Development of a methodology for the quantification of particle number and gaseous concentrations in a bidirectional bus tunnel and the derivation of emission factors

Particle number, NOx and CO concentrations were measured simultaneously at the air entry portal and at the mid-point of a 511 m bidirectional road tunnel, used entirely by urban public transport buses. The aim of this study was to provide information on concentrations of these pollutants inside a unique bus tunnel, and to develop a viable methodology for determining emission factors for on-road vehicles. Measurements were made continuously over a period of five days that included a complete weekend. Traffic flow rate and air flow rate were also monitored. The mean particle number concentration at mid-tunnel was 4.1 × 104 cm−3, which was over four times higher than the urban background concentration. The mean concentrations of NOx and CO at mid-tunnel were 464 ppb and 802 ppb, respectively. All these values were between 2 and 4 times higher than at the air entry portal. Median concentrations during selected time segments coinciding with the morning and evening rush hours, mid-day during weekdays and full day during the weekends were determined and the corresponding bus emission factors of each of the three parameters was calculated. Mean emission factors found for particle number, NOx and CO were 7.1 × 1014 particles km−1, 8.1 g km−1 and 15.9 g km−1, respectively. These values compared well with previous studies, showing that the methodology adopted was sound and viable.