The COVID-19 coronavirus outbreak has caused widespread alarm, travel bans, and the quarantine of multiple cities across the world.

But there's also been an unexpected effect on the environment, in the form of a notable drop in nitrogen dioxide emissions levels across China.

Data collected from the Tropospheric Monitoring Instrument (TROPOMI) on ESA's Sentinel-5 satellite shows a significant drop of nitrogen dioxide – a gas mainly emitted by cars, trucks, power plants and some industrial plants - between January 1 and February 25.

"This is the first time I have seen such a dramatic drop-off over such a wide area for a specific event," said Fei Liu, an air quality researcher at NASA's Goddard Space Flight Centre.

Although during the end of January and start of February there is usually a drop in nitrogen dioxide across China as businesses close during the Lunar New Year, this year, the reductions have continued long after the end of the festival.

(Joshua Stevens/Copernicus Sentinel 5P/ESA)

A sensor on NASA's Aura satellite – Ozone Monitoring Instrument (OMI) is also making measurements on the drop in pollutants.

"There is always this general slowdown around this time of the year," said Barry Lefer, an air quality scientist at NASA. "Our long-term OMI data allows us to see if these amounts are abnormal and why."

Satellite data isn't the only way we're seeing a drop in China's emissions.

An early analysis published by Carbon Brief in February showed that coal use at power stations is at a four year low, and domestic flights are down 70 percent. Along with a drop in steel manufacturing and oil refinery output, they estimate this might have lowered the country's carbon dioxide emissions by approximately a quarter over the last few weeks.

However, just because industries have lowered their carbon dioxide output and nitrogen dioxide levels are lower above China, it doesn't mean that the air is cleaner in the cities. In mid-February Beijing's air pollution levels were still 10 times the recommended level by the World Health Organisation.