Environ. Sci. Technol. 1997, 31, 3669-3676
Oil Field Hydrogen Sulfide in Texas: Emission Estimates and Fate GARY A. TARVER AND PURNENDU K. DASGUPTA* Department of Chemistry and Biochemistry, Texas Tech University, Lubbock, Texas 79409-1061
Hydrogen sulfide is released into the atmosphere during oil recovery operations. However, little is quantitatively known concerning the total sulfur flux due to these fugitive emissions. A mobile laboratory, equipped to measure atmospheric gases and meteorological parameters, was used to collect quantitative data in the vicinity of various oil field operations. Fugitive emissions of H2S and soil sulfate levels were studied. Concentrations of atmospheric sulfur gases (SO2, H2S, mercaptans) were measured in the oilproducing regions of several west Texas counties. Hydrogen sulfide was by far the dominant reduced sulfur gas in all locations. Except in the vicinity of refining/processing plants, mercaptan levels were below 200 pptv. Sulfur dioxide levels were also very low, typically below 200 pptv. In all locations, a strong diurnal pattern of the ambient H2S concentration was observed. Specific sources, the flux, and the fate of the H2S emitted in oil field operations was studied. Crude oil storage tanks were found to be the major sources of sulfur gas emissions. Soil sulfate levels downwind from oil storage tanks were 20-200 times higher than that in control regions of similar geology where no oil is produced. Even within an oil-producing area, soil sulfate levels immediately downwind from storage tank vents exceeded the corresponding levels of sulfate in soil located upwind by a factor of g100.
Introduction Terrestrial biogeochemical cycles of carbon, nitrogen, and sulfur are believed to have been significantly affected by anthropogenic activities. The flux of components associated with each of these cycles has increased significantly over preindustrial periods (1, 2). In particular, the sulfur cycle has been the most perturbed; some estimates place the sulfur flux increase in excess of 100% (1, 3), the primary reason being the use of fossil fuels as energy sources. The flux of sulfur compounds from some anthropogenic activities is well quantitated. The combustion of fossil fuel and biomass is reported as the main source of the anthropogenic sulfur in the atmosphere, accounting for roughly 90% of the total. Much of the gaseous sulfur (ca. 95%) released from this source is SO2 (1, 3). Only limited attention has been paid to other sources of anthropogenic sulfur. In much of the oil- and gas-producing regions of the world, residents are well acquainted the smell of reduced sulfur gases from fugitive emissions during their production in the field. Because emission estimates are seldom based on direct measurement (4, 5), reliable estimates of the sulfur flux from these sources would be of significant interest in many regions, including the State of Texas, a major producer of oil and * Corresponding author fax: 806-742-3067; e-mail: veppd@ttacs. ttu.edu.
S0013-936X(97)00406-9 CCC: $14.00
1997 American Chemical Society
natural gas. An extensive search of available databases indicate, however, that no such data are available for Texas or, for that matter, any other part of the continental United States. We undertook therefore a study to ascertain the magnitude of fugitive emissions associated with oil and gas recovery operations in selected west Texas counties. The measurement of the emitted gaseous flux from a point source requires multipoint measurements of the concentration of the gas and that of the meteorological vectors, which can then be correlated via a plume dispersion model. The most expedient method to acquire atmospheric trace gas concentration and meteorological data in remote locations is via a mobile platform outfitted with the appropriate instruments. A mobile laboratory outfitted with meteorological instrumentation and a sensitive instrument capable of near real time detection of reduced sulfur gases (6) was used for field measurements. The results of such measurements and the conclusions therefrom constitute the subject of this paper.
