An Unusual Winter Ozone Event in Colorado
Year: 2025
Type: article
Abstract: Surface ozone (O 3 ) mixing ratios exceeding the National Ambient Air Quality Standard 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 O 3 ‐rich lower stratospheric air reached the surface. Instead, the statically stable lower stratospheric air suppressed the growth of the daytime boundary layer and trapped nitrogen oxides (NO x = NO + NO 2 ) 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 Goddard Earth Observing System composition forecast models, to describe the stratospheric intrusion and characterize the boundary layer structure, HYSPLIT back trajectories to show the low‐level transport of O 3 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 O 3 production. The box model showed the O 3 production to be NO x saturated, such that the NO x reductions associated with COVID‐19 exacerbated the event rather than mitigating it.
DOI: 10.1029/2025jd043695
Authors: Andrew O. Langford, Kenneth C. Aikin, Raul J. Alvarez, Sunil Baidar, W. Alan Brewer, Steven S. Brown, Matthew M. Coggon, Patrick D. Cullis, Jessica B. Gilman, Georgios I. Gkatzelis, Detlev Helmig, Bryan J. Johnson, K. Emma Knowland, Rajesh Kumar, Aaron D. Lamplugh, Brandi J. McCarty, Ann M. Middlebrook, Gabriele Pfister, Jeff Peischl, Irina Petropavlovskikh, Pamela S. Rickly, Michael A. Robinson, Andrew W. Rollins, Scott P. Sandberg, Christoph J. Senff, Carsten Warneke
Institutions: NOAA Chemical Sciences Laboratory, Cooperative Institute for Research in Environmental Sciences, University of Colorado Boulder, National Oceanic and Atmospheric Administration, NOAA Global Monitoring Laboratory, Forschungszentrum Jülich, Goddard Space Flight Center, Morgan State University, NSF National Center for Atmospheric Research, NSF NCAR Research Applications Laboratory, Colorado Department of Public Health and Environment, NSF NCAR Atmospheric Chemistry Observations & Modeling
Language: English
Subfield: Global and Planetary Change
Field: Environmental Science
Domain: Physical Sciences
Open Access status: hybrid
Estimated APC paid: $4,070
OpenAlex ID: W4412660348