Analysis of the COVID-19 Lockdown's Impact on Air Quality in the Larger Cities of Spain

During the period of the COVID-19 pandemic, the air quality reached the best levels to be recorded in large cities in Spain. To analyze and demonstrate this improvement in air quality levels, the evolution of the average nitrogen dioxide (NO2) levels in 78 Spanish cities with more than 50,000 inhabitants during the pre-COVID-19 years (2017-2019), the period of the COVID-19 lockdown, and the post-COVID-19 year (2021) was analyzed. The results show an improvement in the air quality in most of the cities analyzed for 2020 due to the COVID-19 restrictions. In addition, in 2021, without the COVID-19 restrictions, the air quality levels of the largest cities in Spain showed important improvements in terms of NO2 concentration compared to the levels in the pre-COVID-19 years (2017-2019). Nevertheless, in 2021, only 11 cities were below the average annual limit of 10 mu g/m(3) NO2 established by the World Health Organization (WHO). In addition, no cities with more than 500,000 inhabitants achieved NO2 levels below the WHO limit. Finally, a detailed monthly analysis indicated that the pre-COVID-19 levels were reached again during the last months of the monitored period.