China’s ozone pollution has shown some signs of stabilizing during the current 14th Five-Year Plan period (2021-2025), but the pollutant continues to pose a significant challenge to improving urban air quality, according to a draft report released in Beijing on Monday. Despite efforts to curb emissions, the overall national situation has not seen a decisive turnaround, the report states.

Compiled by the committee on ozone pollution control of the Chinese Society for Environmental Sciences, the blue book details that over the past decade, urban ozone concentrations have followed a two-stage pattern: a steady rise followed by a period of high-level fluctuations. During the 13th Five-Year Plan (2016-2020), the annual average ozone concentration across 339 major cities increased from 126 micrograms per cubic meter in 2016 to a peak of 144 mcg/m³ in 2019 before slightly declining to 138 mcg/m³ in 2020.

Since 2021, ozone levels have oscillated between 137 and 145 mcg/m³, registering 138 mcg/m³ in 2025. Meanwhile, concentrations of other major air pollutants have continued to decline. The report highlights ozone as a growing constraint in meeting urban air quality targets, with its share as the primary pollutant on days exceeding national standards nearly matching that of PM2.5 fine particulate matter. In 2025, ozone accounted for 48.9 percent of such days nationwide, compared with 50.4 percent for PM2.5.

The impact is more pronounced in key economic regions: ozone was the leading pollutant on 54.7 percent of days exceeding standards in the Beijing-Tianjin-Hebei region, 58 percent in the Yangtze River Delta, and an alarming 96.6 percent in the Pearl River Delta.

Hu Jianlin, a chief author of the report and professor at Nanjing University of Information Science and Technology, emphasized the complexity of ozone pollution. It arises from a combination of emissions, chemical transformations, weather patterns, regional atmospheric transport, and natural background levels. He noted that nitrogen oxides (NOx) and volatile organic compounds (VOCs)—the primary ozone precursors—remain at relatively high levels despite some reductions during the current Five-Year Plan. Industrial and transport sectors continue to be the main emission sources, with little structural change in key regions.

Hu also pointed to climate factors such as rising temperatures, anomalies in the East Asian monsoon, and a strengthening Western Pacific Subtropical High as exacerbating ozone formation. Higher temperatures and increased sunlight accelerate chemical reactions that produce ozone from VOCs and NOx.

Highlighting past policy experiences, Hu explained that earlier efforts targeting primarily nitrogen oxides without sufficient VOC control allowed ozone levels to rise between 2013 and 2017. Only when VOC reduction was prioritized alongside NOx did the situation begin to improve. He called for more differentiated, targeted strategies and stronger, systematic controls on both pollutant groups. He also advocated for integrating ozone governance into the broader framework of China’s green transformation and sustainable development goals.

Chai Fafe, former vice-president of the Chinese Research Academy of Environmental Sciences, described ozone pollution control as a long-term endeavor, citing international precedents in the United States where some cities have been granted compliance timelines as long as two decades. He stressed the importance of sound scientific understanding of ozone sources and the development of effective control technologies to achieve meaningful progress.

The report underscores the ongoing challenge ozone pollution poses amid China’s broader efforts to improve air quality, calling for sustained, science-based policies to address this complex environmental issue.