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Simulations of the atmospheric dispersion of methane emissions were created for a region containing 26 oil and gas production sites in the Permian Basin in Texas. Virtual methane sensors were placed at 24 of the 26 sites, with at most 1 sensor per site. Continuous and intermittent emissions from each of the 26 oil and gas production sites, over 4 week-long meteorological episodes, representative of winter, spring, summer, and fall meteorology, were simulated. The trade-offs between numbers of sensors and precision of sensors required to reliably detect methane emissions of 1 to 10 kg/h were characterized. A total of 15 sensors, able to detect concentration enhancements of 1 ppm, were capable of identifying emissions at all 26 sites in all 4 week-long meteorological episodes, if emissions were continuous at a rate of 10 kg/h. More sensors or sensors with lower detection thresholds were required if emissions were intermittent or if emission rates were lower. The sensitivity of the required number of sensors to site densities in the region, emission dispersion calculation approaches, meteorological conditions, intermittency of the emissions, and emission rates, were examined. The results consistently indicated that, for the conditions in the Permian Basin, a fixed monitoring network with approximately one continuous monitor per site is likely to be capable of consistently detecting site-level methane emissions in the range of 5–10 kg/h. [ABSTRACT FROM AUTHOR]

Four solar-powered methane sensing systems with remote communication capabilities were tested for nine months at an oil and gas production site in west Texas. Sensor performance was evaluated using single blind certified gas challenges and by comparison with a continuously operated quantum cascade tunable infrared laser differential absorption spectrometer (QC-TILDAS) system. Dispersion modelling was used to estimate concentrations that would need to be detected to identify continuous and intermittent emission rates of 5-10 kg/hr from oil and gas production sites within ~50-100 m of the sensors, and these concentration thresholds were used in establishing performance criteria for the sensors. The four sensors demonstrated sufficient precision to allow for detection of emission rates of 5-10 kg/hr and had data capture rates that exceeded 80 % during the 9 months of operation. One sensor had a 100 % data capture rate, despite severe weather conditions and extended local electrical power losses. These results demonstrate that multiple commercially available sensing systems are suitable for long term methane emission monitoring in remote oil and gas production regions. [ABSTRACT FROM AUTHOR]

The Gulf Coast Aerosol Research and Characterization Study ([GC-ARCH], also known as the Houston Fine Particulate Matter [PM] Supersite) examined the spatial and temporal variability in fine PM source contributions and composition and the physical and chemical processes that govern PM formation and transformation in southeastern Texas. This was accomplished through the analysis of data collected in a 16-month field sampling program (August 2000 through November 2001). Three core sites and ∼15 peripheral sites, jointly operated by the study team and the Texas Commission on Environmental Quality (TCEQ), were used. Key scientific findings related to spatial and temporal variability in fine PM concentrations, sizes and composition of the fine PM, the strength of primary emission sources and causes of secondary fine PM formation are reported. [ABSTRACT FROM AUTHOR]

Recent evidence has demonstrated that chlorine radical chemistry can enhance tropospheric hydrocarbon oxidation and has the potential to enhance ozone formation in urban atmospheres. To assess these effects quantitatively, an August-September 2000 photochemical episode in southeast Texas was simulated using the comprehensive air quality model, with extensions (CAMx). During this episode, ambient measurements of a unique marker of atmospheric chlorine chemistry, 1-chloro-3-methyl-3butene-2-one (CMBO), were made and model performance was assessed by comparing modeled and observed CMBO mixing ratios. The model predicted ambient CMBO mixing ratios within the uncertainty limits associated with the emissions inventory, so the model was used to assess the impacts of chlorine chemistry on ozone formation. Based on the current emissions inventory, chlorine emissions have the potential to enhance 1-h-averaged ozone mixing ratios by 70 ppb, in very localized areas, during morning hours. Over wider areas, and at times of day when peak ozone concentrations are observed, the impacts of chlorine emissions on ozone concentrations are typically less than 10 ppb. Chlorine emissions also influenced changes in ozone concentrations due to hydrocarbon and NOx emission controls. [ABSTRACT FROM AUTHOR]

