Evaluation of the DMS flux and its conversion to SO2 over the southern ocean

A total of 16 boundary layer (BL) DMS flux values were derived from flights over the Southern Ocean. DMS flux values were derived from airborne observations recorded during the Aerosol Characterization Experiment (ACE 1). The latitude range covered was 55°S–40°S. The method of evaluation was based on the mass-balance photochemical-modeling (MBPCM) approach. The estimated flux for the above latitude range was 0.4–7.0 μmol m⁻² d⁻¹. The average value from all data analyzed was 2.6±1.8 μmol m⁻² d⁻¹. A comparison of the MBPCM methodology with several other DMS flux methods (e.g., ship and airborne based) revealed reasonably good agreement in some cases and significant disagreement in other cases. Considering the limited number of cases compared and the fact that conditions for the comparisons were far from ideal, it is not possible to conclude that major agreement or differences have been established between these methods. A major result from this study was the finding that DMS oxidation is a major source of BL SO₂ over the Southern Ocean. Model simulations suggest that, on average, the conversion efficiency is 0.7 or higher, given a lifetime for SO₂ of ∼1 d. A comparison of two sulfur case studies, one based on DMS–SO₂ data generated on the NCAR C-130 aircraft, the other based on data recorded on the NOAA ship Discoverer, revealed qualitative agreement in finding that DMS was a major source of Southern Ocean SO_2. On the other hand, significant disagreement was found regarding the DMS/SO₂ conversion efficiency (e.g., 0.3–0.5 versus 0.7–0.9). Although yet unknown factors, such as vertical mixing, may be involved in reducing the level of disagreement, it does appear at this time that some significant portion of this difference may be related to systematic differences in the two different techniques employed to measure SO₂. It would seem prudent, therefore, that further instrument intercomparison SO₂ studies be considered. It also would be desirable to stage new intercomparison activity between the MBPCM flux approach and the air-to-sea gradient as well as other flux methods, but under far more favorable conditions.