Nitrogen dioxide (NO2) is an important air pollutant formed during the combustion of fossil fuels. The reduced human activities in the wake of the COVID-19 pandemic led to sharp reductions in surface NO2 around the globe. Near-real-time NASA computer simulations, combined with surface observations, can be used to track the decline and subsequent recovery of NO2 related to COVID-19 stay-at-home orders around the globe. Since NO2 concentrations are a good proxy for industrial, transportation, and domestic activities, the changes in NO2 derived from NASA models during mandatory quarantine correlate well with officially reported changes in economic output. Observations from 4,778 air quality monitoring sites in 46 countries are used to monitor the change in NO2 as a result of COVID-19 containment measures.

Air pollution in cities can be an acute problem leading to damaging effects on people, animals, plants and property. It is an important topic which is getting increased attention as the human population of cities continues to increase. This year it was the subject the 2018 KDD Cup, the annual data mining and knowledge discovery competition organized by ACM SIGKDD. The burning of fossil fuels for transport and home heating is a major contributor to air pollution in urban environments, creating the pollutant nitrogen dioxide (NO2). This is a secondary pollutant produced by the oxidation of NO. It is a major contributor to respiratory problems. In the European Union, the Cleaner Air For Europe (CAFÉ) Directive 2008/50/EC established an hourly limit of 200 μg/m3 and an annual mean limit of 40 μg/m3 in respect of NO2.