Spatial and temporal variations of the PM_2.5 concentrations in Hanoi metropolitan area, Vietnam, during the COVID-19 lockdown

The Vietnamese government has issued several directives of lockdown in Hanoi, the capital city, for a month (1–30 April 2020) to prevent the human-to-human transmission of COVID-19. This action has affected air pollution due to a decline in transportation. Therefore, this study investigates spatial and temporal change in the PM_2.5 concentrations during the first 4 months of 2020 in Hanoi metropolitan area. Spatial distribution maps of the PM_2.5 concentration in Hanoi were provided for the first time. The average PM_2.5 concentrations at the 22 air monitoring stations were strongly correlated with the population. April had a significantly lower level of PM_2.5 than the other 3 months. In particular, the concentrations of PM_2.5 and NO₂ decreased by 12 and 54%, respectively, between March and April, especially at areas for commercial activities. In April, a higher level of PM_2.5 was recorded between Tuesday–Thursday, which is a reverse trend with that in March. Furthermore, Monday and Friday did not show rush hour peaks for the PM_2.5 levels in April. A decrease in the PM_2.5 concentrations was partly influenced by the long-range transport from the outside to Hanoi. This study implies that a reduction in traffic volumes and public activities may possibly improve the air quality.