Workshop on Agricultural Air Quality The Impact of Winter NH3 Emission Reductions on Inorganic Particulate Matter under Present and Future Regulated Conditions

Atmospheric ammonia contributes to the formation of aerosol nitrate, an important constituent of inorganic fine particulate matter, and atmospheric ammonia is a significant source of acidification and excess nutrient loading to sensitive ecosystems. Recent regulation by the US Environmental Protection Agency requires large-scale emission reductions of NOx and SO2. These regulated emission changes will decrease the concentration of nitrate and sulfate and hence alter the composition of inorganic particulate mater. The complexity of the inorganic aerosol system makes prediction of the importance of ammonia after these emission reductions non-trivial (Ansari and Pandis, 1998). Sulfate has a low vapor pressure and condenses to the aerosol phase. Any available ammonia gas will neutralize the sulfate to form ammonium sulfate aerosol. If additional ammonia is available in excess of what is needed to neutralize the sulfate, it can form aerosol ammonium nitrate in accordance with thermodynamic equilibrium. The inorganic aerosol system has three states, listed in order of increasing ammonia concentration: (1) acidic, where there is insufficient ammonia to neutralize the sulfate, (2) ammonia limited, where all of the sulfate is neutralized, but the formation of ammonium nitrate is limited by scarce ammonia, and (3) nitrate limited, where ammonia is present in excess such that the formation of ammonium nitrate is limited b