scholarly journals Aerosol solar radiative forcing near the Taklimakan Desert based on radiative transfer and regional meteorological simulations during the Dust Aerosol Observation-Kashi campaign

2020 ◽  
Vol 20 (18) ◽  
pp. 10845-10864 ◽  
Author(s):  
Li Li ◽  
Zhengqiang Li ◽  
Wenyuan Chang ◽  
Yang Ou ◽  
Philippe Goloub ◽  
...  
Keyword(s):  
Radiative Transfer ◽  
Solar Irradiance ◽  
Radiative Forcing ◽  
Surface Albedo ◽  
Dust Aerosol ◽  
Dust Particles ◽  
Taklimakan Desert ◽  
Microphysical Properties ◽  
The Taklimakan Desert ◽  
Aerosol Parameters

Abstract. The Taklimakan Desert is a main and continuous source of Asian dust particles causing significant direct radiative effects, which are commonly quantified by the aerosol solar radiative forcing (ASRF). To improve the accuracy of estimates of dust ASRF, the Dust Aerosol Observation-Kashi (DAO-K) campaign was carried out near the Taklimakan Desert in April 2019. The objective of the DAO-K campaign is to provide crucial parameters needed for the calculation of ASRF, such as dust optical and microphysical properties, vertical distribution, and surface albedo. The ASRF was calculated using radiative transfer (RT) simulations based on the observed aerosol parameters, additionally considering the measured atmospheric profiles and diurnal variations of surface albedo. As a result, daily average values of ASRF of −19 W m−2 at the top of the atmosphere and −36 W m−2 at the bottom of the atmosphere were derived from the simulations conducted during the DAO-K campaign. Furthermore, the Weather Research and Forecasting model with Chemistry (WRF-Chem), with assimilation of measurements of the aerosol optical depth and particulate matter (PM) mass concentrations of particles with aerodynamic diameter smaller than 2.5 µm (PM2.5) and 10 µm (PM10), is employed to estimate the dust ASRF for comparison. The results of the ASRF simulations (RT and WRF-Chem) were evaluated using ground-based downward solar irradiance measurements, which have confirmed that the RT simulations are in good agreement with simultaneous observations, whereas the WRF-Chem estimations reveal obvious discrepancies with the solar irradiance measurements.

scholarly journals Solar radiative forcing of aerosol particles near the Taklimakan desert during the Dust Aerosol Observation-Kashi campaign in Spring 2019

2020 ◽  
Author(s):  
Li Li ◽  
Zhengqiang Li ◽  
Wenyuan Chang ◽  
Yang Ou ◽  
Philippe Goloub ◽  
...  
Keyword(s):  
Land Surface ◽  
Radiative Forcing ◽  
Surface Albedo ◽  
Dust Aerosol ◽  
Dust Particles ◽  
Taklimakan Desert ◽  
Aerosol Radiative Forcing ◽  
Microphysical Properties ◽  
The Taklimakan Desert ◽  
Cooling Effects

Abstract. The Taklimakan desert is a main and continuous source of Asian dust particles causing a significant direct aerosol solar radiative forcing (ASRF). In order to improve the accuracy of the estimation of dust radiative forcing effects, the Dust Aerosol Observation-Kashi (DAO-K) campaign was carried out near the Taklimakan desert in April 2019. The objective of the campaign is to provide comprehensive parameters such as: dust optical and microphysical properties, vertical distribution and surface albedo, for the calculation of ASRF. The measurements were employed in radiative transfer (RT) simulations and the estimations are improved by considering the actual measured atmospheric profiles and diurnal variations of land surface albedo in addition to reliable aerosol parameters. The RT model estimates the ASRF results in average daily mean cooling effects of −19 W m−2 at the top of atmosphere and −36 W m−2 at the bottom of atmosphere during the DAO-K campaign. The Weather Research and Forecasting model with Chemistry (WRF-Chem) with assimilations of the aerosol optical depth and PM2.5 and PM10 concentrations measurements is prone to overestimate the radiative forcing effects of dust aerosols. The percent difference of daily mean ASRF between the two simulations are greater than 50 % in heavy dust episode. Ground-based observations of downward irradiances have validated that the RT simulations are in good agreement with simultaneous observations, whereas the WRF-Chem estimations exhibit obvious discrepancy with these independent measurements. Data assimilations can partly reduce the discrepancy, but there is still room for improving the WRF-Chem simulation of dust aerosol radiative forcing.