Experimental Section General. All gas flow rates were controlled by mass flow controllers (FC-280, Tylan Corp., Torrance, CA) operated between 10% and 100% of their rated range. Sample gases for calibration were generated with permeation devices (VICI Metrics, Santa Clara, CA), which were thermostated at 30 °C. Mercaptans, SO2, and H2S were removed from cylinder or ambient air by passage through soda-lime packed columns. Water used for the preparation of all aqueous solutions and for the extraction of soil samples was distilled and deionized and met or exceeded ASTM type 2 reagent water standards. All raw data (meteorological conditions and chemical concentrations) were collected on personal computer (PC) based data acquisition systems. The processed data were tabulated, averaged, and compiled using a spreadsheet (Microsoft Excel, Redmond, WA). Ten minute running averages of the meteorological and chemical data were used in all of the plume dispersion models. Instrumentation. Field studies of atmospheric sulfur concentrations at remote west Texas oil field sites were conducted using a mobile laboratory based on a 25-ft Fleetwood Southwind motorhome. Mercaptans and H2S were preconcentrated with a porous poly(vinylidene fluoride) membrane diffusion scrubber (DS, see ref 7) using a NaOHbased scrubber liquid. The collected gases were then liberated from the scrubber liquid by the on-line addition of phosphoric acid. Using a second membrane-based device, essentially a DS operated in reverse, the liberated gas was introduced into the injection loop of a gas chromatograph with a flame photometric detector (GC-FPD). The instrument and the mobile laboratory have been previously described (6). Sulfur dioxide and particulate sulfate were measured with a wetted parallel plate denuder-vapor condensation particle collection system coupled to an ion chromatograph (8). A common sampling inlet was used for all instruments, and the sampling port was located atop the mobile laboratory, at a height of 5 m above the ground level. Ambient air was brought in through a PTFE conduit that was thoroughly washed between each field campaign. Meteorological data were initially collected with by a Model PCW weather station (Digitar, Hayward CA) installed in a dedicated PC. Wind speed, direction, temperature, and barometric pressure were automatically collected at 2.5-min intervals. Subsequently, and for the bulk of this study, the wind vectors and the temperature data were acquired with a sonic anemometer (SWS-101/2K, Applied Technologies, Inc., Boulder, CO) at a rate of 100 Hz and then integrated over 1
VOL. 31, NO. 12, 1997 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
3669
s. The anemometer sensors were also deployed 5 m above ground level via a telescoping mount at the rear of the mobile laboratory. An ion chromatograph (DX-100, Dionex Corporation, Sunnyvale, CA) was utilized to perform sulfate analysis on soil samples. Dionex columns AS5A-5µ and AG-5 were used as analytical and guard columns, respectively, with 30 mM NaOH eluent. A custom chemical suppressor (9) was used with dilute sulfuric acid as regenerant. All chromatographic data were collected on a PC via a Dionex ACI-450 data acquisition system. Photomediated Oxidation of Atmospheric H2S. To study potential photodecomposition/photooxidation of H2S under typical west Texas conditions, a large reaction chamber was formed from a 75 cm × 90 cm × 51 µm thick clear polyethylene bag. Spectral analysis (Hewlett Packard 8451A spectrophotometer) of the polyethylene material indicated g50% transmittance for all wavelengths from 250 to 800 nm. Taking into consideration that essentially no solar radiation below 300 nm reaches the earth’s surface (10), we judged the polyethylene material suitable for the construction of a reaction chamber for the present purposes. To maintain the chamber in an inflated form, an endoskeleton was formed from two springy metal hoops positioned inside the polyethylene bag. The hoops, 87 cm in diameter, were assembled from three 91-cm lengths of stainless steel welding rods coupled by Teflon tubing. The bag opening was hermetically sealed, and the two hoops were oriented to form a 50 cm × 65 cm × 25 cm semi-rigid reaction chamber. Except as stated, two inlet ports centered at one end of the chamber and two outlet ports centered at the opposite end were provided with PTFE conduits for external connections. A carrier flow of zero air at 40 mL min-1 was used to transfer H2S from a permeation device emitting at 80 ng min-1 into the chamber. Further sample dilution was accomplished with zero air or ambient air (which contained H2S at concentrations negligible relative to the test concentrations used in the chamber) that were brought in by aspiration. The reaction chamber, thus functioning as a flow-through reactor that could be optionally irradiated with sunlight, was sampled at rate of 5.00 L(STP) min-1 (approximately 6.00 L min-1 for the typical ambient conditions of 0.90 atm and 20 °C during these experiments). With a volume of ca. 80 L, the average residence time was ∼13 min under the experimental conditions. The H2S concentration of the chamber influent and effluent samples was measured with the DS-GC-FPD system. Soil as a Source or Sink of Atmospheric Sulfur Gases. A 38.0-L test chamber