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.
