MEASUREMENTS OF DIMETHYL SULFIDE AND H2S OVER THE WESTERN NORTH-ATLANTIC AND THE TROPICAL ATLANTIC

Airborne measurements of DMS and H2S were made off the east coast of t he United States and over the tropical Atlantic off Brazil. Samples we re collected through a fluorinated ethylene propylene Teflon inlet man ifold. Dimethyl sulfide (DMS) was preconcentrated onto gold wool and a nalyzed by gas chromatography/flame photometric detection. H2S was col lected on AgNO3-impregnated filters and determined by fluorescence que nching. Use of a new scrubber material (cotton) to remove negative int erference on DMS measurements was investigated. Comparison with a Na2C O3/Anakrom scrubber gave good overall agreement. Only under extreme co nditions, e.g., on flight 9 (continental air mass, low humidity, high 03, and low DMS values) did Na2CO3 show noticeable loss of DMS compare d to cotton. On most flights, especially in marine air masses with hig h humidity and relatively low O3, the results from both scrubbers agre ed well with each other and with other instruments used during the int ercalibration. Off the U.S. East Coast, DMS levels showed strong depen dence on air mass origin with high values (up to 83 ppt) in marine tro pical air masses and low values (10-20 ppt) in continental and polar a ir. Over the tropical Atlantic, DMS ranged over 20-100 ppt in the mixe d layer. Nighttime values were a factor of 1.6-2.3 higher than daytime levels. DMS decreased with altitude to < 1 ppt at 4000 m. H2S in the mixed layer off the U.S. East Coast ranged from 10 to 200 ppt. Signifi cant influence from terrestrial and pollution sources was evident. H2S in air masses originating over the eastern seaboard was much higher t han in continental polar air or over the remote tropical continents. I n contrast, over the tropical Atlantic, concentrations were very low ( 5-10 ppt), typical of truly marine air. Night/day ratios were about 1. 4. No significant geographical variability was seen in H2S levels over the tropical Atlantic. The correlation of atmospheric Rn-222 and H2S was significant, with both being higher off the U.S. East Coast (H2S, 7-270 ppt; Rn-222, 2-20 pCi m-3) than over the tropical Atlantic (0.5- 10 ppt and 2-4 pCi m-3, respectively).