Short-range evaluation of air pollution near bus and railway stations.

In the early morning, during workdays, intensive activity is observed at both bus and railway stations. This particular time is critical because of the combination of three factors: (1) simultaneous departure of many buses and trains, (2) cold engines, and, quite frequently, (3) stable meteorological conditions. In our approach, we use ARIA Local, a simulation package applying CFD tools to air pollution modeling, to study different scenarios. The CFD model used in this study is the MERCURE model, developed by Electricite de France. For a bus station, we simulate a typical morning peak hour situation and study in detail how the pollution is accumulated in the station courtyard and the impact on the close vicinity. Two scenarios are presented: one with classical diesel engine and one with buses using AQUAZOL or NGV fuel. The definition of the sources inside the Eulerian grid is described as static linear sources. The total emission is averaged over the mean path driven by the bus from the bus stop to the exit of the bus station. For a railway station, we simulate a situation in a real railway station within the city of Paris. The emission from a diesel "locomotive" and its impact on air quality is computed and compared to the impact of other nonmobile emissions. In this case, the definition of sources is described as mobile point sources following the trajectory of the train. These two scenarios are discussed in an urban context, taking into account the flow around buildings and different meteorological conditions.