Characteristics of common ozone exceedance days across the U.S. Intermountain West

The U.S. Intermountain West frequently experiences high O₃ events that exceed the National Ambient Air Quality Standard (NAAQS). This study examines 2009-2024 by combining EPA mean daily maximum 8-h average (MDA8 O₃) data with NOAA Hazard Mapping System smoke products, satellite-based tropospheric column retrievals, and meteorology and trajectory data to characterize the frequency, patterns, and potential drivers of common O₃ exceedances, defined as days with MDA8 O₃ > 70 ppb. Four non-attainment areas (NAAs) are evaluated: Phoenix-Mesa-Scottsdale (Arizona); Las Vegas, Clark County (Nevada); Denver Metro/North Front Range (Colorado); Northern and Southern Wasatch Front (Utah). Across all months, daily MDA8 O₃ is moderate (peaking near 45–55 ppb, with ∼2–3% of days >70 ppb), and 37 days are identified as common exceedances when all four NAAs exceed the threshold simultaneously, occurring in 12 years (2010–2013, 2015–2018, 2020–2021, and 2023–2024). Exceedances typically begin in April–June and end in August–September, peaking in mid-summer (June–August). Common exceedances exhibit substantial heterogeneity within individual NAAs in terms of O₃ levels across monitoring sites, occurring more often on weekdays (Tuesday–Friday) and least frequently on Sunday, suggestive of anthropogenic influence superimposed on potential regional factors. Wildfire smoke coincides with common exceedances, especially in 2018, 2020, and 2024 when smoke coverage simultaneously affected all four NAAs. Exceedances are characterized by enhanced temperatures and solar radiation, seasonally varying boundary layer and pressure anomalies, and generally lower formaldehyde-to-NO₂ ratios. Trajectory analyses suggest potential inter-basin transport, motivating continued research into this factor.