The influence of the sulfur dioxide emissions from a large copper smelter in Tacoma, Washington was studied by measuring the chemical composition of rainwater collected upwind and downwind of the source, before and after permanent closure of the smelter in 1985. Data analysis was based on a statistical model that accounted for variability associated with location of 25 sampling sites within three geographic regions, smelter operation, 10 individual rain events observed over 2 years, and measurement uncertanity. After smelter closure the unwind-downwind differences in mean hydrogen ion and excess sulfate ion concentrations within a 600 km2 region, extending to 25 km downwind of the source, had decreased significantly compared to the preclosure value. No significant decreases in mean ion concentrations were observed farther downwind in the Seattle urban area. During five events sampled prior to smelter closure, we estimate that an average of 4.4% (range 0.7--8.6%) of the sulfur dioxide emitted by the smelter was wet deposited as sulfate ion within a 3600 km2 area extending 60 km downwind of the source. ¿ American Geophysical Union 1988