Sulfur gas measurements in the eastern North Atlantic Ocean during the Atlantic Stratocumulus Transition Experiment/Marine Aerosol and Gas Exchange

Abstract

Results from an intensive measurement program studying the marine stratocumulus regime in the eastern North Atlantic Ocean are reported. We observed generally high mixing ratios of sulfur dioxide (SO2) in the region, typically advected within the marine boundary layer (MBL). SO2 mixing ratios ranged from 19 pptv to 1.3 ppbv. Dimethyl sulfide (DMS) mixing ratios also were quite variable, but airborne DMS measurements were never greater than 140 pptv. The mean carbon disulfide mixing ratio was 6.5 pptv. In two intensive Lagrangian experiments, the budgets for SO2 and DMS in the MBL were examined. The observed overnight increase in DMS and the predicted increase based on a budget analysis (using a simple surface flux model) agree within the precision of the data for the first Lagrangian experiment. The photochemical oxidation rates for DMS derived from the budget analysis range from 2.5 to 4.9 μmol/m2 d. Because daytime mixing ratios of NO were seldom larger than 10–15 pptv during both experiments, nighttime oxidation of DMS by NO3 was negligible compared with daytime losses to OH. A positive surface flux is the major term in the DMS budgets. SO2 mixing ratios in the MBL were largely controlled by advected pollution from continental Europe. Deposition of SO2 to the sea surface was the major loss term in the budget analysis. Terms for photochemical production and loss of SO2 could not be independently determined in this analysis, but it is likely these terms were small compared to the surface flux. Based on the observed loss rate for SO2 in polluted European air masses, the mean lifetime of SO2 in the MBL during ASTEX/MAGE is estimated to be 15–18 hours. Our analysis suggests that the Lagrangian experimental design is a valuable tool for isolating chemical change in a dynamic meteorological system, but a budget analysis to determine photochemical production/loss terms is difficult or impossible in the presence of significant pollution. Lagrangian results highlight the potential dangers to chemical interpretation of Eulerian data sets, where advection is often important and needs to be taken into account.