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SEASONAL AND ANNUAL MODELING OF REDUCED NITROGEN COMPOUNDS OVER THE EASTERN UNITED STATES: EMISSIONS, AMBIENT LEVELS, AND DEPOSITION AMOUNTS
Citation:
MATHUR, R. AND R L. Dennis. SEASONAL AND ANNUAL MODELING OF REDUCED NITROGEN COMPOUNDS OVER THE EASTERN UNITED STATES: EMISSIONS, AMBIENT LEVELS, AND DEPOSITION AMOUNTS. JOURNAL OF GEOPHYSICAL RESEARCH 108(D15):ACH 22-1-ACH 22-19, (2003).
Impact/Purpose:
This task has the following objectives:
Improve modelers' ability to focus on scientific and policy issues in modeling studies by providing software that supports composing, applying, and evaluating complex systems of models.
Improve the understanding of the interaction of the atmosphere and the underlying surface, especially the flux of mass in both directions, and EPA's ability to simulate that interaction.
Contribute to multimedia studies and assessments by applying state-of-the-art atmospheric models, estimating atmospheric contributions to multimedia issues and the sources of those contributions, and evaluating the models' strengths and weaknesses.
Description:
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 towards quantifying the atmospheric budgets of reduced nitrogen compounds (NH2) 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 coupling of atmospheric NOx -SOx -NHx species. To account for uncertainties in magnitude and seasonal variation of 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. Correlations between model predicated 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 1980 - early 1990 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 coupling between reduced and oxidized nitrogen compounds in the troposphere, and the compositional characteristics of inorganic aerosol mass in the region.
This document has been subjected to the U.S. EPA review and approved for publication. Mention of trade names or commercial products does not constitute endorsement or recommendation for use.