Articles | Volume 18, issue 4
Atmos. Chem. Phys., 18, 2809–2820, 2018
https://doi.org/10.5194/acp-18-2809-2018
Atmos. Chem. Phys., 18, 2809–2820, 2018
https://doi.org/10.5194/acp-18-2809-2018

Research article 27 Feb 2018

Research article | 27 Feb 2018

Importance of sulfate radical anion formation and chemistry in heterogeneous OH oxidation of sodium methyl sulfate, the smallest organosulfate

Kai Chung Kwong1, Man Mei Chim1, James F. Davies2,a, Kevin R. Wilson2, and Man Nin Chan1,3 Kai Chung Kwong et al.
  • 1Earth System Science Programme, Faculty of Science, The Chinese University of Hong Kong, Hong Kong, China
  • 2Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA, USA
  • 3The Institute of Environment, Energy and Sustainability, The Chinese University of Hong Kong, Hong Kong, China
  • anow at: Department of Chemistry, University of California Riverside, Riverside, CA, USA

Abstract. Organosulfates are important organosulfur compounds present in atmospheric particles. While the abundance, composition, and formation mechanisms of organosulfates have been extensively investigated, it remains unclear how they transform and evolve throughout their atmospheric lifetime. To acquire a fundamental understanding of how organosulfates chemically transform in the atmosphere, this work investigates the heterogeneous OH radical-initiated oxidation of sodium methyl sulfate (CH3SO4Na) droplets, the smallest organosulfate detected in atmospheric particles, using an aerosol flow tube reactor at a high relative humidity (RH) of 85 %. Aerosol mass spectra measured by a soft atmospheric pressure ionization source (direct analysis in real time, DART) coupled with a high-resolution mass spectrometer showed that neither functionalization nor fragmentation products are detected. Instead, the ion signal intensity of the bisulfate ion (HSO4) has been found to increase significantly after OH oxidation. We postulate that sodium methyl sulfate tends to fragment into a formaldehyde (CH2O) and a sulfate radical anion (SO4 ⋅ −) upon OH oxidation. The formaldehyde is likely partitioned back to the gas phase due to its high volatility. The sulfate radical anion, similar to OH radical, can abstract a hydrogen atom from neighboring sodium methyl sulfate to form the bisulfate ion, contributing to the secondary chemistry. Kinetic measurements show that the heterogeneous OH reaction rate constant, k, is (3.79 ± 0.19)  ×  10−13 cm3 molecule−1 s−1 with an effective OH uptake coefficient, γeff, of 0.17 ± 0.03. While about 40 % of sodium methyl sulfate is being oxidized at the maximum OH exposure (1.27  ×  1012 molecule cm−3 s), only a 3 % decrease in particle diameter is observed. This can be attributed to a small fraction of particle mass lost via the formation and volatilization of formaldehyde. Overall, we firstly demonstrate that the heterogeneous OH oxidation of an organosulfate can lead to the formation of sulfate radical anion and produce inorganic sulfate. Fragmentation processes and sulfate radical anion chemistry play a key role in determining the compositional evolution of sodium methyl sulfate during heterogeneous OH oxidation.

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Short summary
To date, it remains unclear how organosulfates evolve over time in the atmosphere. We demonstrate that heterogeneous OH oxidation of sodium methyl sulfate, the smallest organosulfate found in atmospheric aerosols, is efficient. The oxidation can lead to the formation of sulfate radical anion and produce inorganic sulfate. In addition to OH radicals, sulfate radical anion chemistry can play a role in determining the evolution of sodium methyl sulfate and other organosulfates during oxidation.
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