Sources and composition of PM2.5 in the Colorado Front Range during the DISCOVER‐AQ study

Measurements of PM2.5 chemical composition were carried out in Golden, CO during the Deriving Information on Surface Conditions from Column and Vertically Resolved Observations Relevant to Air Quality (DISCOVER-AQ) field study. Chemical composition was dominated by organic compounds, which comprised an average of 75% of the PM2.5 mass throughout the study. Most of the organic matter was secondary (i.e., secondary organic aerosol, SOA), and appears to derive predominantly from regional sources, rather than the Denver metropolitan area. The concentration and composition of PM2.5 in Golden were strongly influenced by highly regular wind patterns and the site's close proximity to the mountains (~5 km). This second factor may be the cause of distinct differences between observations in Golden and those in downtown Denver, despite a distance between the sites of only ~15 km. Concentrations of aerosol nitrate, ammonium and elemental carbon increased significantly during the daytime when the winds were from the northeast, indicating a strong local source for these compounds. Local sources of dust appeared to minimally impact the Golden site, although this was not likely representative of other conditions in the Colorado Front Range. Conversely, dust that had undergone long-range transport from the southwestern U.S. likely impacted the entire Colorado Front Range, including Golden. During this event, water-soluble Ca2+ concentrations exceeded 1 µg m-3, and the PM2.5/PM10 ratio reached its lowest level throughout the study. The long-range transport of wildfire emissions also impacted the Colorado Front Range for 1-2 days during DISCOVER-AQ. The smoke event was characterized by high concentrations of organics and water-soluble K+. The results show a complex array of sources and atmospheric processes influence summertime PM in the Colorado Front Range.