Urban ozone nonattainment: Interconnected challenges in the Intermountain Western U.S

." Despite decades of declining anthropogenic precursor emissions, four major Intermountain Western U.S. metropolitan areas - Phoenix, Las Vegas, Salt Lake City, and Denver - continue to exceed the 2015 8-hour ozone standard. This synthesis identifies both shared and airshed-specific factors driving persistent nonattainment. Elevated background ozone, wildfire smoke, terrain-driven recirculation, and vertical entrainment all substantially limit the effectiveness of local controls, though the dominant mechanisms differ between areas. For lawmakers and regulators, these issues underscore the need for region-specific strategies, enhanced vertical profiling and speciated precursor monitoring, and improved attribution of controllable versus transported contributions to support technically defensible regulatory decisions.