The Impact of COVID-19 Lockdowns on the Air Quality in Major Cities Across the World

A Brief Review

الملخص

The COVID-19 lockdowns provided the opportunity to ascertain the contribution of anthropocentric activities to local and global air pollution. Major pollutants tackled in published studies were particulate matter (PM), sulfur dioxide (SO₂), nitrogen dioxide (NO₂), ground-level ozone (O₃) and carbon monoxide (CO). Generally, the lockdowns reduced emissions from the transport and manufacturing sectors, thereby resulting in decreased levels of pollutants, except for O₃ which increased in most cities potentially due to the decrease of PM, NO₂ and VOC. Overall, the results of the studies revealed improved air quality during the COVID-19 lockdowns but was not sustained as indicated by the gradual resumption to its pre-COVID level with the easing of restrictions. Moreover, the changes in the pollution level during the lockdowns were also potentially influenced by atmospheric chemistry involved in the formation of secondary pollutants and by prevailing meteorological factors, specifically temperature, humidity and wind speed and direction. Hence, these factors should also be considered in formulating sustainable air pollution control strategies by local, national and international policy makers.