Keß, Kevin (2023) Interaction of dimethyl sulphide (DMS) and hypobromous acid (HOBr) in global ocean surface waters. (Master thesis), Christian-Albrechts-Universität zu Kiel, Kiel, Germany, 82 pp.

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Abstract

In marine environments, the volatile organic sulphur compound dimethyl sulphide (DMS) is produced from its precursor dimethyl sulphoniopropionate (DMSP) via enzymatic cleavage as part of complex food web interactions between bacteria, phytoplankton and zooplankton. DMS is either oxidized to dimethyl sulphoxide (DMSO) by microbial degradation and photolysis or gets transferred to the atmosphere via sea-air exchange. Here, it serves as the main source for atmospheric sulphur, while its oxidation products play a crucial role in the formation of cloud condensation nuclei (CCN), which increase the cloud cover and enhance the Earth’s albedo. The halogenated volatile organic compound bromoform (CHBr3) is produced by phytoplankton and seaweeds via enzymatic oxidation of bromide to hypobromous acid (HOBr) and a subsequent halogenation of ambient organic matter. Sea-air gas transfer leads the majority of CHBr3 to be removed from the oceans and introduced into the atmosphere, where its bromine atoms are released by oxidation and complex follow-up reactions. Bromine takes part in a catalytic cycle of ozone degradation and reduces the production of ozone via the regulation of nitrogen oxides. DMS and CHBr3 share similar characteristics in terms of their origin, chemical properties and atmospheric role. However, recent studies also brought up the hypothesis that HOBr might preferentially react with DMS instead of dissolved organic matter (DOM), its typical reaction partner in the formation of CHBr3. This would result in a possible suppression of CHBr3 production, if enough DMS is available in marine environments. Until now, this was only observed in laboratory experiments, which is why this thesis aims to investigate on the relationship between DMS and CHBr3 in natural surface water from various oceans. 71 samples from the SO287 expedition were analysed for DMSOt (total DMSO) and DMSOd (dissolved DMSO) with a purge-and-trap system coupled with a gas chromatograph and an attached flame photometric detector (GC-FPD). In combination with the respective DMS and CHBr3 concentrations and measurements from seven additional expeditions as well as a time series station, a data set with 550 data points was gathered for this purpose. The results from the SO287 samples indicated that high CHBr3:DMSOt ratios above 2 × 10⁻³ exclusively occur at low DMS levels. Similar trends were observed in most of the additional data sets, even though their clarity strongly varied and the number of samples with an enhanced CHBr3:DMSOt ratio was relatively small in comparison to the total quantity of the combined data set. On the other hand, the correlation of CHBr3 concentrations and CHBr3:DMSOt ratios showed a statistically significant linear relationship in seven out of eight data sets. Both observations support the hypothesis that enhanced DMS concentrations are capable of suppressing the marine CHBr3
production via the reaction with HOBr under the formation of DMSO. As the anthropogenic climate change will probably influence the formation and removal of several marine compounds due to temperature and pH related shifts in biological communities and an impact on sea-air exchange, DMS and CHBr3 as well as their precursors and oxidation products need further investigation to fully understand possible feedback loops and natural cycling processes of sulphur and bromine species.

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