Seasonal and spatial trends in organic carbon (OC) concentrations in fine particulate matter (PM) were examined using data collected through the regulatory fine PM monitoring network in southeast Texas, and data collected during the Gulf Coast Aerosol Research and Characterization Study (GC-ARCH or Houston Supersite). Primary OC concentrations in the aerosol were estimated by establishing a linear relationship between primary OC and elemental carbon (EC) from 24-hour integrated samples. The relationship between primary OC and EC varied by site and season. Secondary OC (SOC) was estimated as the difference between total OC and primary OC and was found to be between 0.65±1.11 and 1.15±1.52 μg m-3 among sites (mass of carbon only), averaged over 2 years. The mean fraction of SOC in PM2.5 was found to be between 0.05±0.06 and 0.10±0.33. Primary OC concentrations were on average higher than SOC concentrations at all locations, and were between 1.50±0.69 and 2.89±1.03 μg m-3 (mass of carbon only). Both primary and SOC have highest monthly mean concentrations in early fall through late winter throughout the region. Spatially, sites that are closer to urban emissions have, on average, higher primary and SOC concentrations. One exception to this trend is a remote site located in a forested area north of Houston, where primary and SOC tend to be as high as at urban/industrial locations. [Copyright &y& Elsevier]

Daily, seasonal, and spatial trends in fine particulate matter concentrations, compositions, and size distributions were examined using data collected through the regulatory fine particulate matter monitoring network in Southeast Texas and during the Gulf Coast Aerosol Research and Characterization Study (GC-ARCH or Houston Supersite). PM 2.5 mass concentrations and compositions are generally spatially homogeneous throughout Southeast Texas when averaged over annual or seasonal time periods. There is relatively little seasonality to mean total PM 2.5 mass and mean PM 2.5 composition throughout the region, although slightly higher concentrations of total mass tend to occur in the spring and late fall. High FRM PM 2.5 mass (>20 μg/m 3 ) occurs both when there is high spatial variability among sites and low spatial variability among sites. This suggests that both local and regional emission sources contribute to PM 2.5 in Southeast Texas. Sulfate ion (32%), organic carbon (30%), and ammonium ion (9%) are the largest components on average of PM 2.5 by mass. Mean diurnal patterns for PM 2.5 mass concentrations throughout the region show a consistent morning peak and a weaker and slightly less consistent peak in the late afternoon to early evening. High hourly averaged PM 2.5 mass concentrations (>40 μg/m 3 ) tend to be associated with daily average PM 2.5 above the annual NAAQS of 15 μg/m 3 . These high hourly PM 2.5 concentrations also tend to occur on days with high diurnal variation, indicative of elevated, short-lived PM 2.5 events. In contrast to mass concentrations, particle size distributions are not spatially homogeneous throughout Southeast Texas. Industrial sites have higher concentrations of freshly emitted, primary particles than more residential sites. Because the freshly emitted particles generally have diameters of 0.1 μm or less, these primary emissions do not have as large an impact on PM 2.5 mass or bulk composition as they have on the number density of fine particles. [ABSTRACT FROM AUTHOR]

A modified version of the Global Biosphere Emissions and Interactions System (GloBEIS) was used to predict biogenic sesquiterpene emissions in the Houston-Galveston Area (HGA) in southeast Texas. The estimates were based on a land cover database containing more than 600 land cover categories at a resolution of approximately one kilometer and emission factors taken from literature. Average sesquiterpene emission fluxes were estimated to be between 0.07 and 0.65 kg carbon/km 2 -h. Fluxes at the lower end of the estimated range are consistent with observed concentrations of geologically modern carbon in Houston aerosol. [ABSTRACT FROM AUTHOR]

The size distributions of organonitrate functional groups in ambient Houston, TX aerosol were determined at 3 sites during an air quality field study conducted during August and September 2000. Samples were collected using a Hering low-pressure impactor and were analyzed, in transmission mode, using Fourier transform infrared spectroscopy. The size distributions of organonitrate groups generally fell into 4 categories. In approximately 25% of the samples, the majority of the organonitrate group mass was found in aerosol of approximately 0.1 μm aerodynamic diameter. In approximately 26% and 12% of the samples, the majority of the mass was found in aerosol of approximately 0.25 and 1 μm diameter, respectively; 21% of the samples displayed both 0.1 and 1 μm size modes. The remainder of the samples had relatively low organonitrate mass or other size distributions. Total organonitrate group absorbances, per m[sup 3] of air sampled, were generally similar to measurements from samples collected in Los Angeles in previous studies, however,some samples had organonitrate absorbances that were an order of magnitude more intense than observed at urban sites in Los Angeles. The events with high organonitrate concentrations were observed at source-dominated (industrial) sites and were not correlated with either high NO[sub x] or high ozone concentrations, suggesting that the high concentrations are due to primary sources. The high organonitrate concentrations were generally accompanied by evidence of high acidity and high organic concentrations. [ABSTRACT FROM AUTHOR]

