Calculation of key optical properties of the main anthropogenic aerosols over the Western French coastal Mediterranean Sea

Abstract The main purpose of this study is to provide new estimations of optical properties for different aerosol species (Elemental (EC) and Organic Carbon (OC), sulfate (S), ammonium (AM) and nitrate (N)) over the Western Mediterranean Sea. This study is based on chemical measurements obtained over the French Mediterranean coastal region which were used to calculate the main optical properties (i.e., Mass Extinction Efficiency (MEE), Single Scattering Albedo (SSA) and asymmetry parameter (g)), which are relevant for Regional Climate Models (RCMs). Our results indicate that EC particles display averaged MEE (with associated uncertainty range) of 5.7 (4.6–6.9) m 2 g −1 (at 550 nm), that is significantly lower than the value generally used (~ 10 m 2 g − 1 ) in RCM simulations conducted over this region. Such differences are mainly due to the inclusion of additional coarse modes in our optical calculations, which are not taken into account in RCMs that generally treat only fine EC particles. Concerning organics aerosols, we obtained a mean dry MEE (with associated uncertainty) of 2.7 (1.8–3.6) m 2 g − 1 (at 550 nm) lower than usually (~ 5–7 m 2 g − 1 ) referenced in RCMs. We also investigated the possible impact of absorbing brown carbon “C brown ” in our calculations showing large changes on dry MEE (from 2.7 to 4.5 m 2 g − 1 , at 550 nm) and SSA (from 0.99 to 0.45, at 550 nm) for pure scattering and absorbing C brown organics, respectively. Nitrate and ammonium particles, which are not well documented over this region, are characterized respectively by dry mean (and associated errors) MEE of 1 (0.8–1.2) m 2 g − 1 and 4 (2.4–5.6) m 2 g − 1 (at 550 nm), together with SSA of 0.89 (0.88–0.90) and 0.97 (0.96–0.98) at 550 nm. Such proposed values could be used to optimize RCMs over the Mediterranean basin for studying “aerosol-climate” interaction through feedbacks of particles on sea-surface fluxes, and hydrological cycle.