFlux-E) show better agreement with the Research Moored Array for African-Asian-Australian Monsoon Analysis (RAMA) and Argo observations than the NCEP forced experiment (NCEP-E). The BoB domain averaged sea surface temperature (SST) simulated in the NCEP-E is consistently cooler than the satellite SST, with a root mean square error (RMSE) of 0.79 C. Moreover, NCEP-E shows a limitation in simulating the observed seasonal cycle of the SST due to substantial underestimation of the pre-monsoon SST peak. These limitations are mostly due to the lower values of the NCEP net hedt flux. The seasonal and interannual variations of SST in the TropFlux-E are better comparable to the observations with correlations and skills more than 0.80 and 0.90 respectively. However, SST is overestimated during summer monsoon periods mainly due to higher net heat flux. The superiority of TropFlux forcing over the NCEP reanalysis can also.be seen when simulating the interannual variabilities of the magnitude and vertical extent of Wyrtki jets at two equatorial RAMA buoy locations. The jet is weaker in the NCEP-E relative to the TropFlux-E and observations. The simulated sea surface height anomalies (SSHA) from both the experiments are able to capture the regions of positive and negative SSHA with respect to satellite-derived altimeter data with better performance in the TropFlux-E. The speed of the westward propagating Rossby wave along 18 N in the TropFlux-E is found to be about 4.7 cm/s, which is close to the theoretical phase speed of Rossby waves.

  • 112.
    Dickens, Gerald R.
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för geologiska vetenskaper.
    Down the Rabbit Hole: toward appropriate discussion of methane release from gas hydrate systems during the Paleocene-Eocene thermal maximum and other past hyperthermal events2011Inngår i: Climate of the Past Discussions, ISSN 1814-9340, E-ISSN 1814-9359, Vol. 7, nr 3, s. 831-846Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Enormous amounts of (13)C-depleted carbon rapidly entered the exogenic carbon cycle during the onset of the Paleocene-Eocene thermal maximum (PETM), as attested to by a prominent negative carbon isotope (delta(13)C) excursion and deep-sea carbonate dissolution. A widely cited explanation for this carbon input has been thermal dissociation of gas hydrate on continental slopes, followed by release of CH(4) from the seafloor and its subsequent oxidation to CO(2) in the ocean or atmosphere. Increasingly, papers have argued against this mechanism, but without fully considering existing ideas and available data. Moreover, other explanations have been presented as plausible alternatives, even though they conflict with geological observations, they raise major conceptual problems, or both. Methane release from gas hydrates remains a congruous explanation for the delta(13)C excursion across the PETM, although it requires an unconventional framework for global carbon and sulfur cycling, and it lacks proof. These issues are addressed here in the hope that they will prompt appropriate discussions regarding the extraordinary carbon injection at the start of the PETM and during other events in Earth's history.

  • 113. Donahue, Neil M.
    et al.
    Robinson, Allen L.
    Trump, Erica R.
    Riipinen, Ilona
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för tillämpad miljövetenskap (ITM).
    Kroll, Jesse H.
    Volatility and Aging of Atmospheric Organic Aerosol2014Inngår i: Atmospheric and aerosol chemistry, Springer, 2014, s. 97-143Kapittel i bok, del av antologi (Fagfellevurdert)
    Abstract [en]

    Organic-aerosol phase partitioning (volatility) and oxidative aging are inextricably linked in the atmosphere because partitioning largely controls the rates and mechanisms of aging reactions as well as the actual amount of organic aerosol. Here we discuss those linkages, describing the basic theory of partitioning thermodynamics as well as the dynamics that may limit the approach to equilibrium under some conditions. We then discuss oxidative aging in three forms: homogeneous gas-phase oxidation, heterogeneous oxidation via uptake of gas-phase oxidants, and aqueous-phase oxidation. We present general scaling arguments to constrain the relative importance of these processes in the atmosphere, compared to each other and compared to the characteristic residence time of particles in the atmosphere.

  • 114.
    Döös, Bo R.
    Stockholms universitet.
    On perturbations of atmospheric motion with regard to exchange of sensible heat with the earth's surface and topographic effects1962Doktoravhandling, med artikler (Annet vitenskapelig)
  • 115.
    Döös, Kristofer
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Jönsson, Bror
    Kjellsson, Joakim
    Evaluation of oceanic and atmospheric trajectory schemes in the TRACMASS trajectory model v6.02017Inngår i: Geoscientific Model Development, ISSN 1991-959X, E-ISSN 1991-9603, Vol. 10, nr 4, s. 1733-1749Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Three different trajectory schemes for oceanic and atmospheric general circulation models are compared in two different experiments. The theories of the trajectory schemes are presented showing the differential equations they solve and why they are mass conserving. One scheme assumes that the velocity fields are stationary for set intervals of time between saved model outputs and solves the trajectory path from a differential equation only as a function of space, i.e. stepwise stationary. The second scheme is a special case of the stepwise-stationary scheme, where velocities are assumed constant between general circulation model (GCM) outputs; it uses hence a fixed GCM time step. The third scheme uses a continuous linear interpolation of the fields in time and solves the trajectory path from a differential equation as a function of both space and time, i.e. a time-dependent scheme. The trajectory schemes are tested offline, i.e. using the already integrated and stored velocity fields from a GCM. The first comparison of the schemes uses trajectories calculated using the velocity fields from a high-resolution ocean general circulation model in the Agulhas region. The second comparison uses trajectories calculated using the wind fields from an atmospheric reanalysis. The study shows that using the time-dependent scheme over the stepwise-stationary scheme greatly improves accuracy with only a small increase in computational time. It is also found that with decreasing time steps the stepwise-stationary scheme becomes increasingly more accurate but at increased computational cost. The time-dependent scheme is therefore preferred over the stepwise-stationary scheme. However, when averaging over large ensembles of trajectories, the two schemes are comparable, as intrinsic variability dominates over numerical errors. The fixed GCM time step scheme is found to be less accurate than the stepwisestationary scheme, even when considering averages over large ensembles.

  • 116.
    Döös, Kristofer
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Nilsson, Johan
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Analysis of the Meridional Energy Transport by Atmospheric Overturning Circulations2011Inngår i: Journal of Atmospheric Sciences, ISSN 0022-4928, E-ISSN 1520-0469, Vol. 68, nr 8, s. 1806-1820Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The atmospheric meridional overturning circulation is computed using the interim European Centre for Medium-Range Weather Forecasts Re-Analysis (ERA-Interim) data. Meridional mass transport streamfunctions are calculated not only using pressure as a vertical coordinate but also using temperature, specific humidity, and geopotential height as generalized vertical coordinates. Moreover, mass transport streamfunctions are calculated using the latent, the dry static, or the moist static energy as generalized vertical coordinates. The total meridional energy transport can be obtained by integrating these streamfunctions vertically over their entire energy range. The time-averaged mass transport streamfunctions are also decomposed into mean-flow and eddy-induced components. The meridional mass transport streamfunctions with temperature and specific humidity as independent variables yield a two-cell structure with a tropical Hadley-like cell and a pronounced extratropical Ferrel-like cell, which carries warm and moist air poleward. These Ferrel-like cells are much stronger than the Eulerian zonal-mean Ferrel cell, a feature that can be understood by considering the residual circulation related to specific humidity or temperature. Regardless of the generalized vertical coordinate, the present meridional mass transport streamfunctions yield essentially a two-layer structure with one poleward and one equatorward branch. The strongest meridional overturning in the midlatitudes is obtained when the specific humidity or the moist static energy is used as the vertical coordinate, indicating that the specific humidity is the variable that best distinguishes between poleward- and equatorward-moving air in the lower troposphere.

