Atmospheric Hydrogen Sulfide Over the Equatorial Pacific (SAGA 3)

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 (lrr) to 14.4 ppt with an average value of 3.6 -+ 2.3 ppt (1 rr, 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 x 10 -8 mol m -2 d -1. From this flux the concentration of free sulfide (HeS + S =) in the surface mixed layer of the ocean is estimated to be in the range of 32 to 67 pmol L -r. In the atmosphere the oxidation of H2S produces SO2 at a rate of 2.1 to 4.4 x 10 -l(cid:127) 3 1 mol m- d- 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.


INTRODUCTION
1Hydrogen sulfide (H2S) is emitted into the atmosphere from a wide variety of marine and terrestrial environments, and its flux constitutes an important component of the atmospheric sulfur budget [Andreae, 1990]. Although the sea-to-air flux of hydrogen sulfide is thought to be a relatively minor source, current estimates are based upon an extremely limited data set [Slatt et al., 1978;Delmas and Servant, 1982 band-pass filters at excitation and emission wavelengths of 500 and 520 nm, respectively. Calibration was done by standard addition of a freshly prepared sulfide standard to extracted blank filters. A 1 mmol L -• stock solution of Na2S was prepared in 0.1 M NaOH. The crystalline Na2S was preweighed and stored under nitrogen in sealed glass ampules. The stock solution was diluted (1:50) to make a working standard. The standard solutions were prepared in glassware that had been conditioned and stored containing sulfide solutions. Aqueous sulfide standards prepared using this technique have been verified using thiosulfate titration during previous studies. The calibration has also been verified against a NIST gas standard during the recent NASA/GTE/CITE 3 experiment [Gregory et al., 1993]. The absolute accuracy of the calibration is estimated to be _+ 10%.
The lower limit of detection for this method is determined from the variability of the blanks within each batch of filters. For this analysis, one batch of filters contained approximately 30 filters that were impregnated and dried at the same time. During this cruise the average variability within any given batch was 13 -+ 15 pmol (ltr, n = 40 batches). This gives a detection limit of 0.2 -+ 0.25 ppt for each filter (150-min sampling time) or 0.4 _+ 0.5 ppt per sample as each sample involves measuring the sulfide found on two filters, a front filter and a backup filter.
Backup filters were used for each sample. Laboratory and field tests have shown that a single silver nitrate filter is >98% efficient for H2S removal [Cooper, 1986]. However, it has been shown that an artifact sulfide is formed on the filters during prolonged sampling periods. This artifact is thought to be caused from the hydrolysis of carbonyl sulfide on the filters and is assumed to generate equal sulfide concentrations on both the front and the back filters [Cooper and Saltzman, 1987]. Therefore the measured sulfide is corrected by subtracting from the front filter the amount of sulfide found on the backup filter. Cooper and Saltzman [1987] reported that a diurnal cycle was seen for the artifact sulfide due to a temperature effect with a maximum reported at noon when the filters became the warmest. During this cruise the filter holders were wrapped in a reflective white tape in an attempt to reduce the effect of the Sun on the temperature inside the holder.
In order to prevent contamination from the ship stack and ventilation ports, samples were collected on the bow of the ship. Sampling was done only while the ship was underway with the relative wind within 90 ø of the bow. The Teflon PFA filter holders were sealed with Teflon TFE plugs when not in use. The washing solution was stored in and delivered from a glass repipet and the extraction was done in sealed PFA vials in order to minimize contact of air with the reagents.
Each air sample was collected and analyzed in duplicate. Samples were presumed to be contaminated and were discarded if the front or back duplicate samples did not agree to within 25%. The results reported here are the averages of each set of uncontaminated duplicate samples. When only one of the samples in a set of duplicates exhibited signs of contamination, for example a high backup filter value, the remaining sample value is the one reported here. The average difference observed between duplicate samples was 0.40 ppt with a standard deviation of 0.28 ppt (n = 39 sets of duplicates). We consider this a good measure of the precision of the method under field conditions. The exposed filters were stored in the Teflon PFA filter holders (sealed with TFE end plugs) or in Teflon PFA vials until the analysis was performed. Storage time ranged from 10 min to 24 hours. Tests using duplicate samples showed that storage of the samples had no effect on the analysis.

RESULTS
The atmospheric concentrations of H2S measured during the SAGA 3 cruise are shown in Figure 2a. The concentrations ranged from below the detection limit (0.4 ppt) to 14.4 ppt with a mean of 3.6 ___ 2.3 ppt (1 tr, n = 72). Air mass trajectories for the cruise showed that the air masses encountered had been over the ocean for at least 7 days prior to sampling [Johnson et al., this issue]. The average concentration of H2S found in this study is lower than those previously reported for North Atlantic marine air. Slatt et al. [ 1978] found concentrations ranging from 5 to 50 ppt over the North Atlantic. Delmas and Servant [1982] reported an average concentration of 14 ppt for the West African coastal upwelling region. Herrmann and Jaeschke [1984] observed concentrations ranging from 11 to 118 ppt. Saltzman and Cooper [1988] reported an average concentration of 8.5 --- 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  surface nitrate concentrations. Although indirect, these observations suggest that there may be a relationship between upwelling and H2S emissions in this region. However, H2S concentrations were generally lower and more constant at approximately 2.3 ppt for the remaining three crossings of the equator, although there was evidence for further upwelling on these transects as well. Therefore the dependence of H2S emissions on upwelling may not be a simple one, and further work is needed to clarify this relationship. this issue] and a 94% production of SO2 from the oxidation ofDMS (k = 6.3 x 10 -12 cm 3 molecule -1 s -l' Hynes et al. [1986]). This result indicates that the H2S oxidation contributes < 1% of the SO2 produced from the oxidation of reduced sulfur gases in remote marine air. This is lower than the proportion reported earlier by Saltzman and Cooper [1988] who found that the oxidation of DMS and H2S contribute 87 and 11%, respectively, of the non-sea-salt sulfate over the tropical Atlantic.  The average atmospheric H2S levels measured in that study were higher (7 -+ 4 ppt) than the SAGA 3 measurements (3.6 _+ 2.3 ppt) and they observed a positive correlation between H2S and 222Rn. This would indicate that the oceans can act as a sink for air masses strongly influenced by continental sources and a source in remote regions.

SUMMARY AND CONCLUSIONS
Atmospheric H2S concentrations over the equatorial Pacific ranged from below the detection limit to 14.4 ppt with an average value of 3.6 _+ 2.3 ppt (1 tr, n = 72). The highest concentrations were found on the easternmost two transects just south of the equator. The average concentration of 3.6 ppt is the lowest reported value for background atmospheric H2S over the tropical ocean.
Model calculations indicate that the atmospheric production of SO2 and sulfate from H2S oxidation is minor (<1%) compared to that from DMS in this region. Diurnal variations are not observed in the data. They are probably obscured by spatial or temporal variability in the source.
The oceans appear to be a source of H2S to the atmosphere over the equatorial Pacific. A sea-to-air flux of 9 to 21 x 10 -8 mol m -3 d -1 is needed to balance the atmospheric oxidation. A surface ocean free sulfide concentration of 32 to 67 pmol L -1 is required to support this flux.