Recent years have brought renewed attention to the effects of drought on emissions of biogenic volatile organic compounds. Variability in environmental inputs that influence isoprene and monoterpene emissions within eastern Texas was quantified by examining seasonal and interannual changes in activity factors intrinsic to the Model of Emissions of Gases and Aerosols from Nature (MEGAN) during years that included average-to-wet conditions (2007) and extreme drought and heat (2006 and 2011). Activity factors are used in MEGAN to multiplicatively adjust emissions rates from an assumed set of standard conditions for temperature, light, leaf area index (LAI), and soil moisture. Temperature was found to be the primary driver of seasonal and interannual variations of isoprene and monoterpene emissions; during drought years, reductions in LAI were dominated by predicted emissions increases caused by much warmer temperatures. The response of biogenic emissions to soil water stress is a major source of uncertainty. Dependent on the specific soil moisture database employed, predicted reductions in isoprene emissions ranged from minimal to −70% during the summer of 2011, a period characterized by all-time record drought in the South Central U.S. [ABSTRACT FROM AUTHOR]

Natural gas use in electricity generation in Texas was estimated, for gas prices ranging from $1.89 to $7.74 per MMBTU, using an optimal power flow model. Hourly estimates of electricity generation, for individual electricity generation units, from the model were used to estimate spatially resolved hourly emissions from electricity generation. Emissions from natural gas production activities in the Barnett Shale region were also estimated, with emissions scaled up or down to match demand in electricity generation as natural gas prices changed. As natural gas use increased, emissions decreased from electricity generation and increased from natural gas production. Overall, NOx and SO2 emissions decreased, while VOC emissions increased as natural gas use increased. To assess the effects of these changes in emissions on ozone and particulate matter concentrations, spatially and temporally resolved emissions were used in a month-long photochemical modeling episode. Over the month-long photochemical modeling episode, decreases in natural gas prices typical of those experienced from 2006 to 2012 led to net regional decreases in ozone (0.2-0.7 ppb) and fine particulate matter (PM) (0.1-0.7 μg/m3). Changes in PM were predominantly due to changes in regional PM sulfate formation. Changes in regional PM and ozone formation are primarily due to decreases in emissions from electricity generation. Increases in emissions from increased natural gas production were offset by decreasing emissions from electricity generation for all the scenarios considered. [ABSTRACT FROM AUTHOR]

Abstract: Alkenes are important in photochemical smog formation in southeast Texas due to their high emissions, especially from industrial sources in and around Houston, and their high reactivities. Therefore, properly characterizing the chemistry of alkenes in condensed mechanisms used in regional photochemical models is important in understanding the formation of ozone and other photochemical air pollutants in Houston. The performance of three versions of the SAPRC condensed chemical mechanism family, for predicting ozone and radical formation, was compared. Simulations were compared to environmental chamber data and ambient data. The analyses showed that separately modeling individual alkenes reactions (especially propene for southeast Texas) has the potential to lead to more accurate simulations of alkene chemistry. Caution must be exercised in un-lumping, however. Testing with different formulations of the 1-butene + O3 reaction demonstrated the complexity and interconnectedness in choices of stoichiometric parameters for un-lumped species and the extent to which lumped mechanisms are un-lumped. [ABSTRACT FROM AUTHOR]

