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The Mediterranean atmosphere is impacted by a variety of natural and anthropogenic aerosols which exert a complex and variable pressure on the regional climate and air quality. This study focuses on the Western Mediterranean Sea (west of longitude 20∘ E) using the full POLarization and Directionality of the Earth's Reflectances version 3 (POLDER-3)/Polarization & Anisotropy of Reflectances for Atmospheric Sciences coupled with Observations from a Lidar (PARASOL) aerosol data record derived from the operational clear-sky ocean algorithm (collection 3) available from March 2005 to October 2013. This 8.5-year satellite data set includes retrievals at 865 nm of the total, fine-, and coarse-mode aerosol optical depth (AOD, AODF, and AODC, respectively), Ångström exponent (AE), and the spherical/non-spherical partition of the coarse-mode AOD (AODCS and AODCNS, respectively), that have been carefully validated over the study region (Formenti et al., 2018). Here, we analyze the spatial distribution, the seasonal cycle, and interannual variability of this ensemble of advanced aerosol products in three latitude bands (34–38, 38–42, and > 42∘ N) and for three sites (Ersa, Barcelona, Lampedusa) distributed on the western basin. POLDER-3 retrieves the high influence of north African desert dust over the region, which largely controls the spatial distributions (south-to-north decreasing gradient) and seasonal cycles (spring/summer maximum) of both AOD and coarse AOD, including its non-spherical component. In contrast, the coarse spherical component of AOD remains relatively homogenously low all year long over the region, whereas fine-mode AODs are generally more elevated in the eastern part of the region of study, especially north of the Adriatic Sea. From 2005 to 2013, annual POLDER-3 AOD evolution shows a decreasing trend of 0.0030 yr−1 in absolute value at 865 nm (0.0060 yr−1 at 550 nm). Such a downward evolution is much more pronounced and spatially extended for AODF (−0.0020 yr−1 at 865 nm) than for AODC. Our analysis also suggests that the North Atlantic Oscillation (NAO) index explains a significant part of the interannual variability of POLDER-3 AODC, reflecting its role on the frequency of Saharan dust transport over the region. Finally, the POLDER-3 data set highlights an improvement of air quality related to the fine aerosol component, with a marked evolution toward more frequent occurrence of clean conditions (≥ 75 % of daily AODF-865 nm.05) at the end of the period of study (2010–2013) over most of the Western Mediterranean Sea, and much less evidence of such a large-scale evolution for the coarse fraction. Therefore, despite the high and variable influence of mostly natural north African dust over the region, the POLDER-3 advanced aerosol data set appears sufficiently accurate to successfully resolve the concurrent downward trend of fine, primarily anthropogenic particles, most likely related to reduced emissions in the surrounding European countries.

The Dark Target aerosol algorithm was developed to exploit the information content available from the observations of Moderate-Resolution Imaging Spectroradiometers (MODIS), to better characterize the global aerosol system. The algorithm is based on measurements of the light scattered by aerosols toward a space-borne sensor against the backdrop of relatively dark Earth scenes, thus giving rise to the name “Dark Target”. Development required nearly a decade of research that included application of MODIS airborne simulators to provide test beds for proto-algorithms and analysis of existing data to form realistic assumptions to constrain surface reflectance and aerosol optical properties. This research in itself played a significant role in expanding our understanding of aerosol properties, even before Terra MODIS launch. Contributing to that understanding were the observations and retrievals of the growing Aerosol Robotic Network (AERONET) of sun-sky radiometers, which has walked hand-in-hand with MODIS and the development of other aerosol algorithms, providing validation of the satellite-retrieved products after launch. The MODIS Dark Target products prompted advances in Earth science and applications across subdisciplines such as climate, transport of aerosols, air quality, and data assimilation systems. Then, as the Terra and Aqua MODIS sensors aged, the challenge was to monitor the effects of calibration drifts on the aerosol products and to differentiate physical trends in the aerosol system from artefacts introduced by instrument characterization. Our intention is to continue to adapt and apply the well-vetted Dark Target algorithms to new instruments, including both polar-orbiting and geosynchronous sensors. The goal is to produce an uninterrupted time series of an aerosol climate data record that begins at the dawn of the 21st century and continues indefinitely into the future.

