NASA has published satellite images which show a dramatic decline in pollution levels over China, which the US space agency says is "partly related" to the coronavirus.It claims there has been a significant decrease in nitrogen dioxide (NO2) - a noxious gas emitted by motor vehicles, power plants and industrial facilities - due to the economic slowdown following the outbreak.COVID-19 started in the Chinese city of Wuhan, which was placed in lockdown from 23 January, with residents not allowed to leave and businesses ordered to close to reduce the spread of the disease.Image:Nitrogen dioxide (NO2) is a noxious gas emitted by motor vehicles, power plants and industrial facilities.

Pic: NASAThe reduction in NO2 pollution was first apparent near Wuhan, but eventually spread across the country, according to NASA scientists.The quarantine there was the first of several put in place across China, and then around the world, in what has become one of the largest such actions in human history."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.She recalls seeing a drop in NO2 over several countries during the economic recession that began in 2008, but says the decrease was gradual.There was also a reduction around Beijing during the 2008 Olympics, but scientists claim the effect was mostly localised around the capital, and that pollution levels rose again once the Games ended.The decrease in NO2 also coincided with the Lunar New Year celebrations in China and much of Asia.Generally, businesses and factories close from the last week in January into early February to mark the event.Previous observations have shown that air pollution usually decreases around this period, but then increases once it is over.But although the Lunar New Year may have played a role in the recent drop-off, researchers believe the decrease is down to more than a holiday effect or weather-related variation.Ms Liu has not seen a rebound in NO2 after the celebrations.She said: "This year, the reduction rate is more significant than in past years and it has lasted longer."I am not surprised because many cities nationwide have taken measures to minimise the spread of the virus."NASA researchers compared NO2 values in 2020 with the average amounts detected at this time of year from 2005-19.In 2020, NO2 levels in eastern and central China were significantly lower (from 10-30% lower) than what is normally observed for this time period.