Current status of fine particulate matter ( PM 2 . 5 ) in Vietnam ’ s 1 most populous city , Ho Chi Minh City 2 3

13 Abstract 14 15 This paper provides insights into fine particulate matter pollution in the urban atmosphere 16 of Ho Chi Minh City (HCMC), the most populous city in Vietnam. Fine particulate matter 17 (PM2.5) samples were collected daily at five exposed sites from March 2017 to March 2018. 18 PM10 data (daily) and real-time PM2.5 (hourly) data were recorded concurrently at a roadside 19 site. Daily particulate pollutant levels (i.e., PM2.5 and PM10) were determined using the 20 gravimetric method using an impact sampler, and real-time PM2.5 data were measured using 21 a continuous monitor. The measured PM2.5 concentrations varied from 10.4 to 110.8 μg m, 22 with an annual mean of 36.3 ± 13.7 μg m. All annual mean concentrations at the exposed 23 sites exceeded the value limits of the Vietnamese standard (25 μg m) and World Health 24 Organization air quality guideline (10 μg m), indicating high health risk at these sites. 25 Although the sampling sites varied in their exposure levels, they exhibited very strong 26 correlations and low differences in PM2.5 levels. Diurnal variation with a pronounced peak 2 27 hours after the morning rush hour was observed. This peak is likely attributable to not only 28 primary sources (e.g., traffic-related sources) but also secondary aerosol formation. The 29 urban atmosphere of HCMC was affected by strong local emission sources, as evidenced by 30 the pronounced peak during morning rush hour as well as the significant negative correlation 31 between PM2.5 and wind speed. In addition, monthly PM2.5 levels exhibited remarkable 32 seasonal variability, with the lowest and highest levels observed during the rainy and dry 33 seasons, respectively. However, elevated PM2.5 levels were observed during the months with 34 heavy rains, highlighting the influence of strong emission sources, likely the biomass 35 burning of rice straw residues in the Mekong Delta area. 36 37