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 PM2.5 and precipitation due mostly to dust. Crustal-derived species concentrations in precipitation exhibit positive relationships with [Formula: see text], [Formula: see text], and Cl-, suggesting that acidic gases likely react with and partition to either crustal particles or hydrometeors enriched with crustal constituents. Concentrations of particulate [Formula: see text] show a statistically significant correlation with rain [Formula: see text] unlike snow [Formula: see text], which may be related to some combination of the vertical distribution of [Formula: see text] (and precursors) and the varying degree to which [Formula: see text]-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 [Formula: see text] : [Formula: see text] ratios exhibit the following features with potential explanations discussed: (i) they are higher in precipitation as compared to PM2.5; (ii) they exhibit the opposite annual cycle compared to particulate [Formula: see text] : [Formula: see text] 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 [Formula: see text] : [Formula: see text] ratio in rain increased at the majority of sites due mostly to air pollution regulations of [Formula: see text] precursors.
Highlights
A2n0d10w; aSteeragaevraialGnabdeilVoiteyscc(cWhiie,on2o0dt1hif0oic;usHeaertpoall.d, 2010; et al., 2012), which both affecInt santdruamreeanffteactteidonby the region’s in acid neutralization
Sofouwthinwteersts’nsocwlimveartseusarseumreDmduaecrtianragiSnthyteosrttheeleamtainvsneucaolnwtraibteurtiboanlsderived species concentrations in precipitation exhibit posi- ance (Cayan et al, 2010) and shortening the duration of snow tive relationships with SO24−, NO−3, and Cl−, suggesting that acidic gases likely react with and partition to either crustal particles or hydrometeors enriched with crustal constituents
The analysis aims to address the following questions: (i) What is the annual profile of rain/snow water accumulation, precipitation pH, and composition of precipitation and aerosol particles?; (ii) What species are best correlated with each other in rain and snow?; (iii) What species are most influential towards rain and snow water pH?; (iv) How well-correlated are common species measured in aerosol and precipitation samples?; (v) What is the nature of the nitrate:sulfate ratio in precipitation and aerosol particles? and (vi) How have aerosol and precipitation species concentrations changed between 1995 and 2010?
Summary
A2n0d10w; aSteeragaevraialGnabdeilVoiteyscc(cWhiie,on2o0dt1hif0oic;usHeaertpoall.d, 2010; et al., 2012), which both affecInt santdruamreeanffteactteidonby the region’s in acid neutralization. The amount of fine and sapnadtiaclodairssteriabeurtoiosnolopfEaprotaitcerlntethsiamlSraayyinsaoltsreosnmaolwtervtihaethaemiroruonlet as cloud condensation nuclei (CSCNc)ieanndciceesnuclei (IN), respectively (e.g Rosenfeld and Givati, 2006) In both desert and montane ecosystems, the deposition of nitrate and sulfate have been shown to be acidifying agents for aquatic ecosystems resources (e.g. FeOnnceteaal.,n20S03c),iewnhicleeexcess nitrogen cipitation as compared to PM2.5; (ii) they exhibit the opposite annual cycle compared to particulate NO−3 : SO24− ratios; and (iii) they are higher in snow relative to rain during the wintertime.
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