The Evolution of Ozone’s Weekday-Weekend Effect Across Colorado's Front Range

Tropospheric ozone (O3) is both a public health and environmental hazard, and is capable of causing long and short term effects including pulmonary conditions, premature death, and reduced agricultural yields. O3 has been designated as a criteria pollutant by the US Environmental Protection Agency and is subject to National Ambient Air Quality Standards (NAAQS). O3 is produced from reactions between Volatile Organic Compounds (VOCs) and Nitrogen Oxides (NOx). Vehicles are an important NOx source with emissions that follow daily and weekly patterns in passenger and truck traffic. O3 in a variety of locations across Colorado’s Front Range routinely exceeds the NAAQS and, despite significant reductions in precursor emissions, no strong decreasing trend in O3 has been observed. In this study we analyze weekday-weekend differences in summer NOx and O3 at monitoring sites across the Front Range between 2005 and 2023 to provide insight into the sensitivity of O3 to NOx emissions. Our findings indicate that as NOx concentrations have decreased due to improved regulations, the Wednesday-Sunday difference in daily maximum 8-hour average O3 concentrations has increased. This suggests an increase in O3’s sensitivity to NOx across the Front Range consistent with a shift from NOx-saturated to NOx-limited conditions. This regime shift suggests that future NOx emissions reductions should be increasingly effective in achieving O3 reduction. Finally, we consider the weekly cycle of VOC concentrations and their impact on O3 production as a platform for future research. Studies such as these can help determine the most effective emission control strategies for reducing O3 pollution in Colorado’s Front Range.