Contribution of marine aerosols in the particle size distributions observed in Mediterranean coastal zone

Results from analysis of aerosol size distributions measured in a Mediterranean coastal zone are presented. This paper focuses on aerosol particles smaller than 10 μm because they specifically represent a mixed contribution of aerosols in a coastal zone. The interpretation is first based on low wind speed periods to identify the background aerosol in the study area. We can note the influence of wind direction on both aerosol concentrations and size distributions that have been measured. The contributions of the various particle sources, continental or marine, from anthropogenic or natural origin have been demonstrated regardless of each aerosol size distribution. The influence of solar irradiation has been observed for low wind speed periods. It induces a bimodal size distribution characterized by the classical fine particle peak around 0.04 μm and an accumulation mode at 0.1 μm Aerosol size distributions measured during high continental wind periods (which correspond to a 25 km fetch) show a strong concentration peak around 0.02 μm The good correlation obtained between this particle concentration and wind speed shows that the smallest particles measured during high continental wind speed periods are probably of marine origin. The interpretation of this large contribution of very small particles has focused on two main hypotheses. The increase of gas transfers (probably the dimethyl sulfide) induced by the whitecap coverage upon the sea surface or a specific production of fine particles as previously suggested by Despiau et al. (Journal of Aerosol Science 1996, 27(3),403–415). The specificities of the smallest particles are also observed if we compare background aerosol and concentrations recorded during high wind speed periods. In contrast with the sea-surface-generated aerosols which decrease during an extended period of low wind speed (Smith et al., Atmospheric Environment 1991, 25A, 547–555), we have observed larger concentrations of 0.1–0.3 ,um particles during the background conditions.