Air pollution from traffic and industrial emissions is one of the key issues for modern metropolises, because of urban population growth. Modelling and understanding pollution dispersion at a large scale will help to improve the design of our cities, buildings and traffic systems and to attain a sustainable environment for areas of dense population. Among the different physical mechanisms to predict, thermal radiation is of primary interest because convection and dispersion of pollutants are driven by thermal effects. The radiation absorption and emission by the atmosphere (mainly due to water vapour and carbon dioxide) or by the building surfaces and the radiation scattering by clouds and fog modify the local energy balance. These phenomena are coupled with the air flow and should be taken into account for an accurate modelling. However, in most pollution dispersion simulations, radiation is not taken into account or is crudely modelled. Radiative transfer calculation is still a computational issue because the radiative intensity field is an intricate function of the wavenumber and the direction of the photons, space and time. Coupled simulations of radiative transfer and urban flows require new efficient numerical methods and physical models for an accurate prediction and a better understanding of pollution dispersion at the neighbourhood scale.
This project aims at understanding the effects of radiative transfer on air pollution dispersion in urban areas at both the street scale and the neighbourhood scale. Numerical models have been developed for that purpose, that will help design and manage our cities, buildings and traffic systems in order to produce sustainable, safer, healthier, and more comfortable urban environments. Radiation modelling have been coupled with fluid dynamics, pollutant transport and environmental conditions to produce the first model able to take into account radiation transport effects at the street scale. Numerical simulations at the street scale have been carried out and have shown substantial effects of radiative transfer in low-wind conditions, for a mid-latitude summer atmosphere. In addition, a novel strategy has been developed to improve the computational efficiency of thermal radiation transport calculations. It consists in optimising the angular resolution in space and for each absorption coefficient class to get the minimum error on the radiative source term. The use of such adaptive method will be key to perform numerical simulations at the neighbourhood scale.
