Decadal Trends and Variability in Intermountain West Surface Ozone near Oil and Gas Extraction Fields

Abstract. Decadal trends in the annual fourth-highest daily maximum 8-hour average (A4DM8HA) ozone (O₃) were studied over 2005–2015 for 13 rural/remote sites in the U.S. Intermountain West. No trends were observed in A4DM8HA O₃ at two reference sites, which are located upwind of and thus minimally influenced by emissions from oil and natural gas (O&NG) basins. Trends, or a lack thereof, varied widely at other 11 sites in/near O&NG basins resulting from different controlling factors rather than a simplistic, uniform one. The decreasing trends at Mesa Verde (−0.76 ppbv/yr) and Canyonlands National Park (−0.54 ppbv/yr) were attributed to a 37 % decrease in natural gas production in the San Juan Basin and 35 % emission reductions in coal-fired electricity generation, respectively. The decreasing trend (−1.21 ppbv/yr) at Wind Cave National Park resulted from reduced solar radiation due to increasingly frequent precipitation weather. The lack of trends at remaining sites was likely caused by the increasing O&NG emissions and decreasing emissions from other activities. Wintertime O₃ stagnant events were associated with the Arctic Oscillation (AO). Box model simulations suggested that both volatile organic compounds (VOCs) and nitrogen oxides emission reductions during negative AO years while VOC emission reductions alone in positive AO years could effectively mitigate high wintertime O₃ within the O&NG basins. Our findings suggest that emissions from O&NG extraction likely played a significant role in shaping long-term trends in surface O₃ near/within O&NG basins and hence warrant consideration in the design of efficient O₃ mitigation strategies for the Intermountain West.