scholarly journals Estimating radiative forcing efficiency of dust aerosol based on direct satellite observations: case studies over the Sahara and Taklimakan Desert

2021 ◽  
Vol 21 (15) ◽  
pp. 11669-11687
Author(s):  
Lin Tian ◽  
Lin Chen ◽  
Peng Zhang ◽  
Lei Bi
Keyword(s):  
Radiative Forcing ◽  
Complex Refractive Index ◽  
Satellite Observations ◽  
Dust Aerosol ◽  
Taklimakan Desert ◽  
Dust Source ◽  
Dust Aerosols ◽  
Microphysical Properties ◽  
Source Regions ◽  
The Taklimakan Desert

Abstract. The direct radiative forcing efficiency of dust aerosol (DRFEdust) is an important indicator to measure the climate effect of dust. The DRFEdust is determined by the microphysical properties of dust, which vary with dust source regions. However, there are only sparse in situ measurements of them, such as the distribution of the dust aerosol particle size and the complex refractive index in the main dust source regions. Furthermore, recent studies have shown that the non-spherical effect of the dust particle is not negligible. The DRFEdust is often evaluated by estimating given microphysical properties of the dust aerosols in the radiative transfer model (RTM). However, considerable uncertainties exist due to the complex and variable dust properties, including the complex refractive index and the shape of the dust. The DRFEdust over the Taklimakan Desert and Sahara is derived from the satellite observations in this paper. The advantage of the proposed satellite-based method is that there is no need to consider the microphysical properties of the dust aerosols in estimating the DRFEdust. For comparison, the observed DRFEdust is compared with that simulated by the RTM. The differences in the dust microphysical properties in these two regions and their impacts on DRFEdust are analyzed. The DRFEdust derived from the satellite observation is -39.6±10.0 W m-2τ-1 in March 2019 over Tamanrasset in the Sahara and -48.6±13.7 W m-2τ-1 in April 2019 over Kashi in the Taklimakan Desert. According to the analyses of their microphysical properties and optical properties, the dust aerosols from the Taklimakan Desert (Kashi) scatter strongly. The RTM-simulated results (−41.5 to −47.4 W m-2τ-1 over Kashi and −32.2 to −44.3 W m-2τ-1 over Tamanrasset) are in good agreement with the results estimated by satellite observations. According to previous studies, the results in this paper are proven to be reasonable and reliable. The results also show that the microphysical properties of the dust can significantly influence the DRFEdust. The satellite-derived results can represent the influence of the dust microphysical properties on the DRFEdust, which can also validate the direct radiative effect of the dust aerosol and the DRFEdust derived from the numerical model more directly.

scholarly journals Estimating Radiative Forcing Efficiency of Dust Aerosol Based on Direct Satellite Observations: Case Studies over the Sahara Desert and Taklimakan Desert

2021 ◽  
Author(s):  
Lin Tian ◽  
Lin Chen ◽  
Peng Zhang ◽  
Lei Bi
Keyword(s):  
Radiative Forcing ◽  
Complex Refractive Index ◽  
Satellite Observations ◽  
Dust Aerosol ◽  
Taklimakan Desert ◽  
Sahara Desert ◽  
Dust Aerosols ◽  
Microphysical Properties ◽  
Source Regions ◽  
The Taklimakan Desert