  • 117.
    Döös, Kristofer
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Rupolo, Volfango
    Brodeau, Laurent
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Dispersion of surface drifters and model-simulated trajectories2011Inngår i: Ocean Modelling, ISSN 1463-5003, E-ISSN 1463-5011, Vol. 39, nr 3-4, s. 301-310Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    From a data set encompassing the years 1990-2008 pairs of surface drifters with maximum initial separations of 5, 10 and 25 km have been identified. Model trajectories have been calculated using the same initial positions and times as the selected pairs of surface drifters. The model trajectories are based on the TRACMASS trajectory code and driven by the ocean general circulation model NEMO. The trajectories are calculated off-line, i.e. with the stored velocity fields from the circulation model. The sensitivity of the trajectory simulations to the frequency of the stored velocity fields was tested for periods of 3 and 6 h as well as 5 days. The relative dispersion of the surface-drifter and model trajectories has been compared, where the latter was found to be too low compared to the relative dispersion of the drifters. Two low-order trajectory sub-grid parameterisations were tested and successfully tuned so that the total amplitude of the relative dispersion of the model trajectories is similar to that associated with the drifter trajectories. These parameterisations are, however, too simple for a correct simulation of Lagrangian properties such as the correlation time scales and the variance of the eddy kinetic energy. The importance of model-grid resolution is quantified by comparing the relative dispersion from an eddy-permitting and a coarse-resolution model, respectively. The dispersion rate is halved with the coarse grid. The consequences of the two-dimensionality of the trajectories is evaluated by comparing the results obtained with the 2D and the Lagrangian 3D trajectories. This shows that the relative dispersion is 15% stronger when the trajectories are freely advected with the 3D velocity field.

  • 118. Eckhardt, S.
    et al.
    Hermansen, O.
    Grythe, Henrik
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för tillämpad miljövetenskap (ITM). NILU Norwegian Institute for Air Research, Norway.
    Fiebig, M.
    Stebel, K.
    Cassiani, M.
    Bäcklund, A.
    Stohl, A.
    The influence of cruise ship emissions on air pollution in Svalbard - a harbinger of a more polluted Arctic?2013Inngår i: Atmospheric Chemistry And Physics, ISSN 1680-7316, E-ISSN 1680-7324, Vol. 13, nr 16, s. 8401-8409Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    In this study we have analyzed whether tourist cruise ships have an influence on measured sulfur dioxide (SO2), ozone (O-3), Aitken mode particle and equivalent black carbon (EBC) concentrations at Ny Alesund and Zeppelin Mountain on Svalbard in the Norwegian Arctic during summer. We separated the measurement data set into periods when ships were present and periods when ships were not present in the Kongsfjord area, according to a long-term record of the number of passengers visiting Ny Alesund. We show that when ships with more than 50 passengers cruise in the Kongsfjord, measured daytime mean concentrations of 60 nm particles and EBC in summer show enhancements of 72 and 45 %, respectively, relative to values when ships are not present. Even larger enhancements of 81 and 72% were found for stagnant conditions. In contrast, O-3 concentrations were 5% lower on average and 7% lower under stagnant conditions, due to titration of O-3 with the emitted nitric oxide (NO). The differences between the two data subsets are largest for the highest measured percentiles, while relatively small differences were found for the median concentrations, indicating that ship plumes are sampled relatively infrequently even when ships are present although they carry high pollutant concentrations. We estimate that the ships increased the total summer mean concentrations of SO2, 60 nm particles and EBC by 15, 18 and 11 %, respectively. Our findings have two important implications. Firstly, even at such a remote Arctic observatory as Zeppelin, the measurements can be influenced by tourist ship emissions. Careful data screening is recommended before summertime Zeppelin data is used for data analysis or for comparison with global chemistry transport models. However, Zeppelin remains as one of the most valuable Arctic observatories, as most other Arctic observatories face even larger local pollution problems. Secondly, given landing statistics of tourist ships on Svalbard, it is suspected that large parts of the Svalbard archipelago are affected by cruise ship emissions. Thus, our results may be taken as a warning signal of future pan-Arctic conditions if Arctic shipping becomes more frequent and emission regulations are not strict enough.

  • 119.
    Egeland, Alv
    Stockholms universitet.
    Studies of auroral reflections in the VHF band.1962Doktoravhandling, monografi (Annet vitenskapelig)
  • 120.
    Ehard, Benedikt
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Achtert, Peggy
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU). Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för tillämpad miljövetenskap (ITM).
    Gumbel, Jörg
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Long-term lidar observations of wintertime gravity wave activity over northern Sweden2014Inngår i: Annales Geophysicae, ISSN 0992-7689, E-ISSN 1432-0576, Vol. 32, nr 11, s. 1395-1405Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    This paper presents an analysis of gravity wave activity over northern Sweden as deduced from 18 years of wintertime lidar measurements at Esrange (68ºN, 21ºE). Gravity wave potential energy density (GWPED) was used to characterize the strength of gravity waves in the altitude regions 30–40km and 40–50 km. The obtained values ex- ceed previous observations reported in the literature. This is suggested to be due to Esrange’s location downwind of the Scandinavian mountain range and due to differences in the various methods that are currently used to retrieve gravity wave parameters. The analysis method restricted the identification of the dominating vertical wavelengths to a range from 2 to 13 km. No preference was found for any wavelength in this window. Monthly mean values of GW- PED show that most of the gravity waves’ energy dissipates well below the stratopause and that higher altitude regions show only small dissipation rates of GWPED. Our analy- sis does not reproduce the previously reported negative trend in gravity wave activity over Esrange. The observed inter-annual variability of GWPED is connected to the occurrence of stratospheric warmings with generally lower wintertime mean GWPED during years with major stratospheric warmings. A bimodal GWPED occurrence frequency indicates that gravitywave activity at Esrange is affected by both ubiq- uitous wave sources and orographic forcing.

