Levels and composition of ambient particulate matter at a mountainous rural site in northern Vietnam

Abstract

Samples of 24 h PM10 and PM2.5 were collected at a mountainous rural site of Tamdao, Vietnam, using collocated dichotomous and MiniVol samplers. The sampling was done in wet season (20 September–8 October 2005), dry season (17 December 2005–18 January 2006) and transitional season (5 April–26 April 2010). A pair of PM2.5 and PM10–2.5 or PM10 samples was collected on a sampling day using quartz and mixed cellulose filters for each size fraction. The samples were analyzed for mass, BC, water soluble inorganic ions and elemental compositions. Higher PM2.5 levels were obtained during dry season, at an average of 51 µg/m3, followed by the transitional season, 33 µg/m3, and the lowest in wet season, 25 µg/m3. The ratios between anions and cations, both in equivalence, were all below 1.0, but was higher for PM2.5 than for PM10–2.5. The ammonium balance showed a good agreement between measured and estimated levels for PM2.5, suggesting that most of sulfate and nitrate were in their ammonium salts. The reconstructed mass showed high contributions from secondary inorganic particles, OM-biomass (estimated based on K+), crustal and soot groups, which accordingly helped to reveal major contributing sources. Specifically, the high K-biomass and OM-biomass mass group in the dry season indicated the important contribution from biomass burning. The regional transport from surrounding territories to the site was investigated using 5-day HYSPLIT backward trajectories. The differences in PM mass and compositional species that were statistically significant between the seasons for similar trajectory patterns, or between patterns within a season, suggested the effects of seasons and/or air mass trajectory pathways on PM pollution. The northeast continental type of air masses observed in the dry season, with the longest continental pathway and continental origin, was associated with the highest levels of PM mass and compositional species.