The relation between sources of sulphur dioxide (SO2) and precipitation sulphate (SO4) concentrations at distant receptors is the subject of intensive empirical and theoretical investigation1,2. This relation provides a probe of atmospheric chemistry, physics and meteorology, and insight into the effectiveness of potential acid-deposition reduction strategies. Large variations in SO2 emissions from copper smelters in the southwestern United States, the major regional sulphur source, between 1980 and 1984 provide a unique opportunity to study the source-receptor relation, as during this interval the National Atmospheric Deposition Program (NADP) monitored wet-precipitation sulphate concentrations on a weekly basis at Rocky Mountain locations. Here we extend our earlier analysis of annual-average data2 by examining monthly sulphate concentration and emission data for 1980-84. We show that monthly data are consistent with a linear relation between emissions and concentration, possessing the expected properties of a source-receptor relation. We then predict concentration changes resulting from the addition of a new smelter at Nacozari, Mexico, expected to be the second-largest source of SO2 in North America.
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