Impact of Smoke Intensity on Size-Resolved Aerosol Composition and Microstructure during the Biomass Burning Season in Northwest Vietnam

Aerosol particles significantly impact the regional environment, including climate change, specifically in periods of extensive biomass burning. The major agricultural and domestic combustion emission sources were assessed in nearsource and ambient monitoring campaigns in northwestern Vietnam during the dry season. The composition and microstructure of on-field burning and cooking emissions were analyzed with a variety of techniques. A wide range of observed PM2.5 mass concentrations was categorized according to the smoke level, supported by the evolution of carbon fractions (OC and EC) as well as ionic species and molecular tracers (K+, levoglucosan, and mannosan). The OC/EC and individual organic compound ratios on days with high smoke levels indicate smoldering combustion of softwood and other local biomass species, impacting aerosol composition at the regional level. Acid and non-acid carbonyls, carboxylates, and aliphatic carbon functionalities in the PM2.5 size fraction evolved with increasing smoke intensity, together with carbonates in coarse (PM1-2.5 and PM2.5-10) size fractions, indicating a large impact of smoke emissions and soil lifted up by the intense fires. Biomass burning influence increased the abundance of soot and organic particles in the submicron fraction from 12% at low to 59% and 68% at moderate and high smoke levels, respectively. Smoke micromarkers of local biomass burning source emissions determined the microstructure of ambient aerosols representative for northern Southeast Asia.