Air quality is improving in countries under coronavirus quarantines, experts say, but it is far too early to speak of long-term change.
Images by the US space agency NASA are clear, in February the concentration of nitrogen dioxide (NO2) fell dramatically in Wuhan, China, the epicentre of the COVID-19 pandemic, passing from an indicator that was red/orange to blue.
NO2 is mainly produced by vehicles, industrial sites and thermal power stations.
As China moves past the peak of its crisis, however, recent images by the European Space Agency (ESA) show a resurgence in NO2 emissions.
A striking reduction has also been observed by the ESA in northern Italy, which has been locked down to fight a spread of the novel coronavirus, which causes COVID-19.
The European Environment Agency (EEA) reports a similar change in Barcelona and Madrid, where Spanish authorities issued confinement orders in mid March.
'NO2 is a short-lived pollutant, with a lifetime in the atmosphere of about one day,' said Mr. Vincent-Henri Peuch, from the EU earth surveillance programme Copernicus.
'As a result, this pollutant stays near the emissions sources and can be used as a proxy of the intensity of activity in different sectors,' he said.
Mr. Fei Liu, an air quality researcher at NASA's Goddard Space Flight Center, noted the change in China, saying: 'This is the first time I have seen such a dramatic drop-off over such a wide area for a specific event.'
Even during the economic crisis more than a decade ago, the decrease in NO2 levels 'was more continuous in time,' according to EEA air quality specialist, Dr. Alberto Gonzalez Ortiz.