Numerical simulation of the low visibility event at the Hong Kong International Airport on 25 December 2009

During the daytime of 25 December 2009, low visibility weather affected the Hong Kong International Airport (HKIA). The visibility as reported at the airport got to as low as a couple of hundred metres only. It was observed during this event that pollutant concentration levels at a station near the airport reached high values, namely, PM2.5 and PM10 were about 150-200 micrograms per metre cube. The low visibility might be related to the appearance of pollutants in the airport area. This low visibility event is studied using WRF (Weather Research and Forecast) meteorological model with the Community Multiscale Air Quality (CMAQ) model. Model simulation is conducted starting at about two days before the event. It is found to reproduce the main features of the event, namely, the change of wind direction and the occurrence of high pollution levels at times of lower visibility. The simulated time series of visibility in Hong Kong as well as a number of places over southern China are correlated reasonably well with the actual observations. Source apportionment would be carried out to find out the reasons for the low visibility. Sensitivity of simulation results to the pollution source inventory and boundary layer parameterization would also be discussed.