Growing wildfire-derived PM2.5 across the contiguous U.S. and implications for air quality regulation

Marissa Childs, Mariana Martins, Andrew Wilson, Minghao Qiu, Sam Heft-Neal, Marshall Burke

Growing wildfire activity across North America generates large amounts of smoke that can travel long distances. Characterizing the influence of this smoke on surface air quality is critical for regulation of air quality and protection of public health. Here we provide granular, daily estimates of smoke fine particulate matter (PM_2.5) concentrations across the contiguous U.S. and use them to assess the influence of wildfire smoke on surface PM_2.5 from 2006 to 2024, using a combination of surface measurements, satellites, and machine learning. Each year from 2020 to 2024, population-average smoke PM_2.5 exposures were 2.6--6.9 times higher than the 2006 to 2019 average and exposure periods were more than twice as long. Despite wildfire smoke being historically more common in the Western U.S., the worst 5 days for population-average smoke exposure in our sample period all occurred in 2023, a year with limited Western exposure. We estimate that 34% of monitoring stations are currently above the recently-updated ambient air quality standards but would be below them absent smoke, necessitating increased use of extreme event exemptions to remain within regulatory limits; we show that such use is already rare on attainment-relevant days and could become increasingly challenging. Absent wildfire smoke, we estimate that PM_2.5 concentrations would have continued to improve throughout the contiguous U.S.