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

Abstract

Photochemical ozone (O 3 ) formation in the atmospheric boundary layer occurs at both the surface and elevated altitudes. Therefore, the O 3 formation sensitivity is needed to be evaluated at different altitudes before formulating an effective O 3 pollution prevention and control strategy. Herein, we explore the vertical evolution of O 3 formation sensitivity via synchronous observations of the vertical profiles of O 3 and proxies for its precursors, formaldehyde (HCHO) and nitrogen dioxide (NO 2 ), 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 2 ratio (FNR) varied with altitude. The VOC-limited regime dominated at the ground level, whereas the contribution of the NO x -limited regime increased with altitude, particularly on heavily polluted days. The NO x -limited and transition regimes played more important roles throughout the entire boundary layer than at the surface. The feasibility of extreme NO x reduction to mitigate the extent of the O 3 pollution was evaluated using the FNR−O 3 curve. Based on the surface sensitivity, the critical NO x reduction percentage for the transition from a VOC-limited to a NO x -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 3 pollution in the YRD and PRD regions will transition to the NO x -limited regime before 2030 and be mitigated with further NO x reduction. KEYWORDS: O 3 formation sensitivity, vertical profile, ground-based remote sensing, formaldehyde to NO 2 ratio (FNR), extreme NO x reduction