| Summary: | Indoor and outdoor ultrafine, accumulation mode, and coarse fractions collected at two preschools (S1 and S2) in Hanoi capital, Vietnam were characterized in terms of mass-size distribution and elemental composition to identify major emission sources. The sampling campaigns were performed simultaneously indoors and outdoors over four consecutive weeks at each school. Indoor average concentrations of CO<sub>2</sub> and CO at both schools were below the limit values recommended by American Society of Heating, Refrigerating and Air-Conditioning Engineers (1000 ppm for CO<sub>2</sub>) and World Health Organization (7 mg/m<sup>3</sup> for CO). Indoor concentrations of PM<sub>2.5</sub> and PM<sub>10</sub> at S1 and S2 were strongly influenced by the presence of children and their activities indoors. The indoor average concentrations of PM<sub>2.5</sub> and PM<sub>10</sub> were 49.4 µg/m<sup>3</sup> and 59.7 µg/m<sup>3</sup> at S1, while those values at S2 were 7.9 and 10.8 µg/m<sup>3</sup>, respectively. Mass-size distribution of indoor and outdoor particles presented similar patterns, in which ultrafine particles accounted for around 15–20% wt/wt while fine particles (PM<sub>2.5</sub>) made up almost 80% wt/wt of PM<sub>10</sub>. PM<sub>2.5–10</sub> did not display regular shapes while smaller factions tended to aggregate to form clusters with fine structures. Oxygen (O) was the most abundant element in all fractions, followed by carbon (C) for indoor and outdoor particles. O accounted for 36.2% (PM<sub>0.5–1</sub>) to 42.4% wt/wt (PM<sub>0.1</sub>) of indoor particles, while those figures for C were in the range of 14.5% (for PM<sub>0.1</sub>) to 18.1% (for PM<sub>1–2.5</sub>). Apart from O and C, mass proportion of other major and minor elements (Al, Ca, Cr, Fe , K, Mg, Si, Ti) could make up to 50%, whereas trace elements (As, Bi, Cd, Co, Cr, Cu, La, Mn, Mo, Ni, Pb, Rb, Sb, Se, Sn, Sr, and Zn) accounted for less than 0.5% of indoor and outdoor airborne particles. There were no significant indoor emission sources of trace and minor elements. Traffic significantly contributed to major and trace elements at S1 and S2.
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