Abstract: Spatial gradients of vehicular emitted air pollutants were measured in the vicinity of three roadways in the Austin, Texas area: (1) State Highway 71 (SH-71), a heavily traveled arterial highway dominated by passenger vehicles; (2) Interstate 35 (I-35), a limited access highway north of Austin in Georgetown; and (3) Farm to Market Road 973 (FM-973), a heavily traveled surface roadway with significant truck traffic. A mobile monitoring platform was used to characterize the gradients of CO and NO x concentrations with increased distance from each roadway, while concentrations of carbonyls in the gas-phase and fine particulate matter mass and composition were measured at stationary sites upwind and at one (I-35 and FM-973) or two (SH-71) downwind sites. Regardless of roadway type or wind direction, concentrations of carbon monoxide (CO), nitric oxide (NO), and oxides of nitrogen (NO x ) returned to background levels within a few hundred meters of the roadway. Under perpendicular wind conditions, CO, NO and NO x concentrations decreased exponentially with increasing distance perpendicular to the roadways. The decay rate for NO was more than a factor of two greater than for CO, and it comprised a larger fraction of NO x closer to the roadways than further downwind suggesting the potential significance of near roadway chemical processing as well as atmospheric dilution. Concentrations of most carbonyl species decreased with distance downwind of SH-71. However, concentrations of acetaldehyde and acrolein increased farther downwind of SH-71, suggesting chemical generation from the oxidation of primary vehicular emissions. The behavior of particle-bound organic species was complex and further investigation of the size-segregated chemical composition of particulate matter (PM) at increasing downwind distances from roadways is warranted. Fine particulate matter (PM2.5) mass concentrations, polycyclic aromatic hydrocarbons (PAHs), hopanes, and elemental carbon (EC) concentrations generally exhibited concentrations that decreased with distance downwind of SH-71. Concentrations of organic carbon (OC) increased from upwind concentrations immediately downwind of SH-71 and continued to increase further downwind from the roadway. This behavior may have primarily resulted from condensation of semi-volatile organic species emitted from vehicle sources with transport downwind of the roadway. [Copyright &y& Elsevier]

It is now generally accepted that industrial emission events are occurring from chemical processing facilities in Houston, Texas with daily frequencies and significant temporal variability. These events have been reported to last from hours to days, and a large fraction are made up of four highly reactive volatile organic compounds (HRVOCs): ethene, propene, butenes, and 1,3-butadiene. The Texas Commission on Environmental Quality (TCEQ) has targeted industrial sources of HRVOCs by imposing annual and hourly limits and creating a market-based HRVOC emissions cap and trade (HECT) program. The HECT program uses the Maximum Incremental Reactivity (MIR) scale to calculate the magnitude needed to trade between HRVOCs. The work reported here used the TCEQ regulatory model to evaluate the HECT program''s use of the MIR scale by simulating a series of hypothetical but observational-based industrial emission events for different VOC species. The magnitude of each release was adjusted based on the MIR scale under the assumption that this would give the same increase in ozone. The regulatory model, however, predicted that o-xylene caused the largest increase in ozone. In every simulation, ozone production was directly related to the amount of hydroxyl radicals produced from the photolysis of formaldehyde and other aldehydes. The sensitivity of ozone production to these hydroxyl radical sources appeared regardless of whether the industrial emission event plume encountered high sources of NO x . The MIR scale was developed for an average urban atmosphere and its failure in equating ozone reactivity here may be due to the extreme levels of NO x and VOC seen in event emissions in Houston. [Copyright &y& Elsevier]

The impacts of alternative regional development patterns on emissions, dry deposition, and air quality were examined using four visions of future land use in Austin, Texas associated with a doubling of the populalion in 20-40 years from 2001. Emissions and their spatial allocation were determined based on the development pattern and used to predict hourly ozone concentrations. Differences in hourly ozone concentrations due to changes in anthropogenic emissions between the future case scenarios and a 2007 base case ranged from -14 to 22 ppb and were primarily associated with the implementation of federal mobile source standards; differences due to biogenic emissions and dry deposition due to urbanization ranged from only -1.4 to 0.7 ppb. These differences in the magnitude of emissions produced greater changes in air quality than differences in regional development patterns between the four scenarios. Differences in hourly ozone concentrations between the future development scenarios and a 2007 base case ranged from -14 to 22 ppb, in contrast to differences of -3 to 5 ppb between the future scenarios. The results imply that although the effects of urbanization patterns are non-negligible, the pattern of urban development is not as significant as reductions in emissions per capita. [ABSTRACT FROM AUTHOR]

Ozone formation in the Houston area is more rapid and efficient than in many other urban areas; these features are due to the interaction of urban emissions with industrial plumes which are associated with both continuous and episodic industrial emissions. To examine the chemistry of interactions of the industrial plumes with urban emissions, a Lagrangian Process Analysis tool was embedded in a gridded photochemical model. The tool successfully isolated the plume so that, within the Process Analysis volume, the dominant process affecting ozone concentrations was chemical production. The analyses showed that the chemistry of the industrial plumes is dependent on emissions encountered downwind, and the extent of radical availability in the morning. For one episode, morning stagnation over the industrial region resulted in enhanced morning radical formation due to aldehyde photolysis. The enhanced morning reactivity led to high ozone concentrations as the plume interacted with urban emissions later in the day. In contrast, during a second episode, the dominant factor influencing ozone concentrations was high volatile organic carbon (VOC)/NO x ratios late in the afternoon, as the plume advected over wooded areas. For emission events, the main perturbation of the ozone formation chemistry occurred during the first few hours of the event due to enhanced production of free radicals from aldehyde photolysis, and more efficient utilization of free radicals due to increased reactive hydrocarbon concentrations. [Copyright &y& Elsevier]

