Estimates of regional contributions to wet acid deposition in western Massachusetts during the summer of 1984

The General Motors mobile Atmospheric Research Laboratory was situated in rural western Massachusetts in the Berkshire Mountains for 62 days during the summer of 1984. One purpose of this study was to determine the source regions of wet acid deposition for this northeastern U.S. location. First, to apportion the precursors (sulfate, sulfur dioxide and nitrate) to source regions, daily ambient air samples were analyzed for the precursors as well as for tracer species that are associated with particular sources. Factor and trajectory analyses were then used to deduce the contributions of the Midwest and the Northeast to these precursors. Finally, the contribution of the precursors to precipitation acidity was estimated by analyzing the chemical constituents in the rain during seven precipitation events. Averaged over the entire duration of the study, the data show that Northeast sources accounted for about 60% of the precipitation sulfate and nitrate, while Midwest sources accounted for about 30%. The balance (~ 10%) was accounted for by background sulfate. A more useful way of examining the data is according to the type of storm that caused the wet deposition. The site was affected by two basic types of storms: coastal low-pressure systems that traveled up the Atlantic Coast, and cold fronts that approached from the west. During the coastal lowpressure events, the Midwestern contribution to precipitation acidity was zero, as easterly flows from the Atlantic Ocean dominated. The cold front events, however, were all associated with southwesterly flows, and the Midwest contributions exceeded the Northeast contributions. During these events, the average Midwest contribution to precipitation acidity was about 50%. For all events, the ratio of sulfate to nitrate was approximately 2:1 on an equivalent basis. During the coastal lows, the relative nitrate contributions were the highest. It was estimated that particulate sulfate scavenging was responsible for about half of the sulfate in the rain, while the other half was due to in-cloud oxidation of gaseous sulfur dioxide.