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 (Ca<sup>2+</sup>, Mg<sup>2+</sup>, K<sup>+</sup>, Na<sup>+</sup>) exhibit their highest concentrations between March and June in both PM<sub>2.5</sub> and precipitation due mostly to dust. Crustal-derived species concentrations in precipitation exhibit positive relationships with SO<sub>4</sub><sup>2−</sup>, NO<sub>3</sub><sup>−</sup>, and Cl<sup>−</sup>, suggesting that acidic gases likely react with and partition to either crustal particles or hydrometeors enriched with crustal constituents. Concentrations of particulate SO<sub>4</sub><sup>2−</sup> show a statistically significant correlation with rain SO<sub>4</sub><sup>2−</sup> unlike snow SO<sub>4</sub><sup>2−</sup>, which may be related to some combination of the vertical distribution of SO<sub>4</sub><sup>2−</sup> (and precursors) and the varying degree to which SO<sub>4</sub><sup>2−</sup>-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 NO<sub>3</sub><sup>−</sup> : SO<sub>4</sub><sup>2−</sup> ratios exhibit the following features with potential explanations discussed: (i) they are higher in precipitation as compared to PM<sub>2.5</sub>; (ii) they exhibit the opposite annual cycle compared to particulate NO<sub>3</sub><sup>−</sup> : SO<sub>4</sub><sup>2−</sup> 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 NO<sub>3</sub><sup>−</sup> : SO<sub>4</sub><sup>2−</sup> ratio in rain increased at the majority of sites due mostly to air pollution regulations of SO<sub>4</sub><sup>2−</sup> precursors.