Receptor model source attributions for Utah's Salt Lake City airshed and the impacts of wintertime secondary ammonium nitrate and ammonium chloride aerosol

摘要

Communities along Utah's Wasatch Front are currently developing strategies to reduce daily average PM_(2.5) levels to below National Ambient Air Quality Standards during wintertime persistent stable atmospheric conditions, or cold-air pools. Speciated PM_(2.5) data from the Wasatch Front airshed indicate that wintertime exceedances of the PM_(2.5) standard are mainly driven by high levels of ammonium nitrate. Stable wintertime conditions foster the formation of ammonium nitrate aerosol when sufficient sources of NO_x, ammonia, and oxidative capacity exist. However, this work demonstrates that secondary ammonium chloride aerosol can also be a significant source of secondary wintertime PM_(2.5) if sufficient sources of atmospheric chlorine exist. Two factor analysis techniques, positive matrix factorization (PMF) and Unmix, were used to identify contributors to PM_(2.5) at three monitoring stations along Utah's Wasatch Front: Bountiful, Lindon, andSalt Lake City. The monitoring data included chemically speciated PM_(2.5) data for 227, 227, and 429 days at each location, respectively, during the period from May 2007 through May 2011. PMF identified 10-12 factors and Unmix identified 4—5 factors for each of the locations. The wintertime PMF and Unmix results showed large contributions from secondary PM_(2.5) when PM_(2.5) concentrations exceeded 20 μg/m~3 . PMF identified both ammonium nitrate and ammonium chloride aerosol as significant secondary contributors to PM_(2.5) (10-15% of total PM_(2.5) from ammonium chloride) during wintertime pollution episodes. Subsequent ion balance analysis of the monitoring data confirmed the presence of significant ammonium chloride aerosol on these highly polluted days at all three monitoring sites. The directly emitted primary PM_(2.5) portions of the source attribution results were further compared to county-level emissions inventories and showed generally good agreement for Salt Lake City and Lindon during wintertime except for wood smoke and fugitive dust, which have higher contributions in the receptor modeling results than in the emissions inventories. Implications: The study suggests that secondary ammonium chloride aerosol can be a significant source of wintertime PM2.5 in an ammonia-rich environment, like the Wasatch Front airshed, if sufficient sources of atmospheric chlorine exist. During wintertime, cold-air-pool events, the source attribution results generally agree with the county emission inventories with the exception of wood smoke and cooking sources. At the Salt Lake City monitoring station, the estimated contributions from wood smoke and cooking are nearly double those of the corresponding inventory, suggesting that they are nearly as important as gasoline emissions. Supplemental Materials: Supplemental materials are available for this paper. Go to the publisher's online edition of the Journal of the Air & Waste Management Association.