Context: According to the experts, air quality is improving in countries under COVID-19 quarantines, but it is far too early to speak of long-term change.

What led to such claims?

  • The images by the U.S. space agency NASA shows that 
    • 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.
    • 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.
    • The European Environment Agency (EEA) reports a similar change in Barcelona and Madrid, where Spanish authorities issued confinement orders in mid March.
      • However, recent images by the European Space Agency (ESA) show a resurgence in NO2 emissions.

Dramatic drop-off:

NO2 is a short-lived pollutant, with a lifetime in the atmosphere of about one day. 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. The pollutant can provoke serious inflammation of the respiratory system.

  • According to the experts, this is the first time that  such a dramatic drop-off over such a wide area for a specific event has been observed.
  • Even during the economic crisis more than a decade ago, the decrease in NO2 levels was more continuous in time.
  • Confinement measures thus protect in two ways, by reducing the risk of COVID-19 infection and by easing pollution from road traffic.

It is nonetheless hard to know how much benefit the world’s population will actually experience because, according to the health experts, what will have more impact is the long term exposure.

Nitrogen oxide (NOx)

  • NOx is a common term used for the various nitrogen oxides produced during combustion.
  • Source: 
    • They are produced mainly in internal combustion engines and coal-burning power plants and also produced naturally by lightning.
      • Oxygen and nitrogen do not react at ambient temperatures. But at high temperatures, they produce various oxides of nitrogen. Such temperatures arise inside an internal combustion engine or a power station boiler.
    • Agricultural fertilisation and the use of nitrogen-fixing plants also contribute to atmospheric NOx, by promoting nitrogen fixation by microorganisms.
    • NO and NO2 (contribute to global cooling) should not be confused with nitrous oxide (N2O – GHG), which is a greenhouse gas and has many uses as an oxidiser.


  • Aggravate asthmatic condition: They are believed to aggravate asthmatic conditions and create many respiratory health issues.
  • Leads to smog and acid rain formation: NOx gases react to form smog and acid rain as well as being central to the formation of tropospheric ozone. 
    • When NOx and volatile organic compounds (VOCs) react in the presence of sunlight, they form photochemical smog.
    • Mono-nitrogen oxides eventually form nitric acid when dissolved in atmospheric moisture, forming a component of acid rain.

The reduction of NOx emissions is one of the most important technical challenges facing biodiesel.