Investigation on vehicle emission factors in urban tunnels through mobile measurements with an optimized estimation method

Urban traffic pollution, driven by vehicle exhaust emissions, necessitates accurate assessment through Emission Factors (EFs). While tunnel measurements are commonly used, previous studies relied on fixed sampling points near tunnel entrances and exits, making results vulnerable to external influences. This study aims to elucidate the spatial patterns of EFs within tunnels and their influencing factors using multi-season mobile measurements, providing a reference for optimizing the selection of fixed testing points. Through the mobile measurements of Lianhuashan Tunnel in Dalian, real-time particulate matter concentration data were collected during vehicular transit, and variations in EFs at different locations were analyzed. The average EFs in spring, summer, and winter were 3.07 ± 0.84, 0.56 ± 0.12, and 0.90 ± 0.41 (mg/veh/km), respectively. Factors influencing the distribution characteristics of EFs within the tunnel include complex traffic conditions, characteristics specific to the EFs calculation equation employed in mobile measurement, and the influence of natural wind direction at the tunnel exit. Congestion or intermittent movement causes uneven EFs distribution. In cases of short sampling distances, small fluctuations in pollutant concentrations may result in overestimated EFs. Reverse winds at the exit may also inflate EFs due to pollutant accumulation. It is suggested that the optimal locations for fixed measurements lie within the range of 30 %–84 % of the tunnel length from the entrance.