Measurements of carbonyl sulfide (COS) in surface seawater and marine air,and estimates of the air-sea flux from observations during two Atlantic cruises

Carbonyl sulfide (COS) was measured in surface seawater and in marine air d uring two Atlantic cruises of the R/V Polarstern between Bremerhaven, Germa ny, and Cape Town, South Africa. The cruises took place in the fall of 1997 and in the summer of 1998. The concentration of COS showed clear diurnal, seasonal, and latitudinal variations, as did its saturation ratio. The conc entration of dissolved COS averaged 14.7 pmol L-1 and 18.1 pmol L-1 for the fall and summer cruises, respectively. On most days, seawater was undersat urated in COS during the late night and early morning but was supersaturate d during the rest of the day, implying that the ocean can act as both a sou rce and a sink for COS on the same day. The COS content in seawater was cor related significantly with the global radiation, the CH3SH concentration, a nd the seawater temperature. The air-sea flux of COS from the open Atlantic Ocean was estimated using exchange coefficients calculated according: to E rickson's stability dependent model for air-sea gas exchange. The largest C OS flux into the atmosphere occurred in productive regions (the Benguela Cu rrent, the West African upwelling area, and the northeastern Atlantic) duri ng the warmer seasons. A small net oceanic uptake of COS was found in the B enguela Current during the southern winter. The average open ocean fluxes w ere 13.5 nmol COS m(-2) d(-1) and 28.6 nmol COS m(-2) d(-1) for the two cru ises, respectively. A global open ocean source of 0.10 Tg: COS yr(-1) is ex trapolated from the measured data. The atmospheric mixing ratio of COS aver aged 474+/-33 and 502+/-38 pptv for the fall and summer cruises: respective ly, and had no significant interhemispheric gradient.