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

摘要

Nucleation in coal-fired power-plant plumes can greatly contribute toparticle number concentrations near source regions. The changing emissionsrates of SO and NO due to pollution-control technologies overrecent decades may have had a significant effect on aerosol formation andgrowth in the plumes with ultimate implications for climate and human health.We use the System for Atmospheric Modeling (SAM) large-eddy simulation modelwith the TwO-Moment Aerosol Sectional (TOMAS) microphysics algorithm to modelthe nucleation in plumes of coal-fired plants. We test a range of cases withvarying emissions to simulate the implementation of emissions-controltechnologies between 1997 and 2010. We start by simulating the W. A. Parishpower plant (near Houston, TX) during this time period, when NOemissions were reduced by ~90% and SO emissions decreased by~30%. Increases in plume OH (due to the reduced NO)produced enhanced SO oxidation and an order-of-magnitude increase inparticle nucleation in the plume despite the reduction in SO emissions.These results suggest that NO emissions could strongly regulateparticle nucleation and growth in power-plant plumes. Next, we test a rangeof cases with varying emissions to simulate the implementation of SOand NO emissions-control technologies. Particle formationgenerally increases with SO emission, while NO shows twodifferent regimes: increasing particle formation with increasing NO under low-NO emissions and decreasing particle formation withincreasing NO under high-NO emissions. Next, wecompare model results with airborne measurements made in the W. A. Parishpower-plant plume in 2000 and 2006, confirming the importance of NO emissions on new particle formation and highlighting the substantialeffect of background aerosol loadings on this process (the more pollutedbackground of the 2006 case caused more than an order-of-magnitude reductionin particle formation in the plume compared to the cleaner test day in 2000).Finally, we calculate particle-formation statistics of 330 coal-fired powerplants in the US in 1997 and 2010, and the model results show a mediandecrease of 19% in particle formation rates from 1997 to 2010 (whereasthe 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 plantspecifically, and it shows that the strong NO controls (90%reduction) implemented at the W. A. Parish plant (with relatively weakSO emissions reductions, 30%) are not representative of most powerplants in the US during the past 15 yr. These results suggest that there maybe important climate implications of power-plant controls due to changes inplume chemistry and microphysics, but the magnitude and sign of the aerosolchanges depend greatly on the relative reductions in NO andSO emissions in each plant. More extensive plume measurements for arange of emissions of SO and NO and in varying backgroundaerosol conditions are needed, however, to better quantify these effects.