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  • 1.
    Glantz, Paul
    Stockholm University, Faculty of Science, Department of Applied Environmental Science (ITM).
    Satellite retrieved cloud optical thickness sensitive to surface wind speed in the subarctic marine boundary layer2010In: Environmental research letters, ISSN 1748-9326, Vol. 5, no 3, 034002- p.Article in journal (Refereed)
    Abstract [en]

    The optical and microphysical properties of low level marine clouds, presented over the Norwegian Sea and Barents Sea, have been investigated for the period 2000-2006. The air masses were transported for more or less seven days over the warmer North Atlantic before they arrived at the area investigated. The main focus in this study is on investigating the relationship between cloud optical thickness (COT) and surface wind speed (U-10 m) using satellite retrievals in combination with operational meteorological data. A relatively strong correlation (R-2 = 0.97) is obtained for wind speeds up to 12 m s(-1), in air masses that were probably to a major degree influenced by wind shears and to a minor degree by buoyancy. The relationship (U-2.5) is also in between those most commonly found in the literature for water vapor (similar to U-1) and sea salt (similar to U-3.4). The present results highlight the magnitude of marine sea-spray influence on COT and their global climatic importance.

  • 2.
    Glantz, Paul
    et al.
    Stockholm University, Faculty of Science, Department of Applied Environmental Science (ITM).
    Bourassa, Adam
    Herber, Andreas
    Iversen, Trond
    Karlsson, Johannes
    Stockholm University, Faculty of Science, Department of Meteorology .
    Kirkevag, Alf
    Maturilli, Marion
    Seland, Oyvind
    Stebel, Kerstin
    Struthers, Hamish
    Tesche, Matthias
    Stockholm University, Faculty of Science, Department of Applied Environmental Science (ITM).
    Thomason, Larry
    Remote sensing of aerosols in the Arctic for an evaluation of global climate model simulations2014In: Journal of Geophysical Research - Atmospheres, ISSN 2169-897X, E-ISSN 2169-8996, Vol. 119, no 13, 8169-8188 p.Article in journal (Refereed)
    Abstract [en]

    In this study Moderate Resolution Imaging Spectroradiometer (MODIS) Aqua retrievals of aerosol optical thickness (AOT) at 555 nm are compared to Sun photometer measurements from Svalbard for a period of 9 years. For the 642 daily coincident measurements that were obtained, MODIS AOT generally varies within the predicted uncertainty of the retrieval over ocean (Delta AOT = +/- 0.03 +/- 0.05 . AOT). The results from the remote sensing have been used to examine the accuracy in estimates of aerosol optical properties in the Arctic, generated by global climate models and from in situ measurements at the Zeppelin station, Svalbard. AOT simulated with the Norwegian Earth System Model/Community Atmosphere Model version 4 Oslo global climate model does not reproduce the observed seasonal variability of the Arctic aerosol. The model overestimates clear-sky AOT by nearly a factor of 2 for the background summer season, while tending to underestimate the values in the spring season. Furthermore, large differences in all-sky AOT of up to 1 order of magnitude are found for the Coupled Model Intercomparison Project phase 5 model ensemble for the spring and summer seasons. Large differences between satellite/ground-based remote sensing of AOT and AOT estimated from dry and humidified scattering coefficients are found for the subarctic marine boundary layer in summer.

  • 3.
    Glantz, Paul
    et al.
    Stockholm University, Faculty of Science, Department of Applied Environmental Science (ITM).
    Kokhanovsky, A.
    von Hoyningen-Huene, W.
    Johansson, Christer
    Stockholm University, Faculty of Science, Department of Applied Environmental Science (ITM).
    Estimating PM2.5 over southern Sweden using space-borne optical measurements2009In: Atmospheric Environment, ISSN 1352-2310, E-ISSN 1873-2844, Vol. 43, no 36, 5838-5846 p.Article in journal (Other academic)
    Abstract [en]

