Synoptic conditions associated with high carbon monoxide episodes at a coastal station in Hong Kong

The effect of different synoptic environments and the associated meteorological conditions on the transport of CO to a coastal station located at the southeastern tip of Hong Kong are examined. Based on the 1994 data, we found that the appearance of a Siberia high pressure, the passage of a cold front, the passage of a trough, and the existence of a tropical cyclone to the east to southeast of Hong Kong frequently gave rise to continental outflow from a northeasterly to a northwesterly direction. This continental airstream, which is highly contaminated with anthropogenic air pollutants, raised the CO concentration at the station to a level significantly above the oceanic value (measured to be around 81 pbbv under persistent maritime flow conditions). We also identified 20 high CO episodes in which the CO concentration of at least two consecutive hourly means in a given month exceeded two standard deviations from the monthly mean. All these episodes were directly associated with one of the above four types of synoptic environments with the winds being moderate to light northwesterly to northeasterly.