  • 121.
    Ekman, Annica
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Engström, Anders
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Wang, C.
    The effect of aerosol composition and concentration on the development and anvil properties of a continental deep convective cloud2007Inngår i: Quarterly Journal of the Royal Meteorological Society, ISSN 0035-9009, Vol. 133, nr 627, s. 1439-1452Artikkel i tidsskrift (Fagfellevurdert)
  • 122.
    Ekman, Annica
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Krejci, Radovan
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Engström, Anders
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Ström, Johan
    Institutionen för tillämpad miljövetenskap (ITM).
    de Reus, Marian
    Williams, Jonathan
    Andreae, Meinrat O.
    Do organics contribute to new particle formation in the Amazonian upper troposphere?2008Inngår i: Geophysical Research Letters, Vol. 35, s. L17810-Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    3-D cloud-resolving model simulations including explicit aerosol physics and chemistry are compared with observations of upper tropospheric (12 km) aerosol size distributions over the Amazon Basin. The model underestimates the aerosol number concentration for all modes, especially the nucleation mode (d< 18nm). We show that a boundary layer SO2 mixing ratio of approximately 5 ppb would be needed in order to reproduce the high nucleation mode number concentrations observed. This high SO2 mixing ratio is very unlikely for the pristine Amazon Basin at this time of the year. Hence, it is suggested that vapours other than H2SO4 participate in the formation and growth of small aerosols. Using activation nucleation theory together with a small (0.4-10%) secondary organic aerosol mass yield, we show that isoprene has the potential of substantially increasing the number of small particles formed as well as reducing the underestimate for the larger aerosol modes.

  • 123.
    Ekman, Annica M. L.
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Do sophisticated parameterizations of aerosol-cloud interactions in CMIP5 models improve the representation of recent observed temperature trends?2014Inngår i: Journal of Geophysical Research: Atmospheres, ISSN 2169-897X, Vol. 119, nr 2, s. 817-832Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Model output from the Coupled Model Intercomparison Project phase 5 (CMIP5) archive was compared with the observed latitudinal distribution of surface temperature trends between the years 1965 and 2004. By comparing model simulations that only consider changes in greenhouse gas forcing (GHG) with simulations that also consider the time evolution of anthropogenic aerosol emissions (GHGAERO), the influence of aerosol forcing on modeled surface temperature trends, and the dependence of the forcing on the model representation of aerosols and aerosol indirect effects, was evaluated. One group of models include sophisticated parameterizations of aerosol activation into cloud droplets; viz., the cloud droplet number concentration (CDNC) is a function of the modeled supersaturation as well as the aerosol concentration. In these models, the temperature trend bias was reduced in GHGAERO compared to GHG in more regions than in the other models. The ratio between high- and low-latitude warming also improved compared to observations. In a second group of models, the CDNC is diagnosed using an empirical relationship between the CDNC and the aerosol concentration. In this group, the temperature trend bias was reduced in more regions than in the model group where no aerosol indirect effects are considered. No clear difference could be found between models that include an explicit aerosol module and the ones that utilize prescribed aerosol. There was also no clear difference between models that include aerosol effects on the precipitation formation rate and the ones that do not. The results indicate that the best representation of recent observed surface temperature trends is obtained if the modeled CDNC is a function of both the aerosol concentration and the supersaturation. Key Points <list list-type=bulleted> <list-item id=jgrd51052-li-0001>CMIP5 GCMs disagree on late 20th century zonal average aerosol forcing <list-item id=jgrd51052-li-0002>Including aerosol indirect effects reduces the zonal mean temperature bias <list-item id=jgrd51052-li-0003>A more sophisticated parameterization of droplet activation is beneficial

  • 124.
    Ekman, Annica M. L.
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Engström, Anders
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Söderberg, Anders
    Impact of Two-Way Aerosol-Cloud Interaction and Changes in Aerosol Size Distribution on Simulated Aerosol-Induced Deep Convective Cloud Sensitivity2011Inngår i: Journal of Atmospheric Sciences, ISSN 0022-4928, E-ISSN 1520-0469, Vol. 68, nr 4, s. 685-698Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Recent cloud-resolving model studies of single (isolated) deep convective clouds have shown contradicting results regarding the response of the deep convection to changes in the aerosol concentration. In the present study, a cloud-resolving model including explicit aerosol physics and chemistry is used to examine how the complexity of the aerosol model, the size of the aerosols, and the aerosol activation parameterization influence the aerosol-induced deep convective cloud sensitivity. Six sensitivity series are conducted. A significant difference in the aerosol-induced deep convective cloud sensitivity is found when using different complexities of the aerosol model and different aerosol activation parameterizations. In particular, graupel impaction scavenging of aerosols appears to be a crucial process because it efficiently may limit the number of cloud condensation nuclei (CCN) at a critical stage of cloud development and thereby dampen the convection. For the simulated case, a 100% increase in aerosol concentration results in a difference in average updraft between the various sensitivity series that is as large as the average updraft increase itself. The change in graupel and rain formation also differs significantly. The sign of the change in precipitation is not always directly proportional to the change in updraft velocity and several of the sensitivity series display a decrease of the rain amount with increasing updraft velocity. This result illustrates the need to account for changes in evaporation processes and subsequent cooling when assessing aerosol effects on deep convective strength. The model simulations also show that an increased number of aerosols in the Aitken mode (here defined by 23 <= d <= 100.0 nm) results in a larger impact on the convective strength compared to an increased number of aerosols in the accumulation mode (here defined by 100 <= d <= 900.0 nm). When accumulation mode aerosols are activated and grow at the beginning of the cloud cycle, the supersaturation near the cloud base is lowered, which to some extent limits further aerosol activation. The simulations indicate a need to better understand and represent the two-way interaction between aerosols and clouds when studying aerosol-induced deep convective cloud sensitivity.

  • 125.
    Ekman, Annica M. L.
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Hermann, Markus
    Gross, Peter
    Heintzenberg, Jost
    Kim, Dongchul
    Wang, Chien
    Sub-micrometer aerosol particles in the upper troposphere/lowermost stratosphere as measured by CARIBIC and modeled using the MIT-CAM3 global climate model2012Inngår i: Journal of Geophysical Research, ISSN 0148-0227, E-ISSN 2156-2202, Vol. 117, s. D11202-Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    In this study, we compare modeled (MIT-CAM3) and observed (CARIBIC) sub-micrometer nucleation (N4-12, 4 <= d <= 12 nm) and Aitken mode (N-12, d > 12 nm) particle number concentrations in the upper troposphere and lowermost stratosphere (UT/LMS). Modeled and observed global median N4-12 and N-12 agree fairly well (within a factor of two) indicating that the relatively simplified binary H2SO4-H2O nucleation parameterization applied in the model produces reasonable results in the UT/LMS. However, a comparison of the spatiotemporal distribution of sub-micrometer particles displays a number of discrepancies between MIT-CAM3 and CARIBIC data: N4-12 is underestimated by the model in the tropics and overestimated in the extra-topics. N-12 is in general overestimated by the model, in particular in the tropics and during summer months. The modeled seasonal variability of N4-12 is in poor agreement with CARIBIC data whereas it agrees rather well for N-12. Modeled particle frequency distributions are in general narrower than the observed ones. The model biases indicate an insufficient diffusive mixing in MIT-CAM3 and a too large vertical transport of carbonaceous aerosols. The overestimated transport is most likely caused by the constant supersaturation threshold applied in the model for the activation of particles into cloud droplets. The annually constant SO2 emissions in the model may also partly explain the poor representation of the N4-12 seasonal cycle. Comparing the MIT-CAM3 with CARIBIC data, it is also clear that care has to be taken regarding the representativeness of the measurement data and the time frequency of the model output.

