Effects of Changes in Sulfate, Ammonia, and Nitric Acid on Participate Nitrate Concentrations 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 NO_3~- to changes in concentrations of HNO_3, NH_3, and SO_4~(2-) in the southeastern United States. The data consisted of daily, 24-hr time resolution measurements from the Aerosol Research Inhalation Epidemiology Study (ARIES) Jefferson Street (Atlanta) site and five other sites of the Southeastern Aerosol Research and Characterization Project (SEARCH). Reductions of total NH_3 (gas-phase NH_3 plus particulate NH_4~+), total NO_3~-(HNO_3 plus particulate NO_3~-), SO_4~(2-), or combined total NO_3~- (HNO_3 plus particulate NO_3~-) with SO_4~(2-) were used to estimate the effects of changing emission levels. The conversion of SO_2 to SO_4~2- and NO_2 to HNO_3 involves additional nonlinear reactions not incorporated into the model. For all sites, fine particulate NO_3~-concentrations decreased in response to reductions of either NH_3 or total NO_3~-, but the particulate NO_3~- decreases were greater for the NH_3 reductions than for the total NO_3~- reductions. Particulate NO_3~- concentrations increased in response to reductions of SO_4~(2-) . For the combined reduction (total NO_3~- plus SO_4~(2-)), the resulting particulate NO_3~- concentrations were on average no different than the base-case NO_3~- levels. Measurements of fine particulate NO_3~- and HNO_3 support the modeling conclusions and indicate that particulate NO_3~- formation is limited by the availability of NH_3 at most times at all SEARCH sites.