VOCs

The average concentrations of VOCs between winter and summer at the three monitoring stations are shown in Figure 4.2g. Winter to summer VOC concentration ratios are listed in Table 4.2a.

Most of the VOC concentrations in the winter were higher than those measured in the summer, with the winter to summer VOC concentration ratios being larger than 1. Since these five pollutants were mainly from vehicle exhaust and the evaporation of fuels, the higher temperatures in summer increased the evaporation of VOCs and solar radiation increased the photochemical reaction rates and thus decreased the concentrations of VOCs.

In general, there were many possible reasons that could have affected the concentrations of VOCs in the winter and in the summer. More rainy days in the summertime caused a washout effect of pollutants, thus accumulation of VOCs was less than that in the wintertime. VOCs can also be removed from the particulates under dry and wet deposition. Chemical removal, especially by Hydroxide (OH) radicals, is important during the summertime. But the distance to the emission source is also a crucial factor influencing the concentration levels. The nearer to the emission source, the less influencing certain factors become, like the washout effect and photochemical reactions. Therefore, it should be noted that the ratio varies depending on whichever factor(s) dominates at that particular station.

Carbonyls

The average concentrations of carbonyls between winter (November to February) and summer (May to August) at the three monitoring stations are shown in Figure 4.2h. Winter to summer carbonyls concentration ratios were listed in Table 4.2b.

There are many potential factors that could affect the concentrations of carbonyls in the winter and in the summer. Both ratios at PolyU station were less than 1, which means the concentrations of carbonyls were higher in summer than those in winter. There might have been a potential source for carbonyls at PolyU station during summertime that resulted in the higher concentrations when compared to wintertime. Acetaldehyde and formaldehyde are very volatile organic compounds and so the higher temperatures in summer may have increased the evaporation of carbonyls. Furthermore, acetaldehyde and formaldehyde might be the by-product of photochemical reactions under high solar radiation in summer. This indicates that chemical photooxidation is one of the most important secondary sources of acetaldehyde and formaldehyde.

PAHs

The average concentrations of PAHs between winter and summer at the three monitoring stations are shown in Figure 4.2i. Winter to summer PAHs concentration ratios are listed in Table 4.2c.

All PAHs concentrations in the winter were higher than those measured in the summer. The winter to summer PAH concentration ratios were larger than 1, the ratios at Central/Western and Tsuen Wan station were even larger than 2. The high concentrations of PAHs during winter at Central/Western and Tsuen Wan station were probably due to less photochemical degradation of PAHs in the atmospheric environment. These reactions are dependent on the intensity of solar radiation received and the prevailing wind speed. More rainy days in the summertime causes the washout effect of pollutants. This effect is significant for PAHs because PAHs are attached to the particulate, therefore the concentrations of PAHs are low after a rainy day, especially when the stations are not near to the source (Central/Western and Tsuen Wan station). For PolyU station, the winter to summer ratios were less than the other two stations. This could have been because it was situated near the source (on-road vehicles) so that the washout effect was minimized.