Chemical states of sulfate at Shenandoah National Park during summer, 1991

Despite the importance of the chemical states of sulfate in understanding of particle growth, acidic impacts, and visibility, relatively few data are available during the intense summer sulfate haze periods characteristic of the eastern US. In order to establish the precise anion associated with sulfates, measurements were made of the chemical state of sulfates by size and time during September 1991, as part of a larger Shenandoah NP aerosol study. Inertial impactors were prepared with Teflon substrates to allow operation of existing IMPROVE analytical protocols on highly size resolved fine aerosol samples with a time resolution of 4 h, Sept. 5 through 19, 1991. The results show that the finest aerosols, below 0.34 mu m aerodynamic diameter, are predominantly ammonium sulfate during the entire period. Intermediate size particles in the 0.34 to 1.15 mu m range varied from almost pure ammonium sulfate to almost pure sulfuric acid. Larger particles, 1.15 to 2.5 mu m, could not be chemically resolved due to the presence of excess hydrogen from organic matter. Due to the high humidity and hygroscopic nature of these species, the aerosols contained considerable water, enhancing their impact upon visibility.