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 (O) formation in the atmospheric boundary layer occurs at both the surface and elevated altitudes. Therefore, the O formation sensitivity is needed to be evaluated at different altitudes before formulating an effective O pollution prevention and control strategy. Herein, we explore the vertical evolution of O formation sensitivity via synchronous observations of the vertical profiles of O and proxies for its precursors, formaldehyde (HCHO) and nitrogen dioxide (NO), 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/NO ratio (FNR) varied with altitude. The VOC-limited regime dominated at the ground level, whereas the contribution of the NO-limited regime increased with altitude, particularly on heavily polluted days. The NO-limited and transition regimes played more important roles throughout the entire boundary layer than at the surface. The feasibility of extreme NO reduction to mitigate the extent of the O pollution was evaluated using the FNR-O curve. Based on the surface sensitivity, the critical NO reduction percentage for the transition from a VOC-limited to a NO-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, O pollution in the YRD and PRD regions will transition to the NO-limited regime before 2030 and be mitigated with further NO reduction.