Segregation and Interpretation of Ozone and Carbon Monoxide Measurements by Air Mass Origin at the TOR Station Mace Head, Ireland from 1987 to 1995

Three independent methods have been used to sort the ozone, carbonmonoxide, and other radiatively important trace gases measured at Mace Head,Ireland, and thereby distinguish clean air masses transported over the NorthAtlantic from the more polluted air masses which have recently travelledfrom the European continent. Over the period April 1987–June 1995 theNorthern Hemisphere surface ozone baseline concentrations exhibited a meanconcentration of 34.8 ppb, with a small positive trend (+0.19 ppbyr^-1), while the corresponding trend in air originating fromthe polluted European areas was negative (–0.39 ppbyr^-1). Carbon monoxide measurements from March 1990 toDecember 1994 showed negative trends for both the unpolluted (–0.17ppb yr^-1) and polluted data (–13.6 ppbyr^-1). Overall the continent of Europe was shown to be a smallnet sink of 2.6 ppb for all occasions when European air was transported tothe North Atlantic.