Abstract Description: Despite emission reductions of ozone precursors, ozone concentrations have increased in several areas in the western states. Several urban areas in the western U.S., including Denver, Salt Lake City, Phoenix and Las Vegas, measured higher ozone in more recent years (e.g., 2020, 2021 and 2024) than previous years at the same time anthropogenic emission reductions are occurring. Most of these areas are classified as Serious ozone nonattainment areas under the 2015 ozone National Ambient Air Quality Standard (NAAQS) and are preparing ozone State Implementation Plans (SIPs) due in early 2026. These ozone SIPs are required to demonstrate attainment of the ozone NAAQS by 2026. This paper discusses the data analysis and photochemical modeling efforts of several western U.S. states to demonstrate 2026 ozone attainment and understand why the observed ozone has been increasing in more recent years. The effects of climate change on ozone formation is discussed, including: increased temperature that speed up photochemical reactions and increase biogenic emissions, producing more frequent and intense meteorological conditions more conducive to ozone formation, and enhanced size and intensity of wildfires that are a source of ozone precursor emissions. The transition from more VOC-sensitive to more NOx-sensitive ozone formation conditions may also influence the lack of ozone reduction progress as the peak ozone follows the “ridge line” of the ozone isopleth. The Denver area is used as a case study demonstrate that NOx and VOC emissions are being reduced based on satellite and ground-based measurements while conditions conducive to ozone formation are more prevalent in recent years. Trends in emissions, ozone, meteorological conditions conducive to ozone formation and wildfires are examined to help explain the lack of ozone progress. The procedures for photochemical modeling for the Denver Serious ozone SIP are discussed along with how wildfires and accounted for and emission control measures are evaluated.
