image: Spatial distribution of ozone formation regimes across 37 cities in the North China Plain during June–August 2021. Daytime relative incremental reactivity (RIR) values for the major O3 precursors in each city were distributed around the map. Cities colors on the map and undertones of the surrounding box plots denote VOC-limited (dark orange) and VOC–NOx co-limited (light orange) regimes, respectively.
Credit: © Likun Xue
The research team, led by Professor Likun Xue from Shandong University and Professor Jian Gao from the Chinese Research Academy of Environmental Sciences reported a major advance in understanding urban ozone (O3) formation in China. Their findings demonstrate that O3 formation regime in most cities across the North China Plain has shifted from a VOC-limited regime to a VOC–NOx co-limited, underscoring the effectiveness of China’s efforts in mitigating O3 pollution.
Ozone pollution in China
In recent years, China has achieved remarkable success in reducing major air pollutants, including particulate matters (PM2.5 and PM10), nitrogen oxides (NOx), sulfur dioxide (SO2), and carbon monoxide (CO). However, O3 has exhibited a persistent upward trend and remains at high levels nationwide, emerging as one of the most significant issues to further improvements in urban air quality. North China Plain (NCP) stands out as the most severe O3 pollution in China.
A fundamental shift in ozone formation mechanisms across NCP cities
One of the greatest challenges in O3 mitigation is the highly nonlinear photochemical relationship between O3 and its precursors—NOx and volatile organic compounds (VOCs). Over the past decade, O3 formation in most Chinese cities was predominantly under the VOC-limited (and NOx-saturated) regime, under which reductions in NOx emissions by a certain amount can exacerbate O3 concentrations.
Through coordinated field observations and unified model analyses across 37 cities in the NCP, the study demonstrates that O3 formation in most cities has now shifted from a VOC-limited regime to a VOC–NOx co-limited regime. This transition is most pronounced in cities and during periods experiencing the most severe O3 pollution, indicating that China has passed through the most challenging phase. This shift is largely attributable to the substantial and sustained reductions in NOx emissions over the past decade. Under the current VOC–NOx co-limited regime, further reductions in either NOx or VOC emissions will be effective in alleviating O3 pollution, offering greater flexibility and effectiveness for future control strategies.
High regional consistency in key VOC species and emission sources
Despite substantial differences in economic structure and emission characteristics among cities, the study reveals striking regional homogeneity in the key VOC species and emission sources driving O3 formation across the NCP. At the species level, highly reactive VOCs such as propylene, isoprene, butenes, and aromatic compounds dominate O3 production. In terms of sources, plant emissions, petrochemical industry, solvent usage, and vehicle exhaust together contribute more than 80% of the total ozone formation potential. This strong remarkable consistency suggests that coordinated regional strategies targeting similar precursor sources can achieve greater efficiency and synergistic benefits in ozone mitigation, thereby enhancing the overall effectiveness of air quality management across the NCP.
Ozone levels are expected to decline under future climate and control policies
Building upon the identified transition in O3 formation mechanisms, the study further evaluated the potential impacts of China’s “Carbon Neutrality” objective. Under the “double-carbon” scenario according to the Dynamic Projection Model for Emissions in China (DPEC), O3 concentrations across NCP cities are likely to decline substantially. By 2030, the control policy is projected to significantly reduce O3 levels. By 2060, O3 formation is expected to shift further toward a NOx-limited regime, potentially leading to a fundamental and sustained alleviation of regional O3 pollution.
The findings represents a crucial milestone in the effort to mitigate O3 pollution. Further reductions in both NOx and VOCs are now expected to be broadly effective, providing greater flexibility, feasibility, and optimism for future O3 mitigation in China.