The effect of coal-fired power-plant SO₂ and NO_x control technologies on aerosol nucleation in the source plumes

Abstract. Nucleation in coal-fired power-plant plumes can greatly contribute to particle number concentrations near source regions. The changing emissions rates of SO₂ and NO_x due to pollution-control technologies over recent decades may have had a significant effect on aerosol formation and growth in the plumes with ultimate implications for climate and human health. We use the System for Atmospheric Modeling (SAM) large-eddy simulation model with the TwO-Moment Aerosol Sectional (TOMAS) microphysics algorithm to model the nucleation in plumes of coal-fired plants. We test a range of cases with varying emissions to simulate the implementation of emissions-control technologies between 1997 and 2010. We start by simulating the W. A. Parish power plant (near Houston, TX) during this time period, when NO_x emissions were reduced by ~90% and SO₂ emissions decreased by ~30%. Increases in plume OH (due to the reduced NO_x) produced enhanced SO₂ oxidation and an order-of-magnitude increase in particle nucleation in the plume despite the reduction in SO₂ emissions. These results suggest that NO_x emissions could strongly regulate particle nucleation and growth in power-plant plumes. Next, we test a range of cases with varying emissions to simulate the implementation of SO₂ and NO_x emissions-control technologies. Particle formation generally increases with SO₂ emission, while NO_x shows two different regimes: increasing particle formation with increasing NO_x under low-NO_x emissions and decreasing particle formation with increasing NO_x under high-NO_x emissions. Next, we compare model results with airborne measurements made in the W. A. Parish power-plant plume in 2000 and 2006, confirming the importance of NO_x emissions on new particle formation and highlighting the substantial effect of background aerosol loadings on this process (the more polluted background of the 2006 case caused more than an order-of-magnitude reduction in particle formation in the plume compared to the cleaner test day in 2000). Finally, we calculate particle-formation statistics of 330 coal-fired power plants in the US in 1997 and 2010, and the model results show a median decrease of 19% in particle formation rates from 1997 to 2010 (whereas the W. A. Parish case study showed an increase). Thus, the US power plants, on average, show a different result than was found for the W. A. Parish plant specifically, and it shows that the strong NO_x controls (90% reduction) implemented at the W. A. Parish plant (with relatively weak SO₂ emissions reductions, 30%) are not representative of most power plants in the US during the past 15 yr. These results suggest that there may be important climate implications of power-plant controls due to changes in plume chemistry and microphysics, but the magnitude and sign of the aerosol changes depend greatly on the relative reductions in NO_x and SO₂ emissions in each plant. More extensive plume measurements for a range of emissions of SO₂ and NO_x and in varying background aerosol conditions are needed, however, to better quantify these effects.