The existing models predict that corner reflection (CR) of laser radiation by simple ice crystals of perfect shape, such as hexagonal columns or plates, can provide a significant contribution to the ice cloud backscattering. However in real clouds the CR effect may be suppressed due to crystal deformation and surface roughness. In contrast to the extinction coefficient, which is spectrally independent, consideration of diffraction associated with CR results in a spectral dependence of the backscattering coefficient. Thus measuring the spectral dependence of the cloud backscattering coefficient, the contribution of CR can be identified. The paper presents the results of profiling of backscattering coefficient (β) and particle depolarization ratio (δ) of ice and mixed-phase clouds over West Africa by means of a two-wavelength polarization Mie-Raman lidar operated at 355 nm and 532 nm during the SHADOW field campaign. The lidar observations were performed at a slant angle of 43 degree off zenith, thus CR from both randomly oriented crystals and oriented plates could be analyzed. For the most of the observations the cloud backscatter color ratio β355/β532 was close to 1.0, and no spectral features that might indicate the presence of CR of randomly oriented crystals were revealed. Still, in two measurement sessions we observed an increase of backscatter color ratio to a value of nearly 1.3 simultaneously with a decrease of the spectral depolarization ratio δ355/δ532 ratio from 1.0 to 0.8 inside the layers containing precipitating ice crystals. We attribute these changes in optical properties to corner reflections by horizontally oriented ice plates.

The spatial and temporal variability of the aerosol Single Scattering Albedo (SSA at 865 nm) has been estimated over clear-sky ocean for 2006 by using measurements acquired by POLDER (Polarization and Directionality of Earth Reflectances). Our estimates are correlated with sun-photometer retrievals (R = 0.63). Differences in SSA are generally around 0.05 and systematically fall below 0.055 for optical thicknesses ≥ 0.3 (at 865 nm) and modeling errors ≤ 3.0 %. Fine absorbing aerosols (radius ≤ 0.16 μm) are detected in many coastal regions. The lowest SSAs are retrieved over the southeast Atlantic during summer (0.80) whereas non-absorbing fine particles (≥ 0.98) are observed over the North Pacific. During winter, fine absorbing aerosols are detected together with mineral dust near the coasts of western Africa (0.90), over the tropical Atlantic (0.88) and around India (0.88). Long–range transport of absorbing species is also detected, as for instance over the Arctic. This study could help to constrain aerosol absorption and radiative forcing in models.

International audience; This paper presents the analysis of nearly a decade of continuous aerosol observations performed at the Mbour site (Senegal) with Sun photometer, Lidar, and Tapered Electromagnetic Oscillating Microbalance. This site is influenced all year-round by desert dust and sporadically, in wintertime, by biomass burning particles. Different patterns are revealed for winter and summer, seasons associated to air masses of different origin. The summer (wet season) is characterized by a high aerosol loading (optical thickness, AOT, around 0.57 at 532 nm) composed of large and weakly absorbing particles (Angstrom exponent, α, of 0.23 and single-scattering albedo, ϖ0, of 0.94 at 532 nm). A lower aerosol loading (AOT = 0.32) is observed during winter (dry season) for finer and absorbing particles (α = 0.48 and ϖ0 = 0.87) revealing the presence of biomass burning aerosols and a greater proportion of local emissions. This latter anthropogenic contribution is visible at weekly and daily scales through AOT cycles. A decrease of about 30% in AOT has been featured in autumn since 2003. The derivation of the extinction profiles highlights a dust transport close to the ground during winter and in an aloft layer (up to 5 km) during summer. Accurate calculations of the daily aerosol radiative effect in clear-sky conditions are finally addressed. From spring to winter, seasonal shortwave radiative forcing averages of 14.15, 11.15, 8.92, and 12.06 W m-2 have been found respectively. Up to 38% of the solar clear-sky atmospheric heating can be attributed to the aerosols in this site.

Aerosol properties above clouds have been retrieved over the South East Atlantic Ocean during the fire season 2006 using satellite observations from POLDER (Polarization and Directionality of Earth Reflectances). From June to October, POLDER has observed a mean Above-Cloud Aerosol Optical Thickness (ACAOT) of 0.28 and a mean Above-Clouds Single Scattering Albedo (ACSSA) of 0.87 at 550 nm. These results have been used to evaluate the simulation of aerosols above clouds in five Aerosol Comparisons between Observations and Models (Goddard Chemistry Aerosol Radiation and Transport (GOCART), Hadley Centre Global Environmental Model 3 (HadGEM3), European Centre Hamburg Model 5-Hamburg Aerosol Module 2 (ECHAM5-HAM2), Oslo-Chemical Transport Model 2 (OsloCTM2), and Spectral Radiation-Transport Model for Aerosol Species (SPRINTARS)). Most models do not reproduce the observed large aerosol load episodes. The comparison highlights the importance of the injection height and the vertical transport parameterizations to simulate the large ACAOT observed by POLDER. Furthermore, POLDER ACSSA is best reproduced by models with a high imaginary part of black carbon refractive index, in accordance with recent recommendations.

