When China enforced strict lockdowns and factory closures to limit the spread of the novel coronavirus earlier this year, the rate of air pollution over the country went down significantly, satellite imagery shows.

That same imagery is now showing the rate of nitrogen dioxide (NO2) ramping back up as those measures are slowly lifted due to a steady drop in COVID-19 cases.

The imagery is based off data from the Copernicus Sentinal-5P, one of several satellites that monitor the Earth’s environment for the European Space Agency.

The rapidly decreasing levels of NO2 seen in the imagery coincided with the strict lockdown of Wuhan and the surrounding Hubei province — the original epicentre of the outbreak — that saw factories close and vehicle traffic come to a halt starting in January.

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The NO2 levels then begin to fade across the rest of China, including its capital, Beijing, as limits on operations and daily life were enforced throughout the country in February.

Claus Zehner, the ESA’s manager of the Copernicus Sentinal-5P mission, said initial estimates show a drop in NO2 levels around 40 per cent around Chinese cities, but said more detailed results would be released in the coming weeks and months.

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The Atmosphere Monitoring Service, which relies on data from the larger Copernicus satellite program, found fine particulate matter also dropped by as much as 30 per cent over much of China during February based on previous years.

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“This is the first time I have ever seen such a dramatic drop-off over such a wide area for a specific event,” NASA air quality researcher Fei Liu told NASA’s Earth Observatory blog.

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Both the ESA and NASA, which reported similar data through its own satellites, point out that while reduced emissions are usually seen during Lunar New Year celebrations around the same time, they normally ramp back up quickly afterwards.

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But this year was different: emissions stayed low until March, according to the imagery, when China began loosening restrictions outside of Hubei as COVID-19 cases began to level off.

Wuhan is now the only city in China still deemed “high-risk,” and travel restrictions remain in place there. But with China now no longer reporting local transmissions of the coronavirus, hints are being thrown by government officials in local media that the quarantine could be gradually lifted if things stay where they are now.

The number of daily new infections in mainland China fell below 100 for the first time on March 6. On Thursday, there were 34 cases, up from 13 a day earlier. All of Thursday’s cases involved travellers from abroad.

Total infections in mainland China stood at 80,928, with the death toll at 3,245 by Wednesday, up eight from a day before. Wuhan accounted for six of the eight new deaths in Hubei.

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The Copernicus satellites also captured similar drops in NO2 levels over Italy, which coincided with that country’s own nationwide lockdown due to a massive outbreak of COVID-19 cases.

Thursday saw Italy’s death toll surpass China’s to become the worldwide leader in fatal cases, at 3,405. However, Italy has far fewer confirmed cases, with 41,035 as of Thursday.

—With files from Reuters