An Unusual Winter Ozone Event in Colorado

Abstract. Surface ozone (O3) mixing ratios exceeding the National Ambient Air Quality Standard (NAAQS) were measured at rural monitors along the Colorado Front Range on 17 April 2020 during the COVID-19 lockdown. This unusual episode followed back-to-back upslope snowstorms and coincided with the presence of a deep stratospheric intrusion, but ground-based lidar and ozonesonde measurements show that little, if any, of the O3-rich lower stratospheric air reached the surface. Instead, the statically stable lower stratospheric air suppressed the growth of the convective boundary layer and trapped nitrogen oxides (NOx = NO + NO2) and volatile organic compounds (VOCs) emitted by motor vehicles and oil and natural gas (O&NG) operations near the ground where the clear skies and extensive snow cover triggered a short-lived photochemical episode similar to those observed in the O&NG producing basins of northeastern Utah and southwestern Wyoming. In this study, we use a combination of lidar, ozonesonde, and surface measurements, together with the WRF-Chem and GEOS-CF models, to describe the stratospheric intrusion and characterize the boundary layer structure, HYSPLIT back trajectories to show the low-level transport of O3 and its precursors to the exceedance sites, and surface measurements of NOx and VOCs together with a 0-D box model to investigate the roles of urban and O&NG emissions and the COVID-19 quarantine in the O3 production. The box model showed the O3 production to be NOx saturated, such that the NOx reductions associated with COVID-19 exacerbated the event rather than mitigating it. Such O3 winter O3 exceedances may become more common in Denver with expected, future NOx reductions.