Reduction and evaluation of a near-explicit dimethyl sulfide and methanethiol oxidation mechanism

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Abstract. Marine sulfur species play a key role in the Earth system, but simulating their complex oxidation mechanisms is a significant challenge for Earth system models. We have taken a comprehensive gas-phase mechanism of the OH and NO3-initiated oxidation of dimethyl sulfide and methanethiol (156 reactions and 55 sulfur species) and generated a series of reduced complexity mechanisms using an automated rates-based approach for future incorporation into global models. Four reduced mechanisms were produced, ranging from 38–91 reactions. While the error in the reduced mechanisms typically increased with fewer reactions, the smallest mechanism, which included 38 reactions and 18 species, had an average error of less than 15 % in the concentrations of key sulfur species compared to the full mechanism. This reduced mechanism was compared to a published mechanism of similar size (38 reactions) used in global modelling, which showed average concentrations of methanesulfonic acid and OCS differed by factors of 15 and 3.6, respectively, across all marine box models. While the most reduced mechanism constitutes a large increase in the number of reactions and species typically used in Earth system models for dimethyl sulfide and methanethiol, it provides a traceable link to the most recent experimental work on the subject. To assess its performance in a global modelling framework, an adapted reduced mechanism from this work was implemented into UKCA/UKESM, where it outperformed the highly simplified StratTrop dimethyl sulfide oxidation mechanism when evaluated against ATom-3 and ATom-4 measurements.

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