The western Mediterranean atmosphere is impacted by a variety of aerosol sources, producing a complex and variable mixture of natural and anthropogenic particles, with different chemical and physical properties. Satellite sensors provide a useful global coverage of aerosol parameters but through indirect measurements that request careful validation. Here we present the results of a long-term regional scale analysis of the full dataset (March 2005 and October 2013) of POLDER-3/PARASOL ocean operational retrievals of the total, fine and coarse aerosol optical depth (AOD, AODF and AODC), Angstrom exponent (AE), and the spherical/non-spherical partition of coarse-mode AOD (AODCS and AODCNS), respectively. The evaluation is performed using data from seventeen coastal and insular ground-based AERONET sites on one side, and airborne vertical profiles of aerosol extinction and number size distribution obtained by the SAFIRE ATR 42 aircraft operated in the area during summer 2012 and 2013 on the other side. This study provides the first regional evaluation of uncertainties of the POLDER-3 products, and highlights their quality. The POLDER-3 Ångström exponent, representing AOD spectral dependence in link with the aerosol particle size distribution, is biased towards small values. This bias, however, does not prevent using AE for classifying the regional aerosol laden air masses. AODF corresponds to particle smaller than 0.6–0.8 µm in diameter and appears suitable to monitor the aerosol submicron fraction from space. We also provide an original validation of POLDER-3 AODC and its spherical/non-spherical partition, which shows agreement within 25 % with AERONET shape retrievals when the aerosol coarse fraction dominates.

International audience; This paper presents what can be achieved with the use of satellite observations in the framework of atmospheric composition studies, including for air quality analysis and forecast. The different measurements currently available are presented with their main strengths and limitations. On the basis of examples from recent studies, the use and the gain obtained by these observations are discussed.This includes analysis of pollutant transport events, refinement of the databases used in the models, modeling of satellite observations and data assimilation. The recent Copernicus service is detailed in this context and the prospects for using these observations are finally explained.; Cet article présente ce qu'apportent les observations satellitaires pour les études de la composition atmosphérique, notamment dans un cadre d'analyse et prévision de la qualité de l'air. Les différentes mesures aujourd'hui disponibles sont présentées avec leurs principales forces et limitations. À partir d'exemples d'études récentes, l'utilisation et le gain obtenu grâce à ces observations sont discutés. Cela inclut l'analyse d'événements de transports de panaches de polluants, le raffinement des bases de données utilisées dans les modèles, la modélisation des observables satellitaires et l'assimilation de données. Le récent service Copernicus atmosphère est détaillé dans ce contexte et les perspectives futures d'utilisation de ces observations sont finalement expliquées.

This study presents a comparison between the retrieval of aerosol above clouds (AAC) optical properties from different techniques developed for the A-Train sensors CALIOP and POLDER/PARASOL. The main objective is to analyse the consistency between the results of the active and the passive measurements. We assess the Aerosol Optical Thickness (AOT) of above optically thick clouds (Cloud Optical Thickness (COT) larger than 3) and their Ångström Exponent (AE). These parameters are retrieved with the CALIOP operational method, the POLDER operational polarization method and the CALIOP-based depolarization ratio method (DRM) – for which we also propose a calibrated version (denominated DRMSODA, SODA as Synergized Optical Depth of Aerosols). We analyse six months of data over three distinctive regions characterized by different types of aerosols and clouds. Additionally, for these regions, we select three case studies: a biomass-burning event over the South Atlantic Ocean, a Saharan dust case over the North Atlantic Ocean and a Siberian biomass-burning event over the North Pacific Ocean. 4.5 years of data are studied over the entire globe for distinct situations where aerosol and cloud layers are in contact or vertically separated. Overall, the regional analysis shows a good correlation between the POLDER and the DRMSODA AOTs when the microphysics of aerosols is dominated by fine-mode particles of biomass-burning aerosols from southern Africa (correlation coefficient (R2) of 0.83) or coarse-mode aerosols of Saharan dust (R2 of 0.82). A good correlation between these methods (R2 of 0.68) is also observed in the global treatment, when the aerosol and cloud layers are well separated. The analysis of detached layers also shows a mean difference in AOT of 0.07 at 532 nm between POLDER and DRMSODA, at a global scale. The correlation between the retrievals decreases when a complex mixture of aerosols is expected (R2 of 0.37) – as in the East Asia region, and when the aerosol-cloud layers are in contact (R2 of 0.36). The correlation coefficient between the CALIOP operational method and POLDER is low, as the CALIOP method largely underestimates the aerosol loading above clouds by a factor that ranges from two to four. Potential biases on the retrieved AOT as a function of cloud properties are also investigated. For different types of scenes, the retrieval of above-cloud AOT from POLDER and from DRM are compared for different underlying cloud properties (droplet effective radius (reff) and COT retrieved with MODIS). The results reveal that DRM AOT vary with reff. When accounting for reff in the DRM algorithm, the consistency between the methods increases. The sensitivity study shows that an additional polarized signal coming from aerosols located within the cloud could affect the polarization method, which leads to an overestimation of the AOT retrieved with POLDER algorithm. In addition, the aerosols attached or within the cloud can potentially impact the DRM retrievals through the modification of the cloud droplet chemical composition and its ability to backscatter light. The next step of this work is to combine POLDER and CALIOP to investigate the impacts of aerosols on clouds and climate when these particles are transported above or within clouds.