    in the present study Bremen aerosol retrieval (BAER) columnar aerosol optical thickness (ACT) data, according to moderate resolution imaging spectroradiometer (MODIS) and medium resolution imaging sensor (MERIS) level 1 calibrated satellite data, have been compared with ACT data obtained with the MODIS and MERIS retrieval algorithms (NASA and ESA, respectively) and by AErosol Robotic NETwork (AERONET). Relatively good agreement is found between these different instruments and algorithms. The R-2 and relative RMSD were 0.86 and 31% for MODIS when comparing with AERONET and 0.92 and 21% for MERIS. The aerosols investigated were influenced by low relative humidity. During this period, a relatively large range of aerosol loadings were detected; from continental background aerosol to particles emitted from agricultural fires. In this study, empirical relationships between BAER columnar AOT and ground-measured PM2.5 have been estimated. Linear relationships, with R-2 values of 0.58 and 0.59, were obtained according to MERIS and MODIS data, respectively. The slopes of the regression of ACT versus PM2.5 are lower than previous studies, but this could easily be explained by considering the effect of hygroscopic growth. The present AOT-PM2.5 relationship has been applied on MERIS full resolution data over the urban area of Stockholm and the results have been compared with particle mass concentrations from dispersion model calculations. it seems that the satellite data with the 300 m resolution can resolve the expected increased concentrations due to emissions along the main highways close to the city. Significant uncertainties in the spatial distribution of PM2.5 across land/ocean boundaries were particularly evident when analyzing the high resolution satellite data.

  • 4.
    Glantz, Paul
    et al.
    Stockholm University, Faculty of Science, Department of Applied Environmental Science (ITM).
    Nilsson, E. D.
    Stockholm University, Faculty of Science, Department of Applied Environmental Science (ITM).
    von Hoyningen-Huene, W.
    Estimating a relationship between aerosol optical thickness and surface wind speed over the ocean (vol 92, pg 58, 2009)2011In: Atmospheric research, ISSN 0169-8095, E-ISSN 1873-2895, Vol. 101, no 4, 956-956 p.Article in journal (Refereed)
  • 5.
    Glantz, Paul
    et al.
    Stockholm University, Faculty of Science, Department of Applied Environmental Science (ITM).
    Nilsson, E.D.
    Stockholm University, Faculty of Science, Department of Applied Environmental Science (ITM).
    von Hoyningen-Huene, W.
    Estimating a relationship between aerosol optical thickness and surface wind speed over the ocean2009In: Atmospheric research, ISSN 0169-8095, E-ISSN 1873-2895, Vol. 92, no 1, 58-68 p.Article in journal (Refereed)
    Abstract [en]

    Retrieved aerosol optical thickness (ACT) based on data obtained by the Sea-viewing Wide Field Sensor (SeaWiFS) is combined with surface wind speed, obtained at the European Centre for Medium-Range Weather Forecasts (ECMWF), over the North Pacific for September 2001. In this study a cloud-screening approach is introduced in an attempt to exclude pixels partly or fully covered by clouds. The relatively broad swath width through which the nadir-viewing SeaWiFS scanned over the North Pacific means that the ACT can be estimated according to a relatively large range of wind speeds for each of the scenes analyzed. The sensitivity in AOT due to sea salt and hygroscopic growth of the marine aerosols was also investigated. The validation approach is based on previous parameterization in combination with the environmental quantities wind speed, RH and boundary layer height (BLH), estimated at the ECMWF. In this study a factor of 2 higher AOT is obtained between the highest wind speed (12 m s(-1)) and the lowest wind speed range (0-4 m s(-1)) for September 2001 over remote ocean areas. This is supported by the validation of the results. The enhancement in AOT is explained by a combination of hygroscopic growth of the marine aerosols (similar to 40%) and an increase in the sea salt particle mass concentrations (similar to 60%), caused by a wind-driven water vapor and sea salt flux respectively. Reasonable agreement (within 1 to 52%) occurs also between satellite-retrieved aerosol optical thickness and AOT observed at two AERONET (AErosol RObotic NETwork) ground-based remote sensing stations. The overall variability is also observed by this comparison. Finally, possible reasons why relatively large standard deviations occur around the mean values of AOT, when all data is taken into consideration in the analyses for September 2001, are discussed.

