An airborne field study of dimethylsulfoxide at tropical latitudes and its relationship to the marine sulfur budget

Dimethylsulfide (CH₃SCH₃ or DMS) released from the ocean surface is the largest source of biogenic sulfur to the atmosphere. The Charlson-Lovelock-Andrea-Warren (CLAW) hypothesis proposed a climate regulating feedback mechanism involving DMS oxidation products. In the atmosphere one of the primary oxidants of DMS is the hydroxyl radical (OH). Its oxidation is a complex process leading to many possible final products. Critical to evaluating the CLAW hypothesis is understanding the product distribution of DMS oxidation. However, few field studies have measured the intermediate as well as final oxidation products. One important intermediate product is dimethylsulfoxide (DMSO).; A chemical ionization mass spectrometer (CIMS) technique using protonated ethanol clusters (C₂H₅OH)_nH+ was developed to measure DMSO. Laboratory tests showed that the reactivity of the (C₂H₅OH)_nH+ cluster ions is dependent on the cluster size with reactivity decreasing as cluster size increases. Small (C₂H₅OH)_nH+ clusters, were found to react at or near the collisional rate with DMSO, ammonia (NH₃), acetone, and methyl vinyl ketone. Only DMSO and NH₃ react with large (C₂H₅OH)_nH + cluster ions. Experiments have shown that large (C₂H ₅OH)_nH+ ion clusters must be used at relative humidities greater than 50% at 20°C to prevent formation and interference from H₃O+ ions. These results demonstrate that the (C₂H₅OH)_nH+ ion chemistry can selectively detect DMSO and NH₃ under most ambient conditions with high sensitivity.; This CIMS technique was used during PEM - Tropics B to provide the first airborne DMSO observations, which have challenged our current understanding of marine atmospheric sulfur chemistry. Critical observations included: DMSO levels that closely paralleled the trend in DMS, and thus were out of phase with model simulations; DMSO levels that were significantly higher than those predicted by model simulations; and finally, DMSO and DMS values from multiple flights that were well correlated in both the boundary and buffer layers. The consistent disagreement between observations and model simulations suggest that there is an additional source of DMSO besides the DMS/OH reaction. This study also suggests that at tropical upwelling latitudes the biogenic sulfur source from the ocean could be factors of two to three times larger than previously estimated.