A mass-balance/photochemical assessment of DMS sea-to-air flux as inferredfrom NASA GTE PEM-West A and B observations

This study reports dimethyl sulfide (DMS) sea-to-air fluxes derived from a mass-balance/photochemical-modeling approach. The region investigated was t he western North Pacific covering the latitude range of 0 degrees-30 degree s N. Two NASA airborne databases were used in this study: PEM-West A in Sep tember-October 1991 and PEM-West B in February-March 1994. A total of 35 bo undary layer (BL) sampling runs were recorded between the two programs. How ever, after filtering these data for pollution impacts and DMS lifetime con siderations, this total was reduced to 13. Input for each analysis consiste d of atmospheric DMS measurements, the equivalent mixing depth (EMD) for DM S, and model estimated values for OH and NO3. The evaluation of the EMD too k into account both DMS within the BL as well as that transported into the overlying atmospheric buffer layer (BuL). DMS fluxes ranged from 0.6 to 3.0 mu mol m(-2) d(-1) for PEM-West A (10 sample runs) and 1.4 to 1.9 mu mol m (-2) d(-1) for PEM-West B (3 sample runs). Sensitivity analyses showed that the photochemically evaluated DMS flux was most influenced by the DMS vert ical profile and the diel profile for OH. A propagation of error analysis r evealed that the uncertainty associated with individual flux determinations ranged from a factor of 1.3 to 1.5. Also assessed were potential systemati c errors. The first of these relates to our noninclusion of large-scale mea n vertical motion as it might appear in the form of atmospheric subsidence or as a convergence. Our estimates here would place this error in the range of 0 to 30%. By far the largest systematic error is that associated with s tochastic events (e.g., those involving major changes in cloud coverage). I n the latter case, sensitivity tests suggested that the error could be as h igh as a factor of 2. With improvements in such areas as BL sampling time, direct observations of OH, improved DMS vertical profiling, direct assessme nt of vertical velocity in the field, and preflight (24 hours) detailed met eorological data, it appears that the uncertainty in this approach could be reduced to +/- 25%.