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Ground‐level ozone (O3) was unusually high in northern Colorado in the summer of 2021 with maximum daily 8‐hr average (MDA8) concentrations 6 to 8 parts‐per‐billion by volume (ppbv) higher than in 2019, 2020, or 2022. One or more of the monitors on the Colorado Front Range exceeded the 2015 U.S. National Ambient Air Quality Standard (NAAQS) of 70 ppbv on 66 of the 122 days from 1 June to 30 September, and this record number of exceedances coincided with the near daily presence of dispersed smoke haze from wildfires in Arizona, California, and the Pacific Northwest. In this paper, we use regulatory and non‐regulatory surface O3 and PM2.5 measurements in conjunction with ground‐based lidar observations to estimate how much O3 was associated with the wildfire smoke. Analyses of the surface measurements suggest that pyrogenic O3 transported to northern Colorado with the smoke increased the surface concentrations in northern Colorado by an average of 8 ppbv in July, 3 ppbv in August, and 2 ppbv in September. Analysis of the lidar measurements showed these contributions to be as large as 12 ppbv on some days. Production of O3 from reactions of pyrogenic VOCs and locally emitted NOx appears to have been minimal (<3 ppbv) in the Boulder area, but may have been much larger in the suburbs southwest of downtown Denver. Plain Language Summary: Northern Colorado experienced unusually poor air quality in the summer of 2021 with frequent high ozone (O3) episodes and hazy skies caused by smoke from wildfires in Arizona, California, and the Pacific Northwest. In this study, we use surface and lidar measurements to explore the connection between the two. Our analysis suggests that the unusually high O3 was caused primarily by a combination of O3 transported to Colorado with the wildfire smoke and enhancement of local photochemical production by unusually clear skies and warm temperatures coupled with weak winds that led to localized O3 accumulations and fewer than normal thunderstorms that might otherwise have dispersed the O3. Production of O3 by reactions of locally emitted NOx with VOCs in the wildfire smoke may also have been significant in Southwest Denver. Key Points: The impacts of the 2021 western wildfires on ozone in the Denver metropolitan area and rural northern Colorado in 2021 are examinedOzone transported in the smoke from distant wildfires increased the 8‐hr concentrations in northern Colorado by an average of 8 ppbv in JulyUnusual meteorology, including fewer thunderstorms, allowed ozone produced locally to accumulate along the foothills west of Denver [ABSTRACT FROM AUTHOR]

The Fires, Asian, and Stratospheric Transport–Las Vegas Ozone Study (FAST-LVOS) was conducted in May and June of 2017 to study the transport of ozone (O 3) to Clark County, Nevada, a marginal non-attainment area in the southwestern United States (SWUS). This 6-week (20 May–30 June 2017) field campaign used lidar, ozonesonde, aircraft, and in situ measurements in conjunction with a variety of models to characterize the distribution of O 3 and related species above southern Nevada and neighboring California and to probe the influence of stratospheric intrusions and wildfires as well as local, regional, and Asian pollution on surface O 3 concentrations in the Las Vegas Valley (≈ 900 m above sea level, a.s.l.). In this paper, we describe the FAST-LVOS campaign and present case studies illustrating the influence of different transport processes on background O 3 in Clark County and southern Nevada. The companion paper by Zhang et al. (2020) describes the use of the AM4 and GEOS-Chem global models to simulate the measurements and estimate the impacts of transported O 3 on surface air quality across the greater southwestern US and Intermountain West. The FAST-LVOS measurements found elevated O 3 layers above Las Vegas on more than 75 % (35 of 45) of the sample days and show that entrainment of these layers contributed to mean 8 h average regional background O 3 concentrations of 50–55 parts per billion by volume (ppbv), or about 85–95 µ g m -3. These high background concentrations constitute 70 %–80 % of the current US National Ambient Air Quality Standard (NAAQS) of 70 ppbv (≈ 120 µ g m -3 at 900 m a.s.l.) for the daily maximum 8 h average (MDA8) and will make attainment of the more stringent standards of 60 or 65 ppbv currently being considered extremely difficult in the interior SWUS. [ABSTRACT FROM AUTHOR]