Lockdown brings “unprecedented” dip in pollution over virus epicenters

As parts of the world began implementing lockdowns to control the spread of COVID-19 earlier this year, scientists observing air quality levels began to record large dips in key air pollutants. Now a fuller picture is beginning to emerge of how the shutdowns are clearing the air in major epicenters of the outbreak, with some measures of air pollution dropping by as much as 60 percent in parts of northern China.

As previous studies demonstrated in China and also in Italy, the lockdowns driven by COVID-19 have caused significant declines in airborne nitrogen dioxide. This particularly harmful pollutant, which can cause respiratory problems like asthma and infections in humans, is produced through the burning of fossil fuels, making its way into the air via cars, trucks, power plants and industrial facilities.

A pair of new studies carried out by scientists in Germany and Belgium have sought to paint a more global picture of how air pollution is changing as the world grapples with the coronavirus pandemic. Using satellite data on air quality to track nitrogen dioxide over major virus epicenters in China, South Korea, Italy, Spain, France, Germany, Iran and the US, the team again found a widespread and substantial decline.

Levels of the pollutant dipped by an average of 40 percent over Chinese cities, and by between 20 and 38 percent in Western Europe and the US, compared to the same period in 2019. An anomaly was Iran, where a decline in nitrogen dioxide as not observed, which the team suspects is because of delayed or ineffective lockdown orders.

As part of a separate study, the authors looked at northern China specifically, and levels of a variety of pollutants, including particulate matter measuring smaller than 2.5 microns. These tiny particles consist of solids and liquid droplets that are compact enough to slip into the lungs when inhaled and can cause a range of harmful effects on the respiratory system.

Pulling data from 800 ground-level air quality stations in northern China, the authors found an average decline in particulate matter pollution of 35 percent, while nitrogen dioxide was found to have declined by an average of 60 percent since the lockdowns were implemented on January 23. According to the authors of the study, a decline of this magnitude is unprecedented since satellite monitoring of air quality began in the 1990s.

“Maybe this unintended experiment could be used to understand better the emission regulations,” says study author Jenny Stavrakou, from the Royal Belgian Institute for Space Aeronomy. “It is some positive news among a very tragic situation.”

But the issue of air pollution is far from straightforward. The steep decline in nitrogen dioxide has simultaneously driven an increase in ground-level ozone, which can be destroyed by nitrogen oxides. Unlike the ozone in the atmosphere that protects us from the Sun’s ultraviolet rays, ground-level ozone is created when heat and sunlight triggers chemical reactions involving nitrogen dioxide and volatile organic compounds (VOCs) with harmful consequences, such as pulmonary and heart problems in humans.

In the same regions surveyed for particulate matter, the team found an increase in ground-level ozone concentration of a factor of 1.5 to 2 over the same period.

“It means that by just reducing the [nitrogen dioxide] and the particles, you won’t solve the ozone problem,” says study author Guy Brasseur, from Germany’s Max Planck Institute for Meteorology.

Both studies were published in the journal Geophysical Research Letters, with PDF versions available here and here.

Source: American Geophysical Union

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