Photochemistry of monochlorodimethyl sulfide (CH3SCH2Cl)

Abstract:

Dimethyl sulfide (CH₃SCH₃, DMS), which is mainly emitted from the oceans, is a major source of sulfur in the atmosphere. The atmospheric oxidation of DMS leads to the formation of sulfur dioxide (SO₂), methane sulfonic acid (CH₃SO₃H, MSA), and sulfate (SO₄^2-) aerosol. In the gas phase, DMS is primarily oxidized by hydroxyl (OH) and reactive halogens (e.g. Cl or BrO) by either H-abstraction or an addition pathway. The reaction of molecular chlorine (Cl₂) with DMS has been proposed as another loss process for DMS. In laboratory experiments, this reaction was shown to lead to the formation of monochlorodimethyl sulfide (CH₃SCH₂Cl, MCDMS) as a reaction product. In addition to reaction with the OH radical and Cl atom, UV photodissociation has been proposed as an important loss process for MCDMS. In this work, the UV absorption spectrum of MCDMS was measured between 200 and 350 nm at room temperature using a CCD spectrometer coupled to single-pass 20 and 50 cm long absorption cells. Absolute absorption cross sections were determined using absolute pressure measurements of dilute mixtures of MCDMS in N₂. The cross sections in the actinic region obtained in this study are significantly less than previously reported values. The large discrepancy in absorption cross sections is attributed to sample impurities in the previous measurements. The impact of the spectrum measured in this work on atmospheric photolysis rates is significant and will be discussed.