  • 126.
    Ekman, Annica M. L.
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Lewinschal, Anna
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Struthers, Hamish
    Can an influence of changing aerosol emissions be detected in thepattern of surface temperature change between 1970 and 2000?Manuskript (preprint) (Annet vitenskapelig)
    Abstract [en]

    The general circulation model CAM-Oslo was used to examine the influence of varyingaerosol and greenhouse gas emissions on the pattern of surface temperature change betweenthe years 1970 and 2000, and whether the temperature response over different regions wasgoverned by local (due to changes in energy fluxes) or far-field (due to changes in large-scale circulation) processes. Circulation changes, originating from precipitation anomaliesmainly over the west/central Pacific and off the east coast of North America, influenced asubstantial part of the northern hemisphere temperature change pattern in CAM-Oslo, inparticular over southern North America, but also over Europe and Asia. The result highlightsthe importance of better understanding zonally asymmetric precipitation changes due todifferent forcing agents. A local response in surface temperature due to net surface radiativeflux (RF) anomalies could also be detected over Europe and Asia, where the differencein all-sky net surface RF was mainly driven by aerosol- or circulation-induced changes inliquid water path and cloud cover. A local anthropogenic aerosol effect on the cloud dropletsize and subsequent short-wave (SW) RF was found over Europe and Asia, but only whenexcluding a change in the greenhouse gas concentration. For clear skies, the SW RF patternwas well-correlated with the aerosol optical depth anomalies. However, this correlation wasat least partly governed by relative humidity fluctuations. Overall, the greenhouse andaerosol effects on surface temperature were in the simulations found to be non-linear with asignificant feedback on the aerosol population from a warming climate.

  • 127.
    Ekström, Sanna
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för tillämpad miljövetenskap (ITM).
    Nozière, Barbara
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för tillämpad miljövetenskap (ITM).
    Alsberg, Tomas
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för tillämpad miljövetenskap (ITM).
    Hultberg, Malin
    Magnér, Jorgen
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för tillämpad miljövetenskap (ITM).
    Nilsson, E. Douglas
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för tillämpad miljövetenskap (ITM).
    Artaxo, Paulo
    Of Bacteria and clouds: when microbial substances trigger cloud formation in Earth’s atmosphere2008Inngår i: American Geophysical Union Fall Meeting 2008, 2008Konferansepaper (Fagfellevurdert)
    Abstract [en]

    This presentation reports the discovery that substances produced by microorganisms might trigger the formation of cloud in the atmosphere, at least under certain conditions.

    The Cloud Condensation Nuclei (CCN) efficiency of substances produced by microorganisms (bacteria, fungi, micro-algae …) that are common at Earth’s surface and in the oceans were studied. Their Köhler curves were determined experimentally by surface tension and osmometry measurements and found to have much lower critical supersaturations than any material studied so far, including inorganic salts.

    The presence of these substances was evidenced in aerosols from four different origins (coastal, marine, temperate forest, and Amazonian forest) by LC/MS/MS analyses and by their unique signature on the surface tension. These substances lowered the surface tension of the aerosols below 40 mN/m, allowing them to be activated into cloud droplets before inorganic particles.

    Microorganisms would thus be able to control cloud formation in Earth’s atmosphere under certain conditions. This would explain many previous observations such as correlations between algae bloom and cloud cover. Most importantly, this work identifies a potentially important component of Earth’s hydrological cycle and a new direct link between biosphere and climate.

  • 128.
    Ekström, Sanna
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för tillämpad miljövetenskap (ITM).
    Nozière, Barbara
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för tillämpad miljövetenskap (ITM).
    Hansson, Hans Christen
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för tillämpad miljövetenskap (ITM).
    First direct measurements of the CCN properties of 2-methyltetrols and polyols2008Inngår i: Geophysical Research Abstracts, 2008Konferansepaper (Fagfellevurdert)
    Abstract [en]

    2-methyltetrols and polyols have received a lot of attention in recent years. 2-methyltetrols have been found in aerosols in various regions are believed to be formed by the oxidation of isoprene. Polyols are produced by fungi and have been measured

    in large concentrations in aerosols. The main reason of interest of both 2-methyltetrols and polyols as efficient cloud condensation nuclei (CCN) is due to their high solubility.

    This presentation will report for the first time the experimental determination of complete Köhler curves for 2-methyltetrols (2-methylerythritol and 2-methylthreitol), C3 to C6 polyols (glycerol, erythritol, arabitol, and mannitol), and for comparison their

    analogue di-acids (malonic acid, succinic acid and, adipic acid). The original Köhler equations were determined from osmolality and tensiometry measurements of the compounds both in water and salt solutions (sodium chloride and ammonium sulphate).

    The results indicate that the polyols generally have similar CCN properties as the dicarboxylic acids. The critical supersaturation for aerosol particles with a 30 nm radius were: 2-methyltetrol; 0.68%, mannitol; 0.62%, arabitol; 0.60%, 2-methylerythritol;

    0.57%, erythritol; 0.56%, glycerol; 0.53%, adipic acid; 0.52%, succinic acid; 0.49%, and malonic acid; 0.44%. Mixtures of salts had lower critical supersaturation than water solutions, especially for the polyols. One exception was 2-methylerythritol, which interestingly was less efficient as CCN in salt solutions.

    The CCN efficiency of the polyols is believed to result mostly from their large water affinity, enforcing the Raoult effect, while organic acids lower the Kelvin effect. The very large solubility of polyols compared to the di-acids mean that they could positive effect in the initial phase of the droplet growth while the di-acids cannot. 2-methyltetrols were found to have both a Kelvin and a Raoult effect.

    In addition, these results establish for the first time that the 3-dimensional structure of molecules can have an effect on their CCN properties. The two isomers of the 2-methyltetrols have significantly different CCN properties that are also influenced

    oppositely in the presence of salts.

  • 129. El-Hames, A. S.
    et al.
    Hannachi, Abdelwaheb
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Al-Ahmadi, M.
    Al-Amri, N.
    Groundwater Quality Zonation Assessment using GIS, EOFs and Hierarchical Clustering2013Inngår i: Water resources management, ISSN 0920-4741, E-ISSN 1573-1650, Vol. 27, nr 7, s. 2465-2481Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Three methods are utilized in this paper to assist in the groundwater clustering, in an arid region aquifer, into similar zones according to its quality. A multiple regression is first applied in order to assess the importance of the different chemical constituents in the amount of total dissolved salt, which shows the dominance of chlorine and sodium. A multivariate analysis based on empirical orthogonal functions and hierarchical clustering (EOFs) is applied to assist in water quality clustering in the studied aquifer. The clustering has produced five distinguished categories of groundwater quality, which agree well with World Health Organisation criteria and limits for water usage. Based on these categories, spatial distribution maps of groundwater quality are produced by Kriging and GIS software.

