The relationship between dimethyl sulfide and particulate sulfate in the mid-atlantic ocean atmosphere

Abstract

Dimethyl sulfide (DMS) and atmospheric aerosols were sampled simultaneously over the Atlantic Ocean in the vicinity of Bermuda using the NOAA King Air research aircraft. Total and fine (50% cutoff at 2 μm diameter) aerosol fractions were sampled using two independent systems. The average nonsea-salt (nss)SO 4 2− concentrations were 1.9 and 1.0 μg m −3 (as SO 4 2−) for the total and the fine fractions in the boundary layer (BL) and 0.53 and 0.27 μg m −3 in the free troposphere (FT). Non-sea-salt SO 4 2− in the two aerosol fractions were highly correlated ( r = 0.90), however a smaller percentage (55%) was found in the fine aerosol near Bermuda relative to that (90%) near the North American continent. The BL SO 4 2− concentrations measured in this study were higher than those measured by others at remote marine locations despite the fact that the 7-day air mass back trajectories indicated little or no continental contact at altitudes of 700 mb and below; the trajectories were over subtropical oceanic areas that are expected to be rich in DMS. DMS concentrations were higher near the ocean surface and decreased with increasing altitude within the BL; the average DMS concentration was 0.13 μg m −3. Trace levels of DMS were also measured in the FT (0.01 μg m −3). Computer simultation of the oxidation and removal of DMS in the marine atmosphere suggests that <50% of the SO 4 2− observed could be related to the natural S cycle.