Flow patterns influencing the seasonal behavior of surface ozone and carbon monoxide at a coastal site near Hong Kong

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Surface O 3 and CO were measured at Cape D’Aguilar, Hong Kong during the period of January 1994 to December1996 in order to understand the temporal variations of surface O 3 and CO in East Asia–West Pacific region. The isentropic backward trajectories were used to isolate different air masses reaching the site and to analyze the long-range transport and photochemical buildup of O 3 on a regional scale. The results show that the diurnal variation of surface O 3 was significant in all seasons with daily O 3 production being about 20 ppbv in fall and 10 ppbv in winter, indicating more active photochemical processes in the subtropical region. The distinct seasonal cycles of O 3 and CO were found with a summer minimum (16 ppbv)–fall maximum (41 ppbv) for O 3 and a summer minimum (116 ppbv)–winter maximum (489 ppbv) for CO. The isentropic backward trajectory cluster analyses suggest that the air masses (associated with regional characteristics) to the site can be categorized into five groups, which are governed by the movement of synoptic weather systems under the influence of the Asian monsoon. For marine-originated air masses (M-SW, M-SE and M-E, standing for marine-southwest, marine-southeast and marine-east, respectively) which always appear in summer and spring, the surface O 3 and CO have relatively lower mixing ratios (18, 16 and 30 ppbv for O 3, 127, 134 and 213 ppbv for CO), while the continental air masses (C-E and C-N, standing for continent-east and continent-north, respectively) usually arrive at the site in winter and fall seasons with higher O 3 (43 and 48 ppbv) and CO (286 and 329 ppbv). The 43 ppbv O 3 and 286 ppbv CO are representative of the regionally polluted continental outflow air mass due to the anthropogenic activity in East Asia, while 17 ppbv O 3 and 131 ppbv CO can be considered as the signature of the approximately clean marine background of South China Sea. The very high CO values (461–508 ppbv) during winter indicate that the long-range transport of air pollutants from China continent is important at the monitoring site. The fall maximum (35–46 ppbv) of surface O 3 was believed to be caused by the effects of the weak slowly moving high-pressure systems which underlie favorable photochemical production conditions and the long-range transport of aged air masses with higher O 3 and its precursors.