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

Despite sustained estimated reductions in anthropogenic precursor emissions, ozone design values remain above the 2015 8-hour National Ambient Air Quality Standard in four large, urban nonattainment areas (NAAs) in the Intermountain Western U.S.: Phoenix-Mesa, Arizona; Las Vegas, Nevada; Wasatch Front, Utah; and Denver Metro/North Front Range, Colorado. We synthesize recent field campaigns, regulatory analyses, and observationally constrained modeling to summarize shared drivers of stalled progress and the region-specific mechanisms that modulate ozone accumulation. Across these areas, elevated background ozone, terrain-driven recirculation, and meteorological conditions favoring stagnation and vertical coupling all reduce the effectiveness of local emission controls. However, many important factors also differ by airshed: Phoenix is strongly influenced by monsoon-modulated photochemistry and boundary-layer dynamics; Las Vegas by frequent entrainment of ozone-rich layers from aloft and a high ozone "floor"; the Wasatch Front by corridor confinement, elevated background concentrations, and multi-day recirculation; and the Front Range by complex terrain flows interacting with spatially heterogeneous precursor sources, including strong within-NAA oil and gas emissions. Together, each of these case studies underscores the need for ozone management strategies tailored to each airshed's structural and dynamical constraints, supported by coordinated collaborative science.Implications: 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 ozone standard. This synthesis identifies factors, both shared and specific to each airshed, driving persistent nonattainment. Elevated background ozone, wildfire smoke, terrain driven recirculation, and vertical entrainment all limit the effectiveness of local controls, though dominant mechanisms differ by area. For lawmakers and regulators, these findings underscore the need for region specific strategies, better vertical and precursor monitoring, and improved attribution of controllable versus transported ozone to support sound, technically defensible regulatory decisions.