| Summary: | In order to achieve a better understanding of the modifications of the physical and chemical properties of marine aerosol particles during transport from offshore to the coast, size distribution and chemical composition were measured concurrently in clean air masses over the open North Atlantic Ocean and at an Irish coastal site. Open ocean sampling was performed on board the oceanographic vessel Celtic Explorer sailing 100ĝ€"300 km off the Irish west coast, while coastal measurements were performed at the Mace Head GAW station. The experiment took place between 11 June and 6 July 2006, during the period of phytoplankton bloom. The number size distribution and size-resolved chemical composition of coastal and open ocean samples were very similar, indicating homogeneous physical and chemical aerosol properties over a wide region in the marine boundary layer. The results also show that submicron chemical and physical aerosol properties measured at the coastal Mace Head Atmospheric Research Station were not unduly influenced by coastal artefacts and are thus representative of open water properties. Greater differences between the coastal site and the open ocean were observed for the aerosol supermicron sea spray components; this could be due to a variety of reasons, ranging from higher local wind speeds at the coastal site over the comparison period, to differences in sampling heights and increased local surf-zone production. Evidence of ageing processes was observed: at the costal site the ratio between non-sea-salt sulphate and methanesulphonic acid was higher, and the aerosol water soluble organic compounds were more oxidized than in the open ocean.
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