Measurements obtained between the 9th and 23rd of April 1991 at the Mace Head remote maritime station are presented. These measurements were obtained as part of a EUROTRAC Air Sea Exchange intensive measurement campaign. Analysis of variations in the aerosol and trace concentrations is based on inter-comparison of the measured species and use of local meteorological data and back trajectories for the period. While air masses of principally maritime origin were encountered throughout the campaign, typical background maritime conditions only occurred during short periods. Other periods were highly modified by mainly ''local'' influences which included local biomass burning and Aitken nuclei (AN) production. The biomass burning was observed to contribute to elevated accumulation mode aerosol (0.1-1.0 mu m), black carbon mass concentration, CO, and CH4 levels. The O-3 concentrations were variable throughout the measurement period. Reasonable correlations were found between the trace gas data except between O-3 and CO during the period inffuenced by biomass burning when the O-3 levels were generally depleted. Very high AN concentrations (>20,000 cm(-3)) were measured on a number of sampling days, with a distinct ''dawn effect'' being observed. The very high AN concentrations occurred during otherwise clean air conditions and also in the presence of combustion aerosols and are principally attributed to photochemical gas-to-particle conversion processes. The conditions under which these events occurred and variations in background conditions are examined. Copyright (C) 1996 Elsevier Science Ltd.