Aerosol optical properties derived from POLDER-3/PARASOL (2005–2013) over the western Mediterranean Sea – Part 2: Spatial distribution and temporal variability

• Main Authors: Chiapello, Isabelle, Formenti, Paola, Mbemba Kabuiku, Lydie, Ducos, Fabrice, Tanré, Didier, Dulac, François
• Format: Text
• Language: English
• Published: 2021
• Subjects: Western Basin; North Atlantic; North Atlantic oscillation
• Online Access: https://doi.org/10.5194/acp-2020-1155; https://acp.copernicus.org/preprints/acp-2020-1155/

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. In this study, we investigate aerosol spatial distribution and temporal evolution over the western Mediterranean Sea (west of longitude 20° E) using the full POLDER-3/PARASOL aerosol data record derived from the operational clear-sky ocean algorithm (collection 3) available from March 2005 to October 2013. This 8.5-yr satellite data set includes retrievals at 865 nm of the total, fine, and coarse mode aerosol optical depth (AOD, AOD F , and AOD C , respectively), Angström exponent (AE), and the spherical/non-spherical partition of the coarse-mode AOD (AOD CS and AOD CNS , respectively). In a previous paper (Formenti et al., 2018), these POLDER-3-derived aerosol properties have been carefully validated over the study region, based on coincident ground-based and airborne aerosol measurements. Here we analyze the spatial distribution, the seasonal cycle and interannual variability of this ensemble of products in three latitude bands (34–38° N, 38–42° N, and > 42° N) and for three sites (Ersa, Barcelona, Lampedusa) distributed on the western basin Overall the POLDER-3 AOD spatial distribution exhibits a well-known south-to-north decreasing gradient, and a seasonal cycle characterized by enhanced aerosol loads in spring and summer, both controlled by Saharan dust. POLDER-3 retrievals of AE, AOD F , AOD C , and fine mode fraction (AOD F /AOD) highlight the influence of coarse particles in the southern part of the region, off the north African coast, and higher relative contribution of fine particles in the northern part, off the south European coast, with all year long persistent elevated loads over the Adriatic Sea. Over the rest of the western Mediterranean Sea, POLDER-3 retrievals show a more homogeneous spatial distribution of fine particles than that of coarse particles, even though climatological means of AOD F highlight seasonal differences in the order of a factor 2 between the cleanest conditions occurring in the southern part of the basin in winter and those most polluted observed in its northern part in Spring. The seasonal and spatial variability of AOD CNS is close to that observed for AOD C , whereas POLDER-3 exhibit relatively low and weakly variable levels of coarse spherical particles (AOD CS < 0.05). Over the whole 2005–2013 period, annual POLDER-3 AOD evolution shows a decreasing trend (≥ 0.003 per year in absolute value). Such a decrease is much more pronounced for AOD F (≥ 0.002 per year) than for AOD C (≤ 0.002 per year). Our analysis also suggests that the North Atlantic Oscillation (NAO) index explains a significant part of the interannual variability of POLDER-3 AOD C , reflecting its role on the frequency of Saharan dust transport over the region. Finally, the POLDER-3 dataset highlights an improvement of air quality related to the fine aerosol component, with an evolution toward more frequent occurrence of clean conditions (≥ 70 % of daily AOD F-865 nm < 0.05) at the end of the period of study (2010–2013) over the western Mediterranean Sea.