A Case Study of the Relative Effects of Power Plant Nitrogen Oxides and Sulfur Dioxide Emission Reductions on Atmospheric Nitrogen Deposition

Abstract

The contrasting effects of point source nitrogen oxides (NO_x) and sulfur dioxide (SO₂) 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 SO₂ controls on nitrogen deposition using air quality modeling results obtained from the simultaneous application of NO_x and SO₂ emission controls is presented and discussed using the results from CMAQ simulations conducted with NO_x-only and SO₂-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 (HNO₃). In such instances, the individual effects of NO_x and SO₂ controls on nitrogen deposition can be approximated by the effects of combined NO_x + SO₂ 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.