Characterizing emission factors and oxidative potential of motorcycle emissions in a real-world tunnel environment

Transportation emissions significantly affect human health, air quality, and climate in urban areas. This study conducted experiments in an urban tunnel in Taipei, Taiwan, to characterize vehicle emissions under real driving conditions, providing emission factors of PM_2.5, eBC, CO, and CO₂. By applying multiple linear regression, it derives individual emission factors for heavy-duty vehicles (HDVs), light-duty vehicles (LDVs), and motorcycles (MCs). Additionally, the oxidative potential using dithiothreitol assay (OP_DTT) was established to understand PM_2.5 toxicity. Results showed HDVs dominated PM_2.5 and eBC concentrations, while LDVs and MCs influenced CO and CO₂ levels. The CO emission factor for transportation inside the tunnel was found to be higher than those in previous studies, likely owing to the increased fraction of MCs, which generally emit higher CO levels. Among the three vehicle types, HDVs exhibited the highest PM_2.5 and eBC emission factors, while CO and CO₂ levels were relatively higher for LDVs and MCs. The OP_DTTm demonstrated that fresh traffic emissions were less toxic than aged aerosols, but higher OP_DTTv indicated the impact on human health cannot be ignored. This study updates emission factors for various vehicle types, aiding in accurate assessment of transportation emissions' effects on air quality and human health, and providing a guideline for formulating mitigation strategies.