VOCs

Monthly variations of each selected VOC at the three monitoring stations were plotted as shown in Figure4.2d. The monthly variations of each species at each of the three monitoring stations were compared. Aside from the actual concentrations, the graphs suggest a similar trend at the three stations for benzene during most of the twelve months (except for during the summer months, May to August, at PolyU). This suggests that the stations were influenced by the same air dispersion factors, pollutant sources and transformation reactions of pollutants. As mentioned earlier, the main source of benzene is on-road vehicles and since there are roads surrounding all three stations, it could be assumed that vehicle emissions contributed as the major source of benzene at all three monitoring stations. A similar situation was found for 1,3-butadiene, except for a slight trend difference at PolyU station, again during the summer months. This divergence between the PolyU and the two EPD stations during the summer months could reflect the fact that higher temperatures and solar radiation increase the photochemical reaction rates and thus decrease the concentrations of VOCs as distance from the source (in this case the road) increases. Another explanation could be that there were other sources of these two VOCs in addition to vehicle emissions at the PolyU station.

For the other three pollutants (tetrachloroethylene, methyl chloride and trichloroethylene), the line patterns were completely different at the three stations. One possible explanation is that additional sources at one or all stations contribute to various degrees, depending on how active and close these sources are to the stations.

Carbonyls

As shown in Figure 4.2e, the variation patterns for acetaldehyde and formaldehyde were different at the three monitoring stations. In fact, there are many different sources for acetaldehyde and formaldehyde in addition to on-road vehicles. Again, additional sources at one or all stations may contribute to various degrees, depending on how active and close these sources are to the stations.

PAHs

Basically, the variation patterns for each pollutant were similar among the three monitoring stations in wintertime, especially between the Central/Western and Tsuen Wan stations (Figure 4.2f). This indicates that they were influenced by the same air dispersion factors in winter. As mentioned before, the main source of PAHs is combustion. Among the three stations, only PolyU station was a roadside station, therefore on-road vehicles was a more dominant source of PAHs at the PolyU station when compared to the two roof-top stations. Hence, it is consistent for similar patterns to occur between Central/Western and Tsuen Wan stations as they might be influenced by the same pollutant sources and are similar vertical distances away from the nearby roads (as both are on rooftops). The monthly variations were significantly influenced by weather patterns (rainfall, wind speed and wind direction etc.), which are consistent with the fact that these factors do affect the dispersion of toxic air pollutants.