EXECUTIVE SUMMARY Important elements in the estimation of the anthropogenic contribution to acidic or total sulfate deposition are estimates of acidic@ or total@ primary sulfates {i.e.@ those sulfur oxides which are converted to acids or sulfates before their release to the atmosphere). Such estimates are also necessary for calculations of the proportion of acidic or total sulfate deposition which can be attributed to local sources@ or to long-range transport. Preliminary estimates of primary sulfate emissions which have been used in modeling exercises have indicated that@ especially in the winter@ a large portion of sulfate deposition in the northeastern U.S. may be due to primary sulfate emissions largely from area sources. However@ the validity of this conclusion depends@ in great part@ upon the level of confidence that can be attached to the primacy sulfate emission factors.