| Popis: |
An urban street canyon represents an environment where pollutants such as particulate matter accumulate to high levels. Regulatory guidelines of particulate matter based on the mass metric have led to the development of clean engine technologies that emit smaller particles (ultrafine «IOOnm) and fine «1 urn) which are believed to have greater health implications upon exposure. The modelling of the transport and transformation of fine and ultrafine aerosols will grant insight into the nature of dispersion within urban canyons and inform future studies. This investigation was done using a Computational Fluid Dynamics platform coupled with the Modal Method. Considering an idealized 2-D framework, using the l" order eddy viscosity turbulence model and treating aerosol particles as an inert scalar, the model is able to account for the dispersion structure of aerosol particle number concentration. Consistent with previous measurement results, modelling studies show that the vertical concentration structure at the leeward side of the canyon is such that there is an increase in aerosol concentration up to approximately 2 m in height, followed by a decrease along the height of the canyon and an even sharper decrease within the turbulent shear layer. The gradient of the structure vary at different locations in the canyon and canyon geometries due to differing extents of advection and turbulent diffusion. It is found that the escape of aerosol particles from canyons (when driven by forced convection from the roof level wind) is dominated by turbulent flux, although natural convection could modify the relative importance of turbulent and advective flux, such that advective flux may be equally important. Assuming a constant exhaust emission within the canyon, a parameterization is developed to relate particulate matter flux (from canyons) with inflow wind speed and turbulent intensity. This parameterization is also able to predict aerosol flux within neighbourhood scales fairly well; suggesting that flux measured within the inertial sublayer is an aggregation of fluxes from individual street canyons in the vicinity. The inert scalar treatment of aerosol particles needs to be evaluated by relating lifetimes of aerosol processes in relation to the residence times of particles within the canyon. For the canyon scale considered, it is found that the inert scalar assumption may be used to describe aerosol number dispersion as effects of coagulation on aerosol properties is found to be insignificant. Condensation is found to be significant only at unrealistically high levels of in-volatile species in the vapour phase and in the abundance of soot. If the content of semivolatile compound within the aerosol population is high, then effects of partitioning on aerosol properties may be important, rendering the inert scalar assumption inadequate to characterize aerosol mass concentration dispersion. In addition, partitioning is found to be sensitive to accommodation coefficient and molecular mass of the in-volatile core within the aerosol particle, pointing towards the need for a more accurate characterisation of these parameters. |