  • 6.
    Glantz, Paul
    et al.
    Stockholm University, Faculty of Science, Department of Applied Environmental Science (ITM).
    Tesche, Matthias
    Stockholm University, Faculty of Science, Department of Applied Environmental Science (ITM).
    Assessment of two aerosol optical thickness retrieval algorithms applied to modis aqua and terra measurements in Europe2012In: Atmospheric measurement techniques, ISSN 1867-1381, Vol. 5, no 7, 1727-1740 p.Article in journal (Refereed)
    Abstract [en]

    The aim of the present study is to validate AOT (aerosol optical thickness) and Angstrom exponent (alpha), obtained from MODIS (MODerate resolution Imaging Spectroradiometer) Aqua and Terra calibrated level 1 data (1 km horizontal resolution at ground) with the SAER (Satellite AErosol Retrieval) algorithm and with MODIS Collection 5 (c005) standard product retrievals (10 km horizontal resolution), against AERONET (AErosol RObotic NETwork) sun photometer observations over land surfaces in Europe. An inter-comparison of AOT at 0.469 nm obtained with the two algorithms has also been performed. The time periods investigated were chosen to enable a validation of the findings of the two algorithms for a maximal possible variation in sun elevation. The satellite retrievals were also performed with a significant variation in the satellite-viewing geometry, since Aqua and Terra passed the investigation area twice a day for several of the cases analyzed. The validation with AERONET shows that the AOT at 0.469 and 0.555 nm obtained with MODIS c005 is within the expected uncertainty of one standard deviation of the MODIS c005 retrievals (Delta AOT = +/- 0.05 +/- 0.15 center dot AOT). The AOT at 0.443 nm retrieved with SAER, but with a much finer spatial resolution, also agreed reasonably well with AERONET measurements. The majority of the SAER AOT values are within the MODIS c005 expected uncertainty range, although somewhat larger average absolute deviation occurs compared to the results obtained with the MODIS c005 algorithm. The discrepancy between AOT from SAER and AERONET is, however, substantially larger for the wavelength 488 nm. This means that the values are, to a larger extent, outside of the expected MODIS uncertainty range. In addition, both satellite retrieval algorithms are unable to estimate alpha accurately, although the MODIS c005 algorithm performs better. Based on the inter-comparison of the SAER and MODIS c005 algorithms, it was found that SAER on the whole is able to obtain results within the expected uncertainty range of MODIS Aqua and Terra observations.

  • 7. Sporre, M. K.
    et al.
    Swietlicki, E.
    Glantz, Paul
    Stockholm University, Faculty of Science, Department of Applied Environmental Science (ITM).
    Kulmala, M.
    A long-term satellite study of aerosol effects on convective clouds in Nordic background air2014In: Atmospheric Chemistry And Physics, ISSN 1680-7316, E-ISSN 1680-7324, Vol. 14, no 4, 2203-2217 p.Article in journal (Refereed)
    Abstract [en]

    Aerosol-cloud interactions constitute a major uncertainty in future climate predictions. This study combines 10 years of ground-based aerosol particle measurements from two Nordic background stations (Vavihill and Hyytiala) with MODIS (Moderate Resolution Imaging Spectroradiometer) satellite data of convective clouds. The merged data are used to examine how aerosols affect cloud droplet sizes and precipitation from convective clouds over the Nordic countries. From the satellite scenes, vertical profiles of cloud droplet effective radius (r(e)) are created by plotting retrieved cloud top r(e) against cloud top temperature for the clouds in a given satellite scene. The profiles have been divided according to aerosol number concentrations but also meteorological reanalysis parameters from the ECMWF (European Centre for Medium-Range Forecasts). Furthermore, weather radar data from the BALTEX (Baltic Sea Experiment) and precipitation data from several ground-based meteorological measurement stations have been investigated to determine whether aerosols affect precipitation intensity and amount. Small r(e) throughout the entire cloud profiles is associated with high aerosol number concentrations at both stations. However, aerosol number concentrations seem to affect neither the cloud optical thickness nor the vertical extent of the clouds in this study. Cloud profiles with no or little precipitation have smaller droplets than those with more precipitation. Moreover, the amount of precipitation that reaches the ground is affected by meteorological conditions such as the vertical extent of the clouds, the atmospheric instability and the relative humidity in the lower atmosphere rather than the aerosol number concentration. However, lower precipitation rates are associated with higher aerosol number concentrations for clouds with similar vertical extent. The combination of these ground-based and remote-sensing datasets provides a unique long-term study of the effects of aerosols on convective clouds over the Nordic countries.