was fabricated from a rectangular container of low-density polyethylene (LDPE) with internal dimensions of 51.0 × 35.5 × 21.0 cm. The container was supplied with a tightly fitting LDPE lid. Inlet and outlet ports were connected with PTFE conduits. Experiments regarding the efficiency of the soil as a sink for H2S were conducted with the open end of the container tightly pressed against the earth, and more soil was placed around the rim and compressed to form a seal. Control experiments were performed with the container in the same orientation, but with the lid on such that there was no exposure to soil. The chamber contents were continuously sampled at a rate of 5.00 L(STP) min-1 from one end of the chamber with zero air and/or analyte introduced at the opposite end. To study the role of soil as a source, only zero air was introduced into chamber. To study the role of soil as a sink, H2S (157 ng min-1) was introduced into the chamber in a carrier stream of 40 standard cm3 zero air, with additional zero air introduced at a second inlet port for dilution. Sulfur Content of the Soil. The uptake of atmospheric H2S by soil was investigated by measuring the sulfate concentration of soil samples. Soil samples were obtained at regular intervals encircling the vent of a crude oil storage tank (vide infra). Surface samples were obtained by collecting
3670
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 31, NO. 12, 1997
FIGURE 1. Map of Texas showing the location of the oil reservoirs and the location of the five counties where the present study was conducted. the top 1.5 cm of surface soil into sealable polyethylene bags. Core samples were obtained by driving a 1-m length of 1.5 in. diameter electrical metallic tubing (EMT) to a 60 cm depth. After the tubing/core assembly was withdrawn, the ends were sealed with food-grade polyethylene wrap, and the core/ tubing aggregate was maintained in a vertical position to preclude mixing of loose core portions during processing. The EMT was severed at 10-cm intervals, providing access to discrete soil samples. All soil samples were dried for 8 h at 130 °C. Typically a 10-100-g portion was taken for analysis. This aliquot was ultrasonically extracted with water for an extended period and filtered through a 0.2-µm syringe-tip membrane filter, and the extracted sulfate was determined by suppressed conductometric ion chromatography. In some cases, initial results called for repeat analysis after sample dilution. All results represent mean values of at least triplicate determinations; the instrument was calibrated at the beginning and end of each day with aqueous standards containing Cl-, NO3-, and SO42-.
Results and Discussion Oil and Gas Production in Texas. The Railroad Commission of Texas (RCT) has the oversight and regulatory responsibility for the oil and natural gas industry in Texas. The map in Figure 1 delineates the Texas Railroad Commission Districts 8 and 8A, which constitute the oil fields of west Texas (11). West Texas Intermediate Crude (WTIC) oil is a benchmark for sweet crude oil and is the reference standard for the pricing of crude oil worldwide. The terms sour and sweet in the oil and gas industry refer to the sulfur content of the oil/gas. Although WTIC is may not be significantly sour on a worldwide standard, intense sulfurous smells are apparent even to a casual traveler passing by a west Texas oil field. The shaded areas in Figure 1 indicate the underground oil reservoirs of the San Andres, Fusselman, Edwards, and Smackover formations (RCT, ref 12) that are associated with the major areas of sour gas production in Texas. Over 50% of the statewide oil production comes from districts 8 and 8A. In 1991, for example, districts 8 and 8A accounted for 342 716 116 of the state total 644 514 016 barrels (ca. 109 m3) recovered (11). Our experimental studies were carried out in Cochran, Garza, Hockley, Lubbock, and Terry counties, also indicated in outline in Figure 1. We cannot claim that the data presented
here are representative of the region on statistically defensible grounds. Many of the production sites are either not easily accessible or permission to access is not easily obtained. Sources of Fugitive Emissions. Initial survey work showed that H2S was the only S(-II) compound present in quantifiable concentrations (>200 pptv). Some lower mercaptans were detectable in the vicinity of oil refineries and natural gas processing plants. However, in many of these operations, natural gas is sweetened (usually by showering with concentrated NaOH solutions), and then a selected blend of mercaptans (the specific blend is generally unique to a given plant, mercaptans have a much lower odor threshold than H2S) is added in a small quantity for the facile detection of inadvertent leaks. Concentration of SO2 measured at these locations also show very low levels, typically well below 100 pptv. Attention was therefore primarily focused on the measurement of H2S. Emissions from oil/gas processing sweetening plants are subject to more stringent regulations than are oil field operations. Furthermore, even though relatively large concentrations of sulfur gases were found near these plants, the number of such plants are minuscule relative to the number of oil field production sites. Consideration of the