Size-segregated marine aerosols were collected at Mace Head Atmospheric Research Station (Ireland) during spring and autumn 2002 corresponding with the phytoplankton bloom periods in the North Atlantic. Strict control of the sampling, air mass back trajectory analysis, and analysis of pollutant tracers allowed the selection of a set of samples representative of clean marine conditions. A comprehensive chemical characterization of both (1) water-soluble and water-insoluble organic fraction and (2) water-soluble inorganic ions was performed. The selected samples illustrated a consistent picture in terms of chemical composition. The supermicron mode predominantly comprises sea-salt aerosol with a mass concentration of 10.16 ¿ 0.80 ¿g m-3, the remainder being non-sea-salt (nss) sulphate, 0.03 ¿ 0.01 ¿g m-3, and nitrate, 0.13 ¿ 0.04 ¿g m-3. By comparison, the mass of sea salt, nss sulphate, and nitrate in the submicron mode is found to be 0.39 ¿ 0.08 ¿g m-3, 0.26 ¿ 0.04 ¿g m-3, and 0.02 ¿ 0.01 ¿g m-3, respectively. Water-soluble organic carbon (WSOC) is observed in the submicron mode with a mass concentration of 0.25 ¿ 0.04 ¿g m-3, comparable to that of nss sulphate, and in the supermicron mode with a mass concentration of 0.17 ¿ 0.04 ¿g m-3. The WSOC to total carbon (TC) ratio is found to be 0.20 ¿ 0.12 for the submicron fraction and 0.29 ¿ 0.08 for the supermicron fraction, while the black carbon (BC) to TC ratio is, on average, 0.032 ¿ 0.001 for both aerosol modes. The remaining carbon, water-insoluble organic carbon, contributes 0.66 ¿ 0.11 ¿g m-3 and 0.26 ¿ 0.06 ¿g m-3 to the submicron and supermicron modes, respectively and, thus, represents the dominant submicron aerosol species. Furthermore, the WSOC chemical composition comprises mainly aliphatic and only partially oxidized species and humic-like substances, resulting in appreciable surface-active properties. The observed organic matter chemical features (size-dependent concentration, hydrophobic nature of a substantial fraction of the organic matter, and low oxidized and surface-active WSOC species) are consistent with the hypothesis of a primary marine source; bubble-bursting processes, occurring at the surface of the North Atlantic Ocean during phytoplankton blooms, effectively transfer organic matter into marine aerosol particles, particularly enriching the fine-aerosol fraction.