  • 8. Sporre, Moa K.
    et al.
    Glantz, Paul
    Stockholm University, Faculty of Science, Department of Applied Environmental Science (ITM).
    Tunved, Peter
    Stockholm University, Faculty of Science, Department of Applied Environmental Science (ITM).
    Swietlicki, Erik
    Kulmala, Markku
    Lihavainen, Heikki
    A study of the indirect aerosol effect on subarctic marine liquid low-level clouds using MODIS cloud data and ground-based aerosol measurements2012In: Atmospheric research, ISSN 0169-8095, E-ISSN 1873-2895, Vol. 116, 56-66 p.Article in journal (Refereed)
    Abstract [en]

    Cloud microphysics is substantially affected by aerosol loading and the resulting changes in the reflective properties of the clouds can significantly affect the global radiation budget. A study of how marine low-level clouds over Barents Sea and the northern parts of the Norwegian Sea are affected by air mass origin has been performed by combining ground-based aerosol measurements with satellite cloud retrievals. Aerosol number size distributions have been obtained from measurement stations in northern Finland, and a trajectory model has been used to estimate the movement of the air masses. To identify anthropogenic influences on the clouds, the dataset has been divided according to aerosol loading. The clean air masses arrived to the investigation area from the north and the polluted air masses arrived from the south. Satellite derived microphysical and optical cloud parameters from the Moderate Resolution Imaging Spectrometer (MODIS) have then been analyzed for days when the trajectories coincided with marine low-level clouds over the investigated area. The cloud optical thickness (tau), cloud depth (H) and droplet number concentration (N-d) were significantly higher for the polluted days compared to the clean conditions, while the opposite was found for the cloud droplet effective radius (r(e)). The H and N-d were derived from the satellite retrievals of tau and r(e). Furthermore, calculations of the aerosol cloud interaction relationship (ACI), relating N-d to boundary layer aerosol concentrations, resulted in a value of 0.17, which is in line with previous remote sensing studies. The results demonstrate that ground-based aerosol measurements can be combined with satellite cloud observations to study the indirect aerosol effect, and that the microphysics of marine sub-polar clouds can be considerably affected by continental aerosols.

  • 9.
    Struthers, Hamish
    et al.
    Stockholm University, Faculty of Science, Department of Applied Environmental Science (ITM).
    Ekman, Annica
    Stockholm University, Faculty of Science, Department of Meteorology .
    Glantz, Paul
    Stockholm University, Faculty of Science, Department of Applied Environmental Science (ITM).
    Iversen, T.
    Kirkevag, A.
    Mårtensson, Monica
    Stockholm University, Faculty of Science, Department of Applied Environmental Science (ITM).
    Seland, O.
    Nilsson, Douglas
    Stockholm University, Faculty of Science, Department of Applied Environmental Science (ITM).
    The effect of sea ice loss on sea salt aerosol concentrations and the = diative balance in the Arctic2011In: ATMOSPHERIC CHEMISTRY AND PHYSICS, ISSN 1680-7316, Vol. 11, no 7, 3459-3477 p.Article in journal (Refereed)
    Abstract [en]

