OBSERVATIONS OF DIMETHYL SULFIDE OVER THE WESTERN NORTH-ATLANTIC OCEAN USING AN AIRBORNE TANDEM MASS-SPECTROMETER

An atmospheric sampling tandem mass spectrometer has been evaluated fo r aircraft monitoring of dimethyl sulfide (DMS) and then used for DMS monitoring during portions of six flights over the western North Atlan tic Ocean. Laboratory evaluations demonstrated that the mass spectrome ter is highly selective for DMS, and responds linearly over a wide ran ge of mixing ratios. The detection limit for DMS is 1-2 ppt in dry air . However, the response is suppressed in the presence of water vapor, so the sample air must be dried or the response must be corrected for this effect. There appeared to be no consistent effect of altitude/pre ssure on instrument response. The mass spectrometer was installed on t he Battelle Gulfstream G-1 research aircraft and used to monitor DMS a nd a number of other chemicals during flights over the western North A tlantic Ocean in August and September, 1992. DMS mixing ratios ranged from <2 ppt to 332 ppt, and were highly variable both horizontally and vertically. Vertical profiles indicated that there are times when the marine boundary layer is stratified by one or more temperature invers ions, and that DMS emitted by surface seawater can be confined near th e surface within a shallow layer a few hundred meters deep. Under such circumstances, the DMS photooxidation products may be removed rapidly by deposition, lessening the potential for cloud nucleation. The mean DMS mixing ratio in the boundary layer below the lowest observed temp erature inversion was 61 ppt, with a range of 7-332 ppt. DMS measureme nts in the free troposphere were lower than the boundary layer values, but high enough to suggest significant transport from the boundary la yer to the free troposphere. Significant horizontal variability was ob served during constant altitude flights in the boundary layer. In one case the DMS mixing ratio was observed to vary with the ocean depth un der the flight path, with higher mixing ratios observed over the shall ower coastal Shelf and undersea banks. In several cases we also observ ed an apparent association between atmospheric DMS mixing ratio at low elevation and sea surface temperature.