The contribution of wildfire to PM_2.5 trends in the USA

Steady improvements in ambient air quality in the USA over the past several decades, in part a result of public policy^1,2, have led to public health benefits^1–4. However, recent trends in ambient concentrations of particulate matter with diameters less than 2.5 μm (PM_2.5), a pollutant regulated under the Clean Air Act¹, have stagnated or begun to reverse throughout much of the USA⁵. Here we use a combination of ground- and satellite-based air pollution data from 2000 to 2022 to quantify the contribution of wildfire smoke to these PM_2.5 trends. We find that since at least 2016, wildfire smoke has influenced trends in average annual PM_2.5 concentrations in nearly three-quarters of states in the contiguous USA, eroding about 25% of previous multi-decadal progress in reducing PM_2.5 concentrations on average in those states, equivalent to 4 years of air quality progress, and more than 50% in many western states. Smoke influence on trends in the number of days with extreme PM_2.5 concentrations is detectable by 2011, but the influence can be detected primarily in western and mid-western states. Wildfire-driven increases in ambient PM_2.5 concentrations are unregulated under current air pollution law⁶ and, in the absence of further interventions, we show that the contribution of wildfire to regional and national air quality trends is likely to grow as the climate continues to warm.