Regional variation of the dimethyl sulfide oxidation mechanism in the summertime marine boundary layer in the Gulf of Maine

Abstract

Mixing ratios of dimethyl sulfide (DMS) and its nighttime oxidant, the nitrate radical (NO3), were measured in the summertime marine boundary layer (MBL) of the Gulf of Maine during the New England Air Quality Study–International Transport and Chemical Transformation campaign in 2004. DMS fluxes from the ocean were derived from simultaneous measurements of the wind speed and DMS in seawater. Day and night DMS oxidation rates were determined from modeled OH and measured NO3 concentrations. The average DMS lifetime with respect to oxidation by OH at noon was 13.5 ± 3.4 (1σ) h, while at night, DMS lifetimes with respect to NO3 oxidation varied by sampling region from 11 min to 28 h. Oxidation by photochemically generated halogen species likely also played a role during the day, although the nature and extent of the halogen species is more difficult to predict due to lack of halogen measurements. Closure of the DMS budget in the MBL required a vertical entrainment velocity of ∼0.4 cm s−1. This study suggests that entrainment of DMS out of the MBL competes with daytime oxidation and that the presence of pollution in the form of NOx and O3 in near‐coastal regions at night results in nearly complete DMS oxidation within the MBL via reaction with NO3, with a much smaller contribution from entrainment. One potential implication of near‐complete DMS oxidation within the MBL is a reduction of the amount of sulfur available for aerosol formation and growth at higher altitudes in the atmosphere.