Characterization of black carbon at roadside sites and along vehicle roadways in the Bangkok Metropolitan Region.

Abstract: To understand the seasonality of concentrations of traffic-related black carbon (BC) in a megacity, BC concentrations in fine particles were monitored at the roadside and on roads during both the wet and dry seasons of 2010 in the city center of Bangkok, Thailand. The BC concentration measured every 2 min by an aethalometer at the Dingdaeng roadside in the dry season was 17.9 ± 6.6 μg m−3, which was 1.6-fold higher than the value (11.5 ± 2.7 μg m−3) during the wet season. This seasonal difference could not be explained by washout by rain but was instead due to more frequent upwind conditions caused by a prevailing wind direction from the monitoring site toward the road in the wet season. When the prevailing wind direction was from the road, the average BC concentration at the roadside increased up to 30 μg m−3 during both seasons. In contrast, when the wind direction was from the site to the road, the BC concentration was reduced to the level of urban background concentrations measured inside Lumphini Park and the Dusit Zoo of Bangkok. Roadside BC concentrations were strongly correlated with NO x concentrations and elemental carbon (EC) concentrations measured in 24-h PM2.5 filter samples. Both relationships exhibited linear determination coefficients of more than 0.80, implying that NO x can be used as an indicator and an alternative for traffic-related BC at this roadside site when real-time BC monitors are not available. The average on-road BC concentration (25.5 μg m−3) was similar to the average at the roadside under downwind conditions (25.5 μg m−3) from morning to evening only. In contrast, the latter value was 1.7-fold higher than the daily average at the roadside (14.7 μg m−3) and 7.3-fold higher than the urban background level during the daytime (3.5 μg m−3). The results of this study suggest that residents who live next to major roads, pedestrians at the roadside, and drivers on the roads experience a high risk of exposure to severe levels of traffic-related air pollutants. [Copyright &y& Elsevier]