The ozone climate penalty, NAAQS attainment, and health equity along the Colorado Front Range

Ground-level ozone, a major component of smog, is a respiratory irritant linked to various acute and chronic health problems. Its formation is driven by photochemical reactions involving volatile organic compounds (VOCs) and nitrogen oxides (NOx), which are exacerbated by heat. The US EPA's National Ambient Air Quality Standards (NAAQS) for ozone have been tightened over time, but the DMNFR region consistently struggles to meet these standards. While local factors like oil and gas emissions contribute to high ozone levels, regional and global factors, such as wildfires and long-range transport of pollutants, also play a role. Importantly, weather patterns significantly influence ozone formation, with warmer temperatures and less wind promoting higher concentrations. This study focuses on the "climate penalty," the additional ozone attributable to climate change, specifically analyzing its impact on the DMNFR from the 1950s to the 2010s, a period of significant climate change acceleration. The study aims to quantify this penalty, its effect on NAAQS attainment, and its disproportionate impact on vulnerable populations.

Existing literature demonstrates a strong link between ground-level ozone and various respiratory illnesses. Studies have quantified the contribution of local emissions sources, like oil and gas activities, to elevated ozone levels in the Front Range. Other research highlights the role of regional and global factors, such as wildfire smoke and transboundary pollution. The impact of weather and climate on ozone formation is well-established, with studies showing a relationship between temperature increases and ozone concentration increases. The concept of a "climate penalty" has emerged, representing the extra ozone caused by climate change. However, while many climate penalty studies focus on future projections, this study analyzes the historical climate penalty to inform present-day planning and resource allocation for improving public health.

The study region included 17 counties along the Colorado Front Range, encompassing both urban and rural areas. Data from multiple weather monitoring sites (covering temperature, wind speed, relative humidity, and sea level pressure) and ozone monitoring sites were used. Daily 8-h maximum ozone mixing ratios from 26 monitors were obtained from the US EPA Air Quality System. Additional geospatial data included elevation, population density, and traffic density. Spatio-temporal land-use regression (LUR) models, specifically generalized additive mixed models (GAMMs), were employed. These models incorporated meteorological variables, geographic features, and sociodemographic factors to predict ozone concentrations. The ozone climate penalty was estimated by comparing the observed 2010s ozone field to a counterfactual 1950s ozone field created by adjusting 2010s weather data to match 1950s climate conditions while keeping other variables constant. The impact on NAAQS attainment was assessed by comparing design values (computed from air pollution monitor observations) under observed and counterfactual climates. Health equity was analyzed by regressing census tract-level ozone climate penalties against sociodemographic and health burden variables.

The ozone climate penalty in the DMNFR during the warm season (May-October) was estimated to be 0.5–1.0 ppb, with the highest penalties observed around major urban centers and later in the summer. The counterfactual analysis showed that the climate penalty increased the DMNFR ozone design values, delaying attainment of the 2008 (75 ppb) standard by approximately 2 years (to 2025) and the 2015 (70 ppb) standard by 2 years (to 2035). Spatial analysis revealed that the highest ozone concentrations and penalties were in areas with high population density and traffic. Health equity analysis indicated a positive association between the ozone climate penalty and several factors: percentage of Hispanic/Latino residents, percentage of children in poverty, and rates of asthma, diabetes, poor health status, and lack of health insurance. The strongest association was found with the percentage of residents with asthma.

The findings highlight the substantial impact of climate change on ozone air quality in the DMNFR, emphasizing the need for policy interventions that consider both climate mitigation and local air quality control. The disproportionate impact on vulnerable populations underscores the importance of addressing environmental justice issues alongside broader climate action. The 2-year delay in NAAQS attainment due to the climate penalty has significant economic consequences. The study's results are consistent with other research linking temperature increases to higher ozone levels. The observed spatial patterns of ozone and the climate penalty reflect the influence of urban development, traffic emissions, and topography.

This study provides crucial evidence that climate change is already worsening air quality in the DMNFR, particularly impacting vulnerable communities. The observed climate penalty delays attainment of ozone standards, incurring substantial health and economic costs. Future research should focus on more detailed health impact assessments, exploring the combined effects of ozone and other pollutants, and improving air quality models to incorporate more complex interactions. Aggressive climate mitigation and targeted interventions to improve health equity in affected communities are urgently needed.

The study assumes spatial smoothness in ozone fields, which might mask local-scale processes. The LUR models rely on existing ozone monitor data, and their predictions may be less accurate in areas with limited monitoring. The analysis extrapolates attainment years based on historical trends; changes in emissions control strategies could influence attainment timelines. Finally, the study focuses on a specific time period and region, limiting its generalizability to other locations and time frames.