Department of Earth System Science

Atmospheric H₂S concentrations were measured over the equatorial Pacific on leg 1 of the third Soviet-American Gases and Aerosols (SAGA 3) cruise during February and March 1990. Five N-S transects were made across the equator between Hawaii and American Samoa. The concentrations ranged from below the detection limit of 0.4 ± 0.5 (1 σ) to 14.4 ppt with an average value of 3.6 ± 2.3 ppt (1σ, n= 72). The highest concentrations were found on the easternmost two transects just south of the equator. The average concentration of 3.6 ppt observed on this cruise is the lowest reported value for background atmospheric H₂S over the tropical oceans. A lack of correlation between ²²²Rn and H₂S rules out a significant continental source. Model calculations indicate that the oceanic source of H₂S in this region is in the range of 9 to 21 × 10⁻⁸ mol m⁻² d⁻¹. From this flux the concentration of free sulfide (H₂S + S⁼ ) in the surface mixed layer of the ocean is estimated to be in the range of 32 to 67 pmol L⁻¹. In the atmosphere the oxidation of H₂S produces SO₂ at a rate of 2.1 to 4.4 × 10⁻¹¹ mol m⁻³ d⁻¹ which is only a small fraction of that estimated from the oxidation of dimethyl sulfide (DMS) in this region. A diurnal cycle was not observed in the H₂S data recorded during this cruise.