Boundary layer ozone in the Northern Colorado Front Range in July–August 2014 during FRAPPE and DISCOVER-AQ from vertical profile measurements

Abstract

Data from ground-based ozone (O3) vertical profiling platforms operated during the FRAPPE/DISCOVER-AQ campaigns in summer 2014 were used to characterize key processes responsible for establishing O3 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 O3 mixing ratio. Photochemical O3 production throughout the boundary layer builds on the mid-morning profile. From late morning to mid-afternoon the continuing O3 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 O3 enhancements in the mixed boundary layer confirming O3 production throughout the entire boundary layer. Continuous O3 measurements from the Boulder Atmospheric Observatory (BAO) tall tower at 6 m and 300 m showed hourly O3 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 O3 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 O3 precursors and are crucial in producing peak O3 events in the NCFR. Higher elevation locations to the west of the NCFR plains regularly experience higher O3 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 O3 exposure assessments.

Spatial patterns in summertime surface ozone in the Southern Front Range of the U.S. Rocky Mountains

“…These observations reflect the controls on diurnal ozone variability discussed in similar studies, including nighttime ozone depletion via surface deposition and NO titration, rapid midmorning ozone increases via boundary layer entrainment followed by in situ photochemical production, and diurnal winds facilitating horizontal advection (Jaffe et al, 2018;Trousdell et al, 2019;Oltmans et al, 2019). These effects can be seen to different degrees across the range of site elevations.…”

“…Meanwhile, rates of ozone increase occur at most sites beginning around 5:00-6:00 MST (Figure 6) due to boundary layer growth and daytime turbulence causing ozone from the residual layer to be mixed with air at the surface across at most sites (Brodin et al, 2010;Fine et al, 2015;Bien and Helmig, 2018;Oltmans et al, 2019;Trousdell et al, 2019). In the NFRMA and the San Joaquin Valley of California, entrainment has been observed to cause morning increases in ozone values with minimal to no input from photochemical production from 6:00 to 9:00 MST (Trousdell et al, 2016;Oltmans et al, 2019), and we assume similar dynamics govern the diurnal profiles in Colorado Springs. Early morning boundary layer mixing followed by daytime photolysis are accompanied by diurnal mountain wind patterns; for example, at NAV and CAS, the nighttime downslope flow shifts to daytime upslope flow between 5:00 and 8:00 MST (Figures 7 and S3).…”

Seasonality and Source Apportionment of Nonmethane Volatile Organic Compounds at Boulder Reservoir, Colorado, Between 2017 and 2019

“…While there have been successful reductions in regional NO x emissions in portions of the NFRMA urban corridor (Bien & Helmig, 2018), episodic high O 3 events have not yet responded notably (i.e., improved) to the NO x reductions (Abeleira & Farmer, 2017). Numerous recent studies point to VOC emissions associated with O&NG operations as a significant contributor to regional ozone production (Cheadle et al., 2017; Evans & Helmig, 2017; Lindaas et al., 2019; McDuffie et al., 2016; Oltmans et al., 2019; Pfister et al., 2017).…”

Air quality impacts from oil and natural gas development in Colorado

“…There are convincing arguments that support the conclusion that the deviation in the Colorado ozone behavior with the national trend is caused by emissions from the O&NG sector, both from O&NG signatures seen in elevated ozone episodes [Cheadle et al, 2017;Oltmans et al, 2019] and from photochemical modeling [Pfister et al, 2017a]. As already pointed out above, biogenic VOC emissions have a relatively minor contribution to regional ozone production; elevated ozone episodes are primarily associated with elevated anthropogenic VOCs [Cheadle et al, 2017;Zaragoza et al, 2017;Lindaas et al, 2019].…”

“…Cheadle et al [2017], analyzing selected cases of observations near Greeley during FRAPPE, estimated that O&NG emissions contributed up to ≈20 ppb to ozone production on high ozone days. Oltmans et al [2019] conducted an in depth analysis of the conditions on high ozone days at BAO. Their analysis showed an association of high ozone days with transport from sectors with intense O&NG production towards the northeast.…”