VOCs

Comparing the concentrations of VOCs at the three sampling locations (Figure 4.2a), much higher average concentrations of benzene (5.05 ugm-3) and 1,3-butadiene (0.55 ugm-3) were detected at the PolyU station than at the other two EPD stations. It is likely that these high benzene levels originate from on-road vehicles and since the other two stations were roof-top monitoring stations (about 18m above ground), it seems logical that their benzene concentrations were lower than at the PolyU station.

Relatively high concentrations of methyl chloride (average concentration of 2.40 ugm-3) and trichloroethylene (average concentration of 1.72 ugm-3) were detected at Tsuen Wan station. Since the station is located near an industrial area, these high levels are consistent with the fact that trichloroethylene is mainly released from factories where it is used to remove grease from metals and that methyl chloride is evaporated during the cleaning processes.

Very high concentrations of tetrachloroethylene were found at Central/Western station with an average concentration of 3.37ugm-3, which was approximately two times more than the other two stations. A main source of tetrachloroethylene is dry cleaning fabrics for metal-degreasing and use of it as a starting material (building block). The relatively high concentration of tetrachlorethene may be due to some of these sources being located near the monitoring station.

Carbonyls

As shown in Figure 4.2b, the average concentrations of acetaldehyde (2.59 ugm-3) and formaldehyde (5.32 ugm-3) detected in Tsuen Wan station were higher than at the other two stations. However, the differences of the concentrations of both pollutants among the three monitoring stations were small. According to literature, the typical source of acetaldehyde and formaldehyde in the outdoor environment is on-road vehicles. However at the Tsuen Wan station, industrial emissions are an additional source of acetaldehyde and formaldehyde.

PAHs

PAHs are produced from fuel combustion. At the PolyU station, the main source of PAHs was mainly vehicle engines. All selected PAHs at the PolyU station, with the exception of benzo(a)anthracene, were higher than at the other two stations. Among the four selected PAHs (anthracene, benzo(a)anthracene, benzo(a)pyrene and chrysene), the highest concentration detected was of anthracene (6.08 ngm-3) at the PolyU station, with an average concentration of 2.57 ngm-3 (Figure 4.2c). The second highest concentration of PAHs detected among the three stations was chrysene (2.11 ngm-3), which also originates mainly from on-road vehicles. Industrial combustion is also a potential additional source of PAHs, therefore the concentrations at Tsuen Wan station were generally higher than at Central/Western station. For instance, Tsuen Wan recorded the highest benzo(a)anthracene concentration of all three stations.