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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. 

Emissions of C₂‐C₅alkanes from the U.S. oil and gas sector have changed rapidly over the last decade. We use a nested GEOS‐Chem simulation driven by updated 2011NEI emissions with aircraft, surface, and column observations to (1) examine spatial patterns in the emissions and observed atmospheric abundances of C₂‐C₅alkanes over the United States and (2) estimate the contribution of emissions from the U.S. oil and gas industry to these patterns. The oil and gas sector in the updated 2011NEI contributes over 80% of the total U.S. emissions of ethane (C₂H₆) and propane (C₃H₈), and emissions of these species are largest in the central United States. Observed mixing ratios of C₂‐C₅alkanes show enhancements over the central United States below 2 km. A nested GEOS‐Chem simulation underpredicts observed C₃H₈mixing ratios in the boundary layer over several U.S. regions, and the relative underprediction is not consistent, suggesting C₃H₈emissions should receive more attention moving forward. Our decision to consider only C₄‐C₅alkane emissions as a single lumped species produces a geographic distribution similar to observations. Due to the increasing importance of oil and gas emissions in the United States, we recommend continued support of existing long‐term measurements of C₂‐C₅alkanes. We suggest additional monitoring of C₂‐C₅alkanes downwind of northeastern Colorado, Wyoming, and western North Dakota to capture changes in these regions. The atmospheric chemistry modeling community should also evaluate whether chemical mechanisms that lump larger alkanes are sufficient to understand air quality issues in regions with large emissions of these species.