The effect of COVID-19 lockdowns on the air quality in major cities across the world: A review

The lockdowns brought about by the COVID-19 pandemic provided the opportunity to ascertain the influence 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, various 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 atmospheric chemistry governing the formation of secondary pollutants as well as meteorological factors potentially influenced the air quality during the lockdowns. Meteorological factors include temperature, humidity, wind speed and wind direction. Hence, these factors should also be understood by local, national and international policy makers in developing sustainable control strategies for air pollution.