Fire-Driven Trend Reversal in U.S. Ozone Exposure and Air Quality Progress

Recent surge in wildfire emissions from the Western USA and Canada significantly elevates the surface ozone levels across the Continental USA, hindering regulatory compliance and undermining public health ^1-5 . Here, we develop a full-coverage, long-term (2003-2023), high-resolution (1 km), daily surface ozone dataset over the Continental USA using deep learning, with strong performance in capturing exceedance events of National Ambient Air Quality Standards. Compared to benchmark Copernicus Atmosphere Monitoring Service reanalysis, the dataset captures 2.5 additional exceedance events and avoids 7.5 false alarms on average per monitoring station per year. It better resolves wildfire-driven ozone peaks spatially and temporally as compared to sparse station measurements, and enables neighborhood-scale compliance assessment, improving upon current county-scale evaluations. Our dataset reveals a reversal in national trends of ozone exposure and associated premature mortality: a decline during 2003-2013 (-0.25 ppb×year ^-1 ; -2.81 deaths×million ^-1 ×year ^-1 ) followed by an increase during 2013-2023 (+0.10 ppb×year ^-1 ; +1.22 deaths×million ^-1 ×year ^-1 ). The reversal is primarily driven by rising fire emissions, with the most severe fire impacts on ozone exposure and associated mortality in 2023 (2.34 ppb; 7100 premature deaths), followed by 2021 (1.64 ppb; 4900 premature deaths) and 2020 (1.49 ppb; 4300 premature deaths). While the Western USA consistently experiences high ozone levels, the fastest increase occurs in the urban Midwest. In 2023, the long-range transport of Canadian fire emissions place additional 78 million people in nonattainment zones, effectively preventing a potential 5-ppb tightening of ozone standard and erasing a decade of mitigation progress. Growing challenge in ozone regulation and public health protection is expected in the coming decades as wildfire emissions increase under a warming climate. ^1,6,7