Data from ground-based ozone (O 3 ) vertical profiling platforms operated during the FRAPPE/DISCOVER-AQ campaigns in summer 2014 were used to characterize key processes responsible for establishing O 3 profile development in the boundary layer in the Northern Colorado Front Range. Morning mixing from the upper boundary layer and lower free troposphere into the lower boundary layer was the key process establishing the mid-morning boundary layer O 3 mixing ratio. Photochemical O 3 production throughout the boundary layer builds on the mid-morning profile. From late morning to mid-afternoon the continuing O 3 increase was nearly uniform through the depth of the profile measured by the tethersonde (~400 m). Ozonesondes flown on a near daily schedule over a four week period with multiple profiles on a number of days captured the full 1500 to 2000 m vertical extent of O 3 enhancements in the mixed boundary layer confirming O 3 production throughout the entire boundary layer. Continuous O 3 measurements from the Boulder Atmospheric Observatory (BAO) tall tower at 6 m and 300 m showed hourly O 3 at the 6 m level ≥75 ppb on 15% of the days. The diurnal variation on these days followed a pattern similar to that seen in the tethersonde profiles. The association of high O 3 days at the BAO tower with transport from sectors with intense oil and natural gas production toward the northeast suggests emissions from this industry were an important source of O 3 precursors and are crucial in producing peak O 3 events in the NCFR. Higher elevation locations to the west of the NCFR plains regularly experience higher O 3 values than those in the lower elevation NCFR locations. Exposure of populations in these areas is not captured by the current regulatory network, and likely underestimated in population O 3 exposure assessments.

High mixing ratios of ozone (O 3 ) in the northern Front Range (NFR) of Colorado are not limited to the urban Denver area but were also observed in rural areas where oil and gas activity is the primary source of O 3 precursors. On individual days, oil and gas O 3 precursors can contribute in excess of 30 ppb to O 3 growth and can lead to exceedances of the EPA O 3 National Ambient Air Quality Standard. Data used in this study were gathered from continuous surface O 3 monitors for June–August 2013–2015 as well as additional flask measurements and mobile laboratories that were part of the FRAPPE/DISCOVER-AQ field campaign of July–August 2014. Overall observed O 3 levels during the summer of 2014 were lower than in 2013, likely due to cooler and damper weather than an average summer. This study determined the median hourly surface O 3 mixing ratio in the NFR on summer days with limited photochemical production to be approximately 45–55 ppb. Mobile laboratory and flask data collected on three days provide representative case studies of different O 3 formation environments in and around Greeley, Colorado. Observations of several gases (including methane, ethane, CO, nitrous oxide) along with O 3 are used to identify sources of O 3 precursor emissions. A July 23 survey demonstrated low O 3 (45–60 ppb) while August 3 and August 13 surveys recorded O 3 levels of 75–80 ppb or more. August 3 exemplifies influence of moderate urban and high oil and gas O 3 precursor emissions. August 13 demonstrates high oil and gas emissions, low agricultural emissions, and CO measurements that were well correlated with ethane from oil and gas, suggesting an oil and gas related activity as a NOx and O 3 precursor source. Low isoprene levels indicated that they were not a significant contributor to O 3 precursors measured during the case studies.