    Understanding Arctic climate change requires knowledge of both the external and the local drivers of Arctic climate as well as local feedbacks within the system. An Arctic feedback mechanism relating changes in sea ice extent to an alteration of the emission of sea salt aerosol and the consequent change in radiative balance is examined. A set of idealized climate model simulations were performed to quantify the radiative effects of changes in sea salt aerosol emissions induced by prescribed changes in sea ice extent. The model was forced using sea ice concentrations consistent with present day conditions and projections of sea ice extent for 2100. Sea salt aerosol emissions increase in response to a decrease in sea ice, the model results showing an annual average increase in number emission over the polar cap (70-90 degrees N) of 86 x 10(6) m(-2) s(-1) (mass emission increase of 23 mu g m(-2) s(-1)). This in turn leads to an increase in the natural aerosol optical depth of approximately 23%. In response to changes in aerosol optical depth, the natural component of the aerosol direct forcing over the Arctic polar cap is estimated to be between -0.2 and -0.4 W M(-2) for the summer months, which results in a negative feedback on the system. The model predicts that the change in first indirect aerosol effect (cloud albedo effect) is approximately a factor of ten greater than the change in direct aerosol forcing although this result is highly uncertain due to the crude representation of Arctic clouds and aerosol-cloud interactions in the model. This study shows that both the natural aerosol direct and first indirect effects are strongly dependent on the surface albedo, highlighting the strong coupling between sea ice, aerosols, Arctic clouds and their radiative effects.

  • 10.
    Struthers, Hamish
    et al.
    Stockholm University, Faculty of Science, Department of Applied Environmental Science (ITM). Stockholm University, Faculty of Science, Department of Meteorology .
    Ekman, Annica
    Stockholm University, Faculty of Science, Department of Meteorology .
    Glantz, Paul
    Stockholm University, Faculty of Science, Department of Applied Environmental Science (ITM).
    Iversen, T.
    Kirkevag, A.
    Seland, O.
    Mårtensson, Sebastian
    Stockholm University, Faculty of Science, Department of Meteorology .
    Noone, Kevin
    Stockholm University, Faculty of Science, Department of Applied Environmental Science (ITM).
    Nilsson, E. D.
    Stockholm University, Faculty of Science, Department of Applied Environmental Science (ITM).
    Climate-induced changes in sea salt aerosol number emissions: 1870 to 21002013In: JOURNAL OF GEOPHYSICAL RESEARCH-ATMOSPHERES, ISSN 2169-897X, Vol. 118, no 2, 670-682 p.Article in journal (Refereed)
    Abstract [en]

    Global climate model output is combined with an emission parameterization to estimate the change in the global and regional sea salt aerosol number emission from 1870 to 2100. Global average results suggest a general increase in sea salt aerosol number emission due to increasing surface wind speed. However, the emission changes are not uniform over the aerosol size spectrum due to an increase in sea surface temperature. From 1870 to 2100 the emission of coarse mode particles (dry diameter D-P > 655 nm) increase by approximately 10 % (global average), whereas no significant change in the emission of ultrafine mode aerosols (dry diameter D-p < 76 nm) was found over the same period. Significant regional differences in the number emission trends were also found. Based on CAM-Oslo global climate model output, no straight-forward relationship was found between the change in the number emissions and changes in the sea salt aerosol burden or optical thickness. This is attributed to a change in the simulated residence time of the sea salt aerosol. For the 21st century, a decrease in the residence time leads to a weaker sea salt aerosol-climate feedback that what would be inferred based on changes in number emissions alone. Finally, quantifying any potential impact on marine stratocumulus cloud microphysical and radiative properties due to changes in sea salt aerosol number emissions is likely to be complicated by commensurate changes in anthropogenic aerosol emissions and changes in meteorology.

