Seasonal and annual modeling of reduced nitrogen compounds over the eastern United States: Emissions, ambient levels, and deposition amounts

Abstract

Detailed description of the distributions and seasonal trends of atmospheric nitrogen compounds is of considerable interest given their role in formation of acidic substances, tropospheric ozone and particulate matter and nutrient loading effects resulting from their deposition to sensitive ecosystems. While the oxidized nitrogen species have received considerable research and regulatory attention over the past several decades, little effort has been devoted toward quantifying the atmospheric budgets of reduced nitrogen compounds (NHx) associated with emissions of ammonia. The Regional Acid Deposition Model is enhanced to include detailed treatments of the physical and chemical processes regulating the fate of ammonia emissions and to model the interaction and chemical and thermodynamic coupling between atmospheric NOx‐SOx‐NHx species. To account for uncertainties in magnitude and seasonal variation of ammonia emissions, regional seasonal correction factors for ammonia emissions in the eastern United States are developed through successive model applications and comparison with measurements from regional networks of ambient concentrations and deposition amounts of various species. The resulting ammonia emissions show a distinct seasonal cycle with a maximum in summer followed by spring, fall, and winter. Our calculations suggest that the range between NH3 emissions during the cool and the warm season is a factor of 3–4. Correlations between model predicted ambient levels, gas/particle partitioning, and deposition amounts with measurements show good agreement on both an annual and seasonal basis with R2 in the 0.4–0.7 range for most species examined. Both model calculations and measurements indicate that during winter, large portions of the eastern U.S. are characterized by aerosols that are fully neutralized. Our model calculations for emission scenarios representative of the late 1980s–early 1990s period also indicate that reduced nitrogen species contribute 47(±8)% of the total nitrogen wet deposition in the eastern U.S.; this is in good agreement with 43(±9)% inferred from deposition measurements. These comparisons suggest that the model can capture the spatial and seasonal variability in distributions of various model species, the chemical interactions between reduced and oxidized nitrogen compounds in the troposphere, and the compositional characteristics of inorganic aerosol mass in the region.