Large-eddy simulation of air quality in Hong Kong

To perform realistic high-resolution air quality modeling in a polluted urban area, the WRF (Weather Research and Forecasting) model was used with an embedded large-eddy simulation (LES) module and with online chemistry. As an illustration, the numerical experiment was conducted in the polluted megacity of Hong Kong, which is characterized by multi-type pollution sources as well as complex topography. The multi-resolution simulations from mesoscale to LES scales were evaluated by comparing the calculated fields with ozone sounding profiles and with the observations at surface monitoring stations. The comparison shows that both mesoscale and LES simulations reproduced well the mean concentrations of the chemical species and their diurnal variations at the background sites; however, the mesoscale simulations largely underestimated the NO_Xconcentrations and overestimated O₃ near roadside stations due to the coarse representation of the traffic emissions. The LES simulations improved the agreements with the measurements near the road traffic, and the LES with highest spatial resolution (33 m) provided the best results. The LES simulations showed more detailed structures of the spatial distributions of chemical species than the mesoscale simulations, indicating the capability of LES of resolving high-resolution photochemical transformations in urban areas like Hong Kong. The LES simulations showed similar trends with the mesoscale model in the evolution of the profiles of the chemical species with the development of the boundary layer over a diurnal cycle. The vertical fluxes of the chemical species are stronger in the 33 m LES than the 100 m LES, because the high-resolution LES can better resolve the turbulent eddies.