  • 11.
    Tesche, Matthias
    et al.
    Stockholm University, Faculty of Science, Department of Environmental Science and Analytical Chemistry.
    Achtert, Peggy
    Stockholm University, Faculty of Science, Department of Environmental Science and Analytical Chemistry.
    Glantz, Paul
    Stockholm University, Faculty of Science, Department of Environmental Science and Analytical Chemistry.
    Noone, Kevin J.
    Stockholm University, Faculty of Science, Department of Environmental Science and Analytical Chemistry.
    Aviation effects on already-existing cirrus clouds2016In: Nature Communications, ISSN 2041-1723, E-ISSN 2041-1723, Vol. 7, 12016Article in journal (Refereed)
    Abstract [en]

    Determining the effects of the formation of contrails within natural cirrus clouds has proven to be challenging. Quantifying any such effects is necessary if we are to properly account for the influence of aviation on climate. Here we quantify the effect of aircraft on the optical thickness of already-existing cirrus clouds by matching actual aircraft flight tracks to satellite lidar measurements. We show that there is a systematic, statistically significant increase in normalized cirrus cloud optical thickness inside mid-latitude flight tracks compared with adjacent areas immediately outside the tracks.

  • 12. Tesche, Matthias
    et al.
    Glantz, Paul
    Stockholm University, Faculty of Science, Department of Environmental Science and Analytical Chemistry.
    Johansson, Christer
    Stockholm University, Faculty of Science, Department of Environmental Science and Analytical Chemistry. Environment and Health Administration, Stockholm, Sweden.
    Spaceborne observations of low surface aerosol concentrations in the Stockholm region2016In: Tellus. Series B, Chemical and physical meteorology, ISSN 0280-6509, E-ISSN 1600-0889, Vol. 68, 28951Article in journal (Refereed)
    Abstract [en]

    This article investigates the feasibility of using spaceborne observations of aerosol optical thickness (AOT) derived with the Moderate Resolution Imaging Spectroradiometer (MODIS) for monitoring of fine particulate matter (PM2.5) in an environment of low aerosol loading. Previous studies of the AOT-to-PM2.5 relationship benefit from the large range of observed values. The Stockholm region features a comprehensive network of ground-based monitoring stations that generally show PM2.5 values <20 mu g m(-3). MODIS AOT at 555nm is usually <0.20 and in good agreement with ground-based sun photometer observations in this region. We use MODIS Collection 5 AOT data with a horizontal resolution of 10km x 10km and ground-based in-situ PM2.5 observations to derive an AOT-to-PM2.5 relationship that can be used to estimate fields of PM2.5. This has been carried out with respect to the months from April to September of the period 2000-2013. Relative average absolute deviations of 33-55 % (mean of 45 %) are obtained between MODIS-retrieved and ground-based PM2.5. The root mean square error is 0.2159 mu gm(-3) between retrieved and measured PM2.5. From spaceborne lidar observations, it is found that elevated aerosol layers are generally sparse in the Stockholm region. This favours remote sensing of PM2.5 from space. The deviations found between measured and retrieved PM2.5 are mainly attributed to infrequent situations of inhomogeneous aerosol layering for which column-integrated observations cannot be connected to surface conditions. Using MODIS Collection 6 data with a resolution of 3km x 3 km in a case study actually gives far fewer results than the coarser Collection 5 product. This is explained by the complex geography of the Stockholm region with a coastline and an abundance of lakes, which seems to induce biases in the retrieval of AOT at higher spatial resolution.

  • 13.
    Tesche, Matthias
    et al.
    Stockholm University, Faculty of Science, Department of Applied Environmental Science (ITM).
    Glantz, Paul
    Stockholm University, Faculty of Science, Department of Applied Environmental Science (ITM).
    Johansson, Christer
    Stockholm University, Faculty of Science, Department of Applied Environmental Science (ITM).
    Norman, M.
    Hiebsch, A.
    Ansmann, A.
    Althausen, D.
    Engelmann, R.
    Seifert, P.
    Volcanic ash over Scandinavia originating from the Grimsvotn eruptions in May 20112012In: Journal of Geophysical Research, ISSN 0148-0227, E-ISSN 2156-2202, Vol. 117, D09201- p.Article in journal (Refereed)
    Abstract [en]

