Assessing the contents of polycyclic aromatic hydrocarbons in the tollbooths of a highway toll station via direct and indirect approaches

The present study was set out to assess the contents of polycyclic aromatic hydrocarbons (PAHs) in three types of tollbooths at a highway toll station via direct and indirect approaches. Direct sampling results show that no significant difference could be found in the PAH homologue distributions for samples collected from the car lane/ticket-payment and car lane/cash-payment tollbooths, but both were significantly different from that for the bus/truck lane tollbooth. The above results could be due to the former two types of tollbooths that were designed for the same type of traffic (i.e., cars and vans), but the latter was designed for a different type of traffic (i.e., buses and trucks). For any given type of tollbooth, the total-PAH content (C_Total-PAHs) found during the day shift (= 9370-15500 ng/m³) were not significantly different from that found during the night shift (= 9550-14900 ng/m³), but both were significantly higher than that found during the late-night shift (= 5560-11100 ng/m³). During any given work shift, we found C_Total-PAHs for the three types of tollbooths as the following: bus/truck lane (= 11100-15500 ng/m³) > car lane/ticket-payment (= 7260-13500 ng/m³) > car lane/cash-payment (= 5560-9550 ng/m³). After conducting multivariate regression analyses, we found that none of the three environmental factors (i.e., wind speed, temperature, and relative humidity), except for the vehicle flow rate (Q_Vehicle), had a significant effect on C_Total-PAHs for any given type of tollbooth. Considering directly measuring PAH contents was labor-consuming and costly, and the above results suggest the possibility or using Q_Vehicle to predict C_Total-PAHs for any given type of tollbooth. After conducting simple linear regression analyses, we found that (1) all resultant regression coefficients were found with positive values indicating that an increase in the Q_Vehicle would lead to an increase in the C_Total-PAHs, (2) from the magnitude of the resultant regression coefficients indicating that an increase in C_Total-PAHs caused by per unit Q_Vehicle for the three types of tollbooths were the following: bus/truck lane < car lane/cash-payment < car lane/ticket-payment, and (3) the resultant R² values fell to the 0.54-0.75 range indicating that the variations in C_Total-PAHs could be explained well by Q_Vehicle for the three types of tollbooths. It is concluded that measuring Q_Vehicle can be regarded as an effective indirect method for estimating PAH contents in various types of tollbooths.