Characterization of CHOS compounds in rainwater from continental and coastal storms by ultrahigh resolution mass spectrometry
Introduction
Elucidating the composition of rainwater dissolved organic matter (DOM) has remained at the forefront of atmospheric research principally because of its central role in a host of fundamentally important atmospheric processes (Mullaugh et al., 2011, Willey et al., 2012). Rainwater DOM is a complex heterogeneous mixture of organic compounds, the composition of which remains to a large extent unknown (Altieri et al., 2012, Mitra et al., 2013). This is particularly true when describing organic entities that contain one or more sulfur atoms (Zhao et al., 2013). Organo-sulfur containing compounds play a pivotal role in the formation of secondary organic aerosols (SOA) (Karambelas et al., 2014, Surratt et al., 2007) leading to high molecular weight oligomers. These compounds are of particular interest in the chemistry of the troposphere because they can enhance hydroscopicity of droplets acting as cloud condensation nuclei (Hung et al., 2014).
Unraveling the composition of organic sulfur species in atmospheric waters is hindered primarily by the difficulty in describing molecular formulas of a suite of analytes present in very low abundances in a complex matrix. Recent analyses of atmospheric waters by Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) demonstrated that individual molecular formulas of organic moieties can be assigned with great accuracy. Altieri et al. (2009) used FT-ICR MS to investigate the provenance and elemental composition of rainwater organic sulfur species in rain from an anthropogenically impacted location in New Jersey, USA. The authors suggested that the organo-sulfates likely contributed to the large percentage of uncharacterized water-soluble organic carbon species in atmospheric organic matter and that several organo-sulfates could be used as tracers for SOA formed under highly acidic conditions. Sulfur containing molecular formulas were also found in cloud water dissolved organic matter were CHOS compounds with reduced forms of S were observed in addition to organo sulfates (Zhao et al., 2013).
Altieri et al. (2009) and Zhao et al. (2013) were important studies because they demonstrated the utility of FT-ICR MS in describing organo-sulfur species in atmospheric waters. One of the most important uncertainties regarding organo-sulfur speciation is how the molecular distribution of the various species is influenced by storm origin as determined by air mass back trajectory. The significance of storm origin on organic matter composition was underscored in a recent study by Mead et al. (2013) utilizing FT-ICR MS to analyze H,C,O compounds in rainwater collected from both continental and coastal derived storms. Approximately 25% of the roughly 2000 assigned CHO molecular formulas were unique to each storm classification indicating the importance of air mass back trajectory on the composition of rainwater DOM. Analysis of the unique molecular formula assignments highlighted distinct groupings of various bio- and geo-molecular classes with coastal storms containing unique formulas representative of carbohydrate-like compounds while continental storms had lipid-like formulas. Mead et al. (2013) also found that continental storms contained condensed aromatic structures derived from fossil fuel combustion sources similar to black carbon which were not present in coastal rain events. The present study represents the first detailed analysis of organ-sulfur containing compounds in rainwater by FT-ICR MS collected on an event basis from a series of continental and coastal storms at one geographical location. The data generated therefore provide the most comprehensive examination to date of the chemical formulas of the organo-sulfur containing components of rainwater DOM from various air-mass back trajectories.
Section snippets
Sample collection
Precipitation event samples were collected on the University of North Carolina Wilmington campus (34°13.9′N, 77°52.7′W) located approximately 8.5 km from the Atlantic Ocean. Automatic Sensing Wet/Dry Precipitation Collectors (four Aerochem-Metrics (ACM) Model 301) were used to collect samples. The precipitation collectors housed 4 L glass beaker placed within a HDPE plastic bucket. All glassware used for rain collection, filtration apparatus and storage containers were baked at 450 °C in a
Rainwater bulk properties
A total of seven rain samples collected from both continental (4) and coastal (3) rain events were analyzed. A description of the chemical characteristics of each rain event and corresponding back trajectories can be found elsewhere (Mead et al., 2013). There was a higher percentage of unique molecular formula assignments in continental derived storms compared to coastal storms with almost double the molecular formula assignments of CHOS compounds (Table 1). The average unique molar ratios of
Implications and conclusions
This study represents the first comprehensive examination of CHOS containing molecular formulas from different air mass back trajectories. Continental rain contained a number of organo-sulfate CHOS compounds based upon the observed O:S ratio. Of particular interest was the occurrence of highly condensed aromatic structures containing only one S and one O in continental derived rainwater. To the best of our knowledge, this is the first study to definitively demonstrate the occurrence of these
Acknowledgments
The authors thank the National Science Foundation for financial support under grant AGS-1003078. Work performed at the National High Magnetic Field Laboratory was supported by NSF Division of Materials Research through DMR-11-57490, BP/The Gulf of Mexico Research Initiative to the Deep-C Consortium, the Future Fuels Institute, and the State of Florida.
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