COVID-19 lockdown effects on air quality by NO2 in the cities of Barcelona and Madrid (Spain).

During the months of March and April 2020 we witnessed the largest-scale experiment in history in terms of air quality in cities. Any prediction of this experiment's results may be obvious to science, as it was totally expected, the air quality has improved substantially. Simply stated, it comes as no surprise. The lockdown has made it possible to quantify the limit of decrease in pollution in light of this drastic reduction in traffic, in Madrid and Barcelona showed a significant decrease of the order of 75%. In the case of Spain's two largest cities, the reductions of NO2 concentrations were 62% and 50%, respectively. Hourly measurements were obtained from 24 and 9 air quality stations from the monitoring networks during the month of March 2020. These results allow us to see the limits that can be achieved by implementing low emission zones (LEZ), as well as the amount of contamination that must be eliminated, which in the cases of Madrid and Barcelona, represent 55%. This value defines the levels of effort and scope of actions to be taken in order to ensure that both cities achieve a clean and healthy atmosphere in terms of NO2.