A case study was carried out to characterize the ambient PM2.5 based on ground-based sampling in Bangi, Selangor, Malaysia in September, 2013 during the southwest monsoon season. We determined the total mass concentration, organic carbon, elemental carbon (EC), and metals in PM2.5 samples. The mean PM2.5 mass concentration was 44.5 mu g m(-3), showing that it exceeded the national air quality standard of 35 mu g m(-3) for 24-hour PM2.5 by the U.S. Environmental Protection Agency. Relatively high OC and EC concentrations of this study compared to those of other Southeast Asian countries were observed, which indicate that significant sources of OC and EC exist. The results of char-EC/soot-EC ratios strongly suggest that biomass burning is the main contributor to ambient EC concentrations compared to coal combustion and motor vehicle emissions. From calculations using the mass closure model, organic matter was the most abundant component in PM2.5 mass at 22.4 +/- 6.65 mu g m(-3), followed by nss-sulfate at 4.84 +/- 2.49 rho g m(-3), and EC at 4.11 +/- 0.916 mu g m(-3). This result indicates that targeting the sources of carbonaceous PM2.5 is a crucial step to improve the air quality in this study field.