Multilayer urban canopy modelling and mapping for traffic pollutant dispersion at high density urban areas.

A semi-empirical multilayer urban canopy model is developed to estimate the vertical dispersion of traffic emissions in high density urban areas. It is motivated by the heterogeneity of urban morphology in real urban cities and the need of quick urban design and planning. The urban canopy is divided into multiple layers, to include the impact of building height variance on pollutant dispersion. The model is derived by mass conservation within each layer through adopting a box model. To validate the model, results in several cases with uniform and non-uniform building height distributions are compared with CFD simulations. The validation study indicates that the assumption of zero pollutant concentration over the modeled canopy and no horizontal pollutant transfer has increasingly negligible influence with increasing urban densities. The new multilayer model performs well to model the vertical pollutant transport, and modelling results can mostly follow the trend of the CFD simulations. The present paper conducts two case studies in metropolitan areas in Singapore and Hong Kong to illustrate how to implement this multilayer urban canopy model in the planning practice. With an in-house GIS team using available data, the multilayer model provides planners a way to understand air pollutant dispersion in high-density urban areas.