Total and polarized radiances provided by the Polarization and Directionality of Earth Reflectances (POLDER) satellite sensor are used to retrieve the microphysical and optical properties of the volcanic plume observed during the Eyjafjallajökull volcano eruption in 2010, over cloud-free and cloudy ocean scenes. We selected two plume conditions, fresh aerosols near the sources (three cases) and a downwind volcanic plume observed over the North Sea 30 h after its injection into the atmosphere (aged aerosols). In the near-source conditions, the aerosol properties depend on the distance to the plume. Within the near-source plume, aerosols are mainly non-spherical and in the coarse mode with an effective radius equal to 1.75 (±0.25) μm and an Ångström exponent (AE) close to 0.0. Far from the plume, in addition to the coarse mode, there are particles retrieved in the accumulation mode, suggesting a mixture of sulfate aerosols and volcanic dust, resulting in an AE around 0.8. The properties of the aerosols also depend on whether the plume is fresh or aged. For the downwind (aged) plume, if non-spherical coarse particles as well as some fine mode particles are retrieved, the AE is higher, around ~0.4. In addition, rather low values for the real part of the refractive index (RR) were retrieved for the fresh plume (1.38 < RR < 1.48). Single scattering albedo (SSA) values ranging between 0.92 and 0.98 were retrieved over some parts of the near-source plume; despite the low accuracy of our retrievals, the derived SSA values suggest that the ash particles are rather absorbing. To consider the particle shape, a combination of spheroid models was used. Although the employed model enabled accurate modeling of the POLDER signal in the case of non-spherical ash, our approach failed to model the signal over the optically thickest parts of the near-source plume. The most probable reason for this is the presence of ice crystals within the plume. For the aerosol above clouds (AAC) scenes, polarized measurements allowed the retrieval of the optical thickness (OT) and the AE of optically thin volcanic ash. We found that all the cloud parameters retrieved by passive sensors were biased due to the presence of the elevated volcanic plumes. Finally, thermal infrared measurements were used to identify the type of multilayer scene (cirrus clouds or volcanic dust above liquid clouds) and the retrieval method also provided the OT of thin cirrus layers above the clouds near Iceland.

The seasonal and spatial varability of Aerosol Above Cloud (AAC) properties are derived from passive satellite data for the year 2008. A significant amount of aerosols are transported above liquid water clouds on the global scale. For particles in the fine mode (i.e., radius smaller than 0.3 m), including both clear sky and AAC retrievals increases the global mean aerosol optical thickness by 25(+/- 6%). The two main regions with man-made AAC are the tropical Southeast Atlantic, for biomass burning aerosols, and the North Pacific, mainly for pollutants. Man-made AAC are also detected over the Arctic during the spring. Mineral dust particles are detected above clouds within the so-called dust belt region (5-40 N). AAC may cause a warming effect and bias the retrieval of the cloud properties. This study will then help to better quantify the impacts of aerosols on clouds and climate.

A 15-channel airborne sun tracking photometer has been developed. The instrument provides aerosol optical depths over a wide spectral range (0.34–2.25 μm) with an accuracy of ΔAOD from 0.005 to 0.01. Doing measurements at different altitudes allow us to derive the aerosol extinction vertical profile. Thanks to the wide spectral range of the instrument, information on the aerosol size distribution along the vertical is also available.

Currently most of experiments pursuing comprehensive characterization of atmosphere include coordinated observations by both lidar and radiometers in order to obtain important complimentary information about aerosol properties. The passive observations by radiometers from ground are mostly sensitive to the properties of aerosol in total atmospheric column and have very limited sensitivity to vertical structure of the atmosphere. Such observations are commonly used for measuring aerosol optical thickness and deriving the information about aerosol microphysics including aerosol particles shape, size distribution, and complex refractive index. In a contrast, lidar observations of atmospheric responses from different altitudes to laser pulses emitted from ground are designed to provide accurate profiling of the atmospheric properties. The interpretation of the lidar observation generally relies on some assumptions about aerosol type and loading. Here we present the GARRLiC algorithm (Generalized Aerosol Retrieval from Radiometer and Lidar Combined data) that simultaneously inverts co-incident lidar and radiometer observations and derives a united set of aerosol parameters. Such synergetic retrieval is expected to result in additional enhancements in derived aerosol properties because the backscattering observations by lidar add some sensitivity to the columnar properties of aerosol, while radiometric observations provide sufficient constraints on aerosol type and loading that generally are missing in lidar signals. GARRLiC is based on AERONET algorithm for inverting combined observations by radiometer and multi-wavelength elastic lidar observations. It is expected that spectral changes of backscattering signal obtained by multi-wavelength lidar at different altitudes provide some sensitivity to the vertical variability of aerosol particle sizes. In order to benefit from this sensitivity the algorithm is set to derive not only the vertical profile of total aerosol concentration but it also differentiates between the contributions of fine and coarse modes of aerosol. The detailed microphysical properties are assumed height independent and different for each mode and expected to be derived as a part of the retrieval. Thus, the GARRLiC inversion algorithm retrieves vertical distribution of both fine and coarse aerosol concentrations as well as the size distribution, complex refractive index and single scattering albedo for each mode. The potential and limitations of the method are demonstrated by the series of sensitivity tests. The practical outcome of the approach is illustrated by applications of the algorithm to the real lidar and radiometer observations obtained over selected AERONET site.

