| Summary: | Size-segregated particulate matter (PM) including the PM<sub>0.1</sub> fraction, particles ≤0.1 µm, was monitored during the rainy and dry seasons at three different cities in Sumatra island, Indonesia in 2018. In order to identify possible emission sources, carbonaceous components in the particles collected by a cascade air sampler that is capable of collecting PM<sub>0.1</sub> particles were analyzed by applying a thermal/optical reflectance (IMPROVE-TOR) protocol. The PM<sub>0.1</sub> levels in the Jambi and Pekanbaru areas were similar to those in large cities in East Asia, such as Bangkok and Hanoi. During the rainy season, local emissions in the form of vehicle combustion were the main sources of PM. The influence of peatland fires in the dry season was more significant in cities that are located on the east coast of Sumatra island because of the larger number of hotspots and air mass trajectories along the coast. A clear increase in the carbonaceous profiles as OC, TC, and OC/EC ratios in the dry season from the rainy season was observed, particularly in fine fractions such as PM<sub>0.5–1</sub>. In both seasons, EC vs. OC/EC correlations and soot-EC/TC ratios showed that the PM<sub>0.1</sub> fraction in Sumatra island was heavily influenced by vehicle emissions, while the effect of biomass burning was more sensitive with respect to the PM<sub>0.5–1</sub> fraction, particularly in Jambi and Pekanbaru sites during the dry season.
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