Aerosol optical properties derived from POLDER-3/PARASOL (2005–2013) over the Western Mediterranean Sea – Part 2: Spatial distribution and temporal variability
Authors: Chiapello Isabelle , Formenti Paola , Mbemba Kabuiku Lydie , Ducos Fabrice , Tanré Didier , Dulac François 
Published: August 2021

Abstract. 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 &amp; 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<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.