Water soluble organic aerosols in the Colorado Rocky Mountains, USA: composition, sources and optical properties.

Mingjie Xie,Mark W Williams,Jason C Neff,Michael P Hannigan,Natalie Mladenov,Joseph Wasswa

doi:10.1038/srep39339

Abstract

Atmospheric aerosols have been shown to be an important input of organic carbon and nutrients to alpine watersheds and influence biogeochemical processes in these remote settings. For many remote, high elevation watersheds, direct evidence of the sources of water soluble organic aerosols and their chemical and optical characteristics is lacking. Here, we show that the concentration of water soluble organic carbon (WSOC) in the total suspended particulate (TSP) load at a high elevation site in the Colorado Rocky Mountains was strongly correlated with UV absorbance at 254 nm (Abs254, r = 0.88 p < 0.01) and organic carbon (OC, r = 0.95 p < 0.01), accounting for >90% of OC on average. According to source apportionment analysis, biomass burning had the highest contribution (50.3%) to average WSOC concentration; SOA formation and motor vehicle emissions dominated the contribution to WSOC in the summer. The source apportionment and backward trajectory analysis results supported the notion that both wildfire and Colorado Front Range pollution sources contribute to the summertime OC peaks observed in wet deposition at high elevation sites in the Colorado Rocky Mountains. These findings have important implications for water quality in remote, high-elevation, mountain catchments considered to be our pristine reference sites.

Highlights

For remote alpine settings, where terrestrial carbon (C) and nutrient inputs from the catchment are less prominent than in forested catchments[1], for example, studies have shown that PM deposition of dust and organic material can be an important source of calcium[2,3], phosphorus[4], organic carbon[5] and bioaerosols[6]
Based on significant correlations between DOC concentrations and sulfate and nitrate in summer wet deposition[1], air mass trajectories that pass through the Front Range of Colorado in the summer[10], and summer upslope conditions that carry air masses above the atmospheric boundary layer, the authors hypothesized that the summertime organic aerosol load in high elevation site in the Colorado Rocky Mountains is substantially contributed by Colorado Front Range derived anthropogenic primary and secondary organic aerosol (SOA) emissions
The water soluble organic carbon (WSOC) concentrations exhibited maxima in the summer and minima in winter (Fig. 2a), which is consistent with seasonal trends for DOC in wet deposition observed by Mladenov et al.[1] at the same sampling site and at other National Atmospheric Deposition Program (NADP) sampling stations in the vicinity (Supplementary Fig. S1)

Summary

Introduction

For remote alpine settings, where terrestrial carbon (C) and nutrient inputs from the catchment are less prominent than in forested catchments[1], for example, studies have shown that PM deposition of dust and organic material can be an important source of calcium[2,3], phosphorus[4], organic carbon[5] and bioaerosols[6]. Based on compound-specific techniques such as gas chromatography coupled with mass spectrometry (GC-MS), several classes of chemical compounds have been identified (e.g., saccharides and dicarboxylic acids) and used as tracers for specific emission sources[22,23,24] Spectroscopic techniques such as ultraviolet (UV)-visible absorbance spectroscopy and excitation-emission matrix (EEM) fluorescence spectroscopy has been widely used to investigate the fluorescent properties of dissolved organic matter (DOM) in aquatic environments[5,6,25] but less so for WSOC in aerosols[26]. Based on significant correlations between DOC concentrations and sulfate and nitrate in summer wet deposition[1], air mass trajectories that pass through the Front Range of Colorado in the summer[10], and summer upslope conditions that carry air masses above the atmospheric boundary layer, the authors hypothesized that the summertime organic aerosol load in high elevation site in the Colorado Rocky Mountains is substantially contributed by Colorado Front Range derived anthropogenic primary and secondary organic aerosol (SOA) emissions

Methods

Results

Conclusion