Critical assessment of restrictive socioeconomic measures taken during the SARS-CoV-2 pandemic and their impact on air quality worldwide

The ongoing global pandemic of the coronavirus disease 2019 has been a public health emergency of international concern. Countries have adopted several restriction measures. Because of this fateful moment, it was possible to assess the effect of anthropogenic activities on air pollutants in an unprecedented way. This work aims to outline changes in the air quality levels of several cities worldwide after the COVID-19 pandemic. Data on the criteria pollutants found in these cities before and during the pandemic were used to evaluate air quality performance. The collection of most of the data was possible thanks to the constant monitoring methods applied in some countries. The severe limitation of people's movements significantly reduced pollutants concentration, mainly due to the traffic of vehicles. Carbon monoxide, sulfur dioxide, nitrogen dioxide, particulate matter 2.5 mu m, and particulate matter 10 mu m (CO, SO2, NO2, PM2.5, and PM10) concentration reductions were observed due to more restrictive or flexible lockdowns. In almost all cities evaluated, WHO's air quality guidelines have been achieved, except for tropospheric ozone, which has been increasing with the reduction of nitric oxides (NOx) emissions. The increment in the concentrations of the pollutants immediately after the end of the restrictions is an indication that control strategies must be implemented to improve air quality.