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Aqueous Secondary Organic Aerosol Formation in Ambient Cloud Water Photo-Oxidations

Category: Aerosol and Atmospheric Chemistry

Article In Press
DOI: 10.4209/aaqr.2017.01.0029
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Misha I. Schurman 1,2, Alexandra Boris1, Yury Desyaterik1, Jeffrey L. Collett Jr.1

  • 1 Department of Atmospheric Science, Colorado State University, Fort Collins, CO 80521, USA
  • 2 Colorado Mountain College, Breckenridge, CO 80424, USA

Highlights

Ambient cloudwater was photooxidized, showing two clear reaction regimes.
Regime 1 gained overall oxidation with volatile products causing organic mass loss.
Regime 2 lost both mass and oxidation, indicating functional group fragmentation.
Mass loss began at a consistent oxidation level of O:C > 0.61 ± 0.05 for all samples.
Cloud aqSOA formation rate may be a parameterizable function of O:C.


Abstract

The current understanding of aqueous secondary organic aerosol (aqSOA) formation is based largely on laboratory investigations of very simple surrogate cloud water solutions that aid mechanistic understanding of aqueous oxidation but may not accurately reflect the influence of the complex ambient matrix present in authentic cloud waters on organic chemistry. In this study, unaltered ambient cloud water and ‘biogenically influenced’ ambient cloud water (with added pinonic acid) were photo-oxidized, atomized, and dried to simulate the formation of aqSOA in clouds, then analyzed using an Aerodyne Aerosol Mass Spectrometer. Two major chemical regimes were identified: in the first, particle organic mass is gained, then lost; sustained increases in highly oxidized fragments indicate overall organic acid formation, while increases in nominally volatile fragments suggest that evaporation may contribute to the observed mass decrease. In the second regime, the oxidation level of cloud water organic matter decreases as mass decreases, suggesting that oxidized functional groups are fragmented and lost to evaporation. Overall, the rate of aqSOA production in unaltered cloud water decreases as oxygenation increases, until organic mass loss beginning at consistent values of f44 > 0.23 ± 0.05 and O:C > 0.61 ± 0.05. We hypothesize that there may be a parameterizable ‘maximum oxidation level’ for cloud water above which functional group fragmentation is dominant. These experiments are among the first to quantify organic mass production in ambient cloud water and employ the most atmospherically relevant oxidant concentrations to date.

Keywords

Secondary organic aerosol Cloud chemistry Photooxidation Aqueous


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