High concentrations of ozone in the Houston/Galveston area are associated with industrial plumes of highly reactive hydrocarbons mixed with NO x . The emissions leading to these plumes can have significant temporal variability, and photochemical modeling indicates that the emissions variability can lead to increases and decreases of 10–50ppb, or more, in ozone concentrations. Therefore, in regions with extensive industrial emissions, accounting for emission variability can be important in accurately predicting peak ozone concentrations, and in assessing the effectiveness of emission-control strategies. This work compares the changes in ozone concentrations associated with two strategies for reducing flare emissions in Houston, Texas. One strategy eliminates the highest emission flow rates, that occur relatively infrequently, and a second strategy reduces emissions that occur at a nearly constant level. When emission variability is accounted for in air quality modeling, these control scenarios are both predicted to be much more effective in reducing the expected value of daily maximum ozone concentrations than if similar reductions in the mass of emissions are made and constant emissions are assumed. The change in the expected value of daily maximum ozone concentration per ton of emissions reduced, when emissions variability is accounted for, is 5–10 times the change predicted when constant (deterministic) inventories are used. Strategies that eliminate the infrequent largest emissions are more effective at reducing the highest localized ozone concentrations than changes in nearly constant emissions. [Copyright &y& Elsevier]

Ambient observations have indicated that high concentrations of ozone observed in the Houston/Galveston area are associated with plumes of highly reactive hydrocarbons, mixed with NO x , from industrial facilities. Ambient observations and industrial process data, such as mass flow rates for industrial flares, indicate that the VOCs associated with these industrial emissions can have significant temporal variability. To characterize the effect of this variability in emissions on ozone formation in Houston, data were collected on the temporal variability of industrial emissions or emission surrogates (e.g., mass flow rates to flares). The observed emissions variability was then used to construct regionwide emission inventories with variable industrial emissions, and the impacts of the variability on ozone formation were examined for two types of meteorological conditions, both of which lead to high ozone concentrations in Houston. The air quality simulations indicate that variability in industrial emissions has the potential to cause increases and decreases of 10–52ppb (13–316%), or more, in ozone concentration. The largest of these differences are restricted to regions of 10–20km2, but the variability also has the potential to increase regionwide maxima in ozone concentrations by up to 12ppb. [Copyright &y& Elsevier]

As part of the State Implementation Plan for attaining the National Ambient Air Quality Standard for ozone, the Texas Commission of Environmental Quality has created a Highly Reactive Volatile Organic Compounds (HRVOC) Emissions Cap and Trade Program for industrial point sources in the Houston/Galveston/Brazoria area. This series of papers examines the potential air quality impacts of this new emission trading program through photochemical modeling of potential trading scenarios; this paper examines the air quality impact of allowing facilities to trade chlorine emission reductions for HRVOC allocations on a reactivity weighted basis. The simulations indicate that trading of anthropogenic chlorine emission reductions for HRVOC allowances at a single facility or between facilities, in general, resulted in improvements in air quality. Decreases in peak 1-h averaged and 8-h averaged ozone concentrations associated with trading chlorine emissions for HRVOC allocations on a Maximum Incremental Reactivity (MIR) basis were up to 0.74 ppb (0.63%) and 0.56 ppb (0.61%), respectively. Air quality metrics based on population exposure decreased by up to 3.3% and 4.1% for 1-h and 8-h averaged concentrations. These changes are small compared to the maximum changes in ozone concentrations due to the VOC emissions from these sources (5–10 ppb for 8-h averages; up to 30 ppb for 1-h averages) and the chlorine emissions from the sources (5–10 ppb for maximum concentrations over wide areas and up to 70 ppb in localized areas). The simulations indicate that the inclusion of chlorine emissions in the trading program is likely to be beneficial to air quality and is unlikely to cause localized increases in ozone concentrations (‘hot spots’). [ABSTRACT FROM AUTHOR]

