Black carbon in PM 2.5 at roadside site in Bangkok, Thailand

Traffic is typically a major source of air pollution in urban areas of developing countries. The exhaust emissions include gaseous substances and particulate matter, which PM 2.5 (particulate matter less than 2.5 micrometers in aerodynamic diameter) is the majority. In urban areas, the composition of PM 2.5 at roadside site dominated by the carbonaceous combustion component, the major constituent wherein was represented as black carbon (BC). This study aimed to investigate the diurnal concentrations of BC related to PM 2.5 by using a seven-wavelength aethalometer. Study site was located near one of congested roads in Bangkok, Thailand. The correlation between traffic volumes and BC concentrations at roadside were also discussed. Results showed that the 24-h average BC concentrations of this site were in the range of 1.5-15 μg/m3. The diurnal pattern of BC levels exhibited two peaks of BC concentrations occurring during 0500 to 0700 LST and 2100 to 2300 LST. The morning peak of BC evidently governed by traffic condition while the evening peak resulted from traffic associated with meteorological effect. In addition, the density of heavy-duty diesel vehicles (HDDVs) played an important role to the diurnal pattern of BC concentrations. Interestingly, the highest BC level from this study site was observed on Sunday, where the lowest was found on Monday and Friday. This involved with traffic volumes caused by particular activity around this study site.