Continuous measurements of white light attenuation (ATN) through a quartz fiber filter (related to aerosol black carbon content) and of carbon monoxide (CO) carried out over the period December 1991 through July 1992 at the Mace Head atmospheric field research station (53¿19'N, 9¿54'W) near Carna, County Galway on the west coast of Ireland are presented. Isentropic air flow back trajectories, condensation nuclei concentration data, and meteorological data were generated as part of the Atmosphere/Ocean Chemistry Experiment. Use is made of this supporting data together with synoptic weather charts in the analysis of the measurements. Measurements of ATN and CO are reported for unmodified marine air from the North Atlantic which yield ATN and ATN/CO values in the range 1--4¿10-3 m-3 and 1--4¿10-5 m-3 ppbv-1. Increasing values of ATN and ATN/CO are obtained for polluted transatlantic air, modified marine air, and polluted air from the UK and/or Europe. The covariance of ATN and CO obtained for these air masses suggests a common source. Transport of air from west of the African Saharan region to Mace Head is accompanied by a rise in optical attenuation purported to be due to absorbing desert dust aerosol without a concurrent rise in CO. ¿ American Geophysical Union 1996