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Heterogeneous reactions have the potential to significantly impact urban ozone formation and total reactivity of the atmosphere. This dissertation used comparisons between photochemical modeling predictions and field measurements to examine heterogeneous N₂O₅ chemistry in Southeast Texas. Heterogeneous reactions of N₂O₅ can lead to two different products: nitric acid (HNO₃) and nitryl chloride (ClNO₂). The formation of HNO₃ results in a loss of reactive nitrogen from the atmosphere. In contrast ClNO₂ photolysis forms Cl radicals and NO₂, both of which promote ozone formation in the troposphere. Preliminary modeling identified key uncertainties and the need to perform more refined modeling which included updated PM emissions estimates, an updated gas-phase N₂O₅ hydrolysis reaction rate constant, updated reactive uptake coefficients, and the inclusion of ClNO₂ as a product of heterogeneous N₂O₅ uptake. Refined modeling which incorporated all of these improvements was carried out and was the first comprehensive modeling of this chemistry performed for an urban air pollution episode. Comparisons of aerosol surface area concentrations, N₂O₅ concentrations, HNO₃ concentrations, and ClNO₂ concentrations with ambient data showed that model predictions were reasonable. The exceptions to this were 1) over-predictions of aerosol surface area concentration peaks at altitudes above 1500 meters and 2) over-prediction of N₂O₅ concentrations in the Houston Ship Channel. Further analysis is needed to identify the reasons for these over-predictions. Other key findings from this modeling include the model prediction of inland chlorine concentrations high enough to form ClNO₂ and the prediction that a large portion of atmospheric chlorine is cycled through ClNO₂, therefore making the inclusion of ClNO₂ into photochemical models essential for properly simulating chlorine chemistry. In addition, modeling suggested that the chemistry leads to significant increases of NO[subscript x] at night, but decreases in daytime NO[subscript x] concentrations and that the overall effect was to decrease ozone concentrations. Further investigation into the effect of ClNO₂ as a chlorine source showed that likely ozone increases in the Houston area caused by the presence of this compound are on the order of several ppb. Further analyses showed that vertical dispersion and local atmospheric composition moderated the effect of nitryl chloride on ozone mixing ratios. / text

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Emissions of air pollutants from non-conventional sources have been on the rise in the North Texas area over the past decade. These include primary pollutants such as volatile organic compound (VOC) and oxides of nitrogen (NOx) which also act as precursors in the formation of ozone. Most of these have been attributed to a significant increase in oil and gas production activities since 2000 within the Barnett Shale region adjacent to the Dallas-Fort Worth metroplex region. In this study, air quality concentrations measured at the Denton Airport and Dallas Hinton monitoring sites operated by the Texas Commission on Environmental Quality (TCEQ) were evaluated. VOC concentration data from canister-based sampling along with continuous measurement of oxides of nitrogen (NOx), ozone (O3), particulate matter (PM2.5), and meteorological conditions at these two sites spanning from 2000 through 2014 were employed in this study. The Dallas site is located within the urban core of one of the fastest growing cities in the United States, while the Denton site is an exurban site with rural characteristics to it. The Denton Airport site was influenced by natural gas pads surrounding it while there are very few natural gas production facilities within close proximity to the Dallas Hinton site. As of 2013, there were 1362 gas pads within a 10 mile radius to the Denton Airport site but there were only 2 within a 10 mile radius to Dallas Hinton site. The Dallas site displayed higher concentrations of NOx and much lower concentrations of VOC than the Denton site. Extremely high levels of VOC measured at the Denton site corresponded with the increase in oil and gas production activities in close proximity to the monitoring site. Ethane and propane are two major contributors to the measured VOC concentration, suggesting the influence of fugitive emissions of natural gas. In Dallas, the mean and maximum values of ozone had decreased since 2000 by about 2% and 25%, respectively. Similarly NOx decreased by 50% and 18% in the mean and maximum values. However, the mean VOC value showed a 21% decrease while the maximum value increased by about 46%. In Denton, the change in percentage of ozone and NOx were similar to Dallas but the mean VOC concentration increased by about 620% while the max value increased 1960%. Source apportionment analysis confirmed the findings by identifying the production of natural gas to be the primary source of VOC emissions in Denton, while traffic sources were more influential near the Dallas site. In light of the recent proposal by EPA to revise the ozone standard, the influence of these new unconventional sources should be further evaluated.

