The mass concentrations of total organic carbon (TC) and its chemical fractions, water-soluble organic carbon (WSOC), water-insoluble organic carbon (WISOC), and black carbon (BC), were determined in marine aerosols, which were collected during the period July 1998-September 1999 at the Mace Head Atmospheric Research Station on the west coast of Ireland. In addition, mass concentrations of major inorganic ions, like non-sea-salt (NSS) sulphate, and mass concentrations of dicarboxylic acids have been measured separately within the water-soluble aerosol fraction. The aerosol samples were classified into 'clean marine aerosol samples' and 'modified marine aerosol samples' according to the origin of air masses arriving at Mace Head, and using BC as a tracer for anthropogenic pollution. Following this classification, the data set was,further divided into samples taken during summertime and wintertime. An average TC mass concentration of 591 +/- 75 ng m(-3) was determined for all aerosol samples taken at Mace Head. WSOC was found to be the dominating fraction in 'modified marine aerosol samples', followed by WISOC and BC, whereas in 'clean marine aerosol samples' WISOC was the main fraction, followed by WSOC, and BC. A comparison of the mass concentration of the water-soluble organic matter to the main water-soluble inorganic component, NSS sulphate, gave an average ratio of 0.72 +/- 0.10, which increased to 1.59 +/- 0.23 when considering solely 'clean marine aerosol samples'. The dominating dicarboxylic acid found in carbonaceous aerosols at Mace Head was oxalic acid, followed by malonic acid. Both diacids were responsible for about 14 +/- 2% of the WSOC (in terms of carbon). (C) 2002 Elsevier Science Ltd. All rights reserved.