Comparisons of urban and rural PM_10−2.5 and PM_2.5 mass concentrations and semi-volatile fractions in northeastern Colorado

Abstract. Coarse (PM_10−2.5) and fine (PM_2.5) particulate matter in the atmosphere adversely affect human health and influence climate. While PM_2.5 is relatively well studied, less is known about the sources and fate of PM_10−2.5. The Colorado Coarse Rural-Urban Sources and Health (CCRUSH) study measured PM_10−2.5 and PM_2.5 mass concentrations, as well as the fraction of semi-volatile material (SVM) in each size regime (SVM_2.5, SVM_10−2.5), from 2009 to early 2012 in Denver and comparatively rural Greeley, Colorado. Agricultural operations east of Greeley appear to have contributed to the peak PM_10−2.5 concentrations there, but concentrations were generally lower in Greeley than in Denver. Traffic-influenced sites in Denver had PM_10−2.5 concentrations that averaged from 14.6 to 19.7 µg m⁻³ and mean PM_10−2.5 ∕ PM₁₀ ratios of 0.56 to 0.70, higher than at residential sites in Denver or Greeley. PM_10−2.5 concentrations were more temporally variable than PM_2.5 concentrations. Concentrations of the two pollutants were not correlated. Spatial correlations of daily averaged PM_10−2.5 concentrations ranged from 0.59 to 0.62 for pairs of sites in Denver and from 0.47 to 0.70 between Denver and Greeley. Compared to PM_10−2.5, concentrations of PM_2.5 were more correlated across sites within Denver and less correlated between Denver and Greeley. PM_10−2.5 concentrations were highest during the summer and early fall, while PM_2.5 and SVM_2.5 concentrations peaked in winter during periodic multi-day inversions. SVM_10−2.5 concentrations were low at all sites. Diurnal peaks in PM_10−2.5 and PM_2.5 concentrations corresponded to morning and afternoon peaks of traffic activity, and were enhanced by boundary layer dynamics. SVM_2.5 concentrations peaked around noon on both weekdays and weekends. PM_10−2.5 concentrations at sites located near highways generally increased with wind speeds above about 3 m s⁻¹. Little wind speed dependence was observed for the residential sites in Denver and Greeley. The mass concentration data reported here are being used in ongoing epidemiologic studies for PM in northeastern Colorado.