Within the ESA Climate Change Initiative (CCI) project Aerosol_cci (2010–2013), algorithms for the production of long-term total column aerosol optical depth (AOD) datasets from European Earth Observation sensors are developed. Starting with eight existing pre-cursor algorithms three analysis steps are conducted to improve and qualify the algorithms: (1) a series of experiments applied to one month of global data to understand several major sensitivities to assumptions needed due to the ill-posed nature of the underlying inversion problem, (2) a round robin exercise of "best" versions of each of these algorithms (defined using the step 1 outcome) applied to four months of global data to identify mature algorithms, and (3) a comprehensive validation exercise applied to one complete year of global data produced by the algorithms selected as mature based on the round robin exercise. The algorithms tested included four using AATSR, three using MERIS and one using PARASOL. This paper summarizes the first step. Three experiments were conducted to assess the potential impact of major assumptions in the various aerosol retrieval algorithms. In the first experiment a common set of four aerosol components was used to provide all algorithms with the same assumptions. The second experiment introduced an aerosol property climatology, derived from a combination of model and sun photometer observations, as a priori information in the retrievals on the occurrence of the common aerosol components. The third experiment assessed the impact of using a common nadir cloud mask for AATSR and MERIS algorithms in order to characterize the sensitivity to remaining cloud contamination in the retrievals against the baseline dataset versions. The impact of the algorithm changes was assessed for one month (September 2008) of data: qualitatively by inspection of monthly mean AOD maps and quantitatively by comparing daily gridded satellite data against daily averaged AERONET sun photometer observations for the different versions of each algorithm globally (land and coastal) and for three regions with different aerosol regimes. The analysis allowed for an assessment of sensitivities of all algorithms, which helped define the best algorithm versions for the subsequent round robin exercise; all algorithms (except for MERIS) showed some, in parts significant, improvement. In particular, using common aerosol components and partly also a priori aerosol-type climatology is beneficial. On the other hand the use of an AATSR-based common cloud mask meant a clear improvement (though with significant reduction of coverage) for the MERIS standard product, but not for the algorithms using AATSR. It is noted that all these observations are mostly consistent for all five analyses (global land, global coastal, three regional), which can be understood well, since the set of aerosol components defined in Sect. 3.1 was explicitly designed to cover different global aerosol regimes (with low and high absorption fine mode, sea salt and dust).

This study evaluates the potential of using aerosol optical depth (τa) measurements to characterize the microphysical and optical properties of atmospheric aerosols. With this aim, we used the recently developed GRASP (Generalized Retrieval of Aerosol and Surface Properties) code for numerical testing of six different aerosol models with three different aerosol loads. We found that bimodal log-normal size distributions serve as useful input assumptions, especially when the measurements have inadequate spectral coverage and/or limited accuracy, such as lunar photometry. The direct numerical simulations indicate that the GRASP-AOD retrieval provides modal aerosol optical depths (fine and coarse) to within 0.01 of the input values. The retrieval of the fine mode radius, width, and volume concentration is stable and precise if the real part of the refractive index is known. The coarse mode properties are less accurate, but they are significantly improved when additional a priori information is available. In addition to these numerical studies, we used optical depth observations at six AERONET locations to validate our results with the standard AERONET inversion products. Differences in the fine mode volume median radii for the GRASP-AOD and AERONET inversions are less than 0.02 μm at sites dominated by the fine mode for all cases, although they are typically less than 0.01 μm when τa(440) > 0.3. The comparison of the coarse mode volume median radii shows larger differences than the fine mode at the same sites, with values typically between 0.2–0.3 μm. The comparison of coarse mode volume median radii between GRASP-AOD and AERONET improves for sites dominated by desert dust aerosol, with differences of less than 0.2 μm in most cases. The retrieved values of the fine-mode τa(500) using GRASP-AOD are generally between those values obtained by the standard AERONET inversion and the values obtained by the advance AERONET Spectral Deconvolution Algorithm (SDA), with differences typically lower than 0.02 between GRASP-AOD and both algorithms. Finally, we present some examples of application of GRASP-AOD inversion using moon-photometry and the airborne PLASMA sun-photometer during ChArMEx summer 2013 campaign in the western Mediterranean.

