Optical, physical and chemical properties of aerosols transported to a coastal site in the Western Mediterranean: Focus on primary marine aerosols

As part of the ChArMEx-ADRIMED campaign (summer 2013), ground-based in-situ observations were conducted at the Ersa site (northern tip of Corsica; 533 m asl) to characterize the physical, optical and chemical properties of aerosols. During the observation period, three different aerosol regimes have been identified, including a dust outbreak (Dust) originating from Algeria/Tunisia, a primary marine aerosols (PMA) event from both the Gulf of Lion and emissions near the sample site, and a pollution period from Eastern Europe, which includes anthropogenic and biomass burning sources (BBP). The chemical, physical and optical properties of the observed aerosols as well as their local shortwave (SW) direct radiative forcing (DRF) in clear-sky conditions are compared for these three periods in order to assess the direct radiative impact of PMA above the Western Mediterranean Basin. The PMA period is characterized by a mean sea salt mass concentration up to 6.5 μg m−3, representing 40 % of the total PM10 mass concentration, and a relatively low ratio of chloride to sodium (average of 0.57) indicating a generally "aged" sea salt aerosol at Ersa. In this work, an original dataset, obtained from on-line real-time instruments (ATOFMS, PILS-IC) have been used to characterize the ageing of PMA. The majority of PMA had surprisingly undergone chemical reactions and were mostly advected from long-range transport. During PMA period, the mixing between fresh and aged PMA originated from both local and regional (Gulf of Lion) emissions. The aerosol optical properties, obtained for the whole atmospheric column and at the surface, indicate a single scattering albedo (SSA) near unity (at 440 nm), indicating almost purely scattering particles, associated to a relatively low aerosol optical depth (AOD) close to 0.1 (at 500 nm), and an aerosol angstrom extinction exponent (AE) equal to 1.3 ± 0.4 (between 440 and 870 nm), suggesting a possible mixing of the PMA with fine particles (probably of continental origin). AERONET retrievals indicate a relatively low local SW DRF during the PMA period with mean values of −11 ± 4 W m−2 at the surface and −8 ± 3 W m−2 at the top of the atmosphere (TOA). In comparison, our results indicate that the dust outbreak observed at our site during the campaign, although of moderate intensity (AOD of 0.3–0.4 at 440 nm and column-integrated SSA of 0.90–0.95), induced a local instantaneous SW DRF nearly three times the forcing calculated during the PMA period, with maximum values up to −40 W m−2 at the surface. On average, the SW DRF was about −21 ± 11 and −14 ± 6 W m−2, at the surface and at TOA, respectively, during this dust outbreak. Finally, the BBP period was characterized by a significant increase of the aerosol PM1 mass concentration (from 3.7 μg m−3 to 7.2 μg m−3) due to the influence of biomass-burning and anthropogenic aerosols transported from Eastern Europe. The influence of polluted/smoke particles led to a significant decrease in SSA (0.90 at 440 nm), showing the important absorbing characteristics of such particles. For this period, the SW DRF at the surface and TOA also exhibit higher mean values compared to the PMA period (with values of −23 ± 6 W m−2 and −15 ± 4 W m−2, respectively) and similar range of values as the Dust period.