Special Issue on COVID-19 Aerosol Drivers, Impacts and Mitigation (XVIII)

Nhung H. Le1, Bich-Thuy Ly This email address is being protected from spambots. You need JavaScript enabled to view it.2, Phong K. Thai3, Gia-Huy Pham4, Ich-Hung Ngo2, Van-Nguyet Do1, Thuy T. Le5, Luan V. Nhu6, Ha Dang Son7, Yen-Lien T. Nguyen8, Duong H. Pham5, Tuan V. Vu9

1 Live & Learn for Environment and Community, Hanoi, Vietnam
2 School of Environmental Science and Technology, Hanoi University of Science and Technology, Hanoi, Vietnam
3 Queensland Alliance for Environmental Health Sciences (QAEHS), The University of Queensland, 4102 QLD, Australia
4 Graduate School of Global Environmental Studies, Kyoto University, Kyoto 606-8501, Japan
5 Hanoi Environmental Protection Agency, Department of Natural Resources and Environment, Hanoi, Vietnam
6 Ministry of Natural Resources and Environment, Northern Center for Environmental Monitoring, Hanoi, Vietnam
7 Centre for Energy and Green Growth Research, Hanoi, Vietnam
8 Faculty of Transport Safety and Environment, University of Transport and Communications, Hanoi, Vietnam
9 Environmental Research Group, School of Public Health, Imperial College London, London W2 1PG, UK

Received: April 11, 2021
Revised: July 5, 2021
Accepted: July 25, 2021

 Copyright The Author(s). This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are cited.

Download Citation: ||https://doi.org/10.4209/aaqr.210081  

Cite this article:

Le, N.H., Ly, B.T., Thai, P.K., Pham, G.H., Ngo, I.H., Do, V.N., Le, T.T., Nhu, L.V., Son, H.D., Nguyen, Y.L.T., Pham, D.H., Vu, T.V. (2021). Assessing the Impact of Traffic Emissions on Fine Particulate Matter and Carbon Monoxide Levels in Hanoi through COVID-19 Social Distancing Periods. Aerosol Air Qual. Res. 21, 210081. https://doi.org/10.4209/aaqr.210081


  • Impacts of social distancing on PM2.5 and CO concentrations in Hanoi were studied.
  • RF model addresses weather impacts in the variations of PM2.5 and CO concentrations.
  • Reductions of PM2.5 and CO were found during the social distancing period.
  • The effect of traffic emission on PM2.5 and CO levels were demonstrated.


Hanoi, Vietnam, is usually ranked as one of the most polluted capital cities in terms of air quality, particularly PM2.5. However, there has not been enough data to determine the main source of this pollution. In this study, we utilized the rare opportunity of the COVID-19 social distancing to assess the contribution of traffic emission to PM2.5 and CO levels when traffic volume was reduced significantly in Hanoi. Hourly PM2.5 and CO concentrations were measured from nine urban and traffic monitoring stations during pre-, soft, hard, and post-social distancing periods. As a result, we observed large reductions in both PM2.5 and CO levels during social distancing periods. PM2.5 concentrations were 14–18% lower during the social distancing than before this period, while CO concentrations had a more considerable drop by 28–41%. It is known that meteorological conditions can have significant effects on the ambient levels of air pollutants. To overcome this challenge, weather normalized concentrations of those pollutants were estimated using the random forest model, a machine learning technique. The normalized weather concentrations showed smaller reductions by 7–10% for PM2.5 and 5–11% for CO, indicating the presence of favorable weather conditions for better air quality during the social distancing period. In further analysis, the apparent improvement of air quality in Hanoi during the social distancing period was in line with reducing traffic emissions while emissions from coal-fired power plants remained relatively stable.

Keywords: COVID-19, Air quality, Hanoi, Southeast Asia, Weather normalization

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