Vertical distribution of dimethylsulfide, sulfur dioxide, aerosol ions, and radon over the Northeast Pacific Ocean

Abstract

Dimethylsulfide (DMS), sulfur dioxide (SO2), methanesulfonate (MSA), nonsea-salt sulfate (nss-SO42-), sodium (Na+), ammonium (NH4+), and nitrate (NO3-) were determined in samples collected by aircraft over the open ocean in postfrontal maritime air masses off the northwest coast of the United States (3-12 May 1985). Measurements of radon daughter concentrations and isentropic trajectory calculations suggested that these air masses had been over the Pacific for 4-8 days since leaving the Asian continent. The DMS and MSA profiles showed very similar structures, with typical concentrations of 0.3-1.2 and 0.25-0.31 nmol m-3 (STP) respectively in the mixed layer, decreasing to 0.01-0.12 and 0.03-0.13 nmol m-3 (STP) at 3.6 km. These low atmospheric DMS concentrations are consistent with low levels of DMS measured in the surface waters of the northeastern Pacific during the study period. The atmospheric SO2 concentrations always increased with altitude from <0.16-0.25 to 0.44-1.31 nmol m-3 (STP). The nonsea-salt sulfate (ns-SO42-) concentrations decreased with altitude in the boundary layer and increased again in the free troposphere. These data suggest that, at least under the conditions prevailing during our flights, the production of SO2 and nss-SO42- from DMS oxidation was significant only within the boundary layer and that transport from Asia dominated the sulfur cycle in the free troposphere. The existence of a 'sea-salt inversion layer' was reflected in the profiles of those aerosol components, e.g., Na+ and NO3-, which were predominantly present as coarse particles. Our results show that long-range transport at mid-tropospheric levels plays an important role in determining the chemical composition of the atmosphere even in apparently 'remote' northern hemispheric regions. © 1988 D. Reidel Publishing Company.