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COVID-19 lockdown highlights ozone chemistry in China

Decreasing NOx emissions driving increased ozone pollution in late winter in China

Date:
March 1, 2021
Source:
Harvard John A. Paulson School of Engineering and Applied Sciences
Summary:
Recently, the ozone season in China has been getting longer, spreading from summer into early spring and late winter. The COVID-19 lockdown can help explain why. Researchers found that decreases in NOx emissions are driving increased ozone pollution in late winter in China.
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FULL STORY

In early 2020, daily life in Northern China slammed to a halt as the region entered a strict period of lockdown to slow the spread of COVID-19. Emissions from transportation and industry plummeted. Emissions of nitrogen oxides (NOx) from fossil fuels fell by 60 to 70 percent.

And yet, environmental researchers noticed that ground-level ozone pollution in Beijing and the Northern China Plain skyrocketed during this time period, despite the decrease of NOx, a component of ozone.

The region is no stranger to severe ozone pollution but until about five years ago, most ozone events occurred during the summer. Recently, the ozone season in China has been getting longer, spreading into early spring and late winter. As it turns out, the COVID-19 lockdown can help explain why.

Researchers from the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) and the Nanjing University of Information Science & Technology (NUIST) have found that another component of ozone, volatile organic compounds (VOCs), may be to blame for the increase in winter ozone.

The research is published in the Proceedings of the National Academy of Sciences (PNAS).

"The COVID-19 lockdown was an involuntary experiment in which the emissions decreased abruptly and a lot of ozone appeared suddenly," said Daniel J. Jacob, the Vasco McCoy Family Professor of Atmospheric Chemistry and Environmental Engineering at SEAS and co-corresponding author of the paper.

Ozone is formed through a series of chemical reactions, starting with the oxidation of VOCs. This reaction forms chemical radicals, which drive reactions between NOx and VOCs to produce ozone in the presence of sunlight. In a previous study, researchers from SEAS and NUIST found that in the summertime, particulate matter (PM2.5) acts like a sponge for the radicals needed to generate ozone pollution, sucking them up and preventing them from producing ozone.

In that paper, the researchers found that air pollution policies instituted by the Chinese government that reduced PM2.5 were causing an increase in harmful ground-level ozone pollution, especially in large cities.

In this research, the team found that NOx plays a similar role in the wintertime, scavenging radicals and preventing them from forming ozone. As NOx levels decrease, either all of a sudden with lockdown or gradually with air pollution controls, there are more radicals available for VOCs to react with. This enhanced oxidation of VOCs by radicals would amplify by producing more radicals themselves, and this process optimizes the ozone production efficiency of NOx.

"The COVID-19 experience helps explain the trend of increasing ozone pollution in the late winter and spring in China," said Ke Li, a postdoctoral fellow at SEAS and first author of the study. "As NOx emissions have decreased, the ozone season in China is getting longer."

The research highlights the need to better understand the sources and species of VOCs and regulate their emissions.

"VOC emission controls would stop the spread of the ozone season and have major benefits on public health, crop production, and particulate pollution," said Hong Liao, Professor at NUIST and co-corresponding author of this work.

The paper was co-authored by Yulu Qiu, Lu Shen, Shixian Zhai, Kelvin H. Bates, Melissa P. Sulprizio, Shaojie Song, Xiao Lu, Qiang Zhang, Bo Zheng, Yuli Zhang, Jinqiang Zhang, Hyun Chul Lee, and Su Keun Ku.

It was supported by NUIST through the Harvard-NUIST Joint Laboratory for Air Quality and Climate (JLAQC).


Story Source:

Materials provided by Harvard John A. Paulson School of Engineering and Applied Sciences. Original written by Leah Burrows. Note: Content may be edited for style and length.


Journal Reference:

  1. Ke Li, Daniel J. Jacob, Hong Liao, Yulu Qiu, Lu Shen, Shixian Zhai, Kelvin H. Bates, Melissa P. Sulprizio, Shaojie Song, Xiao Lu, Qiang Zhang, Bo Zheng, Yuli Zhang, Jinqiang Zhang, Hyun Chul Lee, and Su Keun Kuk. Ozone pollution in the North China Plain spreading into the late-winter haze season. Proceedings of the National Academy of Sciences, 2021; 118 (10): e2015797118 DOI: 10.1073/pnas.2015797118

Cite This Page:

Harvard John A. Paulson School of Engineering and Applied Sciences. "COVID-19 lockdown highlights ozone chemistry in China." ScienceDaily. ScienceDaily, 1 March 2021. <www.sciencedaily.com/releases/2021/03/210301151553.htm>.
Harvard John A. Paulson School of Engineering and Applied Sciences. (2021, March 1). COVID-19 lockdown highlights ozone chemistry in China. ScienceDaily. Retrieved March 27, 2024 from www.sciencedaily.com/releases/2021/03/210301151553.htm
Harvard John A. Paulson School of Engineering and Applied Sciences. "COVID-19 lockdown highlights ozone chemistry in China." ScienceDaily. www.sciencedaily.com/releases/2021/03/210301151553.htm (accessed March 27, 2024).

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