Advancing our understanding of dimethyl sulfide oxidation products by field observations of formaldehyde and photolysis frequencies

Dimethyl sulfide (DMS) from marine biogenic emissions is the largest natural source of sulphur in the atmosphere and a major aerosol precursor (Carslaw et al., 2010). With anthropogenic sources of SO2 decreasing, knowledge of the fate of DMS is critical for understanding the formation of new sulphate aerosol particles, growth of existing aerosols to cloud condensation nuclei (CCN), and the burden/composition of aerosols in the remote marine environment.

The recent discovery of a new molecule, hydroperoxy methylthioformate (HOOCH2SCHO; HPMTF) which is formed during the oxidation of DMS has highlighted that there are uncertainties in our knowledge of the main pathways and products of DMS chemistry (Veres et al., 2020).

To address these key uncertainties, a large field and modelling project (Constraining the role of the marine sulfur cycle in the Earth System (CARES)) has been developed. As part of CARES, a comprehensive suite of measurements will be made on board a research ship and from the FAAM aircraft flying off the west coast of Ireland. Of particular relevance to this studentship, actinic flux observations, coupled with absorption cross sections for the functional groups (aldehydic and hydroperoxide) present in HPMTF will be measured and will enable photolysis rates for HPMTF to be determined and the photolytic products to be derived.

HCHO observations, coupled with HCHO photolysis frequencies and observations of a suite of VOCs, CH4, CO, NOx (from which (a) the effective HCHO yield weighted over all OH reactions, can be derived) will provide a proxy for the OH concentration (Wolfe et al., 2019) and enable the loss of HPMTF via OH oxidation to be determined as well as the DMS oxidation rate via OH. Through these targeted field observations and subsequent modelling studies, an improved understanding of the role DMS plays in the marine sulfur cycle and the impact on the climate system will be achieved.