Product distribution, kinetics, and aerosol formation from the OH oxidation of dimethyl sulfide under different RO2 regimes

Abstract. 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 (τbi) of the peroxy radical
CH3SCH2OO 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 (τbi<0.1 s) and low-NO (τbi>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 SO2 (which in the atmosphere would eventually
oxidize to sulfate or be lost to deposition). Under low-NO conditions,
hydroperoxymethyl thioformate (HPMTF, HOOCH2SCHO), formed from
CH3SCH2OO isomerization, dominates the sulfur budget over the
course of the experiment, suppressing or delaying the formation of SO2
and particulate matter. The isomerization rate constant of
CH3SCH2OO at 295 K is found to be 0.13±0.03 s−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 (kOH+HPMTF=2.1×10-11 cm3 molec.−1 s−1, kOH+MTF=1.35×10-11 cm3 molec.−1 s−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.
