Air pollution in Vietnam during the COVID-19 social isolation, evidence of reduction in human activities

The COVID-19 (COronaVIrus Disease) pandemic has been spreading globally and social isolation has been implemented as an effective solution to limit the spread of the pandemic before vaccines are developed successfully. The reduction of human activities during social isolation is leading to an improvement in air quality being observed worldwide. In this study, we analysed satellite TROPOMI (TROPOspheric Monitoring Instrument) NO2 (Nitrogen Dioxide) and OMI (Ozone Monitoring Instrument) NO2 products obtained from Sentinel-5P and Aura satellites, respectively, during the social isolation from 1 April 2020 to 22 April 2020 in Vietnam where air pollution is a serious environmental issue. Both TROPOMI and OMI NO2 have shown low agreement with local on-ground NO2 concentration (the correlation coefficient r equals 0.4 and 0.3 in 2020, respectively) but have had moderate correlation with each other in 2020 (r equals 0.81 for two annual averaged maps). In addition, the moderate available data ratio over cloudy northern areas were observed, especially for TROPOMI datasets. During social isolation, the national average NO2 decreased by 9.3% for TROPOMI and 7.2% for OMI, respectively, in comparison with the same periods in the previous years. The reduction was stronger in the North than in the South and lasted for one more month after the lockdown period. The decrease of NO2 density during the lockdown was observed strongly in urban areas associated with dense population, traffic density, and industrial zones. Further analysis of NO2 density for Ho Chi Minh city revealed a decreasing trend of NO2 during social isolation (16.62% and 11.77% compared with the previous month and the same period in 2019, respectively). Furthermore, we used a random forest model to estimate the NO2 density (normal life situations) based on daily meteorological data at station. During social isolation, NO2 density observed in Ho Chi Minh city decreased by 26% down to that of normal life situations. The NO2 reduction has shown strong anthropogenic impact on air pollution over the study area. Besides, the first use of TROPOMI and OMI NO2 products together has highlighted potentials of satellite observation for air quality monitoring and analysis in Vietnam where on-ground measurements are very limited. � 2021 Informa UK Limited, trading as Taylor & Francis Group.