Nocturnal Ozone Enhancement Reshapes Urban Fine-Particle Chemistry

Nocturnal ozone enhancement (NOE) extends ozone exposure and may exacerbate risks to human health and ecosystem, yet its impacts on fine-particle matter (PM_2.5) chemistry remain poorly constrained. We characterized NOE occurrence across four Chinese megacities (Beijing, Shanghai, Guangzhou, and Shenzhen; 2019-2024) and integrated year-long in-field observations in Shenzhen with nontarget ultrahigh-resolution mass spectrometry and statistical analysis to examine NOE events. Here we show that NOE events in a megacity occurred with an average frequency ranging from 9.82 to12.37%. In Shenzhen, NOE events were associated with stronger mixing signatures and elevated nocturnal oxidizing capacity. Secondary aerosol components were enhanced and dominated the PM_2.5 response, as indicated by a more than 2-fold increase in the nitrate oxidation ratio, higher organic carbon/elemental carbon(OC/EC) ratio, and elevated particle oxidative potential. Importantly, these oxidizing and aerosol enhancements persisted into the subsequent daytime. At the molecular level, water-soluble organic matter shifted toward highly oxidized, carboxyl-bearing CHO species, and we identified 11 NOE-characteristic compounds dominated by CHO formulas. These results link nocturnal ozone dynamics to particle chemistry and highlight nighttime ozone as a lever for joint control of urban O₃ and PM_2.5.