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 fights over the Southern Ocean. DMS flux values were derived from airborne observa tions recorded during the Aerosol Characterization Experiment (ACE I). The latitude range covered was 55 degreesS-40 degreesS. The method of evaluatio n was based on the mass-balance photochemical-modeling (MBPCM) approach. Th e estimated flux for the above latitude range was 0.4-7.0 mu mol m(-2) d(-1 ). The average value from all data analyzed was 2.6 +/- 1.8 mu mol m(-2) d( -1). A comparison of the MBPCM methodology with several other DMS flux meth ods (e.g., ship and airborne based) revealed reasonably good agreement in s ome cases and significant disagreement in other cases. Considering the limi ted number of cases compared and the fact that conditions for the compariso ns 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, t he conversion efficiency is 0.7 or higher, given a lifetime for SO2 of simi lar to 1d. A comparison of two sulfur case studies, one based on DMS-SO2 da ta generated on the NCAR C-130 aircraft, the other based on data recorded o n the NOAA ship Discoverer, revealed qualitative agreement in finding that DMS was a major source of Southern Ocean SO2. On the other hand, significan t 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 mi xing, may be involved in reducing the level of disagreement, it does appear at this time that some significant portion of this difference may be relat ed to systematic differences in the two different techniques employed to me asure SO2. It would seem prudent, therefore, that further instrument interc omparison SO2 studies be considered. It also would be desirable to stage ne w intercomparison activity between the MBPCM flux approach and the air-to-s ea gradient as well as other flux methods, but under far more favorable con ditions. (C) 2000 Elsevier Science Ltd. All rights reserved.