Measurements of the mass concentration of particulate carbon aerosol at Mace Head atmospheric research station on the west coast of Ireland are reported. The measurements were made using an aethalometer manufactured by Magee Scientific, which operates by measuring the light transmission through a quartz fibre filter, whilst air is drawn through the filter. Mass concentration measurements taken between February 1990 and April 1991 are presented and interpreted in terms of air mass and meteorological parameters. Average carbon mass concentration levels of 38 +/- 11 ng m-3 were found during periods when the clean sector sampled air masses had traversed over the North Atlantic for several days. Mass levels of the order of several hundred nanograms per cubic metre were typical of continental air masses sampled at the station. It is observed that there is generally a strong correlation between measured carbon mass and smaller sized particle number concentration as measured with a Particle Measurement System, Active Scattering Aerosol Spectrometer probe (ASASP-X). A less-consistent correlation is found between carbon mass and Aitken nuclei concentration. Low mass concentration values (of the order of tens of nanograms per cubic metre) obtained during sampling periods dominated by northern air masses are also reported.