overall emission per unit volume of oil/gas processed indicate that, on this basis, the emission is much less for such plants than for the production sites. We therefore concentrated on the oil field emissions. A careful search for H2S sources centered on measuring the ambient H2S concentration near specific oil field equipment, essentially using our instrument as a sniffer. In oil production facilities, the oil is either pumped directly to a pipeline or, far more commonly in rural Texas, pumped into a storage tank and periodically removed by tanker trucks. Of the oil field equipment, the pump jacks, recovery injection wells, and natural gas pumping stations showed no significant H2S leakage. In contrast, storage tank farms were found to be significant sources of H2S. Tank vent emissions were then investigated in detail. Hydrogen sulfide is naturally present in the crude oil and natural gas. In addition, many west Texas oil fields are subjected to water injection for enhancing oil recovery. Introduction of water into the oil-bearing strata further promotes the souring of reservoirs by enhanced growth of microorganisms that produce H2S. Sulfate-reducing bacteria are able to grow in a wide temperature range (10-80 °C); those that survive even higher temperatures (121 °C) have also been identified. Even though the organisms require anaerobic conditions for growth, they are able to remain viable for extended periods in the presence of oxygen such that the introduction of oxygenated water into the reservoir does not significantly disrupt the process of a well turning sour. The water injected into the oil well dissolves and suspends various materials in the reservoir, including hydrogen sulfide, creating a mixture of oil and brackish water in the reservoir that is subsequently pumped to the surface. The recovered mixture is routed to storage tanks where the water and oil separate, permitting re-injection of the recovered water. Re-injection of reclaimed water contaminated with thermophilic sulfatereducing bacteria into the well serves to reinforce the cycle of souring (13). The tanks where the oil/water mixtures are stored typically operate at or near ambient atmospheric pressure with the tank contents vented directly into the atmosphere. The arrangement is such that as a storage tank fills, the vapors accumulated above the rising liquid are exhausted into the atmosphere. The volume of head space vapor exhausted is thus proportional to the quantity of oil and water pumped. Vent emissions are enhanced by the fact that continued heavy pumping in many oil fields has significantly reduced the available reserves, resulting in a pump overcapacity. As a result, oil can be removed from a well much faster than it can
refill the bore. The pump jacks are therefore operated intermittently to allow recovery time for the oil to flow from the oil-bearing strata into the well bores. During the quiescent period, the liquid and the vapor in tank equilibrates, and very high concentrations of H2S in the tank vapor space are reached. Operators are required to measure the H2S concentration in the tank vapor; measurement by colorimetric tubes (14) is recommended (12) and is widely used. For WTIC oil, such measurements indicate that the H2S concentration in the headspace typically reaches between 14 and 18% v/v. As a tank fills, large quantities of H2S escape through the tank vent, the total fugitive emissions thus being directly related to the amount of oil recovered. Especially at night, when solar heating is absent, ground-level emissions of H2S, a heavier than air gas, are not readily elevated aloft and the released H2S fumigates the surrounding areas. Human fatalities due to H2S inhalation in west Texas oil fields are hardly unknown. Point Source Release of H2S. Measurements and Model. Gaussian plume models such as the Pasquill-Gifford-Turner (PGT) model (15) are based upon observations of nonbuoyant, ground-level releases, over flat terrain and at distances less than or equal to about 1 km. With its flat treeless terrain, west Texas is well suited for the study of atmospheric dispersion of oil-related H2S emissions, most of which occur at heights below 20 m. Undiluted H2S is heavier than air and will collect in low lying areas on very calm nights. However, it is quickly dispersed by the relatively constant westerly winds and does not appear to be negatively buoyant at low concentrations. A plume formed by a steady-state release of analyte into a uniform windfield has been modeled (16). Analyte concentration C at a point x,y,z downwind from a point source is given by the generalized Pasquill-Gifford plume equation as
C(x,y,z;H) )
[ ( ) ]{ [ ( ) ] [ ( ) ]}
Q 1 y exp 2πσ,σzu 2 σy
2
exp -
exp -
1 z-H 2 σz
1 z+H 2 σz
2
2
+ (1)
where C is the analyte concentration ratio (mass/volume), x is the downwind distance from source, y is the lateral (crosswind) distance from the plume centerline, z is the vertical height of sample collection, H is the release height, Q is the point source emission rate (mass per second), σy is the horizontal crosswind dispersion coefficient, σz is the vertical dispersion coefficient, and u is the average wind velocity. Standard deviations of analyte distribution in the crosswind and vertical directions are given by σy and σz, respectively, and were introduced by Gifford (17) to provide estimates of analyte dispersion when meteorological information is limited. The values of σy and σz can be calculated from power law approximations given by Lees (18). For an analyte concentration measured along a plume centerline at ground level, the equation reduces to
