Abstract
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.
Highlights
Organosulfur compounds have been found to contribute a significant mass fraction of atmospheric organic compounds
Based on the aerosol speciation data measured at different extents of oxidants such as hydroxyl (OH) oxidation, oxidation kinetics will be determined in Sect. 3.2 and reaction mechanisms will be proposed in Sect. 3.3 to explain the formation of major ions detected in the aerosol mass spectra
This work investigates the heterogeneous OH oxidation of sodium methyl sulfate, the smallest organosulfate found in atmospheric particles
Summary
Organosulfur compounds have been found to contribute a significant mass fraction of atmospheric organic compounds. Using a similar approach, Tolocka and Turpin (2012) estimated that organosulfur compounds contribute up to 5–10 % of the total organic mass in southeastern United States, while Shakya and Peltier (2013) reported that organosulfur compounds account for about 1–2 % of organic carbon in Fairbanks, Alaska Given their high atmospheric abundances, it is crucial to understand the compo-. Organosulfates are primarily present in the particle phase owing to their low volatility (Huang et al, 2015; Estillore et al, 2016) They can continuously react with gas-phase oxidants such as hydroxyl (OH) radicals, ozone (O3), and nitrate (NO3) radicals at or near the particle surface throughout their atmospheric lifetime. The effects of salt (e.g., ammonium and potassium salt) on the heterogeneous oxidative kinetics and chemistry are of atmospheric significance and warrant future study
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
