Vertical Evolution of Ozone Formation Sensitivity Based on Synchronous Vertical Observations of Ozone and Proxies for Its Precursors: Implications for Ozone Pollution Prevention Strategies

Photochemical ozone (O3) formation in the atmospheric boundary layer occurs at both the surface and elevated altitudes. Therefore, the O3formation sensitivity is needed to be evaluated at different altitudes before formulating an effective O3pollution prevention and control strategy. Herein, we explore the vertical evolution of O3formation sensitivity via synchronous observations of the vertical profiles of O3and proxies for its precursors, formaldehyde (HCHO) and nitrogen dioxide (NO2), using multi-axis differential optical absorption spectroscopy (MAX–DOAS) in urban areas of the Beijing–Tianjin–Hebei (BTH), Yangtze River Delta (YRD), and Pearl River Delta (PRD) regions in China. The sensitivity thresholds indicated by the HCHO/NO2ratio (FNR) varied with altitude. The VOC-limited regime dominated at the ground level, whereas the contribution of the NOx-limited regime increased with altitude, particularly on heavily polluted days. The NOx-limited and transition regimes played more important roles throughout the entire boundary layer than at the surface. The feasibility of extreme NOxreduction to mitigate the extent of the O3pollution was evaluated using the FNR–O3curve. Based on the surface sensitivity, the critical NOxreduction percentage for the transition from a VOC-limited to a NOx-limited regime is 45–72%, which will decrease to 27–61% when vertical evolution is considered. With the combined effects of clean air action and carbon neutrality, O3pollution in the YRD and PRD regions will transition to the NOx-limited regime before 2030 and be mitigated with further NOxreduction.