Researchers led by Prof. XIE Pinhua from the Hefei lnstitutes of Physical Science of the Chinese Academy of Sciences have discovered the source and transport mechanisms behind an episode of ozone pollution observed in Hefei, a city located in the Yangtze River Delta (YRD) region of China.

The results, published in Science of the Total Environment, offer potential solutions for the prevention and control of severe ozone pollution.

The Yangtze River Delta region experiences frequent ozone pollution events during the warm seasons. Synoptic patterns affect the transport and photochemical production of ozone at multiple scales, ranging from the local to regional scale. However, research on the three-dimensional transport characteristics and source attribution of ozone in the context of extreme ozone pollution events is still limited.

In this study, the researchers integrated stereoscopic detection techniques with numerical models, focusing on Hefei as a receptor city, to reveal the mechanisms behind an episode of exceptionally high ozone levels.

Based on ozone lidar observations, O₃-rich air masses were transported through the nocturnal residual layer over Hefei on September 5, 2020 and descended to the ground through vertical mixing processes the following day, leading to an extreme ozone peak in Hefei. Numerical model results indicated stagnant meteorological conditions in the Yangtze River Delta region, which hindered the dispersion of pollutants.

Massive photochemical production and accumulation of ozone led to the severe pollution over the Yangtze River Delta region. Initially, O₃-rich air masses were transported and converged over the northern Jiangsu under the southeaster wind. Subsequently, the regional circulation type was changed from southeasterly to northeasterly under the influence of the approaching typhoon, which brought the O₃-rich air masses into the upper atmosphere over Hefei.

The ozone source attribution results underlined that the intra-regional contributions from the Yangtze River Delta region dominated the formation of extreme ozone pollution in Hefei during this event.

"Our results provide insights into the regional ozone transport mechanism in the YRD and the optimization of measures to prevent and control severe ozone pollution at spatio-temporal scales," said Prof. XIE Pinhua.