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SECTION
5 ASSIMILATIVE CAPACITY CAPITAL STOCK
| 5.4 |
Assimilative
Capacity of Air (including Noise) |
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| 5.4.1 |
Existing
Capital Stock and Trends |
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| Natural
Assimilative Capacity
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| 5.4.1.1 |
Hong
Kong's atmosphere is subject to a number
of characteristic airstreams which influence
its assimilative capacity. The prevailing
winds in Hong Kong are north-easterly for
about 70% of the year and winds from the
north are particularly prevalent during
the winter months when they carry significant
levels of pollution from industrial sources
in Guangdong. The atmospheric environment
of Hong Kong is also influenced by several
topographically confined airsheds (see Figure
5.4a) which are defined by areas of
steep terrain and which may restrict the
dispersion of pollutants. The assimilative
capacity of the territory's air (which has
not been precisely determined) is limited
by these topographical and meteorological
factors. In addition, many urbanised areas
of Hong Kong have limited air circulation,
due to the density of high rise buildings
resulting in elevated levels of air pollution
from vehicle emissions (this is known as
the 'street canyon' effect).
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| 5.4.1.2 |
In
the absence of detailed research or empirical
findings with regard to the true assimilative
capacity of the airsheds above Hong Kong,
reference is made in this section to existing
levels of air pollutants in comparison with
the series of established Air Quality Objectives
(AQOs). The degree of compliance of air
quality with these standards has been used
as a proxy means to assess assimilative
capacity of the air. EPD generally recognises
that the AQOs are representative of thresholds
beyond which further accumulation of pollutants
would represent an exceedence of assimilative
capacity. Whilst this approach allows a
degree of quantification of the concept,
it is not within the scope of this report
to attempt an overall estimate of assimilative
capacity of individual airsheds in the territory.
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| 5.4.1.3 |
The
discussion on assimilative capacity of the
air with regard to noise is somewhat different
from other issues in this report because
of the character of noise as a pollutant.
Noise emissions are generally only polluting
where they occur and do not pose a "downstream"
impact or effect elsewhere. The assimilative
capacity of the air is therefore effectively
unlimited although it is obviously desirable
to eradicate or minimise noise at source,
and areas with high ambient noise levels
may be considered to have limited assimilative
capacity in a local context.
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| Air
Quality Monitoring |
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| 5.4.1.4 |
Routine
monitoring of air quality in Hong Kong is
conducted by EPD via a network of fixed stations
which determine concentrations of gaseous
and particulate pollutants at both roadside
stations and general stations, the latter
to determine ambient levels. |
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| 5.4.1.5 |
From
the monitoring data obtained, EPD disseminates
hourly information on air quality in the form
of the Air Pollution Index (API). The index
encompasses RSP, SO2, NO2, CO and O3 and works
by scaling the concentrations of the five
pollutants within a range of 0 to 500. The
results are then presented as categories of
low, medium, high, very high or severe on
a station basis - for urban, new town, roadside
and rural. The API is applied separately to
general and roadside datasets. Whilst the
index is used to provide information on ambient
air quality conditions on any particular day,
the following section discusses trends in
air quality with reference to concentrations
of individual pollutants as compared with
their relevant air quality objective (AQO)
in order to comment on assimilative capacity.
Although results from all monitoring stations
are considered together, it should be recognised
that the stations at Mong Kok, Causeway Bay
and Central are roadside sites and therefore
concentrations of certain pollutants (eg particulates
and nitrogen oxides) can be expected to be
higher than those from ambient monitoring
stations. |
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