The 2014–15 Holuhraun lava-flood eruption of Bárdarbunga volcano (Iceland) has emitted prodigious amounts of sulfur dioxide into the atmosphere. This eruption caused a large-scale episode of air pollution throughout Western Europe in September 2014, the first event of this magnitude recorded in the modern era. We gathered a wealth of complementary observations from satellite sensors (OMI, IASI), ground-based remote sensing (lidar, sunphotometry, differential optical absorption spectroscopy) and ground-level air quality monitoring networks to characterize both the spatial-temporal distributions of volcanic SO2 and sulfate aerosols as well as the dynamics of the planetary boundary layer. Time variations of dynamical and microphysical properties of sulfate aerosols in the aged low-tropospheric volcanic cloud, including loading, vertical distribution, size distribution and single scattering albedo, are provided. Retrospective chemistry-transport simulations capture the correct temporal dynamics of this far-range air pollution event but fail to reproduce the magnitude of SO2 concentration at ground-level. Improving forecasts of large-scale volcanogenic air pollution will require refined emission parameters and adapted model grid resolution to accurately describe both long-range transport and local boundary layer dynamics.

West Africa and the adjacent oceanic regions are very important locations for studying dust properties and their influence on weather and climate. The SHADOW (Study of SaHAran Dust Over West Africa) campaign is performing a multi-scale and multi-laboratory study of aerosol properties and dynamics using a set of in situ and remote sensing instruments at an observation site located at IRD (Institute for Research and Development) Center, Mbour, Senegal (14° N, 17° W). In this paper, we present the results of lidar measurements performed during the first phase of SHADOW which occurred in March-April, 2015. The multiwavelength Mie-Raman lidar acquired 3β + 2α + 1δ measurements during this period. This set of measurements has permitted particle intensive properties such as extinction and backscattering Ångström exponents (BAE) for 355/532 nm wavelengths corresponding lidar ratios and depolarization ratio at 532 nm to be determined. The mean values of dust lidar ratios during the observation period were about 53 sr at both 532 nm and 355 nm, which agrees with the values observed during the SAMUM 1 and SAMUM 2 campaigns held in Morocco and Cape Verde in 2006, 2008. The mean value of particle depolarization ratio at 532 nm was 30 ± 4.5 %, however during strong dust episodes this ratio increased to 35 ± 5 %, which is also in agreement with the results of the SAMUM campaigns. The backscattering Ångström exponent during the dust episodes decreased to ~ −0.7, while the extinction Ångström exponent though being negative, was greater than −0.2. Low values of BAE can likely be explained by an increase in the imaginary part of the dust refractive index at 355 nm compared to 532 nm. The dust extinction and backscattering coefficients at multiple wavelengths were inverted to the particle microphysics using the regularization algorithm and the model of randomly oriented spheroids. The analysis performed has demonstrated that the spectral dependence of the imaginary part of the dust refractive index may significantly influence the inversion results and should be taken into account.

The 2014-2015 Holuhraun lava-flood eruption of Bároarbunga volcano (Iceland) emitted prodigious amounts of sulfur dioxide into the atmosphere. This eruption caused a large-scale episode of air pollution throughout Western Europe in September 2014, the first event of this magnitude recorded in the modern era. We gathered chemistry-transport simulations and a wealth of complementary observations from satellite sensors (OMI, IASI), ground-based remote sensing (lidar, sunphotometry, differential optical absorption spectroscopy) and ground-level air quality monitoring networks to characterize both the spatial-temporal distributions of volcanic SO2 and sulfate aerosols as well as the dynamics of the planetary boundary layer. Time variations of dynamical and microphysical properties of sulfate aerosols in the aged low-tropospheric volcanic cloud, including loading, vertical distribution, size distribution and single scattering albedo, are provided. Retrospective chemistry-transport simulations at low horizontal resolution (25 km × 25 km) capture the correct temporal dynamics of this far-range air pollution event but fail to reproduce the correct magnitude of SO2 concentration at ground-level. Simulations at higher spatial resolution, relying on two nested domains with finest resolution of 7.3 km × 7.3 km, improve substantially the far-range vertical distribution of the volcanic cloud and subsequently the description of ground-level SO2 concentrations. However, remaining discrepancies between model and observations are shown to result from an inaccurate representation of the planetary boundary layer (PBL) dynamics. Comparison with lidar observations points out a systematic under-estimation of the PBL height by the model, whichever the PBL parameterization scheme. Such a shortcoming impedes the capture of the overlying Bároarbunga cloud into the PBL at the right time and in sufficient quantities. This study therefore demonstrates the key role played by the PBL dynamics in accurately modelling large-scale volcanogenic air pollution., SCOPUS: ar.j, info:eu-repo/semantics/published