Abstract. The direct radiative forcing efficiency of the dust aerosol (DRFEdust) is an important indicator to measure the climate effect of the dust. The DRFEdust is determined by the microphysical properties of the dust, which vary with the dust source regions. However, there are only sparse in-situ measurements of them, such as the distribution of the dust aerosol particle size and the complex refractive index in the main dust source regions. Furthermore, recent studies have shown that the non-spherical effect of the dust particle is not negligible. The DRFEdust is often evaluated by estimating given microphysical properties of the dust aerosols in the radiative transfer model (RTM). However, considerable uncertainties exist due to the complex and variable dust properties, including the complex refractive index and the shape of the dust. The DRFEdust over the Taklimakan Desert and the Sahara Desert is derived from the satellite observations in this paper. The advantage of the proposed satellite-based method is that there is no need to consider the microphysical properties of the dust aerosols in estimating the DRFEdust. For comparison, the observed DRFEdust is compared with that simulated by the RTM. The differences in the dust microphysical properties in these two regions and their impacts on DRFEdust are analyzed. The DRFEdust derived from the satellite observation is −39.6 ± 10.0 W m−2 τ−1 in March 2019 over Tamanrasset and −48.6 ± 13.7 W m−2 τ−1 in April 2019 over Kashi. According to the analyses of their microphysical properties and optical properties, the dust aerosols from the Taklimakan desert (Kashi) scatter strongly. The RTM simulated results (−41.5 to −47.4 W m−2 τ−1 in the Taklimakan Desert and −32.2 to −44.3 W m−2 τ−1 in the Sahara Desert) are in good agreement with the results estimated by satellite observations. According to previous studies, the results in this paper are proved to be reasonable and reliable. The results also show that the microphysical properties of the dust can significantly influence the DRFEdust. The satellite-derived results can represent the influence of the dust microphysical properties on the DRFEdust, which can also validate the direct radiative effect of the dust aerosol and the DRFEdust derived from the numerical model more directly.

Comparison of radiative transfer schemes for the calculation of heating rates in the atmosphere of Mars

2021 ◽  
Author(s):  
Hao Chen-Chen ◽  
Santiago Pérez-Hoyos ◽  
Agustín Sánchez-Lavega
Keyword(s):  
Radiative Transfer ◽  
Radiative Forcing ◽  
Dust Aerosol ◽  
Dust Particles ◽  
Atmospheric Effects ◽  
Dynamical Models ◽  
Internal Radiation ◽  
Cooling Rates ◽  
Radiation Fields ◽  
Aerosol Radiative

<p><span>The ubiquitous dust aerosol particles in the atmosphere of Mars play a main role on the behaviour and evolution of its climate. By absorbing and scattering the incoming solar radiation, they modify the atmospheric thermal structure and dynamics. Dust radiative forcing calculations are of high relevance to understand Mars' overall atmospheric dynamics. The accuracy in determining internal radiation fields and the resulting atmospheric heating/cooling rates contribute to the uncertainties in these calculations.</span></p><p><span>Radiative transfer schemes using 2-stream approximations are widely implemented in multiple Mars’ dynamical models and Global Circulation Models (GCMs). The uncertainties associated to this approximation are related to neglecting details of dust particles’ scattering phase function: the higher the number of streams considered, the better the accuracy of the scheme, although there is a persistent trade-off between accuracy and computational cost. The objective of this work is to evaluate the accuracy of dust aerosol radiative forcing estimations in the Martian atmosphere by multiple-stream schemes.</span></p><p><span>Several scenarios covering the different atmospheric conditions during the Martian Year were simulated with different radiative transfer models, as well as other high-opacity dust storm scenarios. The atmosphere was discretised into 50 levels from 0 to 100 km, with atmospheric variables loaded from LMD’s Mars Climate Database (MCD). The visible and infrared spectral regions were divided into 12 bands, covering from 0.24 to 1,000 μm. Gaseous opacities were calculated with the correlated-k method, with absorption data retrieved from HITRAN. Dust aerosol radiative properties were derived using the wavelength-dependent properties reported by Wolff et al. (2006, 2009), with vertical distributions following a Conrath profile, and assuming a well-mixed dust scenario. Particle size (effective radius) and column dust opacity were given values to characterise every scenario. Finally, the calculated internal radiation fields and heating/cooling rates with the two-stream approximation code were compared with 4, 8, 16 and 32-stream solutions using the discrete ordinates method (DISORT).</span></p><p><span>The comparison of the results with respect to the 32-stream model shows heating rate underestimations with average differences of about 2.7, 0.3, 0.1, and 0.1 K/sol for the 2-, 4-, 8-, and 16-stream models, respectively. Such differences tend to be larger when there is more dust is loaded into the atmosphere. On the other hand, the average computational times for 1 sol using the 4-, 8-, 16-, and 32-stream schemes are about 15, 25, 40 and 135 times longer than the 2-stream scheme, respectively.</span></p><p><span>Future research prospects include the implementation of multiple-stream DISORT codes in Mars’ mesoscale dynamical models to investigate the accuracy of simulations of the atmospheric effects generated by local and regional dust storms.</span></p>