Abstract: The Houston/Galveston region has a large number of industrial point sources of volatile organic compounds (VOCs) and NO x , and observational data have shown that high concentrations of ozone are formed rapidly in the plumes of these industrial facilities. Some of the highly reactive VOC emissions from these facilities can be episodic, so in this work, photochemical air quality models, capable of simulating hundreds of possible types of emission events, have been developed and used to assess the impact of event magnitude, duration, timing and composition on ozone formation and accumulation. Simulations showed that ozone formation varied from nearly zero to up to 30ppb per 1000lb of release and that ozone formation depended strongly on the time of day of the release. The time of day that leads to maximum increases in ozone varies from day to day, depending on meteorology and NO x availability. On days and times that were conducive to ozone formation, the maximum additional ozone formation due to emission events was linearly related to emission magnitude, but the slope of that dependence varied based on the composition of the events. Overall, only a small percentage of emission events have the potential to lead to substantial ozone formation. Approximately 1.5% of emission events were projected to produce more than 10ppb of additional ozone, and 0.5% of emission events were projected to produce more than 70ppb of additional ozone, compared to base case simulations with no emission events. [Copyright &y& Elsevier]

Growth in unconventional oil and gas development (UOGD) in the United States has increased airborne emissions, raising environmental and human health concerns. To assess the potential impacts on air quality, we deployed instrumentation in Karnes City, Texas, a rural area in the middle of the Eagle Ford Shale. We measured several episodes of elevated Cl 2 levels, reaching maximum hourly averages of 800 ppt, the highest inland Cl 2 concentration reported to date. Concentrations peak during the day, suggesting a strong local source (given the short photolysis lifetime of Cl 2 ) and/or a photoinitiated production mechanism. Well preproduction activity near the measurement site is a plausible source of these high Cl 2 levels via direct emission and photoactive chemistry. ClNO 2 is also observed, but it peaks overnight, consistent with well-known nocturnal formation processes. Observations of organochlorines in the gas and particle phases reflect the contribution of chlorine chemistry to the formation of secondary pollutants in the area. Box modeling results suggest that the formation of ozone at this location is influenced by chlorine chemistry. These results suggest that UOGD can be an important source of reactive chlorine in the atmosphere, impacting radical budgets and the formation of secondary pollutants in these regions.

Emission rates were estimated for >100 oil and gas production sites with significant liquid-handling equipment (tank battery sites) in the Permian Basin of west Texas. Emission estimates based on equipment counts and emission factors, but not accounting for large uninventoried emission events, led to ensemble average emission rates of 1.8-3.6 kg/h per site. None of the site-specific emission estimates for individual sites, based on equipment counts, exceeded 10 kg/h. On-site drone-based emission measurements led to similar emission estimates for inventoried sources. Multiple aircraft measurement platforms were deployed and reported emissions exceeding 10 kg/h at 14-27% of the sites, and these high-emission rate sites accounted for 80-90% of total emissions for the ensemble of sites. The aircraft measurement systems were deployed asynchronously but within a 5 day period. At least half of the sites with emission rates above 10 kg/h detected by aircraft had emissions that did not persist at a level above 10 kg/h for repeat measurements, suggesting typical high-emission rate durations of a few days or less for many events. The two aircraft systems differed in their estimates of total emissions from the ensembles of sites sampled by more than a factor of 2; however, the normalized distributions of emissions for sites with emission rates of >10 kg/h were comparable for the two aircraft-based methods. The differences between the two aircraft-based platforms are attributed to a combination of factors; however, both aircraft-based emission measurement systems attribute a large fraction of emissions to sites with an emission rate of >10 kg/h.

