On the episodic nature of wet deposited sulphate and acidity.

Daily sampling of acidity and sulphate in rain, air concentrations of SO2 and aerosol SO42−, and climatological variables for the period 1977 to 1981 at a site in southern Scotland are analysed. The properties of frequency distributions for wet deposited acidity, sulphate and rainfall, and the episodicity of each, are discussed. Up to 40% of annual wet deposited acidity occurred on < 4% of rain days, exceeding the episodicity of dry deposited SO2 by a factor of 3. Over the 5 year period, 16 % of rain events were more acid than pH 4.0 and 25 % less acid than pH 5.0; meteorological conditions associated with these extremes of the distribution showed marked differences. The most acid events were associated with large aerosol SO42− concentrations, small windspeeds and rainfall amounts, and 48-h surface geostrophic wind back-trajectories over industrial regions of the U.K. and/or Europe. The least acid events were associated with 48-h back-trajectories over the north Atlantic, larger windspeeds and rainfall amounts. The very acid events are well described by the frequency distribution of rainfall acidity. Artefacts in rainfall chemistry data caused by dry deposition on the collector are discussed with particular reference to the variation of SO42− concentration with rainfall amount.