Factors contributing to elevated concentrations of PM sub(2.5) during wintertime near Boise, Idaho

Wintertime chemical composition of water-soluble particulate matter with aerodynamic diameter less than 2.5 mu m (PM sub(2.5)) was monitored in the Treasure Valley region near Boise, Idaho. Aerosol was sampled using a Particle Into Liquid Sampler (PILS) and subsequently analyzed using ion exchange chromatography and a total organic carbon analyzer. During the two-month sampling campaign, the region experienced varying meteorological regimes, with an extended atmospheric stagnation event towards the end of the study. The increases in particulate nitrate did not correlate with an equivalent molar increase of ammonium concentration, ruling out solid ammonium nitrate formation as the dominant source. Instead, our analysis using an online aerosol thermodynamic model suggests that the condensation of gas phase nitric acid was possible within the meteorological conditions experienced during the study. Currently this region is in attainment with the National Ambient Air Quality Standards (NAAQS) for PM sub(2.5); however, with the projected increase in population and economic growth, and the subsequent increase in NOX emissions, these episodic increases in particulate nitrate have the potential of pushing the area to non-attainment status.