Abstract: By making use of the 2005 hourly data of visibility at Chek Lap Kok and suspended particulate (PM2.5) at Tung Chung, PM2.5 concentration and visibility (excluding cases with mist, fog, rain or relative humidity≥95%) are found to have a reciprocal relationship with a correlation coefficient about 0.8. Besides, similar seasonal trends are exhibited in both the number of hours of reduced visibility (visibility below 8 km and excluding cases with mist, fog, rain or relative humidity≥95%) and PM2.5 concentration, i.e. with higher value attained in winter and lower value in summer. Backward trajectory analysis using HYSPLIT indicates that this phenomenon is related to the source of air mass affecting Hong Kong. For continental trajectories, the average daily occurrence of reduced visibility and the daily mean PM2.5 concentration were much higher than the corresponding occurrence of reduced visibility and mean PM2.5 concentration for maritime trajectories. A case study on an event with a tropical cyclone approaching Hong Kong is included to demonstrate the significance of meteorological conditions in determining the visibility and PM2.5 concentration.