Vehicular emissions in roadway tunnels: A critical review

Traffic-induced emissions are a major source of air pollutants, particularly in urban areas. Adequate characterization and quantification of the contribution of this source depend to a large extent on the use of reliable pollutant emission factors. Recently, in addition to characterizing vehicle-induced pollutants, tunnels have been used to evaluate on-road emission factors. Accurate determination of those factors depends on the reliability of field measurements and the use of appropriate mathematical models to simulate the behavior of pollutants inside a tunnel. Different models, with various degrees of complexity, have been developed to estimate vehicle emission factors, and to simulate the fate and transport of pollutant emissions in tunnels. This article presents a critical review of research work conducted to characterize and simulate vehicle-induced pollutants in tunnels. Air-quality standards in tunnels, tunnel ventilation, and factors affecting pollutant concentration in tunnels with corresponding control measures are first addressed. Limitations and research needs are discussed in the context of the current state of the art in tunnel air-quality modeling.