Sulfuric acid plays a major role in the formation of aerosol particles and clouds. Measurements at the west coast of Ireland reveal that oxidation of SO2 by OH explains only 20%, on average, of H2SO4 formation in coastal marine air. Additional sources may be (a) oxidation by Criegee intermediates produced photolytically and/or (b) formation from SO3 instead of SO2 in the oxidation of dimethyl sulfide, suggesting an important role of marine emissions in the self-cleaning power of the atmosphere.