FIELD-STUDY OF DIMETHYLSULFIDE OXIDATION IN THE BOUNDARY-LAYER - VARIATIONS OF DIMETHYLSULFIDE, METHANESULFONIC-ACID, SULFUR-DIOXIDE, NON-SEASALT SULFATE AND AITKEN NUCLEI AT A COASTAL SITE

Measurements of atmospheric dimethylsulfide (DMS) and its oxidation products, sulfur dioxide (SO2), methanesulfonic acid (MSA) and non-sea-salt sulfate (nss-SO42-) were monitored during the period June 9-26, 1989 at a coastal site in Brittany. As indicated by the radon (Rn-222) activities and the high concentrations of NOx the air masses, for most of the experiment, were continental in origin. The observed concentrations range from 1.9 to 65 nmol/m3 for DMS (n = 157), 0.6 to 94.2 nmol/m3 for SO2 (n = 50), 0.6 to 11.6 nmol/m3 for MSA (n = 44) and 42 to 350 nmol/m 3 for nss-SO42- (n = 44). Aitken nuclei reached values as high as 4.5 x 10(5) particles/m3. When continental conditions predominated, the measured SO2 concentrations were lower than those expected from a consideration of the observed DMS concentrations and the existence Of S02 background of the continental air masses. Similarly, compared to the MSA/DMS ratio in the marine atmosphere, higher concentrations of MSA were observed than those expected from the measured levels of DMS. The presence of enhanced levels of MSA was also endorsed by the observation that the measured mean MSA/nss-SO42- ratio of 6 +/- 3% was similar to the mean value of 6.9% observed in the marine atmosphere. These above observations are in line with recent laboratory findings by Barnes et al. (1988), which show an increase of the MSA/DMS yield with a simultaneous decrease of the SO2/DMS yield in the presence of NOx.