Aerosol and precipitation chemistry in the southwestern United States: spatiotemporal trends and interrelationships

Abstract. This study characterizes the spatial and temporal patterns of aerosol and precipitation composition at six sites across the United States Southwest between 1995 and 2010. Precipitation accumulation occurs mostly during the wintertime (December–February) and during the monsoon season (July–September). Rain and snow pH levels are usually between 5–6, with crustal-derived species playing a major role in acid neutralization. These species (Ca²⁺, Mg²⁺, K⁺, Na⁺) exhibit their highest concentrations between March and June in both PM_2.5 and precipitation due mostly to dust. Crustal-derived species concentrations in precipitation exhibit positive relationships with SO₄²⁻, NO₃⁻, and Cl⁻, suggesting that acidic gases likely react with and partition to either crustal particles or hydrometeors enriched with crustal constituents. Concentrations of particulate SO₄²⁻ show a statistically significant correlation with rain SO₄²⁻ unlike snow SO₄²⁻, which may be related to some combination of the vertical distribution of SO₄²⁻ (and precursors) and the varying degree to which SO₄²⁻-enriched particles act as cloud condensation nuclei versus ice nuclei in the region. The coarse : fine aerosol mass ratio was correlated with crustal species concentrations in snow unlike rain, suggestive of a preferential role of coarse particles (mainly dust) as ice nuclei in the region. Precipitation NO₃⁻ : SO₄²⁻ ratios exhibit the following features with potential explanations discussed: (i) they are higher in precipitation as compared to PM_2.5; (ii) they exhibit the opposite annual cycle compared to particulate NO₃⁻ : SO₄²⁻ ratios; and (iii) they are higher in snow relative to rain during the wintertime. Long-term trend analysis for the monsoon season shows that the NO₃⁻ : SO₄²⁻ ratio in rain increased at the majority of sites due mostly to air pollution regulations of SO₄²⁻ precursors.