A severe air pollution episode occurred over much of the western part of Hong Kong (HK) between 28 and 30 December 1999. In this study, multiple analyses were made to understand the cause of this episode; first, the main emission source of the pollution episode was identified using remote sensing techniques from satellite pictures and was located about 100 km to the northeast of HK. Second, analyses were made on PM10 chemical species characterization in four air quality monitor stations (AQMSs), and a high concentration of potassium ions (K+) on the episode days indicates the significant impact of biomass burning. The ratio of elemental carbon and organic carbon values in two western AQMSs indicated that regional transport, rather than the local vehicle emission and power plant, played an important role in this episode. This result is in contrast with the early finding by H. Liu and J. C. L. Chan that this pollution episode is solely due to local sources. Third, to study the transport, diffusion, and trapping of pollutants during this air pollution episode, two numerical models were coupled in this study: a three-dimensional mesoscale meteorological model MM5 and a three-dimensional Lagrangian particle dispersion model. With the help of the Lagrangian dispersion model we show that this severe air pollution event was associated with transport of pollutants over HK within the boundary layer. These pollutants were subsequently brought down to the surface by the local land-sea breeze circulation and trapped over the western part of HK during the day by the same land-sea breeze.