Radical ozone chemistry

Last year’s lockdowns resulted in a drop in air pollution around the globe. But while other pollutants decreased, Beijing and northern China recorded a significant increase in ground-level ozone. A paper published in the Proceedings of the National Academy of Sciences explains ozone chemistry and why this may have happened.

Ozone (O₃) occurs naturally in the upper atmosphere, but artificial amounts at ground level have been increasing. Unlike O₂, it’s highly reactive and can have harmful health effects.

Ozone can be made by common pollutants mixing with the air. The cause is highly reactive molecules called chemical radicals. Volatile organic compounds, or VOCs, are produced from fossil fuels and can form chemical radicals, which start long sequences of reactions in the atmosphere. Nitrogen oxides, also made from fossil fuels, exacerbate these reactions. High levels of VOCs and nitrogen oxides in the air are linked to high levels of ozone.

But while nitrogen oxide gases dropped during the 2020 lockdowns, ozone levels shot up. “The COVID-19 lockdown was an involuntary experiment in which the emissions decreased abruptly and a lot of ozone appeared suddenly,” says Daniel J Jacob, a professor at Harvard University, US, and author on the paper.

The research found that while they can make ozone, nitrogen oxides also act as a sort of sponge, soaking up chemical radicals and preventing them from reacting further. At lower amounts, however, this sponging effect vanishes and nitrogen oxides just contribute to an increase in ozone. The authors suggest that the COVID-19 lockdowns created an optimal amount of nitrogen oxides in the air for producing ozone.

Ozone levels were already on the rise in China prior to the 2020 pandemic. Previous research from the same lab had found that particulate matter in the air had a similar sponging effect, soaking up free radicals. Clean air policies in China, which have reduced the amount of particulate matter over the past decade, have corresponded with a rise in ozone in the summer months.

This research, identifying nitrogen oxides as another sponge, explains the rising ozone in winter and particularly the jump during COVID-19 lockdowns.

The authors suggest that focussing on the original source of all this ozone – VOCs – is the best candidate for reducing air pollution.

“VOC emission controls would stop the spread of the ozone season and have major benefits on public health, crop production, and particulate pollution,” says Hong Liao, a professor at Nanjing University of Information Science and Technology, and an author on the paper.