Chemical characterization of the boundary layer outflow of air pollution to Hong Kong during February–April 2001

As a cooperative effort with the TRACE‐P and ACE‐Asia intensive in the spring of 2001, trace gases and aerosols were measured at a relatively remote coastal site (Hok Tsui) in southeastern Hong Kong. The main objective of the measurement program was to provide continuous ground‐based data in the subtropical region of eastern Asia and to characterize the southward outflow of continental pollution that prevails in the lower atmosphere during early spring. In this paper, we present the results for ozone, CO, NO, NO y , SO 2 , 222 Radon, methane and C 2 –C 8 nonmethane hydrocarbons (NMHCs), C 1 –C 2 halocarbons, and C 1 –C 5 alkyl nitrate measurements obtained between 19 February and 30 April 2001. The average mixing ratios of O 3 , CO, SO 2 , and NO y were 45 ppbv, 404 ppbv, 1.8 ppbv, and 10.4 ppbv, respectively. The two dominant NMHCs were ethane (mean: 2368 pptv) and ethyne (mean: 1402 pptv), followed by propane (814 pptv), toluene (540 pptv), benzene (492 pptv), ethene (498 pptv), and n ‐butane (326 pptv). The most abundant halocarbon was CH 3 Cl (mean: 821 pptv), while 2‐BuONO 2 and i ‐PrONO 2 were the two dominant alkyl nitrates species with a mean mixing ratio of 20 pptv and 19 pptv, respectively. The levels of trace gases were strongly influenced by the outflow of continental air masses initiated by the passage of cold fronts. The data are segregated into four air mass groups according to the levels of 222 Rn and wind direction, representing fresh continental outflow, coastal, perturbed maritime, and local urban air. Ozone and CO showed a moderate positive correlation ( r 2 = 0.4) in the marine air group, characterized by low 222 Rn and CO levels, but they were poorly correlated in the other air mass groups. SO 2 and NO y exhibited good correlations ( r 2 > 0.6) with each other but were poorly correlated with CO, indicating differences in their emission sources and/or removal processes. CO very strongly correlated with ethyne and benzene ( r 2 > 0.85) and also showed good correlations with several other NMHCs. Moreover, CO correlated moderately with a biomass burning tracer (CH 3 Cl) and an urban/industrial tracer (C 2 Cl 4 ) indicating the impact of mixed pollution from urban and biomass burning sources. The relationship of CO, SO 2 , and NO y with the indicator of atmospheric processing, ethyne/CO and propane/ethane, were also examined. The 2001 data were compared to the results obtained in the same period in 1994 during PEM‐West B. The mean ozone level in the spring of 2001 was much higher than during PEM‐West B. SO 2 also had higher concentrations during TRACE‐P, while CO and NO y were comparable during the two campaigns. The observed difference has been discussed in the context of emission changes and variations in meteorology. Although it is difficult to draw definitive conclusions about the extent of the influence of these two factors, it appears that clearer skies and drier conditions may have been responsible for the higher ozone concentrations during the TRACE‐P period.