PM2.5 and PM10-2.5 Compositions during Wintertime Episodes of Elevated PM Concentrations across the Midwestern USA

Northern cities in the Midwest experience frequent PM concentrations in excess of the Federal PM2.5 and PM10 National Ambient Air Quality Standards (NAAQS) during winter, which increasingly outnumber the summertime excesses the further north the cities are located. In contrast, cities located south of the Great Lakes tended to experience a greater number of excesses during summer. In order to ascertain the key components of PM contributing to these winter exceedances, we examined a regional elevated PM concentration episode which occurred between January 31, 2005 and February 6, 2005. The episode was caused by meteorological stagnation which affected 9 states in the Midwestern US (MN, WI, MI, IA, IL, IN, OH, MO, and KY). The temporal and spatial characteristics of PM2.5 and PM10-2.5 mass and bulk chemistry (nitrate, sulfate, ammonium, organic matter) were measured during this and other wintertime elevated PM episodes at Milwaukee, WI, and compared to Chemical Speciation Network (CSN) data. The observed elevated PM episodes demonstrated that nitrate was the driving anthropogenic component of the wintertime exceedances. This led to the conclusion that winter NOx controls on mobile and stationary sources, in combination with sufficient SO2, VOC and NH3 controls, should be further examined for the mitigation of wintertime PM2.5 episodes in the Midwest. The north-south gradient in exceedances of the PM2.5 NAAQS indicated that strategies for avoidance of violations of the 24-hour standard, and to a lesser degree the annual standard, would benefit from an emphasis on different seasons, and therefore different PM components, depending how far north or south the city is located in the Midwest.