The contrasting effects of point source nitrogen oxides (NO x ) and sulfur dioxide (SO 2 ) air emission reductions on regional atmospheric nitrogen deposition are analyzed for the case study of a coal-fired power plant in the southeastern United States. The effect of potential emission reductions at the plant on nitrogen deposition to Escambia Bay and its watershed on the Florida-Alabama border is simulated using the three-dimensional Eulerian Community Multiscale Air Quality (CMAQ) model. A method to quantify the relative and individual effects of NO x versus SO 2 controls on nitrogen deposition using air quality modeling results obtained from the simultaneous application of NO x and SO 2 emission controls is presented and discussed using the results from CMAQ simulations conducted with NO x -only and SO 2 -only emission reductions; the method applies only to cases in which ambient inorganic nitrate is present mostly in the gas phase; that is, in the form of gaseous nitric acid (HNO 3 ). In such instances, the individual effects of NO x and SO 2 controls on nitrogen deposition can be approximated by the effects of combined NO x ϩ SO 2 controls on the deposition of NO y (the sum of oxidized nitrogen species) and reduced nitrogen species (NH x ), respectively. The benefit of controls at the plant in terms of the decrease in nitrogen deposition to Escambia Bay and watershed is less than 6% of the overall benefit due to regional Clean Air Interstate Rule (CAIR) controls.
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