Characteristics of Chemical Components in Fine Particles (PM2.5) and Ultrafine Particles (PM0.1) in Hanoi, Vietnam: a Case Study in Two Seasons with Different Humidity

The variations of the concentration and characteristics of particulate matter (PM) are influenced by several factors, such as emission sources and meteorological conditions. In this study, fine particles (PM 2.5 ) and ultrafine particles (PM 0.1 ) were simultaneously collected during summer and winter in Hanoi, Vietnam, to clarify the behavior of PM 0.1 (which is rarely observed in this region) by characterizing the chemical components of each particle size in order to clarify particle formation and growth in two seasons with different humidity. The results showed that organic carbon (OC) and elemental carbon (EC) were the major compounds in both size ranges of PM, accounting for up to 56% and 80% of the analyzed components in PM 2.5 and PM 0.1 , respectively. Secondary organic carbon (SOC) accounted for 36–41% and 37–47% of the total OC in PM 2.5 and PM 0.1 , respectively, indicating an important contribution of secondary sources to OC. The strong correlations between water-soluble organic carbon (WSOC) and both sulfate (SO 4 2− ) and nitrate (NO 3 − ) showed that photochemical reactions contributed considerably to WSOC formation. Furthermore, the correlations between absolute humidity and other chemical components suggest that the PM 2.5 collected in Hanoi was formed by secondary processes in the aqueous phase on the aerosol surface. In contrast, no correlation was found between the relative humidity and other chemical components. These observations indicate that particle growth is dependent on the number of water molecules above a certain level under drizzle-like weather conditions particular to the study region, whereas occasional heavy rain during the sampling period might have hindered those processes.