Size-segregated Chemical Compositions of Particulate Matter Including PM0.1 in Northern Vietnam, a Highly Polluted Area Where Notable Seasonal Episodes Occur

In October 2017 and June 2018, measurements of size-segregated atmospheric particles containing ultrafine particles (PM0.1) were carried out using a Nanosampler in northern Vietnam (urban, peri-urban and rural areas). This study shows that the variations of PM concentration at the three sites were very similar and that air pollution is widespread in northern Vietnam. However, a concentration difference between cities and a change of composition at high PM concentration were confirmed, and there were some characteristic regional, particle-size, and component trends. From the carbon-fraction distribution, the carbon components of atmospheric particles in Vietnam were affected by vehicles (OC3) and biomass combustion (OC3, EC1), and PM0.1 was affected strongly by vehicles (EC2). The water-soluble ionic component of fine particles in Vietnam is characterized by the highest amount of nitrate next to sulfate. The nitrate formation path is analyzed from the degree of neutralization with ammonium. In coarse and fine particles, particle formation of nitrate due to heterogeneous reaction is dominant, and the contribution of HNO3 adsorption by soil particles and N2O5 hydrolysis is high. For PM0.1, the formation of ammonium nitrate by a homogeneous reaction is the main production path. High-concentration events with increased nitrate were observed in late October, characterized by the effects of long-distance transportation and the promotion of nocturnal N2O5 hydrolysis with increased humidity. In June, when there was an increase in PM concentration derived from biomass combustion, PM0.1 indicated characteristic behavior, suggesting that it is possible to estimate local sources from PM0.1 components and variations.