  • 130.
    Elihn, Karine
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för tillämpad miljövetenskap (ITM).
    Berg, Peter
    Ultrafine particle characteristics in seven industrial plants.2009Inngår i: Annals of Occupational Hygiene, ISSN 0003-4878, E-ISSN 1475-3162, Vol. 53, nr 5, s. 475-484Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Ultrafine particles are considered as a possible cause of some of the adverse health effects caused by airborne particles. In this study, the particle characteristics were measured in seven Swedish industrial plants, with a special focus on the ultrafine particle fraction. Number concentration, size distribution, surface area concentration, and mass concentration were measured at 10 different job activities, including fettling, laser cutting, welding, smelting, core making, moulding, concreting, grinding, sieving powders, and washing machine goods. A thorough particle characterization is necessary in workplaces since it is not clear yet which choice of ultrafine particle metric is the best to measure in relation to health effects. Job activities were given a different order of rank depending on what particle metric was measured. An especially high number concentration (130 x 10(3) cm(-3)) and percentage of ultrafine particles (96%) were found at fettling of aluminium, whereas the highest surface area concentration (up to 3800 mum(2) cm(-3)) as well as high PM10 (up to 1 mg m(-3)) and PM1 (up to 0.8 mg m(-3)) were found at welding and laser cutting of steel. The smallest geometric mean diameter (22 nm) was found at core making (geometric standard deviation: 1.9).

  • 131.
    Elihn, Karine
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för tillämpad miljövetenskap (ITM).
    Berg, Peter
    Lidén, Göran
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för tillämpad miljövetenskap (ITM).
    Correlation between airborne particle concentrations in seven industrial plants and estimated respiratory tract deposition by number, mass and elemental composition2011Inngår i: Journal of Aerosol Science, ISSN 0021-8502, E-ISSN 1879-1964, Vol. 42, nr 2, s. 127-141Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The number and mass distribution of airborne particles were recorded in several industrial plants. From the data obtained, particle deposition was estimated in three regions of the respiratory tract using the ICRP grand average deposition model based on Hinds' (1999) parameterization. The median diameter was 30-70 nm (number distributions), and >4 mu m (mass distributions) near most work activities, resulting in linear relationships between the deposited number/mass concentrations and the number/mass concentrations in the air. Welding and laser cutting produced particles in the 200-500-nm range; total deposition was small, not in accordance with the linear relationship observed for the other work activities. The elemental content varied between particle sizes in some workplaces, causing different elements to deposit in different respiratory regions. Iron was the most abundant element in the particles in many of the workplaces; in an iron foundry, however, Fe was most abundant only in the micron-sized particles whereas the nanoparticles mainly comprised Pb and Sb.

  • 132. Eneroth, Kristina
    Atmospheric transport of carbon dioxide and other trace species in high northern latitudes2003Doktoravhandling, med artikler (Annet vitenskapelig)
  • 133.
    Engardt, Magnuz
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Climate change and carbon dioxide fluxes in the high latitude northern hemisphere1997Doktoravhandling, med artikler (Annet vitenskapelig)
  • 134. Enger, Leif
    et al.
    Tjernström, Michael
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Estimating the effects on the regional precipitation climate in a semiarid region caused by an artificial lake using a mesoscale model1991Inngår i: Journal of applied meteorology (1988), ISSN 0894-8763, E-ISSN 1520-0450, Vol. 30, s. 227-250Artikkel i tidsskrift (Fagfellevurdert)
  • 135.
    Engström, Anders
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Aerosol-cloud interaction from an observational and modeling perspective2011Doktoravhandling, med artikler (Annet vitenskapelig)
    Abstract [en]

    Clouds may respond strongly to changes in the atmospheric aerosol population, and the response of clouds to an increased global aerosol burden could to some extent mask the warming caused by enhanced greenhouse gas concentrations. However, estimates of the impact of aerosols on cloud properties are associated with large uncertainties, both because of difficulties representing the aerosol-cloud interaction within models, and because of problems of unequivocally isolating the effect of aerosols on cloud properties in observational data. This thesis focuses in part on underlying meteorological factors that significantly correlate with both aerosol and cloud properties, and on how sensitive clouds are to small variations in meteorological conditions. It was found that meteorological covariations must be taken into account when estimating the strength of the relationship between aerosols and cloud properties. By studying the response of shallow convective clouds to perturbations in meteorological conditions and aerosol concentration, it was further concluded that variations in meteorological conditions can enhance or mask the relationship between aerosols and cloud properties, making it difficult to isolate the aerosol signature from small meteorological differences. Additionally, the impact of deep convective clouds on the redistribution of aerosols within a cloud life cycle is examined. It was found that mid-tropospheric aerosols can have a substantial source in evaporating cloud droplets within deep convection. Lastly, this thesis focuses on the implications of meteorological analysis uncertainties, in part related to the difficulties of constraining meteorological variability in observational data of clouds and aerosols, but mainly the impact of analysis errors on atmospheric trajectory calculations. A method is presented to consistently estimate the uncertainty in trajectory calculations. It was concluded that the spatial and temporal trajectory error can be substantially underestimated if the analysis error is not taken into account.

  • 136.
    Engström, Anders
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Bender, Frida A. -M.
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Charlson, Robert J.
    Wood, Robert
    Geographically coherent patterns of albedo enhancement and suppression associated with aerosol sources and sinks2015Inngår i: Tellus. Series B, Chemical and physical meteorology, ISSN 0280-6509, E-ISSN 1600-0889, Vol. 67, s. 1-9, artikkel-id 26442Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Earth's albedo is the primary determinant of the amount of energy absorbed by the Earth-atmosphere system. It is a function of the fractional cloud cover and the cloudy-and clear-sky albedos, and thereby of the aerosol loading of the atmosphere. Here, we introduce a method by which we can examine the spatial distribution of the albedo variability that is independent of variations in the two dominant factors of albedo: cloud fraction and liquid water path (LWP). The analysis is based on data simultaneously retrieved from the CERES and MODIS instruments carried on board the Aqua satellite. We analysed the daily overpass data between July 2002 and June 2014 and showed that perturbations in albedo, accounting for variations induced by cloud fraction and LWP, display a coherent geographical pattern. Positive deviations occur in proximity to known anthropogenic aerosol sources, and negative deviations coincide with areas of intense precipitation, acting as aerosol sinks. A simple multiplication of the observed positive perturbations in albedo with the solar flux of 340Wm(-2) yields a magnitude of that effect of several watts per square meter locally. While the location and scale of the geographical pattern might suggest an anthropogenic contribution to the positive albedo perturbations, it is imperative to first carefully examine all other possible causal factors behind the perturbations. Finally, although we have not attempted a full calculation of detection limits, the analysis is capable of sensing very small changes in average albedo of the order of 0.003 out of a total albedo of the order of 0.3. Hence, the applied method might find utilisation in a variety of situations where there is a need to quantify small perturbations of a dependent variable in noisy global data sets.