    A volcanic ash plume that originated from the eruptions of Iceland's Grimsvotn volcano in May 2011 was observed over the Nordic countries using a combination of satellite observations and ground-based measurements. The dispersion of the plume was investigated using London VAAC ash forecasts and MODIS observations. Hourly PM10 concentrations at air quality monitoring stations in the southern parts of Norway, Sweden, and Finland exceeded 100 mu g/m(3) for several hours. The FLEXPART dispersion model has been used to confirm the Icelandic origin of the sampled air masses. Column-integrated quantities from a Sun photometer and vertical profiles from a Raman lidar were used to estimate the ash concentration within an elevated layer over Stockholm. A lofted layer with an optical thickness of 0.3 at 532 nm passed Stockholm in the morning hours of 25 May 2011. Considering a realistic range of coarse-mode fractions and specific ash extinctions from the literature, an estimated range of maximum ash mass concentration of 150-340 mu g/m(3) was derived from the lidar measurements at an altitude of 2.8 km. The lower estimate of the lidar-derived ash mass concentrations within the planetary boundary layer was found to be in good agreement with surface observations of PM10.

  • 14.
    Tesche, Matthias
    et al.
    Stockholm University, Faculty of Science, Department of Applied Environmental Science (ITM).
    Zieger, Paul
    Stockholm University, Faculty of Science, Department of Applied Environmental Science (ITM).
    Rastak, Narges
    Stockholm University, Faculty of Science, Department of Applied Environmental Science (ITM).
    Charlson, R. J.
    Glantz, Paul
    Stockholm University, Faculty of Science, Department of Applied Environmental Science (ITM).
    Tunved, Peter
    Stockholm University, Faculty of Science, Department of Applied Environmental Science (ITM).
    Hansson, Hans-Christen
    Stockholm University, Faculty of Science, Department of Applied Environmental Science (ITM).
    Reconciling aerosol light extinction measurements from spaceborne lidar observations and in situ measurements in the Arctic2014In: Atmospheric Chemistry And Physics, ISSN 1680-7316, E-ISSN 1680-7324, Vol. 14, no 15, 7869-7882 p.Article in journal (Refereed)
    Abstract [en]

    In this study we investigate to what degree it is possible to reconcile continuously recorded particle light extinction coefficients derived from dry in situ measurements at Zeppelin station (78.92 degrees N, 11.85 degrees E; 475 m above sea level), Ny-lesund, Svalbard, that are recalculated to ambient relative humidity, as well as simultaneous ambient observations with the Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) aboard the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) satellite. To our knowledge, this represents the first study that compares spaceborne lidar measurements to optical aerosol properties from short-term in situ observations (averaged over 5 h) on a case-by-case basis. Finding suitable comparison cases requires an elaborate screening and matching of the CALIOP data with respect to the location of Zeppelin station as well as the selection of temporal and spatial averaging intervals for both the ground-based and spaceborne observations. Reliable reconciliation of these data cannot be achieved with the closest-approach method, which is often used in matching CALIOP observations to those taken at ground sites. This is due to the transport pathways of the air parcels that were sampled. The use of trajectories allowed us to establish a connection between spaceborne and ground-based observations for 57 individual overpasses out of a total of 2018 that occurred in our region of interest around Svalbard (0 to 25 degrees E, 75 to 82 degrees N) in the considered year of 2008. Matches could only be established during winter and spring, since the low aerosol load during summer in connection with the strong solar background and the high occurrence rate of clouds strongly influences the performance and reliability of CALIOP observations. Extinction coefficients in the range of 2 to 130 Mm(-1) at 532 nm were found for successful matches with a difference of a factor of 1.47 (median value for a range from 0.26 to 11.2) between the findings of in situ and spaceborne observations (the latter being generally larger than the former). The remaining difference is likely to be due to the natural variability in aerosol concentration and ambient relative humidity, an insufficient representation of aerosol particle growth, or a misclassification of aerosol type (i.e., choice of lidar ratio) in the CALIPSO retrieval.

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