West Africa and the adjacent oceanic regions are very important locations for studying dust properties and their influence on weather and climate. The SHADOW (Study of SaHAran Dust Over West Africa) campaign is performing a multi-scale and multi-laboratory study of aerosol properties and dynamics using a set of in situ and remote sensing instruments at an observation site located at IRD (Institute for Research and Development) Center, Mbour, Senegal (14°N, 17°W). In this paper, we present the results of lidar measurements performed during the first phase of SHADOW which occurred in March-April, 2015. The multiwavelength Mie-Raman lidar acquired 3β+2α+1δ measurements during this period. This set of measurements has permitted particle intensive properties such as extinction and backscattering Angstrom exponents (BAE) for 355/532 nm wavelengths corresponding lidar ratios and depolarization ratio at 532 nm to be determined. The backscattering Angstrom exponent during the dust episodes decreased to ~-0.7, while the extinction Angstrom exponent though being negative, was greater than -0.2. Low values of BAE can likely be explained by an increase in the imaginary part of the dust refractive index at 355 nm compared to 532 nm.

The Maritime Aerosol Network (MAN) data set provides high-quality ground truth to validate the Moderate Resolution Imaging Spectroradiometer (MODIS) aerosol product over open ocean. Prior validation of the ocean aerosol product has been limited to coastal and island sites. Comparing MODIS Collection 5 ocean aerosol retrieval products with collocated MAN measurements from ships shows that MODIS is meeting the prelaunch uncertainty estimates for aerosol optical depth (AOD) with 64% and 67% of retrievals at 550 nm and 74% and 78% of retrievals at 870 nm, falling within expected uncertainty for Terra and Aqua, respectively. Angstrom exponent comparisons show a high correlation between MODIS retrievals and shipboard measurements (R = 0.85 for Terra and 0.83 for Aqua), although the MODIS aerosol algorithm tends to underestimate particle size for large particles and overestimate size for small particles, as seen in earlier collections. Prior analysis noted an offset between Terra and Aqua ocean AODs, without concluding which sensor was more accurate. The simple linear regression reported here is consistent with other anecdotal evidence that Aqua agreement with the Aerosol Robotic Network is marginally better. However, we cannot claim based on the current study that the better Aqua comparison is statistically significant. A systematic increase of error as a function of wind speed is noted in both Terra and Aqua retrievals. This wind speed dependence enters the retrieval when winds deviate from the 6-m/s value assumed in the rough ocean surface and white cap parameterizations. Wind speed dependence in the results can be mitigated by using auxiliary National Centers for Environmental Prediction wind speed information in the retrieval process.

The proposed development is an attempt to enhance aerosol retrieval by emphasizing statistical optimization in inversion of advanced satellite observations. This optimization concept improves retrieval accuracy relying on the knowledge of measurement error distribution. Efficient application of such optimization requires pronounced data redundancy (excess of the measurements number over number of unknowns) that is not common in satellite observations. The POLDER imager on board of the PARASOL micro-satellite registers spectral polarimetric characteristics of the reflected atmospheric radiation at up to 16 viewing directions over each observed pixel. The completeness of such observations is notably higher than for most currently operating passive satellite aerosol sensors. This provides an opportunity for profound utilization of statistical optimization principles in satellite data inversion. The proposed retrieval scheme is designed as statistically optimized multi-variable fitting of the all available angular observations of total and polarized radiances obtained by POLDER sensor in the window spectral channels where absorption by gaseous is minimal. The total number of such observations by PARASOL always exceeds a hundred over each pixel and the statistical optimization concept promises to be efficient even if the algorithm retrieves several tens of aerosol parameters. Based on this idea, the proposed algorithm uses a large number of unknowns and is aimed on retrieval of extended set of parameters affecting measured radiation. The algorithm is designed to retrieve complete aerosol properties globally. Over land, the algorithm retrieves the parameters of underlying surface simultaneously with aerosol. In all situations, the approach is anticipated to achieve a robust retrieval of complete aerosol properties including information about aerosol particle sizes, shape, absorption and composition (refractive index). In order to achieve reliable retrieval from PARASOL observations even over very reflective desert surfaces, the algorithm was designed as simultaneous inversion of a large group of pixels within one or several images. Such, multi-pixel retrieval regime takes advantage from known limitations on spatial and temporal variability in both aerosol and surface properties. Specifically the variations of the retrieved parameters horizontally from pixel-to-pixel and/or temporary from day-to-day are enforced to be smooth by additional appropriately set a priori constraints. This concept is expected to provide satellite retrieval of higher consistency, because the retrieval over each single pixel will be benefiting from co-incident aerosol information from neighboring pixels, as well, from the information about surface reflectance (over land) obtained in preceding and consequent observations over the same pixel. The paper provides in depth description of the proposed inversion concept, illustrates the algorithm performance by a series of numerical tests and presents the examples of preliminary retrieval results obtained from actual PARASOL observations. It is should be noted that many aspects of the described algorithm design considerably benefited from experience accumulated in the preceding effort on developments of currently operating AERONET and PARASOL retrievals, as well as, several core software components were inherited from those earlier algorithms.

