China’s progress in reducing fine particulate pollution has exposed a more elusive threat in the nation’s atmosphere: ozone. A new article in Engineering warns that surface-level ozone is becoming an increasingly persistent pollutant across major urban regions, even as concentrations of PM₂.₅ continue to decline. Drawing on long-term monitoring, atmospheric chamber experiments, chemical modelling and field tests, the authors describe how ozone pollution is evolving and propose a two-part response: coordinated reductions in the gases that create ozone, combined with catalytic coatings capable of breaking down ozone directly on urban surfaces.
Since the launch of China’s Clean Air Action Plan, national PM₂.₅ concentrations have fallen substantially. Yet the improvement in particulate pollution has not produced an equivalent decline in ozone. In many major urban agglomerations, including the expanded Beijing–Tianjin–Hebei region and surrounding cities, ozone has become the leading pollutant on a growing number of days when air quality standards are exceeded. The pollutant of concern is not the protective ozone layer high in the stratosphere, but ozone near the ground, where it can irritate the respiratory system, damage vegetation and contribute to a range of harmful atmospheric reactions.
The article explains that ground-level ozone is not emitted directly in large quantities by a single source. Instead, it forms through sunlight-driven chemical reactions involving nitrogen oxides, known collectively as NOₓ, and volatile organic compounds, or VOCs. Under ultraviolet radiation, nitrogen dioxide can split and generate oxygen atoms that combine with molecular oxygen to produce ozone. VOCs participate in radical reactions that recycle nitrogen oxides and allow ozone production to continue. Because these reactions are nonlinear, reducing one precursor does not always lower ozone immediately. In some circumstances, cutting NOₓ can even increase ozone temporarily by weakening the reaction in which freshly emitted nitric oxide removes ozone, a process known as ozone titration.
According to the researchers, the balance between NOₓ and VOCs divides regions into three chemical regimes: NOₓ-limited, VOC-limited and transitional. In NOₓ-limited areas, additional nitrogen oxides tend to increase ozone formation, so reducing NOₓ is particularly effective. In VOC-limited areas, ozone responds more strongly to reductions in organic precursors. The authors use the empirical kinetic modelling approach, commonly called EKMA, to explain how these regimes are identified and why control strategies must be adapted to local atmospheric chemistry. Their assessment indicates that many Chinese cities and industrial regions remain VOC-limited, while rural areas are more often NOₓ-limited.
This uneven chemistry is closely linked to the country’s changing emissions profile. Between 2013 and 2017, anthropogenic NOₓ emissions declined significantly, while VOC reductions progressed more slowly. The resulting imbalance weakened the ozone-removal effect of nitric oxide in some urban environments and helped sustain or intensify ozone pollution. VOCs also come from a far more dispersed collection of sources, including solvent use, fuel evaporation, industrial processes, consumer products, chemical manufacturing and vegetation. These sources are difficult to measure and control uniformly, and many VOC treatment technologies remain less mature than established systems for reducing NOₓ from power plants and vehicles.
The relationship between ozone and PM₂.₅ has added another layer of complexity. As particulate concentrations fall, more sunlight can penetrate the lower atmosphere, increasing the radiation available to drive photochemical reactions. Particles can also remove reactive radicals from the atmosphere through surface reactions; when fewer particles are present, these radicals may remain available to promote ozone formation. The result is sometimes described as a chemical seesaw between PM₂.₅ and ozone, although the relationship varies by location and season. In parts of southern China, the earlier contrast between the two pollutants has weakened, creating conditions in which coordinated control of both may become increasingly practical. Rising temperatures could intensify the problem by accelerating chemical reaction rates and increasing emissions of biogenic VOCs and volatile chemical products.
The authors argue that deep NOₓ reductions offer a comparatively achievable short-term route to changing the chemistry of heavily polluted urban areas. Most NOₓ comes from combustion in power stations, industrial facilities, ships, construction equipment and vehicle engines, where mature control technologies are already available. These include ammonia-based selective catalytic reduction systems in coal-fired power plants, three-way catalytic converters in gasoline vehicles and urea-based selective catalytic reduction in diesel engines. Atmospheric simulations and smog-chamber experiments indicate that substantial NOₓ reductions can move urban regions away from VOC-limited conditions and toward a state in which ozone begins to decline. Observations during COVID-19 lockdowns, when traffic and industrial nitrogen dioxide emissions fell sharply, provided a real-world demonstration that abrupt changes in NOₓ can alter ozone trends.
The long-term solution, however, is unlikely to be a single nationwide formula. The paper calls for region-specific reductions in both NOₓ and VOCs, calibrated to the precursor ratio and chemical regime of each area. Cutting VOCs is essential in urban locations where ozone formation is VOC-limited, but broad VOC control is technically difficult because emissions are diffuse and chemically diverse. Controls must also account for transport between cities and provinces, since ozone and its precursors can travel substantial distances before chemical reactions are complete. The researchers therefore describe coordinated NOₓ–VOC management as the most scientifically effective strategy, while identifying deep NOₓ cuts as the more immediately deployable component.
Alongside emission controls, the article presents a direct-removal technology designed for the surfaces of cities. Functional coatings containing low-cost transition-metal catalysts can be applied to building exteriors and other artificial surfaces. At ambient temperature and humidity, the catalyst promotes the decomposition of ozone into molecular oxygen without requiring an external energy supply. Laboratory and field tests reported by the authors found that the coatings retained ozone-decomposition activity at different distances from treated surfaces, suggesting that their influence is not restricted to a microscopic layer directly touching the paint. The coatings also add only a modest projected cost compared with conventional exterior paint, making large-scale application conceivable in dense urban environments.
The proposed coatings are not presented as a replacement for controlling emissions at their sources. Their likely impact would be local, depending on the amount of treated surface, airflow, ozone concentration and the durability of the catalytic material under real weather conditions. Instead, the technology is positioned as a supplementary layer of protection within an “environmental catalytic city,” in which buildings and infrastructure actively participate in pollutant removal. If combined with better monitoring, targeted precursor reductions and low-emission urban planning, catalytic surfaces could help reduce ozone exposure in hotspots such as traffic corridors, industrial districts and densely populated neighborhoods. The authors say this integrated approach could support China’s green-building and dual-carbon goals while addressing an air-quality problem that has become more visible precisely because other pollutants are being brought under control.
Subject of Research: Ozone pollution in China and atmospheric ozone control technologies
Article Title: Ozone Pollution in China: Current Status and Control Strategies
Web References: https://doi.org/10.1016/j.eng.2025.06.044; https://www.sciencedirect.com/journal/engineering
References: Chen, T., Chu, B., Ma, J., Ma, Q., Liu, Q., Wang, S., He, K., Zhao, J., and He, H. “Ozone Pollution in China: Current Status and Control Strategies.” Engineering. DOI: 10.1016/j.eng.2025.06.044
Image Credits: Tianzeng Chen, Biwu Chu, Jinzhu Ma, Qingxin Ma, Qian Liu, Shuxiao Wang, Kebin He, Jincai Zhao and Hong He
Keywords
Ozone pollution, China air quality, NOₓ, volatile organic compounds, atmospheric chemistry, PM₂.₅, catalytic coatings, ozone decomposition, environmental catalysis, urban air pollution
