We report aerosol SO42- measurements from 1979 to 1996 at Whiteface Mountain located in the Adirondack Mountains and from 1983 at Mayville, in western New York State. From 1981 to 1991 SO42- decreased, ~3% per year. Reductions of nearly 47% in SO42- and total sulfur concentrations at Whiteface Mountain, and about 30% at Mayville, were observed during 1995 to 96 compared with the mean levels during 1981--91, while the upwind SO2 emissions in the Midwestern United States decreased by 36%. Linear relationships between SO42- and total sulfur concentrations and the SO2 emissions were observed for the entire period. The mean ratio of SO42- concentrations to SO2 emissions for the eighteen year period at Whiteface Mountain is 0.11¿0.014 and for thirteen years at Mayville is 0.23¿0.014 &mgr;g SO42-/m3 per 103 metric tons SO2 emitted per day. The data suggest that the observed relationship could be used to predict decreases in atmospheric SO42- levels based on SO2 emissions several years hence. Furthermore, it appears feasible to develop an empirical source-receptor relationship if similar data were acquired from a network of strategically located sampling sites. ¿ 1998 American Geophysical Union