# Critical transition of urban ozone formation regime in the North China Plain

## TL;DR

Ozone pollution in North China has shifted from being limited by VOCs alone to being limited by both VOCs and NOx, offering new opportunities for targeted emission reductions.

## Contribution

The study identifies a critical transition in urban ozone formation regime and highlights the potential of VOC emission reductions under China’s carbon neutrality strategy.

## Key findings

- Ozone formation in North China shifted from VOC-limited to VOC–NOx co-limited between the 2010s and 2020s.
- Targeted VOC emission reductions from major sources offer the greatest mitigation potential for ozone pollution.
- A national carbon neutrality strategy could effectively alleviate ozone pollution in Chinese cities.

## Abstract

Over the past decade, China has achieved remarkable progress in mitigating aerosol pollution. However, ozone (O3) pollution still shows a worsening trend, implying that China’s control measures may have been less effective in tackling O3 pollution. Herein, we conduct synchronized observations of O3 and its precursors across 37 cities in the heavily polluted North China Plain (NCP) during the summer of 2021 and apply a unified observation-based model to diagnose O3 formation mechanisms. Our results reveal a significant transition in the urban O3 formation regime, shifting from being primarily volatile organic compound (VOC) limited to VOC–nitrogen oxides (NOx) co-limited across the NCP between the 2010s and the 2020s. Notably, the primary VOC species, their respective sources, and the optimal VOCs/NOx reduction ratios exhibit remarkable regional consistency. Modeling analyses further indicate that a long-term national ‘carbon neutrality’ strategy could effectively alleviate O3 pollution, with targeted VOC emission reductions from major anthropogenic sources offering the greatest mitigation potential. These findings underscore the efficacy of China’s endeavors in mitigating O3 pollution, although the effects are not immediately evident from ambient O3 concentrations. O3 pollution control in Chinese cities has reached a critical inflection point, offering considerable flexibility and feasibility in formulating future control policies.

Urban ozone formation in North China has reached a critical turning point, shifting from a predominantly VOC-limited regime to a VOCNOx co-limited regime across cities. This transition indicates that targeted reductions in key anthropogenic VOC emissions, aligned with the ``Carbon Neutrality’ strategy, can enable more effective and flexible O3 pollution control.