Radiative forcing over the Arctic of aerosols from the August 2017 Canadian and Greenlandic wildfires

2021 ◽  
Author(s):  
Filippo Calì Quaglia ◽  
Daniela Meloni ◽  
Alcide Giorgio di Sarra ◽  
Tatiana Di Iorio ◽  
Virginia Ciardini ◽  
...  
Keyword(s):  
Radiative Transfer ◽  
Radiative Forcing ◽  
Solar Zenith Angle ◽  
Surface Albedo ◽  
High Arctic ◽  
The Arctic ◽  
Radiative Transfer Model ◽  
Transfer Model ◽  
Aerosol Layer ◽  
Radiative Effect

<p>Extended and intense wildfires occurred in Northern Canada and, unexpectedly, on the Greenlandic West coast during summer 2017. The thick smoke plume emitted into the atmosphere was transported to the high Arctic, producing one of the largest impacts ever observed in the region. Evidence of Canadian and Greenlandic wildfires was recorded at the Thule High Arctic Atmospheric Observatory (THAAO, 76.5°N, 68.8°W, www.thuleatmos-it.it) by a suite of instruments managed by ENEA, INGV, Univ. of Florence, and NCAR. Ground-based observations of the radiation budget have allowed quantification of the surface radiative forcing at THAAO. </p><p>Excess biomass burning chemical tracers such as CO, HCN, H2CO, C2H6, and NH3 were  measured in the air column above Thule starting from August 19 until August 23. The aerosol optical depth (AOD) reached a peak value of about 0.9 on August 21, while an enhancement of wildfire compounds was  detected in PM10. The measured shortwave radiative forcing was -36.7 W/m2 at 78° solar zenith angle (SZA) for AOD=0.626.</p><p>MODTRAN6.0 radiative transfer model (Berk et al., 2014) was used to estimate the aerosol radiative effect and the heating rate profiles at 78° SZA. Measured temperature profiles, integrated water vapour, surface albedo, spectral AOD and aerosol extinction profiles from CALIOP onboard CALIPSO were used as model input. The peak  aerosol heating rate (+0.5 K/day) was  reached within the aerosol layer between 8 and 12 km, while the maximum radiative effect (-45.4 W/m2) is found at 3 km, below the largest aerosol layer.</p><p>The regional impact of the event that occurred on August 21 was investigated using a combination of atmospheric radiative transfer modelling with measurements of AOD and ground surface albedo from MODIS. The aerosol properties used in the radiative transfer model were constrained by in situ measurements from THAAO. Albedo data over the ocean have been obtained from Jin et al. (2004). Backward trajectories produced through HYSPLIT simulations (Stein et al., 2015) were also employed to trace biomass burning plumes.</p><p>The radiative forcing efficiency (RFE) over land and ocean was derived, finding values spanning from -3 W/m2 to -132 W/m2, depending on surface albedo and solar zenith angle. The fire plume covered a vast portion of the Arctic, with large values of the daily shortwave RF (< -50 W/m2) lasting for a few days. This large amount of aerosol is expected to influence cloud properties in the Arctic, producing significant indirect radiative effects.</p>

