EUROPEAN SOURCE STRENGTHS AND NORTHERN-HEMISPHERE BASE-LINE CONCENTRATIONS OF RADIATIVELY ACTIVE TRACE GASES AT MACE-HEAD, IRELAND

Greenhouse gas measurements have been made continuously with high freq uency and precision at the remote baseline monitoring station at Mace Head on the Atlantic Ocean coastline of Ireland since 1987. By using t hree independent methods, the two-hourly observations have been sorted by air mass origins into those from ''unpolluted'' or Northern Hemisp here baseline air masses and those from ''polluted'' European air mass es. Northern Hemisphere baseline methane, nitrous oxide and carbon dio xide concentrations have risen throughout the 10 year period and their respective mid-1996 levels are the highest mid-year levels recorded s o far. For ozone, the mid-1996 annual mean concentrations was within 0 .1 ppb of the highest mid-year level. The elevated greenhouse gas conc entrations found in ''polluted'' air masses provide clear evidence for the presence of substantial emission sources in Europe. Using a simpl e climatological long-range transport model and a sophisticated Lagran gian dispersion model, it has been possible to estimate the magnitudes of the greenhouse gas emissions required to support the observations. Estimates of European source strengths of methane, nitrous oxide, car bon monoxide and halocarbons agree well with the available emission in ventories. Using the Mace Head observations, significant additional co ntributions to global climate change have been identified from the tra ce gases: methane, nitrous oxide and tropospheric ozone, over and abov e that driven by carbon dioxide. The radiative forcing consequences of a range of HCFCs and HFCs will only become of significance should the ir emissions grow to become comparable with those of the CFCs that the y have replaced. (C) 1998 Elsevier Science Ltd. All rights reserved.