Long-term trends in concentrations of halocarbons and radiatively active trace gases in Atlantic and European air masses monitored at Mace Head, Ireland from 1987-1994

Long-term trends in trace gas concentrations over the period 1987-1994 are reported here for air masses advected to the Mace Head monitoring station on the remote west coast of Ireland. The trace gases covered include the principal halocarbons: CFC-11, CFC-12, CFC-113, CCl₄, and methyl chloroform; the radiatively active trace gases: carbon dioxide, methane, nitrous oxide and ozone; together with carbon monoxide, the major photochemical ozone precursor. By careful sorting using two independent techniques, it is possible to distinguish air masses that arrive at Mace Head from over the North Atlantic Ocean and those that have recently travelled over polluted European continental land areas. Concentration trends have been derived for each trace gas in polluted European continental and baseline North Atlantic maritime air and they appear to be distinctly different. Using a simple long-range transport model, estimates have been made of the European source strengths required to sustain the observed concentrations of each trace gas and their recent trends. These are compared with published emission inventories where they are available. The European continent appears to be a significant source of methane, carbon dioxide and nitrous oxide, a net sink for ozone and a declining source of the principal halocarbons and carbon monoxide.