AGGREGATION OF SELECTED 3-DAY PERIODS TO ESTIMATE ANNUAL AND SEASONAL WET DEPOSITION TOTALS FOR SULFATE, NITRATE, AND ACIDITY .2. SELECTION OF EVENTS, DEPOSITION TOTALS, AND SOURCE-RECEPTOR RELATIONSHIPS

A method for deriving estimates of long-term acidic deposition over eastern North America based on a limited number of Regional Acid Deposition Model runs has been developed. The main components of this method are the identification of a representative sample of events for model simulation and the aggregation of the deposition totals associated with the events. Meteorological categories, defined according to 3-day progressions of 850-mb wind flow over eastern North America, were used to guide the selection of events. This paper describes how events were selected from the categories and how they were combined (aggregated) to estimate long-term deposition. The effectiveness of the category-based approach was compared against alternate aggregation approaches and it was found to provide the best sample-based estimates of long-term wet sulfate deposition across eastern North America. Thirty events from the 1982-85 time period were selected using a set of predetermined criteria and aggregated to estimate seasonal and annual SO42-, NO3-, and H+ deposition at 20 Utility Acid Precipitation Study Program sites. The accuracy of the estimates varied geographically and depending upon whether they were for the annual or seasonal time periods. Over the main area of interest (a smaller 13-site region), the mean rms errors for annual deposition were 10%, 15%, and 12% for sulfate, nitrate, and acidity, respectively. Source-receptor relationships associated with the 30 events were examined for three sites located in Michigan, North Carolina, and upstate New York. It was found that the amount of time that transport was to these areas from the U.S. Midwest (an area of high SO2 emissions) was represented to within 20%.