The Barnett shale basin, the largest onshore gas field in the state of Texas, mainly produces natural gas. The basin’s oil and gas productions have dramatically increased over the past two decades with the enhancement via shale fracturing (fracking) technology. However, recent studies suggest that air emissions from shale fracking have significantly contributed to the growing air pollution problem in North Texas. In this study, air emissions from the Barnett shale basin during the production phase of the oil and gas activities (once the product is collected from the wells) are quantified. Oil and gas production data were acquired from the Texas Railroad Commission for the baseline years of 2010 through 2013. Methodology from prior studies on shale basins approved by the Texas Commission on Environmental Quality was employed in this study and the emission inventories from the production phase sources were quantified. Accordingly, the counties with the most gas operations in the basin, Tarrant, Johnson, Denton and Wise, were found to be the highest emitters of air pollutants. Tarrant County was responsible for the highest emitted NOx (42,566 tons) and CO (17,698 tons) in the basin, while Montague County released the maximum VOC emissions (87,601 tons) during the study period. Amongst the concerned emitted pollutants, VOC was the largest emitted pollutant during the study period (417,804 tons), followed by NOx (126,691 tons) and CO (47,884 tons). Significant Sources of air emissions include: storage tanks, wellhead compressor engines, and pneumatic devices. Storage tanks and pneumatic devices contributed to about 62% and 28% of the total VOC emissions, respectively. Whereas, wellhead compressor engines are primarily responsible for about 97% of the total NOx emissions. Finally, in Tarrant, Wise and Denton counties, the emissions increased during the study period due to increase in the oil and gas production, while Johnson County’s emission contribution declined throughout the study period.

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Ambient observations have indicated that ozone formation in the Houston area is frequently faster and more efficient, with respect to NOx consumed, than other urban areas in the country. It is believed that these unique characteristics of ozone formation in the Houston area are associated with the plumes of reactive hydrocarbons, emanating from the industrial Houston Ship Channel area. Thus, accurate quantification of industrial emissions, particularly of reactive hydrocarbons, is critical to effectively address the rapid ozone formation and the consequent high levels of ozone in the area. Industrial emissions of hydrocarbons have significant temporal variability as evidenced by various measurements, but they have been assumed to be continuous at constant levels for air quality regulation and photochemical modeling studies. This thesis examines the effect of emission variability from industrial sources on ozone formation in the HoustonGalveston area. Both discrete emission events and variability in continuous emissions are examined; new air quality modeling tools have been developed to perform these analyses. Also, this thesis evaluates the impact of emission variability on the effectiveness of emission control strategies in the Houston-Galveston area. Overall, the results indicate that industrial emission variability plays a substantial role in ozone formation and that controlling emission variability can be effective in ozone reduction. These results suggest that a quantitative treatment of emission variability should be included in the development of air quality plans for regions with extensive industrial activity, such as Houston.

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Elemental carbon (EC), a product of incomplete combustion of fossil fuels and biomass, contributes to climate warming and poor air quality. In urban areas, diesel fuel trucks are the main source of EC emissions from mobile sources. After emission, EC is deposited to receptor surfaces via two main pathways: precipitation (wet deposition) and directly as particles (dry deposition). Urban trees may play an important role in removing EC from the atmosphere by intercepting and delivering it directly to the soil. The goal of this research was to quantify the magnitude of EC retention in leaf waxes (in-wax EC) and EC fluxes to the soil via leaf litterfall in the City of Denton, Texas. Denton is a rapidly growing urban location in the Dallas-Fort Worth metropolitan area. A foliar extraction technique was used to determine EC retention in leaf waxes. Foliar samples were collected monthly, from April through July, from pairs of Quercus stellata (post oak, n=10) and Quercus virginiana (live oak, n = 10) trees. Samples were rinsed with water and chloroform in a two-step process to determine EC retained in leaf waxes. A Sunset OC/EC aerosol analyzer was utilized to analyze the EC content of extracts filtered onto quartz-fiber filters. From April through July, leaf litter was collected bi-weekly under 35 trees (20 post oak, 15 live oak), and oven dried to determine dry weight. EC retained by tree canopies was estimated by multiplying in-wax EC by canopy leaf area index, while EC flux to soil was estimated by multiplying in-wax EC by leaf litterfall mass. This study shows that through retention of EC in leaf waxes, urban tree canopies represent important short-term sinks for soot in urban areas.