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

by C. R. Lonsdale, R. G. Stevens, C. A. Brock, P. A. Makar, E. M. Knipping, J. R. Pierce
Atmospheric Chemistry and Physics ()
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Nucleation in coal-fired power-plant plumes can greatly contribute to particle number concentrations near source regions. The changing emissions rates of SO 2 and NO x due to pollution-control technologies over recent decades may have had a significant effect on aerosol for-mation and growth in the plumes with ultimate implications for climate and human health. We use the System for At-mospheric Modeling (SAM) large-eddy simulation model with the TwO-Moment Aerosol Sectional (TOMAS) micro-physics algorithm to model the nucleation in plumes of coal-fired plants. We test a range of cases with varying emis-sions to simulate the implementation of emissions-control technologies between 1997 and 2010. We start by simulat-ing the W. A. Parish power plant (near Houston, TX) dur-ing this time period, when NO x emissions were reduced by ∼90 % and SO 2 emissions decreased by ∼30 %. Increases in plume OH (due to the reduced NO x) produced enhanced SO 2 oxidation and an order-of-magnitude increase in par-ticle nucleation in the plume despite the reduction in SO 2 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 SO 2 and NO x emissions-control technologies. Particle formation generally increases with SO 2 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 parti-cle formation and highlighting the substantial effect of back-ground 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 com-pared 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 me-dian decrease of 19 % in particle formation rates from 1997 to 2010 (whereas the W. A. Parish case study showed an in-crease). Thus, the US power plants, on average, show a dif-ferent result than was found for the W. A. Parish plant specif-ically, and it shows that the strong NO x controls (90 % reduc-tion) implemented at the W. A. Parish plant (with relatively weak SO 2 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 implica-tions of power-plant controls due to changes in plume chem-istry and microphysics, but the magnitude and sign of the aerosol changes depend greatly on the relative reductions in NO x and SO 2 emissions in each plant. More extensive plume measurements for a range of emissions of SO 2 and NO x and in varying background aerosol conditions are needed, how-ever, to better quantify these effects.

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