Shifts of the Three-Dimensional O3 Photochemical Regime of the North China Plain within the Boundary Layer

Delineation of sensitive areas for O₃ formation is essential for the scientific control of O₃ pollution. Most previous studies have primarily focused on characterizing the surface O₃ photochemical regime. Recent studies show significant vertical gradients in ozone precursor concentration ratios within the boundary layer, with weaker VOC-driven radical cycles and enhanced NO_x-mediated photochemistry in the upper boundary layer. Given vertical transport of O₃ within the boundary layer, quantifying a three-dimensional photochemical regime is critical. In this study, we employed the chemical transport model GEOS-Chem combined with the photochemical indicator to investigate changes in the O₃ photochemical regime over the North China Plain (NCP) from 2013 to 2017. The analysis indicates that VOC-limited or transitional regimes prevail near the surface, whereas regions closer to the top of the boundary layer exhibit greater sensitivity to NO_x. Notably, emission reduction policies have driven the NCP region to become increasingly sensitive to NO_x, both horizontally and vertically. This shift stems from HO₂ radicals increasing more significantly than OH radicals, causing p_H2O2 to rise more than p_HNO3. In turn, the elevation of radical concentrations and the transition of control regimes jointly boost daytime O₃ formation and curbs nighttime O₃ consumption, leading to O₃ increases in the NCP.