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Discussion papers | Copyright
https://doi.org/10.5194/acp-2018-462
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 12 Jun 2018

Research article | 12 Jun 2018

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This discussion paper is a preprint. It is a manuscript under review for the journal Atmospheric Chemistry and Physics (ACP).

Influence of Relative Humidity on the Heterogeneous Oxidation of Secondary Organic Aerosol

Ziyue Li1, Katherine A. Smith2, and Christopher D. Cappa2 Ziyue Li et al.
  • 1Atmospheric Sciences Graduate Group, University of California, Davis, USA
  • 2Dept. of Civil and Environmental Engineering, University of California, Davis, USA

Abstract. Secondary organic aerosol (SOA) is a complex mixture of hundreds of semi volatile to extremely low volatility organic compounds that are chemically processed in the atmosphere, including via heterogeneous oxidation by gas phase radicals. Relative humidity (RH) has a substantial impact on particle phase, which can affect how SOA evolves in the atmosphere. In this study, SOA from dark α-pinene ozonolysis is heterogeneously aged by OH radicals in a flowtube at low and high RH. At high RH (RH=89%) there is substantial loss of particle volume (~60%) at an equivalent atmospheric OH exposure of 3 weeks. In contrast, at low RH (RH=25%) there is little mass loss (<20%) at the same OH exposure. Mass spectra of the SOA particles were measured as a function of OH exposure using a vacuum ultraviolet aerosol mass spectrometer (VUV-AMS). The mass spectra observed at low RH overall exhibit minor changes with oxidation and negligible further changes above an OH exposure=2×1012moleculecm−3s, suggesting limited impact of oxidation on the particle composition. In contrast, the mass spectra observed at high RH exhibit substantial and continuous changes as a function of OH exposure. Further, at high RH clusters of peaks in the mass spectra exhibit unique decay patterns, suggesting different responses of various species to oxidation. A model of heterogeneous oxidation has been developed to understand the origin of the difference in aging between the low and high RH experiments. Differences in diffusivity of the SOA between the low and high RH experiments alone can explain the difference in compositional change but cannot explain the difference in mass loss. Instead, the difference in mass loss is attributable to RH dependent differences in the OH uptake coefficient and/or the net probability of fragmentation, with either or both larger at high RH compared to low RH. These results illustrate the important impact of relative humidity on the fate of SOA in the atmosphere.

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Influence of Relative Humidity on the Heterogeneous Oxidation of Secondary Organic Aerosol Z. Li, K. Smith, and C. D. Cappa https://doi.org/10.25338/B8ZK5X

Ziyue Li et al.
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Short summary
We investigated the influence of relative humidity on the heterogeneous oxidation of secondary organic aerosol (SOA) particles by OH radicals. We observe significantly faster volume loss and compositional change of SOA at high RH, show that viscosity differences determine compositional changes, but that variability in either the uptake coefficient or the fragmentation probability are required to explain the difference in volume loss between low and high RH.
We investigated the influence of relative humidity on the heterogeneous oxidation of secondary...
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