Background ozone O3 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 O3 perturbation is correlated with variations of CO2, CO, CH4, H2 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 Niño event. Over the 16 years of observations background O3 has increased by about 8 ppb (∼24%) in the clean oceanic sectors representative of the Northern Hemisphere marine boundary layer.