Fate of ammonia emissions at the local to regional scale as simulated by the Community Multiscale Air Quality model

Abstract

Atmospheric deposition of nitrogen contributes to eutrophication of estuarine waters and acidification of lakes and streams. Ammonia also contributes to fine particle formation in the atmosphere and associated health effects. Model projections suggest that NH3 deposition may become the major source of nitrogen deposition in the future. The regional transport of NH3 contributes to nitrogen deposition. Conventional wisdom for many is that a large fraction, or even all, of the NH3 emissions deposit locally, near their source as dry deposition, which we believe is incorrect. In this study we use a regional atmospheric model, the Community Multiscale Air Quality (CMAQ) model to identify the dominant processes that dictate the fate of NH3 and address the questions of how much NH3 deposits locally and what is the range of influence of NH3 emissions. The CMAQ simulation is for June 2002 with a 12–km grid size, covering the eastern half of the U.S. We study three different NH3 dry deposition formulations, including one that represents bi–directional NH3 air–surface exchange, to represent uncertainty in the NH3 dry deposition estimates. We find for 12–km cells with high NH3 emissions from confined animal operations that the local budget is dominated by turbulent transport away from the surface and that from 8–15% of a cell’s NH3 emissions dry deposit locally back within the same cell. The CMAQ estimates are consistent with local, semi–empirical budget studies of NH3 emissions. The range of influence of a single cell’s emissions varies from 180 to 380 kilometers, depending on the dry deposition formulation. At the regional scale, wet deposition is the major loss pathway for NH3; nonetheless, about a quarter of the NH3 emissions are estimated to transport off the North American continent, an estimate that is not sensitive to the uncertainty in dry deposition.