O 3 , CH 4 , CO 2 , CO, NO 2 and NMHC aircraft measurements in the Uinta Basin oil and gas region under low and high ozone conditions in winter 2012 and 2013

Instrumented aircraft measuring air composition in the Uinta Basin, Utah, during February 2012 and January-February 2013 documented dramatically different atmospheric ozone (O 3 ) mole fractions. In 2012 O 3 remained near levels of ∼40 ppb in a well-mixed 500–1000 m deep boundary layer while in 2013, O 3 mole fractions >140 ppb were measured in a shallow (∼200 m) boundary layer. In contrast to 2012 when mole fractions of emissions from oil and gas production such as methane (CH 4 ), non-methane hydrocarbons (NMHCs) and combustion products such as carbon dioxide (CO 2 ) were moderately elevated, in winter 2013 very high mole fractions were observed. Snow cover in 2013 helped produce and maintain strong temperature inversions that capped a shallow cold pool layer. In 2012, O 3 and CH 4 and associated NMHCs mole fractions were not closely related. In 2013, O 3 mole fractions were correlated with CH 4 and a suite of NMHCs identifying the gas field as the primary source of the O 3 precursor NMHC emissions. In 2013 there was a strong positive correlation between CH 4 and CO 2 suggesting combustion from oil and natural gas processing activities. The presence of O 3 precursor NMHCs through the depth of the boundary layer in 2013 led to O 3 production throughout the layer. In 2013, O 3 mole fractions increased over the course of the week-long episodes indicating O 3 photochemical production was larger than dilution and deposition rates, while CH 4 mole fractions began to level off after 3 days indicative of some air being mixed out of the boundary layer. The plume of a coal-fired power plant located east of the main gas field was not an important contributor to O 3 or O 3 precursors in the boundary layer in 2013.