Ozone formation due to episodic industrial emissions was modeled for the Houston-Galveston-Brazoria (HGB) region in Texas. A total of more than 1000 synthetic emission scenarios were modeled, accounting for different sources, emission magnitudes, emission compositions, duration and timing of the emission events, meteorology at the time of the emissions, and location of the emission source. Episodic emissions consistent with the characteristics of flaring resulted in the greatest amount of ozone formation. An index, based on incremental reactivity, for characterizing the ozone formation potential of flaring emissions was developed using the synthetic scenarios and was applied to the ~4500 flares that report emissions in the state of Texas. The flares with the highest ranking index in the HGB were identified and additional modeling was performed on the ozone formation potential of these flares. If these flares are modeled with their annual average emission rates, the maximum increase in maximum daily 8-h average ozone (MDA8) formation associated with flaring emissions is <1 ppb. If, however, it is assumed that emissions from flaring are episodic, rather than continuous, and temporal patterns consistent with previous data collection on flare emission variability in the HGB are applied, MDA8 enhancements associated with flaring emissions are >10 ppb for multiple scenarios. This analysis suggests that, for flares emissions that have high ozone formation potential, the temporal patterns of flare emissions should be accounted for in developing ozone mitigation plans., Competing Interests: Declaration of competing interest The authors declare the following competing financial interest(s): This work was supported by the 8-Hour Coalition, a group of companies operating facilities, including flares, in the Houston area. One of the authors (DTA) has served and is serving on the Environmental Protection Agency's Science Advisory Board; in this role, he is a paid Special Governmental Employee; DTA has current research support from the National Science Foundation, the Department of Energy, the Texas Commission on Environmental Quality, the Gas Technology Institute's Collaboratory to Advance Methane Science, the National Institute of Clean and Low Carbon Energy (NICE), ExxonMobil Upstream Research Company, Pioneer Natural Resources, and Environmental Defense Fund. He has also worked on air quality projects that have been supported by oil and gas producers and Environmental Defense Fund. DTA has done work as a consultant for multiple companies, including British Petroleum, Cheniere, Eastern Research Group, ExxonMobil, KeyLogic, and SLR International. Ling Huang has also served as a consultant, through Ramboll, to multiple companies., (Copyright © 2022 Elsevier B.V. All rights reserved.)

The methane emission intensity (methane emitted/gas produced or methane emitted/methane produced) of individual unconventional oil and gas production sites in the United States has a characteristic temporal behavior, exhibiting a brief period of decrease followed by a steady increase, with intensities after 10 years of production reaching levels that are 2-10 times the 10 year production-weighted average. Temporal patterns for methane emission intensity for entire production regions are more complex. Historical production data and facility data were used with a detailed basin-wide methane emission model to simulate the collective behavior of tens of thousands of wells and associated midstream facilities. For production regions with few to no new wells being brought to production, and existing wells having reached a mature stage, as in the Barnett Shale production region in north central Texas, the methane emission intensity gradually increases, as natural gas production decreases faster than emissions decrease, following the general pattern exhibited by individual wells. In production regions that are rapidly evolving, either with large numbers of new wells being put into production or with the introduction of source-specific regulations, the behavior is more complex. In the Eagle Ford Shale, which has had both a large number of new wells and the introduction of source-specific regulations, the methane emission intensity stays within relatively narrow bounds but the distribution of sources varies. As source distributions vary, basin-wide propane-to-methane and ethane-to-methane emission ratios vary, impacting methods used in source attribution.

Spatially resolved emission inventories were used with an atmospheric dispersion model to predict ambient concentrations of methane, ethane, and propane in the Eagle Ford oil and gas production region in south central Texas; predicted concentrations were compared to ground level observations. Using a base case inventory, predicted median propane/ethane concentration ratios were 106% higher (95% CI: 83% higher-226% higher) than observations, while median ethane/methane concentration ratios were 112% higher (95% CI: 17% higher-228% higher) than observations. Predicted median propane and ethane concentrations were factors of 6.9 (95% CI: 3-15.2) and 3.4 (95% CI: 1.4-9) larger than observations, respectively. Predicted median methane concentrations were 7% higher (95% CI: 39% lower-37% higher) than observations. These comparisons indicate that sources of emissions with high propane/ethane ratios (condensate tank flashing) were likely overestimated in the inventories. Because sources of propane and ethane emissions are also sources of methane emissions, the results also suggest that sources of emissions with low ethane/methane ratios (midstream sources) were underestimated. This analysis demonstrates the value of using multiple light alkanes in attributing sources of methane emissions and evaluating the performance of methane emission inventories for oil and natural gas production regions.

