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Many previous studies have focused on air quality in road tunnels, which is a major concern for ventilation design. However, few researchers have investigated the air quality in tunnels in high-altitude areas. The ambient environment in high-altitude areas featuring low oxygen content and ambient pressure can influence the vehicle pollutant emissions inside the tunnels. As a result, field measurements were carried out considering environmental parameters and gaseous pollutant concentrations in four road tunnels at different altitudes to determine exact vehicle emissions. Results show that ambient pressure is closely related to the altitude and that the oxygen content will decrease when the altitude increases due to the lower air density. The air velocity induced by the vehicle fleet is only affected by the traffic parameters and not influenced by the air density. The CO and NO emission factors in test tunnels at different altitudes were calculated according to pollutant mass balance models. We can conclude that an environment featuring low oxygen content and ambient pressure in high-altitude areas contributes to incomplete combustion of the engines and performance degradation of the catalytic converter, which leads to higher toxic pollutant emissions inside the tunnels. In this study, the CO and NO emission factors were 0.701 and 0.403 g km−1 vehicle-1 respectively at an altitude of 500 m, which then increased to 1.567 and 0.886 g km−1 vehicle-1, respectively, at an altitude of 3850 m. The NO altitude coefficient is introduced, similar to the CO altitude coefficient in the guidelines, to reflect the influence of altitude on the NO emission factor, and we can determine that the impact of altitude on the CO emission factor is greater. We hope our work will provide a reference for road tunnel ventilation design in high-altitude areas. |
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