Divergent Urban Ozone Responses to Straw Burning in Northern China from Observational Data: Roles of Meteorology and Photochemistry

Open burning of crop residue is a major source of air pollutants in China. While a nationwide straw burning ban implemented in 2016 has proven effective in reducing primary emissions, its impact on ozone (O3), a key pollutant detrimental to human health, remain poorly quantified. This study aims to assess the impact of straw burning on downwind urban O3 pollution and to investigate the complex mechanisms governing O3 changes resulting from transported agricultural fire plumes. Here, using multi-satellite data and ground observations from 2013 to 2020, this study developed a method to identify smoke-affected days and estimate plume transport time over northern China. The results show that the straw burning ban effectively reduced peak concentrations of particulate matter (PM2.5) during harvest seasons. However, O3 responses on smoke-affected days were heterogeneous, showing both increases and decreases. The random forest model revealed the meteorological and chemical drivers of O3 variability. Elevated formaldehyde (HCHO) and temperatures promote O3 production, while higher NO2 and relative humidity enhance its titration. Trajectory analysis further decoupled the mechanisms that O3 and HCHO enhancements were primarily driven by local photochemistry, whereas NO2 increases were attributable to regional transport and mixing with anthropogenic pollution. This study underscores the necessity for integrated air quality management strategies that account for the complex trade-offs between PM2.5 and O3 to fully realize the public health benefits of emission control policies.