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 SO₄^2−, 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 SO₄^2− show a statistically significant correlation with rain SO₄^2− unlike snow SO₄^2−, which may be related to some combination of the vertical distribution of SO₄^2− (and precursors) and the varying degree to which SO₄^2−-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^− : SO₄^2− 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^− : SO₄^2− 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^− : SO₄^2− ratio in rain decreased at the majority of sites due mostly to air pollution regulations of SO₄^2− precursors

Aerosol Chemical Profile of Near-Source Biomass Burning Smoke in Sonla, Vietnam during 7-SEAS Campaigns in 2012 and 2013

“…The slight variation in NO 3 -suggested the lack of gas-to-particle transformation at the Sonla site because of near BB sources, as reported by Lee et al (2011). Another plausible explanation for the lack of NO 3 -enhancement is the nitric acid phase partitioning to coarse particles such as crustal dust and sea salt (Sorooshian et al, 2013;Prabhakar et al, 2014), which exceeded the upper size limit detected in the present study.…”

Laboratory Studies of the Cloud Droplet Activation Properties and Corresponding Chemistry of Saline Playa Dust

“…7−9 Further, the dust from the Lordsburg and Willcox playas have been detected in precipitation collected in the southwestern United States highlighting the potential significance of playa dusts as contributors to precipitation. 73,74 Notably, more than half of the samples measured in this study were defined as "highly hygroscopic" or "slightly hygroscopic" demonstrating that playas can emit fine dust particles that do not require chemical aging in order to efficiently seed cloud droplets. This finding indicates that the commonly held assumption that all dust is nonhygroscopic should be re-examined.…”

Daily evolution in dust and black carbon content, snow grain size, and snow albedo during snowmelt, Rocky Mountains, Colorado

“…7). This is likely due to the increased atmospheric dust loading in this region, which peaks in April-May-June (Painter and others, 2012b;Sorooshian and others, 2013), coinciding with peak dust emission in the Colorado Plateau source regions.…”