Mobile measurements of ozone, its precursors, and air toxics at the local and regional scale during the Colorado Front Range OZone Éxperiment (FROZÉ 2025)

Abstract

The Colorado Denver Metro/North Front Range (DMNFR) area has been designated as nonattainment for ozone (O₃) by the Environmental Protection Agency, as this region frequently exceeds the National Ambient Air Quality Standard (NAAQS) for O₃ in the summer. Despite significant reductions in emissions of precursor nitrogen oxides (NOₓ = NO + NO₂) and volatile organic compounds (VOCs) in recent years, the area continues to struggle to reach attainment. O₃ and NO₂ are criteria air pollutants, and can both cause significant health concerns. In addition to O₃ nonattainment, the region faces concerns about air toxics exposure to residents. Emissions of air toxics come from many sources, including traffic, oil and gas (O&G) development and production, utilities, manufacturing, and commercial/institutional sectors. With both O&G operations and the region’s population growing, air toxics are an increasing concern for the health of residents. This work focuses on mobile measurements of O₃, NO₂, and VOCs from the Colorado State University (CSU) Plume Tracker during the Colorado Front Range OZone Éxperiment (FROZÉ) in summer 2025. The CSU Plume Tracker completed 13 driving intensives in the Northern Front Range, continuously measuring O₃ and NO₂, and collecting whole air grab canister samples for detailed VOC speciation in 12 disproportionately impacted communities (DICs). We find that air toxics concentrations in DICs were below acute health guideline values at the time of sampling. However, toluene, ethylbenzene, xylenes, and n-hexane were significantly higher in some DICs when compared to the mean value of same-hour measurements in non-DIC communities from stationary monitors. This finding suggests that existing stationary measurements in non-DIC communities do not always reflect air toxics exposures occurring in DICs, and additional monitoring in these communities should be prioritized. Among criteria air pollutants in DICs, we find that O₃ levels were generally lower, especially during daytime sampling, in DICs when compared to the mean of the stationary sites. Elevated NO₂ levels were also observed in 2 of 12 DICs, relative to the mean value found at stationary monitoring sites. Regional NO₂ hotspots generally aligned with combustion sources near population centers, O&G development, and heavily-trafficked highways, and were highly dependent on meteorological conditions. Finally, we examine the spatial variability in O₃ concentrations across the DMNFR O₃ NAA, finding considerable variability sometimes apparent over relatively short spatial scales. Two distinct O₃ regimes were present: regional O₃ formation, where O₃ concentrations climb steadily throughout the daytime across a broad region, and local O₃ modification, where changes in emissions and/or meteorology can lead to rapid changes in O₃ concentrations over short temporal and spatial scales.