Impacts of Intensified Heatwaves on Escalation of Ozone and Reactive Nitrogen in Summer

Global warming has intensified the frequency and severity of heatwave events (HWs); however, the research on the interaction between O₃ and reactive nitrogen during HWs is far from sufficient. In this study, three types covering nine HWs events were identified during summer 2023 (June-August) in Beijing, including daytime HWs (DHWs, 4 days), nighttime HWs (NHWs, 8 nights), and compound HWs (CHWs, 27 days). Twenty-eight days of O₃ pollution (OP) occurred during the whole summer, among which 64% (18 out of 28) appeared during HWs. HWs elevated O₃ concentrations by 27.3% comparing to non-HWs, while CHWs significantly amplified diurnal O₃ fluctuations accompanied by the accumulation of reactive nitrogen (NOx, NOz, and NOy), indicting a stronger tendency toward O₃-reactive nitrogen combined pollution. During HWs, NOx/NOy ratios were reduced, and secondary oxidized nitrogen NOz levels were elevated, which accelerated photochemical O₃ production and promoted VOC-driven chain reactions, thereby intensifying and prolonging O₃ pollution. O₃ suppression was observed at temperatures of ∼38 °C, indicating a nonlinear and complex influence of HWs on the O₃ formation. This work elucidates synergistic O₃-reactive nitrogen transformation mechanisms under extreme HWs, providing a scientific foundation for refined regional pollution modeling and precision mitigation strategies.