SPATIAL VARIATION IN ACIDIC SULFATE AND AMMONIA CONCENTRATIONS WITHINMETROPOLITAN PHILADELPHIA

Acidic sulfate concentrations were measured in metropolitan Philadelph ia during the summers of 1992 and 1993, as part of a continuing effort to characterize particle concentrations in urban environments. Sampli ng was performed simultaneously at eight sites located within and arou nd metropolitan Philadelphia. Sites were selected based on their popul ation density and on their distance and direction from the city center . Air pollution sampling was conducted every other day during the summ er of 1992 and every day during the summer of 1993. All samples were c ollected for 24-h periods beginning at 9 a.m. (EDT). All acidic sulfat e and ammonia samples were collected using modified Harvard-EPA Annula r Denuder Systems (HEADS). In this paper, we examine the spatial varia tion in acidic sulfate and ammonia concentrations within the metropoli tan Philadelphia area. We also identify factors that may influence the ir variation and develop models to predict their concentrations. Outdo or sulfate (SO42-) concentrations were uniform within metropolitan Phi ladelphia; however, aerosol strong acidity (H+) concentrations varied spatially. This variation generally was independent of wind direction, but was related to local factors, such as the NH3 concentration, popu lation density, and distance from the center of the city. Physico-chem ical models, which were developed using data collected during the summ er of 1992, were excellent predictors of 24-h and mean summertime H+ c oncentrations measured during the summer of 1993. Models accounted for 78% of the variation in 24-h H+ levels. Results suggest that a single stationary ambient (SAM) monitor would be sufficient to estimate SO42 - exposures for populations living in Philadelphia. For H+, however, m ultiple monitoring sites or models should be used to determine the out door H+ exposures of populations Living in urban environments, althoug h a single SAM site may provide an excellent index of H+ variation ove r time.