The report described the EPA's multi-year program to investigate the specific issues surrounding human exposures to aerosol activity. Philadelphia, a large metropolitan area in the heart of the northeastern seaboard afflicted with photochemical regional smog during the summertime, was chosen as the first city in the program. A pilot study of ambient concentrations was conducted in July 1991. An annular denuder system (ADS) sampler was operated for two weeks near downtown Philadelphia, with a second unit operated in central, suburban New Jersey, the same location of measurements in past years. The Philadelphia site was found to have higher concentrations of most major aerosol species, ammonia and acidic particles than in New Jersey, showing that aerosol neutralization within the urban center will not necessarily totally eliminate acidic particle exposures.