Maximum ozone concentrations in the southwestern US and Texas: implications of the growing predominance of the background contribution

Abstract. We utilize a simple, observation-based model to quantitatively estimate the US anthropogenic, background and wildfire contributions to the temporal and spatial distributions of maximum ozone concentrations throughout the southwestern US, including Texas and parts of California. The very different temporal variations in the separate contributions provide the basis for this analysis: over the past 4 decades the anthropogenic contribution has decreased at an approximately exponential rate by a factor of ∼ 6.3, while the US background concentration rose significantly through the 1980s and 1990s, reached a maximum in the mid-2000s, and has since slowly decreased. We primarily analyze ozone design values (ODVs), the statistic upon which the US National Ambient Air Quality Standards (NAAQS) are based. The ODV is an extreme value statistic that quantifies the relatively rare maximum observed ozone concentrations; thus, ODV time series provide spatially and temporally resolved records of maximum ozone concentrations throughout the country. Recent contributions of US background ozone to ODVs (primarily due to transported baseline ozone) are 64–70 ppb (parts per billion) over most of the southwestern US, and wildfires (also generally considered a background contribution) add further enhancements of 2–6 ppb in southwestern US urban areas. US anthropogenic emissions from urban and industrial sectors now produce only relatively modest enhancements to ODVs (less than ∼ 6 ppb in 2020) outside of the three largest urban areas considered (Dallas, Houston and Los Angeles), where the 2020 enhancements were in the 17–30 ppb range. As a consequence, US background ozone concentrations now dominate over US anthropogenic contributions in the western US, including the Los Angeles urban basin, where the largest US ozone concentrations are observed. In the southwestern US, this predominance is so pronounced that the US background plus wildfire contributions to ODVs approach or exceed the US NAAQS threshold for ozone of 70 ppb (implemented in 2015) and 75 ppb (implemented in 2008); consequently, NAAQS achievement has been precluded in this region. The large background contribution in this region has led to a pronounced shift in the spatial distribution of maximum US ozone concentrations; once ubiquitous nearly nationwide, ODVs of 75 ppb or greater have nearly disappeared in the eastern US, but such values are still frequent in the southwestern US. By 2021, the trend in maximum ODVs in two of the more highly populated eastern urban areas (i.e., New York City and Atlanta) had decreased to the point that they were smaller than those in significantly less populated southwestern US urban areas and nearly as small as ODVs recorded at isolated rural southwestern US sites. Two implications arise from these findings. First, alternate emission control strategies may provide more effective approaches to ozone air quality improvement; as background ozone makes the dominant contribution to even the highest observed concentrations, an international effort to reduce northern midlatitude baseline ozone concentrations could be pursued, or a standard based on the anthropogenic increment above the regionally varying US background ozone concentration could be considered to provide a regionally uniform emission reduction challenge. Second, the predominant contribution of US background ozone across the southwestern US presents a profound challenge for air quality modeling, as a manifold of stratospheric and tropospheric processes occurring at small spatial scales but over hemisphere-wide distances must be accurately treated in detail to predict present and future background contributions to daily maximum ozone concentrations at local scales.