A high ozone episode in winter 2013 in the Uinta Basin oil and gas region characterized by aircraft measurements

Abstract. During the winter of 2012–2013 atmospheric surface ozone mole fractions exceeded the US 8 h standard of 75 ppb on 39 days in the Uinta Basin of Utah. As part of the Uinta Basin Winter Ozone Study (UBWOS) aircraft flights were conducted throughout the basin with continuous measurements of ozone (O₃), methane (CH₄), carbon dioxide (CO₂), carbon monoxide (CO), nitrogen dioxide (NO₂), and discrete whole air flask samples for determination of ∼50 trace gases including a number of non-methane hydrocarbons (NMHCs). During the course of seven flights conducted between 31 January and 7 February 2013, coinciding with strong, multi-day temperature inversions, O₃ levels gradually built up in the shallow boundary layer from ∼45 ppb to ∼140 ppb. Near-surface CH₄ mole fractions increased during the episode from near background levels of ∼2 ppm to over 10 ppm. Based on elevated levels of CH₄ across the basin and high correlations of CH₄ with NMHCs from the discrete air samples, O₃ precursor NMHCs were also inferred to be elevated throughout the basin. Discrete plumes of high NO₂ were observed in the gas production region of the basin suggesting that gas processing plants and compressor facilities were important point sources of reactive nitrogen oxides (NO_x). Vertical profiles obtained during the flights showed that the high O₃ mole fractions (as well as other elevated constituents) were confined to a shallow layer from near the ground to 300–400 m above ground level (m a.g.l.) capped by a strong temperature inversion. The highest mole fractions of the measured constituents during the study period were in an isothermal cold layer that varied from ∼300 m depth on 4 February to ∼150 m on 5 February. A gradient layer with declining mole fractions with altitude extended above the isothermal layer to ∼1900 m a.s.l. (300–400 m a.g.l.) indicative of some mixing of air out of the boundary layer. O₃ mole fractions continued to increase within the basin as the high O₃ episode developed over the course of a week. CH₄ mole fractions, on the other hand, leveled off after several days. On several flights, the aircraft sampled the plume of a coal-fired power plant (located east of the main gas field) flowing above the inversion layer. These measurements ruled out the effluents of the power plant as a significant source of NO_x for O₃ production beneath the temperature inversion in the basin. The presence of elevated O₃ precursors within the basin and the rapid daytime production of O₃ in the atmosphere beneath the temperature inversion both indicated that O₃ was being produced from precursors emitted within the basin beneath the temperature inversion. Although observations show that horizontal winds in the surface layer were relatively light during the high ozone event, they were sufficient to disperse precursors up to 80 km from primary sources in the main gas field in the southeast quadrant to the balance of the Uinta Basin.