Acid aerosol events in Uniontown, Pennsylvania, were studied during the summer of 1990. Size-fractionated acid aerosols were collected using a microorifice cascade impactor. The collected samples were analyzed for hydrogen, sulfate, nitrate, and ammonium ion concentrations. These measurements show evidence of a relationship between the mass median geometric aerodynamic diameter of sulfate particles and both particle acid content (H+/SO=4) and ambient relative humidity. Sulfate particle size increases with both increasing acid content and relative humidity. Considering the uncertainties involved, the growth of atmospheric sulfate particles observed in this study was found to be in good agreement with that predicted on the basis of laboratory-derived data. To our knowledge, this is the first time that such a relationship has been observed under real atmospheric conditions. Therefore the results of this study suggest that laboratory experiments and thermodynamic calculations can be used in order to understand the chemistry of atmospheric sulfates and their impacts on visibility reduction. In addition, we present evidence to suggest that the chemical composition of acid aerosol particles may be related to the ambient relative humidity. ¿American Geophysical Union 1993