A Regional Modeling Analysis of Reduced Nitrogen Cycling in the Eastern United States

A Regional Modeling Analysis of Reduced Nitrogen Cycling in the Eastern United States    

Rohit Mathur Environmental Program–North Carolina Supercomputing Center P.O. Box 12889 Research Triangle Park, NC 27709-2889

and

Robin L. Dennis Atmospheric Sciences Modeling Division Air Resources Laboratory National Oceanic and Atmospheric Administration Research Triangle Park, NC 27711    

Abstract

While much attention has been devoted to studying the role of oxides of nitrogen in the atmosphere and towards reducing their emissions, there has been little focus on the cycling of reduced nitrogen compounds (denoted NHx ) in the atmosphere and its impacts on atmospheric aerosols and acidic substances. The primary confounding factors limiting such investigations has been the limited understanding of the sources, sinks, and chemical coupling of NHx compounds in the atmosphere. The anthropogenic emissions of NH3 are still rather poorly quantified. Because NH3 emissions tend to be on the local to regional scale and the lifetime of NH3 is on the order of hours, the modeling framework adopted to study its cycling must have sufficient resolution. A model for the atmospheric behavior of NHx must also account for its interactions with aerosols and be able to describe both the atmospheric transport of NH3 near a source and the transport of NH4+ over long distances. In order to synthesize the current knowledge of the processes governing the fate of NHx in a consistent modeling framework, the Regional Acid Deposition Model (RADM) was enhanced by adding several additional modules to represent the various atmospheric physical and chemical pathways governing the fate of emitted NH3. The resulting version of the model is referred to as the Extended-RADM. The model has the ability to dynamically represent the various competing process that interact to influence the cycling of reduced and oxidized forms of nitrogen and their interactions. Model applications over the eastern United States will be discussed. Preliminary predictions of the gas/aerosol partitioning of reduced and oxidized nitrogen will be given. Model performance evaluations based on comparisons of model predictions (of both ambient levels as well as wet deposition amounts) with measurements and previous model simulations will be presented. The degree of neutralization of acidic aerosols over the eastern U.S. will be discussed based on both model and observed estimates. Preliminary estimates of the range of influence of oxidized and reduced nitrogen, for selected emission regions will be presented.