Quasi-Lagrangian investigation into dimethyl sulfide oxidation in maritimeair using a combination of measurements and model

Using a combination of field measurement data and a modified photochemical box model, strong evidence is presented to suggest that the rate of daytime oxidation of dimethyl sulfide (DMS) by OH radicals is insufficient to desc ribe the measured conversion. Quasi-Lagrangian measurements were made at tw o sites in the eastern Atlantic (Research Vessel and Mace Head Research Sta tion, Ireland) as part of the Atmospheric Chemistry Studies in the Oceanic Environment (ACSOE) program, Periods of connected flow between the two site s were identified, air parcel transit times were estimated, and measurement s of the main DMS oxidation products (MSA, SO2, and nss-SO42-) were compare d with model predictions to establish whether the model's chemical mechanis m could account for observed changes. The main finding was that during dayt ime periods with maritime air masses, the model failed to predict a suffici ent increase in DMS oxidation products during the estimated transit time. T his was despite a tendency to overprediction of the progress of nitrogen ch emistry during air mass advection, and independent checks on the model esti mates of hydroxyl radical concentrations through measurements. In the light of this, the involvement of halogen species (most probably halogen oxides) or heterogeneous oxidation processes is tentatively suggested as the cause of enhanced daytime DMS oxidation in the marine boundary layer(MBL). Incre asing the rate constant for the OH + DMS reaction by a factor of 3.3 (as a crude way of simulating parallel channels of DMS oxidation) permitted model results to reproduce the measurements very much more closely.