Table 1.

Previous studies analyzing the impact of COVID-19 on air quality of different locations.The text in bold represents the contributions of the present study.

Study	Study location	Pollutants types	Key observations
Zambrano-Monserrate and Ruano (2020)	Quito, Ecuador	PM2.5, NO2 and O3	PM2.5 and NO2 concentration decreased while O3 concentration increased.
Collivignarelli et al. (2020)	Milan, Italy	PM10, PM2.5, BC, CO benzene, SO2 and NOx	All pollutants except SO2 show a significant reduction.
Lian et al. (2020)	Wuhan, China	PM2.5, PM10, SO2, CO, NO2 and O3	The AQI of Wuhan City decreased significantly, the higher the population density the more significant the decline.
Baldasano (2020)	Barcelona and Madrid (Spain)	NO2	The NO2 concentrations in Barcelona and Madrid (Spain) were reduced by % and 62%, respectively.
Gautam (2020b)	India, Italy, Spain, France	NO2	NO2 concentration reduced during lockdown period.
Adams (2020)	Ontario, Canada	PM2.5, NO2, NOx and O3	PM2.5 concentrations did not change, while a moderate reduction in O3 concentration was observed. NO2 and NOx levels were strongly reduced.
Selvam et al. (2020)	Gujarat, India	AQI, PM2.5, PM10, SO2, CO, NO2 and O3	NO2 was reduced by 30–84%, while O3 increased by 16–58%. Overall, AQI improved by 58% as compared to 2019.
Kumari and Toshniwal (2020)	Delhi, Mumbai, Singrauli (India)	PM2.5, PM10, SO2, NO2 and O3	The concentration of PM10, PM2.5, NO2 and SO2 reduced by 55%, 49%, 60% and 19% and 44%, 37%, 78% and 39% for Delhi and Mumbai, respectively.
Pacheco et al. (2020)	Ecuador	NO2	NO2 concentration was reduced by up to 23%.
This study	Beijing, Bengaluru, Delhi, Las Vegas, Lima, London, Madrid, Moscow, Mumbai, Rome, Sao Paulo, and Wuhan	PM2.5, PM10, SO2, NO2and O3	PM2.5, PM10and NO2reduced significantly during lockdown. However, SO2and O3showed a mixed trend. Moreover, observed results showed that the achieved air quality improvement is temporary.