Secondary pollutants such as sulfate and nitrate aerosols are major components of visibility degradation in Class I areas in the United States. Because sulfate preferentially scavenges ammonia over nitrate in the formation of aerosols, the amount of nitrate produced may be limited by the lack of sufficient atmospheric ammonia. In the CALPUFF model, the default method of treating ammonia is to assume a spatially uniform constant background concentration that is available for forming ammonium sulfate and then the remainder for ammonium nitrate on a puff-by-puff basis. A more refined method has been recently developed in CALPUFF to better estimate the non-linear impacts of ammonia-limiting effects on the formation of nitrate aerosol. This method has the advantage of allowing sulfate and nitrate from background sources to be accounted for in a manner consistent with observational measurements of these pollutants. The background concentrations of sulfate, nitrate, and nitric acid measured at the CASTNET monitoring network are used to estimate time-varying total available ammonia concentrations. Additional ammonia contributed by new anthropogenic sources, such as the ammonia slip from NOx control devices, is explicitly modeled by CALPUFF. The total (background + new source) sulfate and nitrate compete for the available ammonia, and if sufficient ammonia is not available, the formation of nitrate aerosols may be inhibited.

The impact of ammonia limiting effects on regulatory Class I visibility analyses for new sources of SOx and NOx is discussed. Also the sensitivity of the technique to different assumptions regarding potential nitrate volatilization from the Teflon CASTNET filters is quantified. Comparisons of CASTNET and IMPROVE monitoring data (which is less susceptible to volatilization of nitrate) are made at sites in the Eastern United States. Conclusions regarding the need to evaluate ammonia-limiting effects in Class I impact assessments are provided.