Spatio-temporal variability in black carbon concentrations at highway toll plaza: Comparison between manual and electronic toll lanes

The quantification of the differences in the magnitudes of black carbon (BC) concentrations between electronic and manual toll lanes is necessary to understand the impacts of toll plazas to air pollution. This study investigated the space-time distribution of BC concentrations and compared the BC pollution profiles between traffics and toll collection lanes of an urban highway toll plaza in Shanghai. Experiments were carried out between August 15 and August 22, 2019. The mean BC concentrations at east-westbound manual toll collection (MTC) lanes were 17.8 and 14.8 μg m⁻³, respectively, while the counterpart values for the electronic toll collection (ETC) lanes were 7.7 and 9.2 μg m⁻³, respectively. Moreover, parallel measurements at urban background site yielded mean BC level ranges between 3.4 and 2.8 μg m⁻³. The traffic volumes through the east-westbound ETC and MTC lanes were estimated as 489 and 347 vehicles h⁻¹, respectively. Although ETC-lane traffic was ~1.4 times higher than that of the MTC lanes, owing to the toll collection method, the BC concentrations at the MTC lanes were 1.6–2.3 times higher than those at the ETC lanes and 4.8–5.9 times higher than that of the urban background site. As per multivariate regression analysis, it was found that the variations in BC concentrations were caused by traffic emissions and influenced by the local background BC levels, influences of wind speed and humidity were also determined. This study found that at highway toll plazas, when compared with MTC lanes, ETC lanes can contribute to reducing BC emissions by ~50%.