Seasonal Variation of Black Carbon in Fine Particulate Matter in Semi-urban and Agricultural Areas of Thailand

Black carbon (BC) mass concentrations, associated with fine particulate matter (PM_2.5), were monitored for a year at semi-urban and agricultural areas in Thailand to study and compare their seasonal variation and contribution to fine particulate matter. The measurements were based on two simultaneously operating samplers and an optical transmissometer. PM_2.5 varied from 60.7 to 80.3 μg m⁻³ and 36.6 to 48.4 μg m⁻³ for the semi-urban and agricultural sites, respectively. PM_2.5 showed similar trends for both sites with high values during the dry season and low values during the wet season. The BC levels during the dry season ranged from 15.6 to 29.0 μg m⁻³ compared to 14.5 to 23.8 μg m⁻³ in the wet season in a semi-urban site, whereas at an agricultural site, values ranged from 11.1 to 15.4 μg m⁻³ and 6.8 to 13.5 μg m⁻³ for the dry and wet seasons, respectively. The annual BC accounted for 22–38% and 19–36% of PM_2.5 at the semi-urban and agricultural sites, respectively. Based on backward trajectory analysis, it can be concluded that both local and regional sources of BC are important. These results indicated that the local sources such as road traffic and agricultural burning played a significant role in BC levels. However, the impacts of distant sources include areas, where there is extensive open biomass burning which were not negligible. The model indicated the contribution of BC from the northern part of Thailand and Cambodia during days of high BC levels. The findings of this study aid policymakers in developing effective air quality control strategies based on complex transboundary problems.