Background ozone O-3 observations at Mace Head on the west coast of Ireland since 1987 show a significant positive trend of 0.49 +/- 0.19 ppb year(-1) through to 2003. Increasing trends are observed for all seasons, with the largest trends during the winter season, 0.63 +/- 0.31 ppb year(-1) and the smallest trends during the summer, 0.39 +/- 0.25 ppb year(-1). However, this growth rate has not been consistent over time with a major anomaly evident in 1998-1999. This major O-3 perturbation is correlated with variations of CO2, CO, CH4, H-2 and CH3Cl, which are likely due to large-scale biomass burning events in tropical and boreal regions during 1997-1999 coupled with an intense El Ni (n) over tildeo event. Over the 16 years of observations background O-3 has increased by about 8 ppb (similar to24%) in the clean oceanic sectors representative of the Northern Hemisphere marine boundary layer. (C) 2004 Elsevier Ltd. All rights reserved.