Effects of Changes in Sulfate, Ammonia, and Nitric Acid on Particle Composition in the Southeastern United States

摘要

A thermodynamic equilibrium model, "Simulating Composition of Atmospheric Particles at Equilibrium" (SCAPE2), was used to investigate the response of fine particulate nitrate to changes in concentrations of nitric acid, ammonia, and sulfate in the southeastern United States. The data consisted of daily, 24-hour time resolution measurements from the Aerosol Research Inhalation Epidemiology Study (ARIES) Jefferson Street (Atlanta) site and five sites of the Southeastern Aerosol Research and Characterization Project (SEARCH). Reductions of total ammonia (gas-phase ammonia plus particulate ammonium, 20 percent decrease), total nitrate (nitric acid plus particulate nitrate, 20 percent decrease), sulfate (20 and 40 percent decreases), or combined total nitrate (nitric acid plus particulate nitrate, 20 percent decrease) with sulfate (40 percent decrease) were used to estimate the effects of changing emission levels. The conversion of SO2 to sulfate and NO2 to nitric acid involves additional nonlinear reactions not incorporated into the model. For all sites, fine particulate nitrate concentrations decreased in response to reductions of either ammonia or total nitrate, but the particulate nitrate decreases were greater in response to the ammonia reductions than in response to the total nitrate reductions. Particulate nitrate concentrations increased in response to either 20 or 40 percent reductions of sulfate. For the combined reduction (20 percent total nitrate with 40 percent sulfate), the resulting particulate nitrate concentrations were on average no different than the base-case nitrate. Measurements of fine particulate nitrate and HNO3 support the modeling conclusions and indicate that particulate nitrate formation is limited by the availability of ammonia at most times at all SEARCH sites.