Influence of transboundary pollution of biomass burning on ionic compounds in urban particulates

Abstract

Transboundary pollution of biomass burning smoke is a recurrent issue affecting air quality at receptor site, such as Singapore. In this study, daily PM2.5 collected in Singapore in 2006, 2008, and 2009, and weekly size segregated particles sampled in 4 weeks of 2008 were chemically characterized. Back trajectory analysis along with satellite images (haze maps) was employed to differentiate between smoke episodes vs. non-smoke episodes. With a strong influence of transboundary smoke, PM2.5 was linearly correlated with visibility (R2=0.85), and PM10 to PM2.5 ratio substantially decreased. Mass size distribution of particulates appeared in bimodal distribution, peaking at cut-off size of 0.38 and 4.1 ?m. Transboundary pollution of biomass burning smoke reinforced size distribution with higher concentration, rather than altering distribution pattern. Oxalates in PM2.5 were enhanced by 2.5, 1.6, and 0.6 times due to the transboundary smoke in 2006, 2008, and 2009 respectively. Chemical characterization of size segregated particles showed that NH4+, K+, SO42- and oxalates dominated in fine mode particles (0.384 ?m), whereas Mg2+, Ca2+, Cl-, NO3- occupied in coarse mode particles. The concentration of major ions changed in the order of SO42- > NH4+ > NO3- > Cl- > K+ > Na+ > Ca2+ > Mg2+. Appearance of both NH4+ and SO42- in droplet mode (0.384 ?m) was attributed to condensation of NH3 on surface of acidic particles, and an unusual appearance of NH4+ in 0.029 ?m (nuclei mode) in week 2 is observed. K+ was highly affected by biomass burning smoke with higher fine-coarse ratio of K+ during smoke episodes. Mg2+ and Ca2+ were mostly found in coarse mode particles; and significant enhanced under influence of transboundary pollution. It indicated Mg2+ and Ca2+ were contributed from crustal particles which were re-suspended during burning process. Cl- and NO3- concentrated in coarse mode particles during the transboundary smoke episode. Oxalates showed unimodal distribution in background condition, bimodal distribution under partial effects of transboundary pollution, and trimodal distribution with full effect of transboundary pollution. A high correlation coefficient (R2>0.82) between oxalates and SO42- in 4 sampling weeks suggested their in-cloud process formation pathway. Enrichment of oxalates in 2.4 ?m particles was attributed to heterogeneous formation on surface of sea salt particles, while their appearance in nuclei mode (0.057 ?m) could be due to photochemical reactions of hydrocarbons with oxidants followed by gas-particle condensation.