Late Winter Ozone and PM 2.5 NAAQS Exceedance in the Northern Colorado Front Range in Relation to Oil and Natural Gas Emissions

There has been a notable increase in winter and early spring ozone exceedance occurrences in enclosed air basins that contain abundant oil and natural gas (O&NG) activity. High ozone levels can arise from emissions of nitrogen oxides (NO_x) and volatile organic compounds (VOCs) combined with enhanced photochemical reactions. The wintertime temperature inversions trap these emissions near the ground and cause their concentrations to increase, and the high albedo of snow accelerates the photochemical reactions. Although these events have been noted in rural high elevation O&NG development basins in the western United States, here, we report on and analyze a late winter high ozone and particulate matter (PM) event in the densely populated Northern Colorado Front Range (NCFR) in an unenclosed air basin with abundant O&NG production. On 19–20 March 2021, the 8-hr average ozone reached 82 ppb, and 24-hr-averaged PM 2.5 concentrations reached 77 μg m⁻³, resulting in the year's highest PM pollution event. Using a combination of in-situ chemical and meteorological measurements, we show that the buildup of ozone precursors was likely attributable to O&NG emissions in a stable boundary layer in conjunction with Denver Cyclone conditions, a mesoscale feature common in the NCFR that recirculates pollutants. The concurrent PM pollution was likely driven by animal feedlot emissions and diverse sources of NO_x, including O&NG. This event raises concern about the potential of wintertime air quality standard exceedances and cumulative adverse health effects considering an expanding O&NG industry within a densely populated urban and suburban area.