Tighter Control of AVOCs/NOx atop Carbon Neutrality Mitigations for China to Meet WHO Ambient Ozone Standard

Abstract. Tropospheric ozone (O₃) remains a persistent air quality challenge in China, and it is uncertain whether deep precursor emission cuts aligned with carbon neutrality can secure the World Health Organization (WHO) ozone guideline of 60 μg m⁻³ across all major urban agglomerations. Here we simulate summertime MDA8 O₃ over China’s five largest urban agglomerations using CMAQv5.5 under a range of emission scenarios, including a national synergistic emission reduction strategy (NSERS) that reduces NOₓ and anthropogenic VOCs by 40 % and 30 % in 2030, and by 90 % and 50 % in 2060, relative to 2020. To address remaining exceedances, we introduce an exceedance-driven local emission homogenization (O₃-LEH) method that constrains precursor emissions in non-attainment grid cells by the mean of nearby compliant cells. NSERS lowers cluster-mean MDA8O₃ to 41–72 μg m⁻³ by 2060, but residual exceedances above the WHO guideline persist in high-emission hotspots, mainly in the Beijing-Tianjin-Hebei and Yangtze River Delta regions. The O₃-LEH adjustments further reduce O₃ in these hotspots, yet only partially eliminate exceedances. Relative incremental reactivity diagnostics reveal a systematic transition from VOC-limited regimes in 2020 toward NOₓ-limited regimes in 2060, accompanied by a sharp increase in the AVOC/NOₓ ratio. Our results indicate that uniform national emission reductions alone cannot guarantee full attainment of the WHO ozone guideline, and that spatially differentiated, hotspot-targeted controls are a necessary complement to deep decarbonization.