scholarly journals Hygroscopicity of Different Types of Aerosol Particles: Case Studies Using Multi-Instrument Data in Megacity Beijing, China

2020 ◽  
Vol 12 (5) ◽  
pp. 785 ◽  
Author(s):  
Tong Wu ◽  
Zhanqing Li ◽  
Jun Chen ◽  
Yuying Wang ◽  
Hao Wu ◽  
...  
Keyword(s):  
Enhancement Factor ◽  
Mineral Dust ◽  
Aerosol Particles ◽  
Dust Aerosol ◽  
Dust Particles ◽  
Chemical Components ◽  
Hygroscopic Growth ◽  
Microphysical Properties ◽  
Fine Mode ◽  
Fine Mode Aerosol

Water uptake by aerosol particles alters its light-scattering characteristics significantly. However, the hygroscopicities of different aerosol particles are not the same due to their different chemical and physical properties. Such differences are explored by making use of extensive measurements concerning aerosol optical and microphysical properties made during a field experiment from December 2018 to March 2019 in Beijing. The aerosol hygroscopic growth was captured by the aerosol optical characteristics obtained from micropulse lidar, aerosol chemical composition, and aerosol particle size distribution information from ground monitoring, together with conventional meteorological measurements. Aerosol hygroscopicity behaves rather distinctly for mineral dust coarse-mode aerosol (Case I) and non-dust fine-mode aerosol (Case II) in terms of the hygroscopic enhancement factor, f β ( R H , λ 532 ) , calculated for the same humidity range. The two types of aerosols were identified by applying the polarization lidar photometer networking method (POLIPHON). The hygroscopicity for non-dust aerosol was much higher than that for dust conditions with the f β ( R H , λ 532 ) being around 1.4 and 3.1, respectively, at the relative humidity of 86% for the two cases identified in this study. To study the effect of dust particles on the hygroscopicity of the overall atmospheric aerosol, the two types of aerosols were identified and separated by applying the polarization lidar photometer networking method in Case I. The hygroscopic enhancement factor of separated non-dust fine-mode particles in Case I had been significantly strengthened, getting closer to that of the total aerosol in Case II. These results were verified by the hygroscopicity parameter, κ (Case I non-dust particles: 0.357 ± 0.024; Case II total: 0.344 ± 0.026), based on the chemical components obtained by an aerosol chemical speciation instrument, both of which showed strong hygroscopicity. It was found that non-dust fine-mode aerosol contributes more during hygroscopic growth and that non-hygroscopic mineral dust aerosol may reduce the total hygroscopicity per unit volume in Beijing.

scholarly journals Improved estimate of global dust radiative forcing using a coupled chemical transport-radiative transfer model

2013 ◽  
Vol 13 (1) ◽  
pp. 2415-2456 ◽  
Author(s):  
L. Zhang ◽  
Q. B. Li ◽  
Y. Gu ◽  
K. N. Liou ◽  
B. Meland
Keyword(s):  
Radiative Transfer ◽  
Vertical Profile ◽  
Radiative Forcing ◽  
Chemical Transport ◽  
Dust Particles ◽  
Radiative Transfer Model ◽  
Transfer Model ◽  
Vertical Profiles ◽  
Dust Source ◽  
Source Regions

