Abstract
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.
Highlights
The model does not include horizontal transport or downward mixing of H2 S from the free troposphere. These processes should play a minor role in the H2S budget for this study, in view of the distance of the cruise track from terrestrial sources and the lackofcorrelatiobnetweenH2Sand222RnT.heH2Slifetime in the boundary layer, as determined from the OH concentrations calculated for this study, is of the order of a day
A boundary layer height of 2 km was assumed, based on radiosonde profiles measured during this cruise
In this zero-dimensional kinetic model, a fixed time step (1 min) Runge-Kutta routine is used to solve the rate expressionsfor the reactions of H2S and dimethyl sulfide (DMS) with OH radicals in the marine boundary layer
Summary
This work represents an extension of the data base to the equatorial Pacific Ocean, a region which should exhibit minimal atmospheric input from terrestrial and coastal sources. A similar average concentration of 7 --- 4 ppt (ltr) was reported for remote marine air masses over the temperate western North Atlantic Ocean by Andreae et al [1991]. An average H2S level (2.4 ppt) similar to that measured during this study has been reported for remote marine air masses over the South Atlantic Ocean [Cooper and Saltzman, 1993]. Of these previous measurements, those made before 1987 may be high due to the formation of artifact sulfide from the hydrolysis of OCS on the AgNO 3 filters [Cooper and Saltzman, 1987]. Atmospheric H2S and seawater dimethyl sulfide (DMS) [Johnson, this issue] both exhibit a general increase i i i
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