Aerosol optical properties derived from POLDER-3/PARASOL (2005–2013) over the western Mediterranean Sea – Part 1: Quality assessment with AERONET and in situ airborne observations

Abstract

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 require 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, AOD F , and AOD C ), Ångström exponent (AE), and the spherical or non-spherical partition of coarse-mode AOD (AOD CS and AOD CNS ), respectively. The evaluation is performed using data from 17 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. AOD F corresponds to particles smaller than 0.6-0.8 µm in di-ameter and appears suitable to monitor the aerosol submicron fraction from space. We also provide an original validation of POLDER-3 AOD C and its spherical or non-spherical partition, which shows agreement within 25 % with AERONET shape retrievals when the aerosol coarse fraction dominates.

Summertime surface PM<sub>1</sub> aerosol composition and size by source region at the Lampedusa island in the central Mediterranean Sea

“…This calibration was done after the campaign but with similar tuning conditions. The sea salt SO 2− 4 (ss-SO 2− 4 ) was calculated as 0.252 • 0.3 • [seasalt], where 0.252 is the mass ratio of SO 2− 4 to Na + in sea salt and 0.3 is the mass ratio of Na + to sea salt (Ghahremaninezhad et al, 2016). Given these assumptions, the uncertainty in the sea salt concentrations is likely to be significantly higher than the typical 20 %, although the total contribution of sea salt to the PM 1 fraction was very small (0.30 µg m −3 ; <4 %).…”

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

“…This algorithm, described in details by Herman et al (2005) and Tanré et al (2011), has been slightly improved in collection 3 regarding non-spherical particles in the coarse mode (Formenti et al, 2018). Briefly, it is based on the total and polarized radiances measured at 670 and 865 nm.…”

“…CC BY 4.0 License. microphysical models (described in Table S1 in the Supplement of Formenti et al, 2018), the algorithm recalculates for each clear sky pixel the observed polarized radiances at several observational angles. Importantly, in the aerosol models used for the inversion, aerosols are considered as non-absorbing (the imagery part of the refractive index is assumed as zero) and the real part of their refractive index is invariant between 670 and 865 nm.…”

“…The coarse mode includes a non-spherical fraction based on the spheroidal model from Dubovik et al (2006), whereas a Mie model for homogeneous spherical particles is used to calculate multi-spectral and multi-angle polarized radiances. As an improvement compared to former versions of the algorithm, the effective diameter of the spheroidal model is allowed to take two values (namely 2.96 and 4.92 µm) in collection 3 (Table S1 of Formenti et al, 2018). Within the coarse mode, the non-spherical fraction is set to 5 discrete values (0.00, 0.25, 0.50, 0.75, and 1.00, Tanré et al (2011)).…”

“…Georgoulias et al, 2016). Despite the increasing number of satellite-based aerosol studies, especially in the East part of the Mediterranean area (Georgoulias et al, 2016;Shaheen et al, 2020), it is noticeable that no investigation of POLDER-3/PARASOL aerosol products (Herman et al, 2005;Tanré et al, 2011) has been performed yet over this region, despite its potential for monitoring the size-resolved aerosol properties over sea surfaces over its almost 9 years period of operation (Formenti et al, 2018).…”