Abstract. Atmospheric mineral dust particles exert significant direct radiative forcings and are critical drivers of climate change. Here, we use the GEOS-Chem global three-dimensional chemical transport model (3-D CTM) coupled online with the Fu-Liou-Gu (FLG) radiative transfer model (RTM) to investigate the dust radiative forcing and heating rates based on different dust vertical profiles. The coupled calculations using a realistic dust vertical profile simulated by GEOS-Chem minimize the physical inconsistencies between 3-D CTM aerosol fields and the RTM. The use of GEOS-Chem simulated aerosol optical depth (AOD) vertical profiles as opposed to the FLG prescribed AOD vertical profiles leads to greater and more spatially heterogeneous changes in estimated radiative forcing and heating rate produced by dust. Both changes can be attributed to a different vertical structure between dust and non-dust source regions. Values of the dust AOD are much larger in the middle troposphere, though smaller at the surface when the GEOS-Chem simulated AOD vertical profile is used, which leads to a much stronger heating rate in the middle troposphere. Compared to FLG vertical profile, the use of GEOS-Chem vertical profile reduces the solar radiative forcing effect by about 0.2–0.25 W m−2 and the Infrared (IR) radiative forcing over the African and Asia dust source regions by about 0.1–0.2 W m−2. Differences in the solar radiative forcing at the surface between using the GEOS-Chem vertical profile and the FLG vertical profile are most significant over the Gobi desert with a value of about 1.1 W m−2. The radiative forcing effect of dust particles is more pronounced at the surface over the Sahara and Gobi deserts by using FLG vertical profile, while it is less significant over the downwind area of Eastern Asia.

scholarly journals Dust aerosol radiative effects during summer 2012 simulated with a coupled regional aerosol–atmosphere–ocean model over the Mediterranean

2014 ◽  
Vol 14 (18) ◽  
pp. 25351-25410 ◽  
Author(s):  
P. Nabat ◽  
S. Somot ◽  
M. Mallet ◽  
M. Michou ◽  
F. Sevault ◽  
...  
Keyword(s):  
Land Surface ◽  
Radiative Forcing ◽  
Dust Aerosol ◽  
Dust Particles ◽  
Spatial And Temporal Variability ◽  
Surface Solar Radiation ◽  
Climate System Model ◽  
The Mediterranean ◽  
Daily Variability ◽  
Aerosol Radiative

Abstract. The present study investigates the effects of aerosols on the Mediterranean climate daily variability during summer 2012. Simulations have been carried out using the coupled regional climate system model CNRM-RCSM5 which includes prognostic aerosols, namely desert dust, sea salt, organic, black-carbon and sulfate particles, in addition to the atmosphere, land surface and ocean components. An evaluation of the dust aerosol scheme of CNRM-RCSM5 has been performed against in-situ and satellite measurements. This scheme shows its ability to reproduce the spatial and temporal variability of aerosol optical depth (AOD) over the Mediterranean region in summer 2012. Observations from the TRAQA/ChArMEx campaign also show that the model correctly represents dust vertical and size distributions. Thus CNRM-RCSM5 can be used for aerosol–climate studies over the Mediterranean. Here we focus on the effects of dust particles on surface temperature and radiation daily variability. Surface shortwave aerosol radiative forcing variability is found to be more than twice higher over regions affected by dust aerosols, when using a prognostic aerosol scheme instead of a monthly climatology. In this case downward surface solar radiation is also found to be better reproduced according to a comparison with several stations across the Mediterranean. Moreover, the radiative forcing due to the dust outbreaks also causes an extra cooling in land and sea surface temperatures. A composite study has been carried out for 14 stations across the Mediterranean to identify more precisely the differences between dusty days and the set of all the days. Observations show that dusty days receive less radiation at the surface and are warmer than average because of southwesterly fluxes often generating dust outbreaks. Only the simulation using the prognostic aerosol scheme is found to reproduce the observed intensity of the dimming and warming on dusty days. Otherwise, the dimming is underestimated and the warming overestimated.

scholarly journals Retrieval of aerosol microphysical and optical properties above liquid clouds from POLDER/PARASOL polarization measurements

