DMS oxidation in the Antarctic marine boundary layer:: Comparison of model simulations and field observations of DMS, DMSO, DMSO2, H2SO4(g), MSA(g), and MSA(p)

A sulfur field study (SCATE) at Palmer Station Antarctica (January 18 to February 25) has revealed several major new findings concerning (dimethyl sulfide) DMS oxidation chemistry and the cycling of sulfur within the Antarctic environment. Significant evidence was found supporting the notion that the OH/DMS addition reaction is a major source of dimethyl sulfoxide (DMSO), Methane sulfonic acid (MSA(g)) levels were also found to be consistent with an OH/DMS addition mechanism involving the sequential oxidation of the products DMSO and methane sulfinic acid (MSIA). Evidence supporting the hypothesis that the OH/DMS addition reaction, as well as follow-on reactions involving OH/DMSO, are a major source of SO2 was significant, but not conclusive. No evidence could be found supporting the notion that reactive intermediates (i.e., SO3) other than SO2 were an important source of H2SO4. Quite clearly, one of the major findings of SCATE was the recognition that a large fraction of the Antarctic oxidative cycle for DMS (near Palmer Station) took place above the boundary layer (BL) in what we have labeled here as the atmospheric buffer layer (BuL). Although still speculative in places, the overall picture emerging from the SCATE field/modeling results is one involving major coupling between chemistry and dynamics in the Antarctic. At Palmer the evidence points to frequent episodes of rapid vertical transport from a very shallow marine BL into the overlying BuL. Due to the combination of a long photochemical lifetime for DMS and the frequency of shallow convective events, a large fraction of ocean released DMS is transported into the BuL while still in its unoxidized state. There, in the presence of elevated OH and low aerosol scavenging, high levels of oxidized sulfur accumulate. Parcels of this BuL air are then episodically entrained back into the BL, thereby providing a controlling influence on BL SO2, DMSO, and DMSO2. Additionally, because SO? and DMSO are major precursors to H2SO4 and MSA, BuL chemistry, in conjunction with vertical transport, also act to control BL levels of the latter species. Although many uncertainties remain in our understanding of Antarctic DMS chemistry, the above picture already suggests that previous chemical interpretations of Antarctic field data may need to be altered.