Aerosol and Precipitation Chemistry in the Southwestern United States : Spatiotemporal Trends

16 17 This study characterizes the spatial and temporal patterns of aerosol and precipitation 18 composition at six sites across the United States Southwest between 1995 and 2010. Precipitation 19 accumulation occurs mostly during the wintertime (December February) and during the 20 monsoon season (July September). Rain and snow pH levels are usually between 5 – 6, with 21 crustal-derived species playing a major role in acid neutralization. These species (Ca 2+ , Mg 2+ , 22 K + , Na + ) exhibit their highest concentrations between March and June in both PM2.5 and 23 precipitation due mostly to dust. Crustal-derived species concentrations in precipitation exhibit 24 positive relationships with SO4 2, NO3 , and Cl , suggesting that acidic gases likely react with and 25 partition to either crustal particles or hydrometeors enriched with crustal constituents. 26 Concentrations of particulate SO4 2show a statistically significant correlation with rain SO4 227 unlike snow SO4 2, which may be related to some combination of the vertical distribution of 28 SO4 2(and precursors) and the varying degree to which SO4 2-enriched particles act as cloud 29 condensation nuclei versus ice nuclei in the region. The coarse:fine aerosol mass ratio was 30 correlated with crustal species concentrations in snow unlike rain, suggestive of a preferential 31 role of coarse particles (mainly dust) as ice nuclei in the region. Precipitation NO3 :SO4 2ratios 32 exhibit the following features with potential explanations discussed: (i) they are higher in 33 precipitation as compared to PM2.5; (ii) they exhibit the opposite annual cycle compared to 34 particulate NO3 :SO4 2ratios; and (iii) they are higher in snow relative to rain during the 35 wintertime. Long-term trend analysis for the monsoon season shows that the NO3 :SO4 2ratio in 36 rain increased at the majority of sites due mostly to air pollution regulations of SO4 2precursors. 37 38