O3, CO, NOy, and SO2 were measured at a coastal site in Hong Kong (22¿13'N, 114¿15'E, 60 m MSL) during the Pacific Exploratory Mission-West B (PEM-West B) in February and March 1994. Average concentrations determined in this study were 34¿14 ppbv for O3, 458¿130 ppbv for CO, 9.33¿7.84 ppbv for NOy, and 1.31¿1.46 ppbv for SO2. Their high and variable levels suggest that the study site was often under the impact of fresh continental emissions (including urban Hong Kong) during the season of continental outflow. Concentrations of these species were strongly influenced by the passage of cold fronts and troughs which periodically brought high levels of pollutants from the north. Outflow of continental air was indicated by dramatic changes in meteorological parameters and in the levels of trace gas species. CO appeared to be a good chemical indicator of changes of air mass type, and its variability may be attributed to the relative strength of the outflow and to the transport of urban plumes. Variations of NOy and SO2 appeared to be mainly dominated by local sources. O3 was poorly and often negatively correlated with CO and NOy, suggesting that air masses sampled in the study period were highly inhomogeneous with respect to the chemical signatures and that O3 was chemically titrated by anthropogenic pollutants during the early stages of continental outflow. Calculated isentropic trajectories captured large-scale changes of air masses, indicated also by surface meteorological and chemical data. Trajectory results offering finer resolutions would yield more insight into the histories of smaller-scale air masses. Finally, the reasons for apparent disagreement between trajectory results, surface winds, and sometimes chemical data require further investigation. ¿ 1997 American Geophysical Union