Large improvements of air quality in China during the lockdown have been widely reported, but new research shows that two pollutants harmful to human health—fine particulate matter (PM_2.5) and ozone—were only slightly reduced.

The study, by scientists from the University of Leeds, UK and the Southern University of Science and Technology, China, analyzed air pollutant concentrations from China’s national network of around 1,300 monitoring stations to quantify the response of air pollution across China during the COVID-19 lockdown.

They found that the falls in some air pollutants such as nitrogen dioxide (NO₂) were substantial whereas other pollutants such as PM_2.5 and ozone were only slightly reduced or barely affected. The open-access study is published in the IOP Publishing journal Environmental Research Letters.

Time series of the relative anomaly (%) during 2020 in the 7 d residual mean concentration of (a) NO₂, (b) PM_2.5, (c) PM₁₀ and (d) O₃. This is calculated by dividing the 7 d mean of the residual component of by the sum of the seasonal, trend and Lunar New Year components. The black line shows the median across all stations, with the coloured lines showing the medians across regions. The 'lockdown period,' defined as 23 January to 31 March, is shaded. Silver et al.

To understand the impact of the control measures during the COVID-19 outbreak, the researchers compared pollutant concentrations in 2020 with expected concentrations had the COVID-19 outbreak not occurred.

They used a time series of China-wide measurements of key pollutant concentrations from January 2015 to April 2020 to isolate the changes during the lockdown period, compared with concentrations otherwise expected based on recent trends, seasonality, and the effects of the Lunar New Year (the precise date of which changes from year to year).

During the lockdown period in China, defined as January 23rd to March 31st, 2020, we found that the largest reductions occurred in NO₂, with concentrations 27% lower on average across China. The largest reductions were in Hubei province, where NO₂ concentrations were 50.5% lower than expected during the lockdown.

Much smaller reductions were observed for other pollutants. PM_2.5—fine particles measuring less than 2.5 µm—had a modest reduction of 11% across China, and was not reduced in north-east China. These particles are the most harmful constituent of air pollution, as they travel deep into the lungs and bloodstream and damage the lungs and heart. Ozone can irritate breathing, affect lung function and worsen lung conditions such as asthma. We found almost no change in ozone concentrations because of the pandemic control measures.

—Lead author Ben Silver, from the University of Leeds

Chinese NO_x emissions are dominated by transport (35%), industry (35%), and power generation (19%), all of which are likely to have been affected by the lockdown. Reduction in emissions from these dominant sectors and short lifetime explain the larger reduction in NO₂ compared to other pollutants.

PM_2.5 concentrations in China are heavily influenced by residential emissions, which are likely to have been less affected by the control measures. The larger relative reductions in PM₁₀ and CO (carbon monoxide) compared to PM_2.5, may be due to a greater reduction in primary emission sources and the greater contribution of secondary aerosol to PM_2.5. Reductions in emissions of volatile organic compounds and NO_x, combined with changes in PM concentrations, resulted in little overall change in ozone concentrations.

—Co-author Xinyue He, from the Southern University of Science and Technology

Resources

• Ben Silver, Xinyue He, Steve R Arnold, and Dominick V Spracklen (2020) “The impact of COVID-19 control measures on air quality in China” Environment Research Letters doi: 10.1088/1748-9326/aba3a2