Measurements of dimethyl sulfide and H 2 S over the western North Atlantic and the tropical Atlantic

Abstract

Airborne measurements of DMS and H 2 S were made off the east coast of the United States and over the tropical Atlantic off Brazil. Samples were collected through a fluorinated ethylene propylene Teflon inlet manifold. Dimethyl sulfide (DMS) was preconcentrated onto gold wool and analyzed by gas chromatography/flame photometric detection. H 2 S was collected on AgNO 3 ‐impregnated filters and determined by fluorescence quenching. Use of a new scrubber material (cotton) to remove negative interference on DMS measurements was investigated. Comparison with a Na 2 CO 3 /Anakrom scrubber gave good overall agreement. Only under extreme conditions, e.g., on flight 9 (continental air mass, low humidity, high O 3 , and low DMS values) did Na 2 CO 3 show noticeable loss of DMS compared to cotton. On most flights, especially in marine air masses with high humidity and relatively low O 3 , the results from both scrubbers agreed well with each other and with other instruments used during the intercalibration. Off the U.S. East Coast, DMS levels showed strong dependence on air mass origin with high values (up to 83 ppt) in marine tropical air masses and low values (10–20 ppt) in continental and polar air. Over the tropical Atlantic, DMS ranged over 20–100 ppt in the mixed 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. H 2 S in the mixed layer off the U.S. East Coast ranged from 10 to 200 ppt. Significant influence from terrestrial and pollution sources was evident. H 2 S in air masses originating over the eastern seaboard was much higher than in continental polar air or over the remote tropical continents. In 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 H 2 S levels over the tropical Atlantic. The correlation of atmospheric 222 Rn and H 2 S was significant, with both being higher off the U.S. East Coast (H 2 S, 7–270 ppt; 222 Rn, 2–20 pCi m −3 ) than over the tropical Atlantic (0.5‐10 ppt and 2–4 pCi m −3 , respectively).