Model Study of the Photooxidation of CH₃SO₂SCH₃ at Atmospheric Pressure: Thermal Decomposition of the CH₃SO₂ Radical

Abstract

The knowledge of the rate of CH₃SO₂ decomposition is critical for understanding the distribution of the end products of the tropospheric oxidation of organic sulphur-containing compounds, such as DMS, CH₃SSCH₃ (DMDS) or CH₃SH. A global mechanism of their oxidation was developed a decade ago (Yin et al., 1990). The core of this mechanism is a series of oxidation steps leading to a branching between formation of methane sulfonic acid, CH₃SO₃H (MSA), and sulphate, SO₄ ⁻², originating from SO₂. Since SO₂ and MSA are produced mainly from CH₃SO₂, the competition between CH₃SO₂ decomposition and further oxidation to CH₃SO₃ will determine their yield distribution. Rapid unimolecular dissociation of CH₃SO₂ to CH₃ and SO₂ favours production of sulphuric acid, which leads to formation of new cloud condensation nuclei. Atmospheric MSA is absorbed by pre-existing aerosols and droplets and washed out with precipitation. Therefore, the decomposition-to-oxidation branching of CH₃SO₂ is an important factor in determining the composition of the marine atmospheric aerosol.