  • 137.
    Engström, Anders
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Bender, Frida A.-M.
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Charlson, R. J.
    Wood, R.
    The nonlinear relationship between albedo and cloud fraction on near-global, monthly mean scale in observations and in the CMIP5 model ensemble2015Inngår i: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 42, nr 21, s. 9571-9578Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We study the relation between monthly mean albedo and cloud fraction over ocean,60∘S–60∘N. Satellite obser vations indicate that these clouds all fall on the same near-exponential curve,with a monotonic distribution over the ranges of cloud fractions and albedo. Using these observational dataas a reference, we examine the degree to which 26 climate models capture this feature of the near-globalmarine cloud population. Models show a general increase in albedo with increasing cloud fraction, but noneof them display a relation that is as well defined as that characterizing the observations. Models t ypicallydisplay larger albedo variability at a given cloud fraction, larger sensitivity in albedo to changes in cloudfraction, and lower cloud fractions. Several models also show branched distributions, contrasting with thesmooth observational relation. In the models the present-day cloud scenes are more reflective than thepreindustrial, demonstrating the simulated impact of anthropogenic aerosols on planetary albedo.

  • 138.
    Engström, Anders
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Ekman, Annica M.L.
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Impact of meteorological factors on the correlation between aerosol optical depth and cloud fraction2010Inngår i: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 37, s. L18814-Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The aerosol optical depth has in several recent studies been found to correlate with cloud fraction. This study examines the global distribution of the total correlation between aerosol optical depth, cloud fraction and meteorological conditions using satellite observations together with atmospheric re-analysis data from the ECMWF. The results show large regional differences in the correlation between aerosol optical depth and cloud fraction, where a higher correlation is found over remote ocean. The one meteorological variable that correlates significantly with both aerosol optical depth and cloud fraction is the 10-meter wind speed. Constructing the partial correlation between aerosol optical depth and cloud fraction, with the impact from 10-meter wind speed removed, yields a significant difference compared to the total correlation. In several regions the remaining partial correlation is reduced from 0.4 to below 0.1. The results highlight the need to investigate all possible correlations between meteorological variables, cloud properties and aerosols. Citation: Engstrom, A., and A. M. L. Ekman (2010), Impact of meteorological factors on the correlation between aerosol optical depth and cloud fraction

  • 139.
    Engström, Anders
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Ekman, Annica M.L.
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Modeled response in radiative properties of isolated shallow convective clouds due to perturbations in meteorological state variables and atmospheric aerosol loadingManuskript (preprint) (Annet vitenskapelig)
    Abstract [en]

    The aim of the present study is to estimate the range of variability in observational data of meteorological variables permitted to determine a clear and unambiguous signal in cloud fraction and cloud albedo due to changes in aerosol concentrations. Three different cases of tropical shallow convection have been simulated using a two-dimensional cloud resolving model with aerosol-cloud interactions. For each case, 30 different small perturbations were added to the initial meteorological profiles of zonal wind, potential temperature and specific humidity. For each meteorological per- turbation an additional 14 simulations were performed with different boundary layer accumulation mode aerosol concentrations ranging from clean (50 cm-3) to polluted (1350 cm-3) conditions. A total of 5040 simulations were performed to elucidate the impact of aerosols on the simulated cloud fraction and cloud albedo and to compare this to the corresponding impact to the influence of small meteorological perturbations. For the simulated cases, we find that for cloud fraction, the aerosol signal is in general much weaker than the change due to small changes in meteorology. This was especially true for parameters that influence the relative humidity of the environment, i.e. tempera- ture and specific humidity. For cloud albedo, the aerosol signal surpasses that of the relatively small meteorological perturbations. We find up to 40% difference in cloud albedo going from clean to polluted conditions. The corresponding maximum change due to the meteorological perturbations was only 14%. We conclude that for the simulated shallow convective clouds, isolating an aerosol effect on cloud fraction is not possible if using meteorological analysis data containing errors of the same order of magnitude as the imposed meteorological perturbations. However, the meteorological constraints may be sufficient to isolate an aerosol effect on cloud albedo.

  • 140.
    Engström, Anders
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Ekman, Annica M.L.
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Krejci, Radovan
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Ström, Johan
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för tillämpad miljövetenskap (ITM).
    de Reus, Marian
    Wang, Chien
    Observational and modelling evidence of tropical deep convective clouds as a source of mid-tropospheric accumulation mode aerosols2008Inngår i: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 35, s. L23813-Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    High concentrations (up to 550 cm−3 STP) of aerosols in the accumulation mode (>0.12 μm) were observed by aircraft above 7.5 km altitude in the dynamically active regions of several deep convective clouds during the INDOEX campaign. Using a coupled 3-D aerosol-cloud-resolving model, we find that significant evaporation of hydrometeors due to strong updrafts and exchange with ambient air occurs at the boundaries and within the cloud tower. Assuming that each evaporated hydrometeor release an aerosol, an increase in the aerosol concentration by up to 600 cm−3 STP is found in the model at altitudes between 6 and 10 km. The evaporation and release of aerosols occur as the cloud develops, suggesting that deep convective clouds are important sources of mid-tropospheric aerosols during their active lifetime. This source may significantly impact the vertical distribution as well as long-range transport of aerosols in the free troposphere.

  • 141.
    Engström, Anders
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Karlsson, Johannes
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Svensson, Gunilla
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    The Importance of Representing Mixed-Phase Clouds for Simulating Distinctive Atmospheric States in the Arctic2014Inngår i: Journal of Climate, ISSN 0894-8755, E-ISSN 1520-0442, Vol. 27, nr 1, s. 265-272Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Observations from the Surface Heat Budget of the Arctic Ocean experiment (SHEBA) suggest that the Arctic Basin is characterized by two distinctly different preferred atmospheric states during wintertime. These states appear as two peaks in the frequency distribution of surface downwelling longwave radiation (LWD), representing radiatively clear and opaque conditions. Here, the authors have investigated the occurrence and representation of these states in the widely used ECMWF Interim Re-Analysis (ERA-Interim) dataset. An interannually recurring bimodal distribution of LWD values is not a clearly observable feature in the reanalysis data. However, large differences in the simulated liquid water content of clouds in ERA-Interim compared to observations are identified and these are linked to the lack of a radiatively opaque peak in the reanalysis. Using a single-column model, dynamically controlled by data from ERA-Interim, the authors show that, by tuning the glaciation speed of supercooled liquid clouds, it is possible to reach a very good agreement between the model and observations from the SHEBA campaign in terms of LWD. The results suggest that the presence of two preferred Arctic states, as observed during SHEBA, is a recurring feature of the Arctic climate system during winter [December–March (DJFM)]. The mean increase in LWD during the Arctic winter compared to ERA-Interim is 15 W m−2. This has a substantial bearing on climate model evaluation in the Arctic as it indicates the importance of representing Arctic states in climate models and reanalysis data and that doing so could have a significant impact on winter ice thickness and surface temperatures in the Arctic.

