Comparison of aerosol acidity in urban and semi-rural environments

During the summer of 1990 acid aerosol measurements were conducted simultaneously in Pittsburgh, Uniontown, and State College, Pennsylvania. Acid aerosols and ammonia concentrations were compared at these cities. Aerosol acidity was found to be lower in the urban area (Pittsburgh) than the semi-rural areas. Ammonia levels were higher at the urban site than in the semi-rural environments, probably due to the higher population density at the urban site. During acid episodes, the dominating sulfur species in Pittsburgh were found to be a mixture of totally and partially neutralized sulfates, i.e. (NH4)2SO4 and NH4HSO4, while in State College and Uniontown sulfur species were primarily a mixture of NH4HSO4 and H2SO4. Diurnal variation was found for H+ and SO42−, while NH3 exhibited no diurnal pattern. High spatial correlations were found for SO42− levels between Pittsburgh and the semi-rural areas, implying a regional transport. In spite of the significant absolute differences, H+ concentrations also were found to be significantly correlated across sites, as well as with each other at individual sites.