A research team led by Prof. Xie Pinhua from Hefei Institutes of Physical Science (HFIPS) of the Chinese Academy of Sciences (CAS), studied the ratio of HCHO and NO₂ during a typical pollution episode in Hefei, a city in east central China, from Sept. 3 to 11, 2020, and found the characteristics of ozone formation control factors.

Their findings were published in Science of The Total Environment.

With the help of 2D Multi-Axis differential optical absorption spectroscopy (2D-MAX-DOAS) and light detection and ranging (LIDAR) in detecting spatial information of atmospheric pollutants, the researchers conducted an experiment of key pollutants and an ozone formation study in Hefei from September 1 to 20, 2020.

2D-MAX-DOAS is an optical telemetry method. It can retrieve the spatial distribution characteristics of various gaseous pollutants in the atmosphere by collecting solar scattered light at different azimuth and elevation angles.

"The most important discovery is that the O₃ formation controlling factors changed with altitudes," said Ren Bo, first author of the paper. He further explained the feature of main control factors affected by the tropospheric bottom layer (< 2km) on the formation of ozone.

The researchers found that the change of volatile organic compounds (VOCs) concentration in the near-surface atmosphere affected the concentration of O₃. But the control factors affecting ozone formation in the atmosphere were nitrogen oxides (such as NO, NO₂, etc.) at the height of 400m, then changed into VOCs and nitrogen oxides at the height of 1 km, and finally VOCs at the height of 1.6km.

"It can be seen from the observation that when the ratio of HCHO to NO₂ concentration in the atmosphere is about 5.50, ozone concentration decreases significantly," said Ren.

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