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      Understanding high wintertime ozone pollution events in an oil- and natural gas-producing region of the western US

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          Recent increases in oil and natural gas (NG) production throughout the western US have come with scientific and public interest in emission rates, air quality and climate impacts related to this industry. This study uses a regional-scale air quality model (WRF-Chem) to simulate high ozone (O<sub>3</sub>) episodes during the winter of 2013 over the Uinta Basin (UB) in northeastern Utah, which is densely populated by thousands of oil and NG wells. The high-resolution meteorological simulations are able qualitatively to reproduce the wintertime cold pool conditions that occurred in 2013, allowing the model to reproduce the observed multi-day buildup of atmospheric pollutants and the accompanying rapid photochemical ozone formation in the UB. <br><br> Two different emission scenarios for the oil and NG sector were employed in this study. The first emission scenario (bottom-up) was based on the US Environmental Protection Agency (EPA) National Emission Inventory (NEI) (2011, version 1) for the oil and NG sector for the UB. The second emission scenario (top-down) was based on estimates of methane (CH<sub>4</sub>) emissions derived from in situ aircraft measurements and a regression analysis for multiple species relative to CH<sub>4</sub> concentration measurements in the UB. Evaluation of the model results shows greater underestimates of CH<sub>4</sub> and other volatile organic compounds (VOCs) in the simulation with the NEI-2011 inventory than in the case when the top-down emission scenario was used. Unlike VOCs, the NEI-2011 inventory significantly overestimates the emissions of nitrogen oxides (NO<sub>x</sub>), while the top-down emission scenario results in a moderate negative bias. The model simulation using the top-down emission case captures the buildup and afternoon peaks observed during high O<sub>3</sub> episodes. In contrast, the simulation using the bottom-up inventory is not able to reproduce any of the observed high O<sub>3</sub> concentrations in the UB. Simple emission reduction scenarios show that O<sub>3</sub> production is VOC sensitive and NO<sub>x</sub> insensitive within the UB. The model results show a disproportionate contribution of aromatic VOCs to O<sub>3</sub> formation relative to all other VOC emissions. The model analysis reveals that the major factors driving high wintertime O<sub>3</sub> in the UB are shallow boundary layers with light winds, high emissions of VOCs from oil and NG operations compared to NO<sub>x</sub> emissions, enhancement of photolysis fluxes and reduction of O<sub>3</sub> loss from deposition due to snow cover.

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          Author and article information

          Atmospheric Chemistry and Physics
          Atmos. Chem. Phys.
          Copernicus GmbH
          January 2015
          : 15
          : 1
          : 411-429
          © 2015



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