How emissions, climate, and land use change will impact mid-century air quality over the United States: A focus on effects at national parks

We show that, when stringent domestic emission controls are applied, air quality is predicted to improve across the US, except surface O₃ over the western and central US under RCP8.5 conditions, where rising background ozone counteracts domestic emission reductions.

Under the RCP4.5 scenario, surface O₃ is substantially reduced (about 5 ppb), with daily maximum 8 h averages below the primary US Environmental Protection Agency (EPA) National Ambient Air Quality Standards (NAAQS) of 75 ppb (and even 65 ppb) in all the NPs. PM2.5 is significantly reduced in both scenarios (4 μg m⁻³; ~50%), with levels below the annual US EPA NAAQS of 12 μg m⁻³ across all the NPs; visibility is also improved (10-15 dv; >75 km in visibility range), although some western US parks with Class I status (40-74 % of total sites in the US) are still above the 2050 planned target level to reach the goal of natural visibility conditions by 2064.