Monitoring of Ambient Volatile Organic Compounds at Two Urban Sites in Hong Kong from 1997 to 1998

Abstract

Volatile organic compounds (VOCs) in ambient air were measured at two urban sites (Central & Western and Tsuen Wan) in Hong Kong. Twenty-four-hour whole air samples were collected into Summa passivated stainless steel canisters approximately every 7 days at the two sites from July 1997 to June 1998. A gas chromatogra phy mass spectrometry (GC/MS) technique was em ployed for the analysis of 42 targeted VOCs as listed in the USEPA Method TO-14. At the method sensitivity of 0.10-0.20 ppbv, 16 and 18 listed compounds were de tected at Central & Western and Tsuen Wan, respective ly. BTEX (benzene, toluene, ethylbenzene and xylenes) and some halohydrocarbons including trichlorofluoro methane (Freon 11), dichlorodifluoromethane (Freon 12), methylene chloride and trichloroethene were the predominant VOCs found in over 75% of the ambient samples collected at the two sites. Styrene and 1,3,5- trimethylbenzene were intermittently found in a few samples at the Tsuen Wan site only. Four other com pounds (chloromethane, 1,1,1-trichloroethane, tetra chloroethene, and 4-ethyltoluene) were quantified in less than half of the total samples. The annual average con centration of the measurable VOCs was well within the range of 0.20-5.0 ppbv and was comparable to other major American and European cities. However, individu al measurements of toluene concentration occasionally exceeded 20 ppbv. Good correlations (r = 0.65-0.98) were found to exist between individual members of BTEX; correlation between BTEX and the halohydrocar bons could not be established. This indicates that am bient BTEX in the two sites should come from the same origin. In addition, the ambient concentrations of BTEX found at the two Hong Kong sites were comparable to other urban cities and showed significant seasonal varia tion, while the halohydrocarbons did not. The ambient concentrations of BTEX tended to peak in the relatively dry winter months in Hong Kong. The ratio of toluene/ benzene was relatively higher than the range of values reported in the literature for some American and Euro pean cities, but was in line with the findings reported for two South East Asian cities. The present work is the first of its kind describing the systematic setting up of a moni toring network for measuring ambient VOCs distribution and will act as a foundation for the establishment of a database of ambient VOCs in the region for future envi ronmental management and planning activities.