A Views & Comments article published in Engineering systematically unpacks the growing ozone pollution challenge across China and lays out two complementary technical frameworks to stabilise and lower tropospheric ozone concentrations, drawing on multi-year atmospheric monitoring, chamber simulation and field test evidence from national environmental research teams.
Since the launch of China's Clean Air Action Plan, nationwide ambient PM₂.₅ levels have maintained steady declines, yet surface ozone has emerged as a persistent secondary air pollutant with fluctuating upward trends across major urban agglomerations including the expanded 2+36 city cluster around Beijing, Tianjin and Hebei. Ozone now outpaces fine particulate matter as the primary pollutant on more air quality monitoring days, with regional average daily maximum 8-hour ozone readings consistently exceeding the secondary grade threshold set by China's National Ambient Air Quality Standard, and sitting far above the long-term exposure safety benchmark issued by the World Health Organization. The paper clarifies that ozone forms via nonlinear photochemical coupling cycles between nitrogen oxides (NOₓ) and volatile organic compounds (VOCs), with formation sensitivity split into three distinct regimes: NOₓ-limited, VOC-limited and transitional zones, differentiated by the regional ratio of the two precursor emissions as evaluated through empirical kinetic modelling approach (EKMA).
The study traces the core drivers of sustained ozone elevation to mismatched emission reduction progress between the two precursors. Anthropogenic NOₓ emissions recorded a notable drop between 2013 and 2017, while VOC abatement lagged significantly; uneven cuts weakened NO-driven titration effects and lifted ozone levels, with most Chinese urban and industrial zones currently operating under VOC-limited ozone formation conditions, while rural areas fall under NOₓ-limited chemistry. Meteorological shifts compound this trend: falling PM₂.₅ burdens increase surface solar radiation flux and reduce radical quenching by particulate surfaces, creating a chemical "seesaw relationship" between PM₂.₅ and ozone that has already broken down into positive pollutant correlation across southern China, allowing coordinated control of both pollutants. Warming temperatures further accelerate photochemical reaction rates and boost emissions of biogenic volatile organics and volatile chemical products, amplifying ozone generation amid China's dense industrial and urban precursor loadings.
For mitigation, the paper identifies large-scale NOₓ reduction as a more practically deliverable short-term precursor control strategy compared to broad VOC cuts. VOC sources are widely dispersed, mixing biogenic and diffuse anthropogenic streams with immature end-of-pipe treatment technologies, whereas NOₓ mainly stems from stationary combustion facilities and vehicle engines with mature removal systems including coal plant NH₃-selective catalytic reduction and automotive three-way and urea-SCR devices. Smog chamber and box model simulations confirm urban ozone will shift to NOₓ-limited status and begin declining only after deep NOₓ emission cuts, a pattern validated by COVID-19 lockdown observational data where extreme NO₂ suppression reversed earlier ozone growth trends. The paper notes region-tailored coordinated NOₓ-VOC reduction at optimal ratios remains the most theoretically effective long-term control route, yet deep NOₓ cuts represent the more actionable near-term pathway.
As a supplementary technical solution, the paper outlines ambient ozone direct catalytic decomposition coatings engineered for urban artificial surfaces including building exteriors. Low-cost transition metal catalysts blended into standard exterior coatings enable spontaneous ozone breakdown into oxygen under ambient temperature and humidity without external energy input. Field trials confirm consistent ozone decomposition activity across different distances from coated surfaces, and the technology carries modest cost increments over conventional building paint. Widespread deployment across China's dense built environments can deliver localized ozone removal at a far lower projected economic cost than joint NOₓ-VOC precursor abatement, forming the technical foundation of the proposed "environmental catalytic city" and self-purifying urban design frameworks aligned with national green building and dual-carbon development targets.
The paper "Ozone Pollution in China: Current Status and Control Strategies," is authored by Tianzeng Chen, Biwu Chu, Jinzhu Ma, Qingxin Ma, Qian Liu, Shuxiao Wang, Kebin He, Jincai Zhao, Hong He. Full text of the open access paper: https://doi.org/10.1016/j.eng.2025.06.044
