Airshed Domains for Modeling Atmospheric Deposition of Oxidized and Reduced Nitrogen to the Neuse/Pamlico System of North Carolina

1 ABSTRACT Atmospheric deposition is important to nutrient loadings to coastal estuaries. Atmospheric emissions of nitrogen travel hundreds of kilometers as they are removed via atmospheric deposition. Long-range transport from outside the Neuse/Pamlico system in North Carolina is an important contributor to the total (wet + dry) deposition of nitrogen to the watershed, estuary and Sound. We need to delimit the extent of long-range transport that significantly contributes to deposition and thus loadings. Since airsheds do not have natural boundaries, in contrast to watersheds, an approach to define principal airsheds has been developed using simulations of the Extended Regional Acid Deposition Model. Principal airsheds for the deposition of oxidized nitrogen (nitrates/nitric acid), and reduced nitrogen (ammonium/ammonia) are defined and characterized. The principal airshed for oxidized nitrogen is 665,600 km 2 and for reduced nitrogen it is 406,400 km 2 , 25 and 15 times larger than the drainage area, respectively. Nitrogen oxide emissions from within the oxidized nitrogen principal airshed are estimated to explain 63% of the oxidized nitrogen deposition to the Neuse/Pamlico system. Ammonia emissions from within the reduced nitrogen principal airshed are estimated to explain 60% of the reduced nitrogen deposition to the system for 1996 emissions. North Carolina emissions contribute a large share of the reduced nitrogen deposition, but a small share of the oxidized nitrogen deposition, estimated to be 45% and 20%, respectively. Thus, a large regional atmospheric perspective is necessary for multi-media modeling involving nutrient deposition to coastal estuaries.