Product distribution, kinetics, and aerosol formation from the OH oxidation of dimethyl sulfide under different RO ₂ regimes

The atmospheric oxidation of dimethyl sulfide (DMS) represents a major natural source of atmospheric sulfate aerosols. However, there remain large uncertainties in our understanding of the underlying chemistry that governs the product distribution and sulfate yield from DMS oxidation. Here, chamber experiments were conducted to simulate gas-phase OH-initiated oxidation of DMS under a range of reaction conditions. Most importantly, the bimolecular lifetime (inline-formulaτ_bi) of the peroxy radical CHinline-formula₃SCHinline-formula₂OO was varied over several orders of magnitude, enabling the examination of the role of peroxy radical isomerization reactions on product formation. An array of analytical instruments was used to measure nearly all sulfur-containing species in the reaction mixture, and results were compared with a near-explicit chemical mechanism. When relative humidity was low, “sulfur closure” was achieved under both high-NO (inline-formulaτ_bi<0.1 s) and low-NO (inline-formulaτ_bi>10 s) conditions, though product distributions were substantially different in the two cases. Under high-NO conditions, approximately half the product sulfur was in the particle phase, as methane sulfonic acid (MSA) and sulfate, with most of the remainder as SOinline-formula₂ (which in the atmosphere would eventually oxidize to sulfate or be lost to deposition). Under low-NO conditions, hydroperoxymethyl thioformate (HPMTF, HOOCHinline-formula₂SCHO), formed from CHinline-formula₃SCHinline-formula₂OO isomerization, dominates the sulfur budget over the course of the experiment, suppressing or delaying the formation of SOinline-formula₂ and particulate matter. The isomerization rate constant of CHinline-formula₃SCHinline-formula₂OO at 295 K is found to be inline-formula0.13±0.03 sinline-formula⁻¹, in broad agreement with other recent laboratory measurements. The rate constants for the OH oxidation of key first-generation oxidation products (HPMTF and methyl thioformate, MTF) were also determined (inline-formula $M16inlinescrollmathml{k}_{\mathrm{normal\; OH}+\mathrm{normal\; HPMTF}}=\mathrm{normal\; 2.1}\times {\mathrm{normal\; 10}}^{-\mathrm{normal\; 11}}$ 119pt16ptsvg-formulamathimg6cbeb0cc360810694ee519608e80e378 acp-22-16003-2022-ie00001.svg119pt16ptacp-22-16003-2022-ie00001.png  cminline-formula³ molec.inline-formula⁻¹ sinline-formula⁻¹, inline-formula $M20inlinescrollmathml{k}_{\mathrm{normal\; OH}+\mathrm{normal\; MTF}}=\mathrm{normal\; 1.35}\times {\mathrm{normal\; 10}}^{-\mathrm{normal\; 11}}$ 114pt16ptsvg-formulamathimgf8b91c755c1ad005524b87bb54c8db7c acp-22-16003-2022-ie00002.svg114pt16ptacp-22-16003-2022-ie00002.png  cminline-formula³ molec.inline-formula⁻¹ sinline-formula⁻¹). Product measurements agree reasonably well with mechanistic predictions in terms of total sulfur distribution and concentrations of most individual species, though the mechanism overpredicts sulfate and underpredicts MSA under high-NO conditions. Lastly, results from high-relative-humidity conditions suggest efficient heterogenous loss of at least some gas-phase products.