Valley Cold Pools, Snow Cover, and Speciated PM2.5 in Western US cities

Western US urban areas frequently experience prolonged periods with light winds and strong temperature inversions in winter months, especially when snow cover is present. This can lead to levels of fine particulate matter (PM2.5) in excess of the 24-hour national Ambient Air Quality Standard (NNAQS) of 35 µg/m3. Chen et al., 2012 found that in Reno, Nevada exceedances of the PM2.5 standard have occurred only during periods of snow cover and were due mostly to enhanced formation of ammonium nitrate particles due to colder temperatures and higher relative humidity on days with snow cover. They also found that PM2.5 concentration was strongly related to the strength of the temperature inversion. In this study the analysis is extended to additional western US valley locations that often experience wintertime valley cold air pools, including Salt Lake City and Logan, Utah, Missoula Montana, Boise, Idaho, and Spokane, Washington. The analyses include relationship between PM2.5 and inversion strength, segregated by days with and without snow cover. The fractional contribution of each major component of PM2.5 (organic carbon, elemental carbon, ammonium, nitrate, sulfate) is also compared for days with and without snow cover. It was found that the fractional contribution of nitrate to PM2.5 was enhanced for days with snow cover. Control of sources contributing to ammonium nitrate during these conditions would reduce the PM2.5 levels and health effects. Chen, L.-W. A., Watson, J. G., Chow, J. C., Green, M. C., Inouye, D., and Dick, K.: Wintertime particulate pollution episodes in an urban valley of the Western US: a case study, Atmos. Chem. Phys., 12, 10051-10064, doi:10.5194/acp-12-10051-2012, 2012.