Synthesis of Satellite and Surface Measurements, Model Results, and FRAPPÉ Study Findings to Assess the Impacts of Oil and Gas Emissions Reductions on Maximum Ozone in the Denver Metro and Northern Front Range Region in Colorado

Abstract

The Denver Metro/Northern Front Range area has been designated a severe nonattainment area (NAA) for O3. Previous research associated with the 2014 Front Range Air Pollution and Photochemistry Éxperiment field campaign and a 2017 O3 source apportionment modeling analysis point to north‐south gradients in oil and gas (O&G) impacts on O3, with greater contributions to the north and a dominance of mobile sources to the south. Recent analysis of Atmospheric Infrared Sounder satellite methane enhancement and ethane trends in the area show ∼50% and ∼70% reductions in methane and ethane, respectively, from 2013 to 2020. These are consistent with reductions in O&G volatile organic compound (VOC) emissions estimated in the most recent O3 State Implementation Plan but inconsistent with a timeline of top‐down methane flux estimates for limited periods from 2012 to 2021. Reductions in annual fourth maximum eight‐hour O3 concentrations from 2012 to 2019 are 5 ppb greater at Fort Collins West (FTCW) to the north than at Chatfield Reservoir to the south. Bayesian correlation analyses and partial correlations between FTCW O3 and ethane conditioned for tropospheric NO2 from the Aura satellite Ozone Monitoring Instrument suggest that most of the 9‐ppb reduction here is due to declines in O&G emissions, and 9 ppb is comparable to estimated O&G VOC contributions on high‐concentration days in the northern portion of the study area. The gradient in trends suggests that significant O&G emissions reductions have benefited locations in the central and northern portions of the NAA.