Methanesulfonic acid and sulfuric acid aerosol formed through oxidation of reduced sulfur compounds in a humid environment

Authors: VAN ROOY, PAUL - University Of California; DROVER, RYAN - Claremont Colleges; CRESS, TANNER - Claremont Colleges; MICHAEL, CARA - Claremont Colleges; PURVIS-ROBERTS, KATIE - Claremont Colleges; Silva, Philip - Phil; NEE, MATTHEW - Western Kentucky University; COCKER, DAVID - University Of California, Riverside

Submitted to: Atmospheric Environment
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/19/2021
Publication Date: 6/19/2021
Citation: Van Rooy, P., Drover, R., Cress, T., Michael, C., Purvis-Roberts, K., Silva, P.J., Nee, M., Cocker, D. 2021. Methanesulfonic acid and sulfuric acid aerosol formed through oxidation of reduced sulfur compounds in a humid environment. Atmospheric Environment. 261. Article 118504. https://doi.org/10.1016/j.atmosenv.2021.118504.
DOI: https://doi.org/10.1016/j.atmosenv.2021.118504

Interpretive Summary: Organic sulfur compounds are common emissions from waste management in agriculture. In the atmosphere they are important reactants but the products formed from their reaction and the pathways are not well understood. A series of laboratory studies were conducted to understand the reaction pathways of two of these compounds, dimethylsulfide and dimethyldisulfide. In previous work these compounds were studied under dry conditions to understand their basic oxidation chemistry while this study added in realistic conditions of humidity to ascertain the importance of water. In an environmental chamber, the two sulfur compounds were exposed to atmospheric oxidation conditions with reactants at concentrations typical of the atmosphere. Two main sulfur reaction products in the particulate phase were identified as sulfuric acid, which is a common particulate component and methanesulfonic acid, which is typically thought of as a component observed only over the oceans. Higher levels of water and the presence of nitrogen oxides increased the production of methanesulfonic acid relative to sulfuric acid. Nitrate radical oxidant when combined with humidity produced nearly complete conversion of the sulfur compounds to methanesulfonic acid. These observations suggest that methanesulfonic acid may be underestimated as a particulate component over the continent.

Technical Abstract: Particulate sulfuric acid and methanesulfonic acid (MSA) are known to form through oxidation of reduced sulfur compounds, however, the mechanisms by which these compounds form are not well understood. Additionally, the aerosol yields and ratio of MSA to sulfuric acid particulate formation are not well documented, making it difficult to estimate the health and climate impacts of reduced sulfur compounds. To investigate these unknowns, dimethylsulfide (DMS) and dimethyldisulfide (DMDS) were oxidized, using a variety of oxidants, in a 37.5 cubic meter Teflon environmental chamber with relative humidity ranging from 2% to 55%. The mass fraction of particulate MSA was estimated based on unique aerosol fragments, at m/z 79 and 96, measured by a High Resolution Time-of-Flight Mass Spectrometer. MSA to sulfuric acid particulate ratios varied depending initial conditions. This study revealed that substantial water vapor is necessary to form MSA. The mass fraction of MSA increases in the presence of nitrogen oxides. Nitrate radical oxidation of DMS and DMDS resulted in nearly 100% of the aerosol estimated to be MSA, suggesting nighttime chemistry may play an important role in ambient MSA formation. This study builds upon results from similar experiments, presented in Van Rooy et al. (2021), that were conducted under extreme dry conditions.