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Discussion papers
https://doi.org/10.5194/acp-2019-218
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/acp-2019-218
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 07 Mar 2019

Research article | 07 Mar 2019

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

Effects of Inorganic Salts on the Heterogeneous OH Oxidation of Organic Compounds: Insights from Methylglutaric Acid-Ammonium Sulfate

Hoi Ki Lam1, Sze Man Shum1, James F. Davies2, Mijung Song3, Andreas Zuend4, and Man Nin Chan1,5 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, Chonbuk National University, Jeollabuk-do, Republic of Korea
  • 4Department of Atmospheric and Oceanic Sciences, McGill University, Montreal, Québec, Canada
  • 5The Institute of Environment, Energy, and Sustainability, The Chinese University of Hong Kong, Hong Kong, China

Abstract. Atmospheric particles, consisting of inorganic salts, organic compounds and a varying amount of water, can continuously undergo heterogeneous oxidation initiated by gas-phase oxidants at the particle surface, changing the composition and properties of particles over time. To date, most studies focus on the chemical evolution of pure organic particles upon oxidation. To gain more fundamental insights into the effects of inorganic salts on the heterogeneous kinetics and chemistry of organic compounds, we investigate the heterogeneous OH oxidation of 3-methylglutaric acid (3-MGA) particles and particles containing both 3-MGA and ammonium sulfate (AS) in an organic-to-inorganic mass ratio of 2 in an aerosol flow tube reactor at a high relative humidity of 85.0 %. The molecular information of the particles before and after OH oxidation is obtained using the Direct Analysis in Real Time (DART), a soft atmospheric pressure ionization source, coupled to a high-resolution mass spectrometer. Optical microscopy measurements reveal that 3-MGA-AS particles are in a single liquid phase prior to oxidation at high relative humidity. Particle mass spectra show that C6 hydroxyl and C6 ketone functionalization products are the major products formed upon OH oxidation in the absence and presence of AS, suggesting that the dissolved salt does not significantly affect reaction pathways. The dominance of C6 hydroxyl products over C6 ketone products could be explained by the intermolecular hydrogen abstraction by tertiary alkoxy radicals formed at the methyl-substituted tertiary carbon site. On the other hand, kinetic measurements show that the effective OH uptake coefficient, γeff, for 3-MGA-AS particles (0.99 ± 0.05) is smaller than that for 3-MGA particles (2.41 ± 0.13) by about a factor of ~ 2.4. A smaller reactivity observed in 3-MGA-AS particles might be attributed to a higher surface concentration of water molecules, and the presence of ammonium and sulfate ions, which are chemically inert to OH radicals, at the particle surface. This could lower the collision probability between the 3-MGA and OH radicals, resulting in a smaller overall reaction rate. Our results suggest that inorganic salts likely alter the overall heterogeneous reactivity of organic compounds with gas-phase OH radicals rather than reaction mechanisms in well-mixed aqueous organic-inorganic particles.

Hoi Ki Lam et al.
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Short summary
We show the presence of dissolved inorganic salts could reduce the overall heterogeneous reactivity of organic compounds with gas-phase OH radicals at the surface by lowering the surface concentration of organic compounds. Until recently, the kinetic parameters reported in the literature were mostly measured based on experiments with pure organic particles. The lifetime of organic compounds or chemical tracers against heterogeneous OH reaction in the atmosphere could be longer than expected.
We show the presence of dissolved inorganic salts could reduce the overall heterogeneous...
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