Wildfire Smoke Challenges Attainment of the Strengthened U.S. Annual PM 2.5 Standard through Regional and Transboundary Contributions

High Resolution Image Download MS PowerPoint Slide The recently strengthened U.S. annual standard for fine particulate matter (PM 2.5 ) of 9 μg m –3 narrows compliance margins nationwide, yet the implications of increasing wildfire activity for meeting this standard remain uncertain. Here, we use high-resolution nested GEOS-Chem simulations with emission sensitivity experiments for 2021–2023 to attribute annual PM 2.5 sources across the contiguous U.S. We find that severe fires in the western U.S. in 2021 produce substantial regional PM 2.5 enhancements and state-level exceedances. In 2023, simulations show that the unprecedented Canadian wildfire season leads to widespread exceedances across the Northeast and Midwest, exposing more than 150 million people to concentrations above the current standard. Sensitivity simulations indicate that reductions in domestic anthropogenic emissions alone are insufficient to eliminate exceedances in smoke-influenced years. PM 2.5 composition exhibits strong seasonal contrasts, with nitrate aerosols dominating wintertime concentrations, while organic aerosols become regionally prominent in summer during wildfire smoke episodes. Comparison with surface observations indicates that the model captures the spatial variability of PM 2.5 but exhibits a persistent low bias in annual mean concentrations. Overall, these results show that wildfire activity can offset gains from anthropogenic emission controls and increasingly constrain future attainment of the strengthened annual PM 2.5 standard.