Unlabelled: Accurate estimates of biogenic emissions are required for air quality models that support the development of air quality management plans and attainment demonstrations. Land cover characterization is an essential driving input for most biogenic emissions models. This work contrasted the global Moderate Resolution Imaging Spectroradiometer (MODIS) land cover product against a regional land cover product developed for the Texas Commissions on Environmental Quality (TCEQ) over four climate regions in eastern Texas, where biogenic emissions comprise a large fraction of the total inventory of volatile organic compounds (VOCs) and land cover is highly diverse. The Model of Emissions of Gases and Aerosols from Nature (MEGAN) was utilized to investigate the influences of land cover characterization on modeled isoprene and monoterpene emissions through changes in the standard emission potential and emission activity factor, both separately and simultaneously. In Central Texas, forest coverage was significantly lower in the MODIS land cover product relative to the TCEQ data, which resulted in substantially lower estimates of isoprene and monoterpene emissions by as much as 90%. Differences in predicted isoprene and monoterpene emissions associated with variability in land cover characterization were primarily caused by differences in the standard emission potential, which is dependent on plant functional type. Photochemical modeling was conducted to investigate the effects of differences in estimated biogenic emissions associated with land cover characterization on predicted ozone concentrations using the Comprehensive Air Quality Model with Extensions (CAMx). Mean differences in maximum daily average 8-hour (MDA8) ozone concentrations were 2 to 6 ppb with maximum differences exceeding 20 ppb. Continued focus should be on reducing uncertainties in the representation of land cover through field validation., Implications: Uncertainties in the estimation of biogenic emissions associated with the characterization of land cover in global and regional data products were examined in eastern Texas. Misclassification between trees and low-growing vegetation in central Texas resulted in substantial differences in isoprene and monoterpene emission estimates and predicted ground-level ozone concentrations. Results from this study indicate the importance of land cover validation at regional scales.

The combined emissions and air quality impacts of electricity generation in the Texas grid and natural gas production in the Eagle Ford shale were estimated at various natural gas price points for the power sector. The increased use of natural gas in the power sector, in place of coal-fired power generation, drove reductions in average daily maximum 8 h ozone concentration of 0.6-1.3 ppb in northeastern Texas for a high ozone episode used in air quality planning. The associated increase in Eagle Ford upstream oil and gas production nitrogen oxide (NOx) emissions caused an estimated local increase, in south Texas, of 0.3-0.7 ppb in the same ozone metric. In addition, the potential ozone impacts of Eagle Ford emissions on nearby urban areas were estimated. On the basis of evidence from this work and a previous study on the Barnett shale, the combined ozone impact of increased natural gas development and use in the power sector is likely to vary regionally and must be analyzed on a case by case basis.

The impacts of alternative regional development patterns on emissions, dry deposition, and air quality were examined using four visions of future land use in Austin, Texas associated with a doubling of the population in 20-40 years from 2001. Emissions and their spatial allocation were determined based on the development pattern and used to predict hourly ozone concentrations. Differences in hourly ozone concentrations due to changes in anthropogenic emissions between the future case scenarios and a 2007 base case ranged from -14 to 22 ppb and were primarily associated with the implementation of federal mobile source standards; differences due to biogenic emissions and dry deposition due to urbanization ranged from only -1.4 to 0.7 ppb. These differences in the magnitude of emissions produced greater changes in air quality than differences in regional development patterns between the four scenarios. Differences in hourly ozone concentrations between the future development scenarios and a 2007 base case ranged from -14 to 22 ppb, in contrast to differences of -3 to 5 ppb between the future scenarios. The results imply that although the effects of urbanization patterns are non-negligible, the pattern of urban development is not as significant as reductions in emissions per capita.

Emission trading is a market-based approach designed to improve the efficiency and economic viability of emission control programs; emission trading has typically been confined to trades among single pollutants. Interpollutant trading (IPT), as described in this work, allows for trades among emissions of different compounds that affect the same air quality end point, in this work, ambient ozone (O3) concentrations. Because emissions of different compounds impact air quality end points differently, weighting factors or trading ratios (tons of emissions of nitrogen oxides (NO(x)) equivalent to a ton of emissions of volatile organic compounds [VOCs]) must be developed to allow for IPT. In this work, IPT indices based on reductions in O3 concentrations and based on reductions in population exposures to O3 were developed and evaluated using a three-dimensional gridded photochemical model for Austin, TX, a city currently on the cusp of nonattainment with the National Ambient Air Quality Standards for O3 concentrations averaged over 8 hr. Emissions of VOC and NO(x) from area and mobile sources in Austin are larger than emissions from point sources. The analysis indicated that mobile and area sources exhibited similar impacts. Trading ratios based on maximum O3 concentration or population exposure were similar. In contrast, the trading ratios did exhibit significant (more than a factor of two) day-to-day variability. Analysis of the air quality modeling indicated that the daily variability in trading ratios could be attributed to daily variations in both emissions and meteorology.