  • 142.
    Engström, Anders
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Magnusson, Linus
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Estimating trajectory uncertainties due to flow dependent errors in the atmospheric analysis2009Inngår i: Atmospheric Chemistry And Physics, ISSN 1680-7316, E-ISSN 1680-7324, Vol. 9, nr 22, s. 8857-8867Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The uncertainty of a calculated trajectory is dependent on the uncertainty in the atmospheric analysis. Using the Ensemble Transform method (originally adapted for ensemble forecasting) we sample the analysis uncertainty in order to create an ensemble of analyses where a trajectory is started from each perturbed analysis. This method, called the Ensemble analysis method (EA), is compared to the Initial spread method (IS), where the trajectory receptor point is perturbed in the horizontal and vertical direction to create a set of trajectories used to estimate the trajectory uncertainty. The deviation growth is examined for one summer and one winter month and for 15 different geographical locations. We find up to a 40% increase in trajectory deviation in the mid-latitudes using the EA method. A simple model for trajectory deviation growth speed is set up and validated. It is shown that the EA method result in a faster error growth compared to the IS method. In addition, two case studies are examined to qualitatively illustrate how the flow dependent analysis uncertainty can impact the trajectory calculations. We find a more irregular behavior for the EA trajectories compared to the IS trajectories and a significantly increased uncertainty in the trajectory origin. We conclude that by perturbing the analysis in agreement with the analysis uncertainties the error in backward trajectory calculations can be more consistently estimated.

  • 143.
    Engström, Erik J.
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Characterization of soot in air and rain over southern Asia2009Doktoravhandling, med artikler (Annet vitenskapelig)
    Abstract [en]

    Filter-based optical measurements of light absorbing particulate matter at awavelength of about 550 nm, here referred to as soot, in air and rainwaterhave been performed during the period from 1st June 2005 to 31th May 2009at Godavari in Nepal, Sinhagad in India and Hanimaadhoo in The Maldives.A method for determination of water-insoluble light absorbing matter inrainwater has been developed. Analysis of environmental samples has beensuccessfully performed with the described method on samples collected atHanimaadhoo and Godavari. At Hanimaadhoo the average soot concentrationin rainwater was 48 mgl-1 and at Godavari 86 mgl-1.In order to reduce systematic errors at optical determination of soot due tothe light scattering of non-absorbing particles co-deposited on the filter, suchas inorganic salts and mineral dust, an additional sensor recording backscatteredlight was used. Two alternative protocols of corrections (optical andchemical) were applied to the samples. Simultaneous measurements of sootand inorganic ions in aerosol and precipitation at Hanimaadhoo during theperiod May 2005 to February 2007 made it possible to calculate the washoutratio (WR) of these components as a measure of how efficiently they are scavengedby precipitation. During the monsoon season the WR for soot was similarto that of sulphate and other fine mode aerosol components, indicating thatsoot containing particles in these situations were efficient as cloud condensationnuclei. During the polluted winter days, on the other hand, the WR forsoot was 3 times smaller than that of sulphate, showing that the soot containingparticles had retained a hydrophobic character even after a travel time ofseveral days.The Indian monsoon circulation with its two annual phases in combinationwith the location of the main combustion source areas dominated the observedpatterns of soot at the observatories in India and Maldives. Godavari in Nepalwas however mainly influenced by combustion sources all year around concealingpossible variability related to the monsoon circulation. At Hanimaadhoo,peak values in the soot concentration occurred during the winter season(December to April) when air was transported from the polluted Indian subcontinentout over the Indian Ocean. At least a factor of ten lower values wererecorded in air that had spent more than 10-days over the Indian Ocean duringthe monsoon season (July to September).

     

  • 144.
    Engström, Erik J.
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Leck, Caroline
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Reducing uncertainties associated with filter-based optical measurements of soot aerosol particles with chemical information.Inngår i: Atmospheric Chemistry And Physics, ISSN 1680-7316, E-ISSN 1680-7324Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Of the many identified and potential effects ofatmospheric aerosol particles on climate, those of soot particlesare the most uncertain, in that analytical techniques concerningsoot are far from satisfactory. One concern whenapplying filter-based optical measurements of soot is thatthey suffer from systematic errors due to the light scatteringof non-absorbing particles co-deposited on the filter, suchas inorganic salts and mineral dust. In addition to an opticalcorrection of the non-absorbing material this study providesa protocol for correction of light scattering based onthe chemical quantification of the material, which is a novelty.A newly designed Particle Soot Absorption Photometerwas constructed to measure light transmission on particleaccumulating filters, which includes an additional sensorrecording backscattered light. The choice of polycarbonatemembrane filters avoided high chemical blank values and reducederrors associated with length of the light path throughthe filter.Two protocols of corrections were applied to aerosol samplescollected at the Maldives Climate Observatory Hanimaadhooduring episodes with either continentally influencedair from the Indian/Arabian subcontinents (wintermonsoon) or pristine air from the Southern Indian Ocean(summer monsoon). The two ways of correction (optical andchemical) lowered the particle light absorption of soot by 63 to 61 %, respectively, for data from the Arabian Sea sourcedgroup, resulting in median soot absorption coefficients of 4.2 and 3.5 Mm-1. Corresponding values for the South IndianOcean data were 69 and 97 % (0.38 and 0.02 Mm-1). A comparison with other studies in the area indicated anoverestimation of their soot levels, by up to two orders ofmagnitude. This raises the necessity for chemical correctionprotocols on optical filter-based determinations of soot, before even the sign on the radiative forcing based on their effectscan be assessed.

  • 145.
    Engström, Erik
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Leck, Caroline
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Determination of water-insoluble light absorbing matter in rainwater using polycarbonate membrane lters and photometricdetection.Artikkel i tidsskrift (Fagfellevurdert)
  • 146.
    Engström, Erik
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Leck, Caroline
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Seasonal variability in light absorption particulate matter or soot in air at three stations in the South-Asian region situated in Nepal, India and MaldivesManuskript (preprint) (Annet vitenskapelig)
    Abstract [en]

    Filter-based optical measurements of light absorbing particulate matter at awavelength of about 550 nm, referred to as soot, in air have been performed during theperiod from 1st June 2005 to 31th May 2009 at Godavari in Nepal, Sinhagad in India andHanimaadhoo in the Maldives. In order to reduce systematic errors due to the lightscattering of non-absorbing particles co-deposited on the filter, such as inorganic saltsand mineral dust, an additional sensor recording backscattered light was implemented.Two protocols of corrections (optical and chemical) were applied to the samplescollected at the observatories. The Indian monsoon circulation with its two annualphases in combination with the location of the combustion sources and their contributionrelative other non-anthropogenic sources dominated the observed patterns of soot at theobservatories in India and Maldives. The observatory in Nepal was however mainlyinfluenced by combustion sources all year around concealing possible variability relatedto the monsoon circulation. At the receptor observatory in the Maldives, peak values inthe soot absorption coefficient occurred during the winter season (December to April)when air was transported from the polluted Indian subcontinent out over the IndianOcean. A close to two orders of magnitude lower values were recorded in air that hadspent more than 10-days over the Indian Ocean during the monsoon season (July toSeptember), suggested to be dominated by particulate matter from remote marinebiogenic sources.