2013 ◽  
Vol 6 (4) ◽  
pp. 991-1016 ◽  
Author(s):  
F. Waquet ◽  
C. Cornet ◽  
J.-L. Deuzé ◽  
O. Dubovik ◽  
F. Ducos ◽  
...  
Keyword(s):  
Biomass Burning ◽  
Radiative Forcing ◽  
Mineral Dust ◽  
Spherical Particles ◽  
Dust Particles ◽  
Polarization Measurements ◽  
Microphysical Properties ◽  
Coarse Mode ◽  
Fine Mode ◽  
Aerosol Properties

Abstract. Most of the current aerosol retrievals from passive sensors are restricted to cloud-free scenes, which strongly reduces our ability to monitor the aerosol properties at a global scale and to estimate their radiative forcing. The presence of aerosol above clouds (AAC) affects the polarized light reflected by the cloud layer, as shown by the spaceborne measurements provided by the POlarization and Directionality of Earth Reflectances (POLDER) instrument on the PARASOL satellite. In a previous work, a first retrieval method was developed for AAC scenes and evaluated for biomass-burning aerosols transported over stratocumulus clouds. The method was restricted to the use of observations acquired at forward scattering angles (90–120°) where polarized measurements are highly sensitive to fine-mode particle scattering. Non-spherical particles in the coarse mode, such as mineral dust particles, do not much polarize light and cannot be handled with this method. In this paper, we present new developments that allow retrieving also the properties of mineral dust particles above clouds. These particles do not much polarize light but strongly reduce the polarized cloud bow generated by the liquid cloud layer beneath and observed for scattering angles around 140°. The spectral attenuation can be used to qualitatively identify the nature of the particles (i.e. accumulation mode versus coarse mode, i.e. mineral dust particles versus biomass-burning aerosols), whereas the magnitude of the attenuation is related to the optical thickness of the aerosol layer. We also use the polarized measurements acquired in the cloud bow to improve the retrieval of both the biomass-burning aerosol properties and the cloud microphysical properties. We provide accurate polarized radiance calculations for AAC scenes and evaluate the contribution of the POLDER polarization measurements for the simultaneous retrieval of the aerosol and cloud properties. We investigate various scenes with mineral dust particles and biomass-burning aerosols above clouds. For clouds, our results confirm that the droplet size distribution is narrow in high-latitude ocean regions and that the droplet effective radii retrieved from both polarization measurements and from total radiance measurements are generally close for AAC scenes (departures smaller than 2 μm). We found that the magnitude of the primary cloud bow cannot be accurately estimated with a plane parallel transfer radiative code. The errors for the modeling of the polarized cloud bow are between 4 and 8% for homogenous cloudy scenes, as shown by a 3-D radiative transfer code. These effects only weakly impact the retrieval of the Aerosol Optical Thickness (AOT) performed with a mineral dust particle model for which the microphysical properties are entirely known (relative error smaller than 6%). We show that the POLDER polarization measurements allow retrieving the AOT, the fine-mode particle size, the Ångström exponent and the fraction of spherical particles. However, the complex refractive index and the coarse-mode particle size cannot be accurately retrieved with the present polarization measurements. Our complete and accurate algorithm cannot be applied to process large amounts of data, so a simpler algorithm was developed to retrieve the AOT and the Ångström exponent above clouds in an operational way. Illustrations are provided for July–August 2008 near the African coast. Large mean AOTs above clouds at 0.865 μm (>0.3) are retrieved for oceanic regions near the coasts of South Africa that correspond to biomass-burning aerosols, whereas even larger mean AOTs above clouds for mineral dust particles (>0.6) are also retrieved near the coasts of Senegal. For these regions and time period, the direct AAC radiative forcing is likely to be significant. The final aim of this work is the global monitoring of the AAC properties and the estimation of the direct aerosol radiative forcing in cloudy scenes.

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