Assessing the impact of traffic emissions on fine particulate matter and carbon monoxide levels in Hanoi through COVID-19 social distancing periods

Nhung H. Le1, Bich-Thuy Ly2, *, Phong K. Thai3, Gia-Huy Pham4, Ich-Hung Ngo2, Van-Nguyet Do1, Thuy T. Le5, Luan V. Nhu7, Ha Dang Son7, Yen-Lien T. Nguyen8, Duong H. Pham5, Tuan V. Vu9 1Live & Learn for Environment and Community, No.24 Lane 45B, Vong Thi, Tay Ho, Hanoi, Vietnam 2School of Environmental Science and Technology, Hanoi University of Science and Technology, No.1 Dai Co Viet Street, Hanoi, Vietnam 3Queensland Alliance for Environmental Health Sciences (QAEHS), The University of Queensland, Woolloongabba, 4102, QLD, Australia 4Graduate School of Global Environmental Studies, Kyoto University, Yoshida-Honmachi, Sakyo-ku, Kyoto, 606-8501, Japan 5Hanoi Environmental Protection Agency, Department of Natural Resources and Environment, No.13 Trung Yen 3, Trung Hoa Street, Cau Giay District, Hanoi, Vietnam 6Ministry of Natural Resources and Environment, Northern Center for Environmental Monitoring, No.556 Nguyen Van Cu Street, Gia Thuy Ward, Long Bien District, Hanoi, Vietnam. 7Centre for Energy and Green Growth Research, 8/44/3 Ham Tu Quan Street, Hoan Kiem District, Hanoi, Vietnam 8Faculty of Transport Safety and Environment, University of Transport and Communications, No.3 Cau Giay, Hanoi, Vietnam 9Environmental Research Group, School of Public Health, Imperial College London, Norfolk Place, London W2 1PG, UK.

Section S1. Meteorological conditions in study periods Fig. S3. presents the meteorological conditions in the study period. In the period, the dominant wind came from the northeast, east and southeast, with wind speed ranged from 0-2 to 6.6-8.9 m/s ( Fig.   6(a)). A small portion of wind came from northwest and north, with wind speed mainly < 4 m/s. The effects of wind on PM2.5 concentrations are further discussed in Section 3.2.1.
The temperature varied and gradually increased from March to April, rarely surpassed 25 o C ( Fig.   6(c)). Since May, the temperature became different, grew to higher than 25 o C. Effects of temperature on PM2.5 concentrations were also demonstrated and discussed in previous studies (Hien et al., 2002, Ly et al., 2018, Ly et al., 2020. Relative humidity ranged between 60 to 95% ( Fig. 6(d)). The relative humidity in May was almost always smaller than 80%. Radiation also gradually increased from March to May (Fig. 6(e)). The radiation in March and April was in the range of approximately 50 to 250 Wm -2 , except for the increase to over than 300 Wm -2 in late April. In May, higher radiation was observed ranging from 150 to approximately 300 Wm -2 .
Regarding precipitation, the heaviest rain (1.5 mm) in the study period happened at the beginning of March ( Fig. 6(f)). Afterward, almost no rainfall occurred for the rest of month. April observed two events of slight rain with less than 0.5 mm. In May, rainfall was higher than in April in both frequency and intensity, though precipitation rate barely reached 0.5 mm (except for one occasion that got 0.5 < depth < 1.5 mm. It is worth noted that the precipitation in this study periods was very small and the effects on air pollutants' concentrations were negligible.

Section S2. Calculation of the reduction of emission from transportation in lockdown period
In order to estimate the effects of transportation on the air quality during the social distance period in Hanoi, the emission of CO and PM2.5 are calculated as follows: Where: Ee,i is the total emission of pollutant i (kg/day); N is the number of vehicles of the estimated vehicle type (e.g. motorcycle, bus); EFi is the emission factor of pollutant i for the estimated vehicle type (g/km); VKT is the daily average vehicle kilometers travelling of the estimated vehicle (km/day).
In this study only the emissions of motorcycle and truck were focused as motorcycle has a highest share in the fleet and bus has a highest daily driving distance (average distance of 212 km/day 3 (Nguyen Thi Kim Oanh & Huynh, 2015)). The data used in estimating emission are presented in Table   S4.
As can be calculated from the emission amount from Table S4, comparing with pre-social distancing period, the amount of air pollutants released from road vehicles in Hanoi in hard social distancing period were impressively reduced, by 49% and 60% for CO and PM2.5, respectively. It implies that the pollutants concentration in ambient air also could be reduced remarkably.           In fact, nearly all particle matters released from the vehicle exhaust are found in the form of fine fraction (PM2.5) (Brizio & Genon, 2008;Ketzel et al., 2007). Therefore, an approximation may estimate EFPM2.5 equal to EFPM10.