Monthly concentration of nitrogen dioxide in select European cities 2019-2023
Lockdowns and other measures caused by the outbreak of the coronavirus in 2020 resulted in lower traffic levels in some of the world's busiest cities. The reduction in traffic led to reduced concentrations of harmful pollutants such as nitrogen dioxide (NO₂). In Madrid, the average concentration of NO₂ in April 2020 was 11.7 micrograms per cubic meter of air (μg/m³). This was a reduction of over 55 percent compared to NO₂ levels in April 2019. Cities such as Rome and London saw similar reductions during this period.