C(x,0,0,H) )
[ ( )]
Q 1 H exp πσyσzu 2 σz
2
(2)
The PGT model is, however, based on some assumptions that are not applicable to the dispersion of released H2S in west Texas. The calcareous soil in this region, for example, is significantly alkaline and cannot possibly be a perfect reflector of an H2S plume, as assumed in the model. In addition, H2S may be photooxidized during transport. A previous study by Rege and Tock (19), using controlled lowlevel release of H2S from a point source over west Texas terrain, had shown that the PGT predictions are many times larger
VOL. 31, NO. 12, 1997 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
3671
FIGURE 2. Comparison of experimental H2S dispersion data with original and modified PGT model predictions for controlled release experiments. than what is observed. These investigators modified the PGT model by adjusting the dispersion coefficients σy and σz as suggested by Turner (15); good agreements with experimental data were observed. In the present work, the applicability of the PGT model for use with H2S release was also tested by releasing pure (99%) cylinder H2S from a tank at a flow rate of 40-60 mL min-1 with a mass flow controller from a height of 3 m. Air samples were obtained at various positions downwind, and meteorological conditions at the release site were recorded to allow calculation of the dispersion coefficients. Ten minute running averages of wind speed, wind direction, and H2S release rate were used. Equation 1 was used to compute the theoretical downwind concentration of the controlled release. In agreement with Rege and Tock (19), the unmodified PGT model was found to greatly overpredict H2S concentrations in the downwind plume. Figure 2 shows a typical data set for the controlled release of hydrogen sulfide with the measured and PGT-predicted concentrations shown respectively as circles and diamonds. With the exception of the data in the 20-m range when a severe crosswind existed, the predicted values are much higher. The cited approach (19) resulted in the best fit coefficients:
σy ) 0.053x1.37
(3)
σz ) 0.130x1.70
(4)
Figure 2 shows the predictions of the corrected model as triangles and the excellent correlation between the measured versus the predicted H2S concentrations. This modified model was used in the following. Measurement of Emissions from a Tank Battery. The emission from several tank batteries were studied; the selection criteria was based on a combination of site accessibility and isolation from other sources. The case for one tank battery is presented below. The particular site described below, located in the Slaughter field of Cochran County (33°28′ N latitude, 102°34′ W longitude), is typical of others studied during a 3-year research program. This site contained a battery of four storage tanks connected to a common vent pipe with an exhaust point 7 m above ground. The vent is not located atop the tanks but is ducted away from the tanks and exhausts from a 6.3-cm diameter port 15
3672
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 31, NO. 12, 1997
FIGURE 3. Measured H2S concentrations (left ordinate) near a tank battery in Cochran County (right ordinate) and the sulfur flux computed from this and simultaneously measured wind vector, using the modified PGT model. m from the nearest tank (typically crosswind relative to the tanks). The site meets the criteria of flux assessment by the PGT model; no major equipment/terrain obstacles near to or downwind from the tank vent were present. H2S concentrations at various distances downwind of the tank vent (during the time tank was being filled) along with blank background levels upwind from the vent were measured. The results of the atmospheric measurements are presented in Figure 3 where the measured ambient concentrations of hydrogen sulfide (circles) and the calculated sulfur flux (triangles) from the vent are presented. The estimated flux was obtained using the modified dispersion coefficients presented in eqs 3 and 4 and are presented in Figure 3; few points that were taken during large crosswind conditions were omitted. The source strength does vary with time as changes in the wind vector causes differing amounts of pressure at the vent point. The integrated average sulfur flux over the period shown is ∼3.1 × 106 g of S/yr. Although there is obviously a great deal of variation in the instantaneous data, similar mean values have been obtained for this tank battery on a number of other measurements. Oil field operators are required to submit forms containing various field details regularly, including the quantity of oil produced. The data from these forms are summarized and reported annually by the Oil and Gas Division of the Railroad Commission (11). Although data for the 2-year period during which this tank farm was intensively studied was not available, the oil production rate from this site has been stable and in the immediately previous year was 69 858 barrels/yr or 1.11 × 105 m3/yr. The