Laboratory investigation of dimethyl sulfide oxidation: Comprehensive product characterization, peroxy radical isomerization, and implications for sulfur distribution

Abstract:

The oxidation of dimethyl sulfide (DMS) from the oceans and the subsequent formation of sulfate aerosol represent a major source of uncertainty in understanding the climate impact of natural aerosols. However, many details of the oxidation mechanism remain poorly constrained, largely due to limitations in measuring the full suite of oxidation products. In this work, we perform laboratory chamber studies aimed at comprehensively characterizing the chemical evolution of a broad range of gas- and particle-phase products from DMS oxidation. Measurements are made using a suite of advanced mass spectrometric techniques that include a proton-transfer reaction mass spectrometer, an iodide chemical ionization mass spectrometer, and an aerosol mass spectrometer. Specifically, hydroperoxymethyl thioformate (HOOCH₂SCHO), the recently discovered compound formed from the isomerization of methylthiomethylperoxy radical (CH₃SCH2OO•), is identified and quantified together with other sulfur containing species such as dimethyl sulfoxide, methanesulfonic acid, methanesulfinic acid and sulfate. Comparing laboratory measurements to results from a 0-D model, the isomerization rate constant of the methylthiomethylperoxy radical is estimated. In addition, the evolution of key properties that reflect fragmentation and functionalization of the chemical system, such as carbon number and sulfur oxidation state, are systemically tracked as the oxidation proceeds. To our knowledge, our work represents one of the most complete experimental studies of DMS oxidation products made to date and thus has important implications for constraining marine sulfur budgets and sulfate aerosol formation.