Semiempirical Model Estimates of Ammonia Dry Deposition to North Carolina Coastal Watersheds

John T. Walker U.S. EPA, National Risk Management Research Laboratory
Rob Austin North Carolina State University, Department of Soil Science Raleigh, NC
Sue Kimbrough U.S. EPA, National Risk Management Research Laboratory
Wayne Robarge North Carolina State University, Department of Soil Science Raleigh, NC

In this poster we present a new method for predicting ambient NH3 concentrations and bi-directional air-surface exchange at field to watershed scales in areas where animal production intensity is high. For development and initial testing, which includes the results presented, the model domain includes the Neuse and Cape Fear River basins in eastern North Carolina. This region of the state contains some of the highest county-scale NH3 emissions in the U.S. The model incorporates a facility-scale NH3 emissions inventory from which ground-level ambient NH3 concentrations are predicted as a nonlinear function of distance from the nearest source. Ammonia air-surface exchange rates are determined using a two-layer canopy compensation point model that takes into account differences in soil, vegetation, and water emission potential, as well as surface physical characteristics, for primary land use categories. Model output includes NH3 emissions from agricultural point sources, atmospheric concentrations, and net air-surface exchange fluxes at 100 m resolution. The model framework is described and estimates of dry + wet deposition of NH3 in the
Cape Fear and Neuse River basins are presented.