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https://doi.org/10.5194/acp-2020-900
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/acp-2020-900
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

  23 Sep 2020

23 Sep 2020

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This preprint is currently under review for the journal ACP.

Effects of Liquid–Liquid Phase Separation and Relative Humidity on the Heterogeneous OH Oxidation of Inorganic–Organic Aerosols: Insights from Methylglutaric Acid/Ammonium Sulfate Particles

Hoi Ki Lam1, Rongshuang Xu1, Jack Choczynski2, James F. Davies2, Dongwan Ham3, Mijung Song3, Andreas Zuend4, Wentao Li5, Ying-Lung Steve Tse5, and Man Nin Chan1,6 Hoi Ki Lam et al.
  • 1Earth System Science Programme, Faculty of Science, The Chinese University of Hong Kong, Hong Kong, China
  • 2Department of Chemistry, University of California Riverside, Riverside, CA, USA
  • 3Department of Earth and Environmental Sciences, Jeonbuk National University, Jeollabuk-do, Republic of Korea
  • 4Department of Atmospheric and Oceanic Sciences, McGill University, Montreal, Québec, Canada
  • 5Departemnt of Chemistry, The Chinese University of Hong Kong, Hong Kong, China
  • 6The Institute of Environment, Energy and Sustainability, The Chinese University of Hong Kong, Hong Kong, China

Abstract. Organic compounds residing near the surface of atmospheric aerosol particles are exposed to chemical reactions initiated by gas-phase oxidants, such as hydroxyl (OH) radicals. Aqueous droplets composed of inorganic salts and organic compounds can undergo phase separation into two liquid phases, depending on aerosol composition and relative humidity (RH). Such phase behavior can govern the surface characteristics and morphology of the aerosols, which in turn affect the heterogeneous reactivity of organic compounds toward gas-phase oxidants. In this work, we used an aerosol flow tube reactor coupled with an atmospheric pressure ionization source (Direct Analysis in Real Time) and a high-resolution mass spectrometer to investigate how phase separation in model aqueous droplets containing an inorganic salt (ammonium sulfate, AS) and an organic acid (3-methyglutaric acid, 3-MGA) with an organic-to-inorganic dry mass ratio (OIR) of 1 alters the heterogeneous OH reactivity. At high RH, 3-MGA/AS aerosols were aqueous droplets with a single liquid phase. When the RH decreased, aqueous 3-MGA/AS droplets underwent phase separation at ~75 % RH. Once the droplets were phase-separated, they exhibited either a core–shell, partially engulfed, or a transition from core–shell to partially engulfed structure, with an organic-rich outer phase and an inorganic-rich inner phase. The kinetics, quantified by an effective heterogenous OH rate constant, was found to increase gradually from 1.01 ± 0.02 × 10e−12 to 1.73 ± 0.02 × 10e−12 cm3 molecule−1 s−1 when the RH decreased from 88 % to 55 %. The heterogeneous reactivity of phase-separated droplets is slightly higher than that of aqueous droplets with a single liquid phase. This could be explained by the finding that when the RH decreases, higher concentrations of organic molecules (i.e. 3-MGA) are present at or near the droplet surface, which are more readily exposed to OH oxidation, as demonstrated by phase separation measurements and model simulations. This could increase the reactive collision probability between 3-MGA molecules and OH radicals dissolved near the droplet surface and secondary chain reactions. Even for phase-separated droplets with a fully established core–shell structure, the diffusion rate of organic molecules across the organic-rich outer shell is predicted to be fast in this system. Thus, the overall rate of reactions is likely governed by the surface concentration of 3-MGA rather than a diffusion limitation. Overall, understanding the aerosol phase state (single liquid phase versus two separate liquid phases) is essential to better probe the heterogenous reactivity under different aerosol chemical composition and environmental conditions (e.g. RH).

Hoi Ki Lam et al.

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Short summary
This work demonstrates that organic compounds present at or near the surface of aerosols can be subjected to oxidation initiated by gas-phase oxidants, such as hydroxyl radicals (OH). The reactivity is sensitive to their surface concentrations, which are determined by the phase separation behavior. This results of this work emphasize the effects of phase separation and potentially distinct aerosol morphologies on the chemical transformation of atmospheric aerosols.
This work demonstrates that organic compounds present at or near the surface of aerosols can be...
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