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 NOx-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 NOx. Notably, emission reduction policies have driven the NCP region to become increasingly sensitive to NOx, both horizontally and vertically. This shift stems from HO2 radicals increasing more significantly than OH radicals, causing pH2O2 to rise more than pHNO3. 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.