Impact of Bromine on Dimethyl Sulphide Oxidation

Tom Breider, Martyn Chipperfield, Ryan Hossaini, Hannah Mantle

Background

The emission of Bromine from sea salt aerosols and macroalgae (seaweed) may provide an additional oxidant for dimethyl sulphide (DMS) in remote marine regions. Most global models currently assume DMS oxidation only takes place via reaction with OH and NO₃.

Modelling of DMS Oxidation by BrO

We have used the TOMCAT chemical transport model coupled to the detailed size resolved aerosol microphysics module, GLOMAP, to study the impact of BrO on DMS oxidation. Organic bromine emissions are taken from Warwick et al. (2006). Sea salt bromine emissions are calculated using the source function described in Yang et al. (2005). This has been extended to include the scheme published in Alexander et al. (2005) allowing bromine emissions to be restricted only to sea salt sizes that have been acidified. Acidification takes place via uptake of SO₂ and HNO₃.

Global Model Simulations

The simulations suggest BrO contributes 16% of the annual global DMS sink. The largest effect is simulated over the Southern Hemisphere oceans where BrO contributes up to 50% of DMS oxidation (Figure 1). 

In addition the model suggests the presence of a feedback mechanism between DMS, sea salt and BrO. As a product of DMS oxidation is SO₂, DMS provides a strong source of aerosol acidity in the remote marine southern hemisphere. This enhances the release of bromine from sea salt and in turn increases the oxidation sink of DMS. This may represent a mechanism by which DMS controls its own lifetime.