Chapter 6.3 Dimethylsulfide (DMS) flux and DMS oxidation over the North Atlantic: Comparison of a top-down and a bottom-up approach

Abstract

Dimethylsulfide (DMS) is a gas that forms in some biologically productive regions of the surface ocean. Only a few percent of the total DMS in surface waters is released to the atmosphere but, because it has the potential to form new CCN on oxidation to sulfate, this flux may be an important variable to understand in a changing climate scenario. DMS fluxes in this study were used in a comparative approach to constrain DMS oxidation pathways. A traditional approach to calculate DMS flux (bottom-up) used DMS concentrations in surface waters and the parameterization of Wanninkhof and McGillis (1999) for piston velocity. The bottom-up surface water DMS fluxes were calculated for the SOLAS SABINA Spring cruise over the North Atlantic: 30–60°N from April 25–May 14, 2003. These fluxes were compared to a top-down approach using a steady-state oxidation model where corresponding DMS mixing ratios in the air over the Atlantic, and boundary layer heights from the NARCM model were incorporated. In the simplest scheme, using OH alone as an oxidant, the top-down and bottom-up approaches showed poor agreement in terms of both the magnitude of DMS flux and its temporal variations. Better sensitivity for DMS flux was achieved using OH plus NO 3 oxidation, but temporal variations were still poorly represented. Adding very small amounts of BrO to the oxidation scheme improved the agreement between the top-down and bottom-up approaches, but only slightly. Transport of DMS was examined during a high atmospheric DMS concentration event using estimates for the spatial distribution of atmospheric DMS from the NARCM model using the Kettle et al. (1999) database and could be responsible for the poor agreement between flux estimates during high wind regimes. However, the poor agreement between the two approaches during low wind regimes suggests that the oxidation pathway for DMS over the North Atlantic is more complex than at low latitudes where OH oxidation is dominant.