This study characterizes the spatial and temporal patterns of aerosol andprecipitation composition at six sites across the United States Southwestbetween 1995 and 2010. Precipitation accumulation occurs mostly during thewintertime (December–February) and during the monsoon season (July–September). Rain and snow pH levels are usually between 5–6, withcrustal-derived species playing a major role in acid neutralization. Thesespecies (Ca, Mg, K, Na) exhibit their highestconcentrations between March and June in both PM and precipitationdue mostly to dust. Crustal-derived species concentrations in precipitationexhibit positive relationships with SO, NO, andCl, suggesting that acidic gases likely react with and partition toeither crustal particles or hydrometeors enriched with crustal constituents.Concentrations of particulate SO show a statisticallysignificant correlation with rain SO unlike snowSO, which may be related to some combination of the verticaldistribution of SO (and precursors) and the varying degree towhich SO-enriched particles act as cloud condensation nucleiversus ice nuclei in the region. The coarse : fine aerosol mass ratio wascorrelated with crustal species concentrations in snow unlike rain,suggestive of a preferential role of coarse particles (mainly dust) as icenuclei in the region. Precipitation NO : SO ratiosexhibit the following features with potential explanations discussed: (i)they are higher in precipitation as compared to PM; (ii) theyexhibit the opposite annual cycle compared to particulateNO : SO ratios; and (iii) they are higher in snowrelative to rain during the wintertime. Long-term trend analysis for themonsoon season shows that the NO : SO ratio in rainincreased at the majority of sites due mostly to air pollution regulationsof SO precursors.
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