Ozone pollution still a big hurdle to better air quality

China's ozone pollution showed signs of easing its upward trend during the 14th Five-Year Plan period (2021-25), but the pollutant remains a major obstacle to improving air quality, according to a draft blue book released in Beijing on Monday.

The report said the national situation has yet to show a decisive and effective turnaround.

Compiled by the committee on ozone pollution control of the Chinese Society for Environmental Sciences, the blue book said the national average ozone concentration in urban areas has followed a two-stage pattern over the past decade: a steady rise followed by a period of high-level fluctuations.

During the 13th Five-Year Plan period (2016-20), the annual average ozone concentration in China's 339 major cities rose from 126 micrograms per cubic meter in 2016 to a decade high of 144 mcg/cu m in 2019, before edging down to 138 mcg/cu m in 2020.

Since the start of the 14th Five-Year Plan, the figure has fluctuated between 137 and 145 mcg/cu m, reaching 138 mcg/cu m in 2025.

By contrast, concentrations of other major air pollutants have continued to decline over the same period. Ozone, however, has become one of the main constraints on meeting urban air-quality targets, and its role as a primary pollutant has become increasingly pronounced, according to the blue book.

In 2025, among the days with air quality exceeding national standards across the country, days with ozone as the primary pollutant accounted for 48.9 percent of the total, while days with PM2.5 particulate matter as the primary pollutant accounted for 50.4 percent.

The proportion was higher in key regions. Ozone was the primary pollutant on 54.7 percent of such days in the Beijing-Tianjin-Hebei cluster, 58 percent in the Yangtze River Delta and 96.6 percent in the Pearl River Delta.

Hu Jianlin, a chief author of the blue book and a professor at the School of Environmental Science and Engineering at Nanjing University of Information Science and Technology, said ozone pollution is a complex issue involving emissions, chemical transformation, meteorological conditions, regional transport and natural background sources.

He said the main precursors of ozone are nitrogen oxides and volatile organic compounds, or VOCs, which remain at relatively high levels despite declines during the 14th Five-Year Plan period.

Hu said the industrial and transport sectors remain the main sources of ozone precursors, with no fundamental change in the emission structure in key regions.

He also noted that rising temperatures, anomalies in the East Asian monsoon and a strengthening Western Pacific Subtropical High have worsened ozone pollution.

Warm temperatures and sunlight accelerate chemical reactions between VOCs and nitrogen oxides, making ozone more likely to form.

Hu said the experience of the past decade shows that sustained control efforts can produce results.

Between 2013 and 2017, reduction policies focused mainly on nitrogen oxides, while VOCs received insufficient attention, resulting in rising ozone concentrations, he said. In the following phase, when VOC control was prioritized alongside nitrogen oxide reduction, tangible progress began to emerge.

Hu called for differentiated and more targeted reduction strategies, with stronger and more systematic controls on both pollutants. He also said ozone governance should be incorporated into the broader framework of green transformation in economic and social development.

Chai Fahe, former vice-president of the Chinese Research Academy of Environmental Sciences, said ozone governance is a long-term task, as international experience shows.

In the United States, some cities that fail to meet ozone standards are allowed compliance periods of up to two decades, he said.

"Winning this long battle hinges on solid science. We need to truly understand the problem and its real sources, while also building better control technologies that deliver clearer, stronger results," he said.

houliqiang@chinadaily.com.cn