Trace gas measurements in coastal Hong Kong during the PEM-WEST

Abstract. 0 3, CO, NO., and SO 2 were measured at a coastal site in Hong Kong (22ø13'N, 114ø15'E, 60 m MSL) c•uring the Pacific Exploratory Mission-West B (PEM-West B) in Febru-ary and March 1994. Average concentrations determined inthis study were 34 + 14 ppbv for 0 3, 458 + 130 ppbv for CO, 9.33 + 7.84 ppbv for NO., and 1.31 + 1.46 ppbv for SO 2. Their high and site was often under the impact of fresh continental emissions variable levels suggest that the study ',v (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 dra-matic 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 f urban plumes. Variations of NOy and SO 2 appeared to be mainly dominated by local sources. 0 3 was poorly and often negatively corre-lated with CO and NOy, suggesting that air masses sampled inthe study period were highly in-homogenous with respect to the chemical signatures and that 0 3 was chemically titrated by an-thropogenic pollutants during the early stages of continental outflow. Calculated isentropic trajec-tories 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 histo-ries of smaller-scale air masses. Finally, the reasons for apparent disagreement between trajectory results, surface winds, and sometimes chemical data require further investigation