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.