concentration of H2S in the tank vapor at this location was 17.7% (v/v). If the pumped oil displaced an equivalent volume of vapor in equilibrium, this would indicate an atmospheric emission of 2.6 × 106 g of S/yr. The fact that this does not take into account the amount of water injected indicates that the agreement with the average experimental value is quite reasonable. More importantly, it suggests that reasonable estimates of the H2S emission rates from such sources can be obtained from values that are readily available, namely, the annual oil production rate and the H2S concentration in headspace in equilibrium with the oil (and re-emergent water) from the particular reservoir. Fate of Released H2S. Diurnal Pattern of H2S Concentration. Several studies were made of atmospheric H2S concentrations in the general area of oil-producing facilities,
FIGURE 4. Typical diurnal variation in H2S concentration. without locating the detection equipment near any specific source. At all locations, H2S concentrations consistently exhibited a strong diurnal pattern, with nighttime maxima in the range of 1-5 ppbv followed by rapid abatement at sunrise. By 10-11 AM, H2S levels fell below the instrument detection limit of 200 pptv. Figure 4 demonstrates a typical diurnal pattern observed in our studies. There are some fluctuations that are caused by shifts in direction and magnitude of the wind, but the overall pattern is evident. A number of investigators have reported on diurnal patterns of H2S. While Jaeschke et al. (20) clearly observe a diurnal pattern and ascribe the daytime removal to oxidation by HO•, Servant and Delpart (21) are far less certain that there is any clear diurnal pattern and ascribe concentration variations to fluctuations in source strengths. More recently, Bartell et al. (22) have looked at biogenic COS and H2S emissions and found that a small portion of the COS deposition to the soil was being re-emitted as H2S. Their data showed no clear diurnal H2S profile. We do not have any information to suggest that H2S emissions from the oil fields increase during the night, nor is there any logical reason to believe so. The reason for the presently observed diurnal profile is therefore not attributable to variations in source strength. Photomediated Oxidation. One obvious reason for the observed diurnal pattern is that H2S is efficiently removed by photomediated oxidation during the daylight hours. The importance of such reactions were studied by introducing H2S and pure air into the photochemical reactor (tR, 13 min). After initial stabilization, the output concentration remained stable and experimentally indistinguishable from the input concentration (16 µg/m3) whether the reactor was maintained in the dark (with an opaque cover) or exposed to direct midday sunlight. The opaque/sunlight irradiated experiment was repeated with ambient air for an uninterrupted 24-h period. Again, no significant difference between input and output concentrations was noticeable. Regarding homogeneous gas phase oxidation, the secondorder reaction rate constants of H2S with photochemical oxidants such as O3, HO•, and HO2• at 298 K are respectively 2 × 10-20, 5.2 × 10-12, and 5 × 10-12 cm3 molecule-1 s-1, respectively (23). The typical concentration of ozone during our experiments was 20-30 ppbv; maximum excursions of 100 ppbv have been observed. Even at 100 ppbv (ca. 2.5 × 1012 molecules/cm3 at 298 K), the half-life of H2S from the ozone-mediated oxidation process will be many months. Rate data are also available for reaction of H2S with atomic oxygen, but the ground-level concentration of the latter is so low that this reaction is not likely to represent a major removal process either. Overall, the removal process by reaction with HO• and HO2• may be the most efficient among these. Recent measurements in summertime rural Ontario (24) suggest that the maximum excursions of ROx (RO2• + HO2• + RO• + HO•) occur during midafternoon on sunlit days to values under 25
pptv. The peak concentration is perhaps 25% as high during a cloudy day. At further southern latitudes where insolation intensity is higher and photooxidation of emission products from vegetation contributes to peroxy radical production, RO2 concentration is an order of magnitude higher (25). At both locations, the radical concentration begins to increase from essentially a zero value to measurable concentrations shortly after sunrise. Based on the range of peroxy radical concentrations at different locations given in ref 26, the exact concentration in west Texas is likely to be between the two results above. Even at 10 pptv radical concentration, with an overall rate constant equal to that for HO2•, the half-life of H2S will be under 10 min. This reaction therefore indeed provides a plausible removal mechanism. However, even if H2S is dominantly oxidized by HO• and/or HO2•, we have not observed SO2 as a significant primary product. As previously noted, the SO2 concentrations in these regions tend to be consistently lowscertainly there is no increase in SO2 concentration with the onset of daylight hours concomitant to the disappearance of the H2S. This is bothersome. In an altogether different region, Spedding and Cope (27) have also observed that the (oxidative) disappearance of low H2S concentrations (
