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

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 degrees 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, AOD(F), and AOD(C), respectively), Angstrom exponent (AE), and the spherical/nonspherical partition of the coarse-mode AOD (AOD(CS) and AOD(CNS), 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 degrees 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 AOD(F-865 nm) < 0.05) 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.

Automated summary

The study examines the aerosol optical properties in the Western Mediterranean Sea using POLDER-3 data, showing a significant influence of North African desert dust on the region. The spatial distribution and seasonal cycles of both total and coarse-mode aerosol optical depth are largely controlled by this desert dust. The study also reveals a decreasing trend in fine-mode aerosol optical depth over the period of 2005-2013, potentially related to reduced emissions in surrounding European countries.