A recent paper by Mishchenko et al. compares near-coincident MISR, MODIS, and AERONET aerosol optical depth (AOD) products, and reports much poorer agreement than that obtained by the instrument teams and others. We trace the reasons for the discrepancies primarily to differences in (1) the treatment of outliers, (2) the application of absolute vs. relative criteria for testing agreement, and (3) the ways in which seasonally varying spatial distributions of coincident retrievals are taken into account.

Remote sensing of aerosol from space is a challenging and typically underdetermined retrieval task, requiring many assumptions to be made with respect to the aerosol and surface models. Therefore, the quality of a priori information plays a central role in any retrieval process (apart from the cloud screening procedure and the forward radiative transfer model, which to be most accurate should include the treatment of light polarization and molecular-aerosol coupling). In this paper the performance of various algorithms with respect to the of spectral aerosol optical thickness determination from optical spaceborne measurements is studied. The algorithms are based on various types of measurements (spectral, angular, polarization, or some combination of these). It is confirmed that multiangular spectropolarimetric measurements provide more powerful constraints compared to spectral intensity measurements alone, particularly those acquired at a single view angle and which rely on a priori assumptions regarding the particle phase function in the retrieval process.

Here we review the methods presently available and expected in the near future for retrieving the tropospheric aerosol properties using remote sensing. Since all aerosol properties cannot be derived from space, measurements performed from the surface of the Earth are used to adjust the parameters that are not directly accessible and to limit the variability of the parameters that present a weaker sensitivity. The aerosol properties derived include the column concentration (expressed by the aerosol optical depth), the size (given by distribution of the aerosol in 2 to 3 size modes or measurement of the Angstrom coefficient), composition (expressed by the refractive index), shape and vertical profile. The article is restricted to aerosols that are within the troposphere since the techniques used for stratospheric aerosols are very specific.

EnglishAs the organization of the scientific community involved in observations of atmospheric components (gas, aerosols, clouds, precipitation) is evolving, it is worthwhile to explain how the ICARE Data and Services Centre has established itself within the French scientific community. Since it was created 15 years ago, the ICARE centre has demonstrated the benefits that scientists can draw from a clear policy of pooling means and sharing resources. francaisAu moment ou l'organisation de la communaute scientifique concernee par les observations des differents composants atmospheriques (gaz, aerosols, nuages, precipitations) est en pleine evolution, il n'est pas inutile de decrire comment le Centre de donnees et services Icare a su trouver sa place dans le paysage francais. Son historique au cours de ces quinze dernieres annees illustre parfaitement les benefices que peuvent tirer les chercheurs d'une politique claire et assumee de mutualisation des moyens.

Lidar sounding is used for the analysis of possible contribution of the corner reflection (CR) effect to the total backscattering in case of ice crystals. Our study is based on observations of mixed phase clouds performed during the SHADOW campaign in Senegal. Mie-Raman lidar allows measurements at 355 nm and 532 nm at 43 dg. off-zenith angle, so the extinction and backscattering Ångström exponents can be evaluated. In some measurements we observed the positive values of backscattering Ångström exponent, which can be attributed to the corner reflection by horizontally oriented ice plates.

Monthly mean infrared (10 μm) dust layer aerosol optical depth (AOD) and mean altitude are simultaneously retrieved over the tropics (30° S–30° N) from almost seven years of Atmospheric Infrared Sounder (AIRS) observations covering the period January 2003 to September 2009. The method developed relies on the construction of look-up-tables computed for a large selection of atmospheric situations and follows two main steps: first, determination of the observed atmospheric thermodynamic situation and, second, determination of the dust properties. A very good agreement is found between AIRS-retrieved AODs and visible optical depths from the Moderate resolution Imaging Spectroradiometer (MODIS/Aqua) during the main (summer) dust season, in particular for three regions of the tropical North Atlantic and one region of the north-western Indian Ocean. Outside this season, differences are mostly due to the sensitivity of MODIS to aerosol species other than dust and to the more specific sensitivity of AIRS to the dust coarse mode. AIRS-retrieved dust layer mean altitudes are compared to the Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP/CALIPSO) aerosol mean layer altitude for the period June 2006 to June 2009. Results for a region of the north tropical Atlantic downwind of the Sahara show a good agreement between the two products (σ≈360 m). Differences observed in the peak-to-trough seasonal amplitude, smaller from AIRS, are principally attributed to the large difference in spatial sampling of the two instruments. They also come from the intrinsic limit in sensitivity of the passive infrared sounders for low altitudes. These results demonstrate the capability of high resolution infrared sounders to measure not only dust aerosol AOD but also the mean dust layer altitude.