Weekend‐Weekday Implications and the Impact of Wildfire Smoke on Ozone and Its Precursors at Boulder Reservoir, Colorado Between 2017 and 2019

The Northern Front Range of Colorado regularly exceeds the ozone (O3) National Ambient Air Quality Standard. This region is unique because it is impacted by emissions from urban activities (i.e., traffic and light industry) as well as from oil and natural gas production. Here, we summarize three years of observations from a new O3 monitoring site at the western edge of the region, located at the Boulder Reservoir. There were 41 days at the Boulder Reservoir in 2017, 2018, and 2019 when the maximum daily 8‐hour average O3 mixing ratio (MDA8 O3) exceeded 70 ppbv. Of these 41 high O3 days, 40 occurred between May and September, two were influenced by stratospheric intrusion events, one was influenced by a local industrial fire, and 13 of the days were impacted by wildfire smoke. Within the smoke‐free subset of data, weekend‐weekday differences in O3 versus nitrogen oxides (NOx) during 2018 and 2019 suggest NOx‐limited O3 production. Enhanced nonmethane volatile organic compounds (NMVOC) to NOx ratios are also observed on weekend days and are driven by weekend reductions in NOx. An analysis of 13 NMVOCs on days that had MDA8 O3 > 70 ppb showed that NMVOC abundances and their relative source contributions are variable at this location. Finally, we compare smoke‐impacted with smoke‐free conditions. At a given temperature, smoke‐impacted days have higher expected O3 mixing ratios than smoke free‐days. On average, benzene, toluene, and hexane are also elevated during smoke‐impacted periods. Plain Language Summary Colorado's Northern Front Range Metropolitan Area has exceeded the U.S. National Ambient Air Quality Standard for ozone (O3) for two decades. The Boulder Reservoir is a new air pollution measurement site nestled at the western edge of this region, and has co‐located continuous monitoring of O3, a subset of volatile organic compounds (VOCs), and nitrogen oxides. Here, we present a multi‐year record of these observations, focused on the summer season. Based on an analysis of differences between weekends and weekdays, the data indicate that local O3 production may be limited by the availability of nitrogen oxides. The VOCs associated with high O3 events are highly variable, with varying contributions from regional sources. There are also notable gas‐phase composition changes in response to the presence of wildfire smoke. When smoke is present, O3 and several hazardous air pollutants are also typically elevated. Key Points High ozone days at Boulder Reservoir