  • 147.
    Engström, J. E.
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Leck, Caroline
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Reducing uncertainties associated with filter-based optical measurements of light absorbing carbon particles with chemical information2011Inngår i: Atmospheric Measurement Techniques, ISSN 1867-1381, E-ISSN 1867-8548, Vol. 4, nr 8, s. 1553-1566Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The presented filter-based optical method for determination of soot (light absorbing carbon or Black Carbon, BC) can be implemented in the field under primitive conditions and at low cost. This enables researchers with small economical means to perform monitoring at remote locations, especially in the Asia where it is much needed. One concern when applying filter-based optical measurements of BC is that they suffer from systematic errors due to the light scattering of non-absorbing particles co-deposited on the filter, such as inorganic salts and mineral dust. In addition to an optical correction of the non-absorbing material this study provides a protocol for correction of light scattering based on the chemical quantification of the material, which is a novelty. A newly designed photometer was implemented to measure light transmission on particle accumulating filters, which includes an additional sensor recording backscattered light. The choice of polycarbonate membrane filters avoided high chemical blank values and reduced errors associated with length of the light path through the filter. Two protocols for corrections were applied to aerosol samples collected at the Maldives Climate Observatory Hanimaadhoo during episodes with either continentally influenced air from the Indian/Arabian subcontinents (winter season) or pristine air from the Southern Indian Ocean (summer monsoon). The two ways of correction (optical and chemical) lowered the particle light absorption of BC by 63 to 61 %, respectively, for data from the Arabian Sea sourced group, resulting in median BC absorption coefficients of 4.2 and 3.5 Mm(-1). Corresponding values for the South Indian Ocean data were 69 and 97% (0.38 and 0.02 Mm(-1)). A comparison with other studies in the area indicated an overestimation of their BC levels, by up to two orders of magnitude. This raises the necessity for chemical correction protocols on optical filter-based determinations of BC, before even the sign on the radiative forcing based on their effects can be assessed.

  • 148.
    Engström, J. Erik
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Leck, Caroline
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Seasonal variability in atmospheric black carbon at three stations in South-Asia2017Inngår i: Tellus. Series B, Chemical and physical meteorology, ISSN 0280-6509, E-ISSN 1600-0889, Vol. 69, artikkel-id 1331102Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Filter-based optical measurements of black carbon in air, a constituent of soot, have been determined with a 528 nm light source during the period from 1 June 2005 to 31 May 2009 on samples taken at Godavari in Nepal, Sinhagad in India and Hanimaadhoo in the Maldives. In order to reduce systematic errors due to the light scattering of non-absorbing particles co-deposited on the filter, such as inorganic salts and mineral dust, an additional sensor recording backscattered light was implemented. Two protocols of corrections (optical and chemical) were applied to the samples collected at the observatories. The Indian monsoon circulation with its two annual phases in combination with the location of the combustion sources and their contribution relative to other non-anthropogenic sources dominated the observed patterns of black carbon at two of the observatories: in India and the Maldives. The observatory in Nepal was however mainly influenced by combustion sources all year around concealing possible variability related to the monsoon circulation. At the receptor observatory in the Maldives, peak values in the black carbon absorption coefficient occurred during the winter season (December to April) when air was transported from the polluted Indian subcontinent out over the Indian Ocean. A close to two orders of magnitude lower values were recorded in air that had spent more than 10-days over the Indian Ocean during the monsoon season (July to September), suggested to be dominated by particulate matter from remote marine biogenic sources and not by combustion sources.

  • 149.
    Engvall, Ann-Christine
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Properties and Origin of Arctic Aerosols2008Doktoravhandling, med artikler (Annet vitenskapelig)
    Abstract [en]

    The present thesis deals with the origin and physics of aerosols in the Arctic atmosphere. These show a large annual variability due to changes of the photochemical and cloud processes as well as of the synoptic-scale atmospheric pressure patterns. High concentrations of anthropogenic trace gases and particles are found in the atmosphere during winter and spring, whereas the summer period is least affected as regards human impact. The thesis is based on a synthesis of aerosol observations from ground stations as well as research aircraft. A major goal was to study the shift that the Arctic aerosol-size distribution undergoes from spring to summer, a transition that takes place during a rather short period of around 10 days. Six years of aerosol, chemical, and transport data are investigated for the April-June period. This analysis indicates that the rapid transition is governed by a delicate balance between insolation and the source and sink processes affecting the aerosol. In-situ observations show that exchange processes between the boundary layer and the free troposphere may be a key component governing the temporal evolution of the aerosol during summer. It has been concluded that air-borne measurements are essential for establishing the vertical distribution of the aerosol (knowledge of which may be essential when analysing long-term and point measurements). As emphasized in the thesis, insights concerning this vertical structure are especially valuable when layers aloft show concentrations of soot or light-absorbing aerosol and, in addition, the environment is highly reflecting, as is the case in the Arctic. Such plumes, transported from lower latitudes and difficult to detect from the surface, are suggested to have contributed to the high-altitude Arctic warming trend observed during the last two decades. The results in this thesis underline that merging long-term observations with aircraft measurements is highly useful when studying aerosol and its effects.

  • 150.
    Engvall, Ann-Christine
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Krejci, Radovan
    Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).
    Ström, Johan
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för tillämpad miljövetenskap (ITM).
    Minikin, Andreas
    Treffeisen, Renate
    Stohl, Andreas
    Herber, Andreas
    In-situ airborne observations of the microphysical properties of the Arctic tropospheric aerosol during late spring and summer2008Inngår i: Tellus. Series B, Chemical and physical meteorology, ISSN 0280-6509, E-ISSN 1600-0889, Vol. 60, nr 3, s. 392-404Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    In-situ aerosol data collected in the Arctic troposphere during a three-week period in 2004 were analysed. The measurements took place during late spring, i.e., at the time of the year when the characteristics of the aerosol distribution change from being accumulation-mode dominated to being primarily of the Aitken-mode type, a process that previously has been observed in the boundary layer. To address the question whether this transition is also detectable in the free troposphere of an aircraft-measured data from the ASTAR 2004 campaign were analysed. In this study, we present vertically as well as temporally results from both ground-based and airborne measurements of the total number concentrations of particles larger than 10 and 260 nm. Aircraft-measured size distributions of the aerosol ranging from 20 to 2200 nm have been evaluated with regard to conditions in the boundary layer as well as in the free troposphere. Furthermore an analysis of the volatile fraction of the aerosol population has been performed both for the integrated and size-distributed results. From these investigations we find that the transition takes place in the entire troposphere.

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