DMS oxidation: effects of halogens and of increased sea salt alkalinity.

The oxidation of DMS is the main source of SO₂, MSA, and non-sea-salt sulfate (nss) in the clean marine boundary layer (MBL). Recently the importance of BrO - in addition to OH and NO₃ - has been suggested as oxidant for DMS. Furthermore HOCl_aq and HOBr_aq can increase the oxidation of S(IV) to S(VI) in aerosol and cloud particles. Chlorine and bromine in the MBL are derived from seasalt aerosol. We investigated the importance of these processes for the chemistry of the MBL and possible climate links with the one-dimensional chemical and microphysical model MISTRA-MPIC. BrO plays a very significant role as oxidant even under very low mixing ratios of 0.5 pmol mol^-1. We found that still significant uncertainty exists in the kinetics of DMS oxidation especially with regard to the endproducts of DMS oxidation. Under most conditions that we studied the net effect of halogens, especially under cloudy conditions, is an increase in particulate sulfur (MSA plus nss) and decrease of precursors for the formation of new CCN. In field experiments at Baring Head, New Zealand, it was found that supermicron sea salt aerosols were enriched several fold in their alkalinity resulting from ocean surface layer biogenic sources. Extrapolation of these Baring Head results to typical open ocean lower MBL conditions indicate that supermicron sea salt aerosol alkalinity is enriched by a factor of 100 - 150% compared to deeper ocean water. This would lead to increased alkalinity in the seasalt aerosol and therefore an enhancement of the importance of the nss production by O_3,aq. Our model runs show a non-linear response of the oxidation by O_3,aq with increasing alkalinity due to saturation effects. In the model runs with increased alkalinity O_3,aq dominates the total nss production.