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More
than 70% of the toxic air pollutants are volatile
organic compounds (VOCs), which play a major role
in urban and regional atmospheric chemistry. VOCs
have received attention not only as precursors
to photochemical gas phase and particulate products,
but also for their possible adverse health effects.
Most of the VOCs are emitted to the atmosphere
from anthropological activities, arising mainly
from fuel combustion and evaporation of VOC vapours.
Five
VOCs (methyl chloride, benzene, 1,3-butadiene,
trichloroethylene and tetrachloroethylene) have
been identified in this study. VOC samples were
collected every six days and a total of 59 samples
were collected (of which 58 samples were valid).
The measured concentrations of VOCs found in this
study were plotted in Figure
4.1a and a summary of the statistical data
is shown in Table 4.1a.
In
this study, benzene was found to exhibit the highest
concentrations out of the five VOC species. The
average concentration of benzene was measured
to be 5.12 ugm-3, with a maximum concentration
of 11.25ugm-3. Methyl chloride was the second
most polluting species with an average concentration
of 2.29ugm-3 (ranging from 0.21 to 5.69 ugm-3)
(Figure 4.1b).
The concentrations of the other three VOC species
(1,3-butadiene, trichloroethylene and tetrachloroethylene)
were comparatively lower than benzene, with concentrations
below the detection limit of GC-MSD.
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Table
4.1a summary of VOC Data at PolyU Station
(n=58)
|
| VOCs
(ugm-3) |
Max. |
Min. |
Average
|
Standard
deviation |
| Methyl
chloride |
5.69 |
0.21 |
2.29 |
1.13 |
| 1,3-Butadiene |
1.51 |
0.24 |
0.58 |
0.35 |
| Benzene |
11.16 |
1.02 |
5.12 |
2.21 |
| Trichloroethylene |
4.31 |
0.55 |
0.95 |
0.77 |
| Tetrachloroethylene |
6.80 |
0.69 |
1.52 |
1.14 |
Monthly
Variations of VOCs
The
monthly variations of VOCs are shown in Figure
4.1c. The trend pattern for benzene and 1,3-butadiene
was similar, suggesting that they might be from
the same source. Peak concentrations were noted
in March 2000. No significant monthly variations
were observed for methyl chloride and trichloroethylene.
Seasonal
Variations of VOCs
In
Hong Kong, the winter season includes November,
December, January and February, while the summer
season includes May, June, July and August. Strong
monsoon winds and dry weather characterize winter
seasons, while summers are hot and humid with
occasional showers and thunderstorms.
The
seasonal variations of VOCs are shown in Figure
4.1d. The concentrations of benzene and methyl
chloride in winter were higher than those in summer,
but for the other three VOCs, the concentrations
in summer were higher than in winter. Further
discussion on the seasonal variation of VOCs is
given in Section 4.2.3.
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