44. Variations of Water Soluble Organic Carbon and Carbon Isotopic Composition in Hong Kong

S. C. Lee(a)*, J. J. Cao(b), K. F. Ho(a)

(a) Research Center for Urban Environmental Technology & Management, Department of Civil & Structural Engineering, The Hong Kong Polytechnic University, Hong Kong

(b) State Key Laboratory of Loess & Quaternary Geology, Institute of Earth Environment, Chinese Academy of Sciences, China

Hong Kong, on the southern tip of the booming Pearl River Delta in Southern China, is also one of the most densely inhabited metropolitan areas in Asia, with the majority of the population of 7 million crowded into only 15% of the total area of 1068 km2. According to The Census and Statistics Department of Hong Kong, there were 517000 registered vehicles on 1904 km of roads in 2000. Diesel trucks with high EC emission accounted for 30% of the total amount of motor vehicles. Motor vehicles are considered as one of the major contributors to air pollution in Hong Kong. Consequently, it is a representative Chinese city to for the characterization and variability assessment of OC and EC in a meso-scale setting.

To determine the levels and variations of organic carbon (OC) and elemental carbon (EC) in Hong Kong urban atmosphere, particulate matter samples were collected by using high volume (Hi-vol.) samplers at three monitoring stations (roadside, urban, urban background) from November 2000 to February 2001 and June 2001 to August 2001. The concentrations of OC, EC and water-soluble organic carbons (WSOC) were analyzed by the thermal/optical reflectance (TOR) method and selective thermal manganese dioxide oxidation (TMO). The respective stable carbon isotopic composition of OC and EC was determined using a Finnigan MAT-251 mass spectrometer after combustion.

Highest concentration levels of OC, EC and WSOC were found at roadside and the lowest levels at urban background site due to the emission source distance and physical dispersion. But the percentages of WSOC in total carbon at these sites were inversely correlated with their concentrations, i.e., highest percentages of WSOC were observed in background site. It is speculated that high WSOC fraction may be associated with aged aerosol because of the secondary formation by photochemical oxidation of organic precursors of anthropogenic pollutants during the transport of pollutants from roadside microenvironment to background atmosphere in an urban scale. The annual average d13C value of OC and EC were –26.9 ± 0.5‰ and -25.6 ± 0.1‰, respectively. There were no notable differences for seasonal distributions of carbon isotopic composition, indicating that mainly motor vehicular sources contributed to carbonaceous particles in the urban atmosphere of Hong Kong. OC isotope composition at the roadside and urban sites were similar, but heavier than those at the background site, implying the presence of other organic sources, such as biogenic sources, at that location.