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 (NO_x = 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 NO_x 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 NO_x saturated, such that the NO_x 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 NO_x reductions.