XN000022-1994-12-15 — Page 23

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* locations close to road traffic in built-up urban areas, * combined commercial and residential districts, and * districts close to industrial areas.

The department currently monitors air quality continuously at eight stations, including the three mentioned. Short-term measurement exercises are also undertaken as and where the need arises.

The reported air pollutants include sulphur dioxide (SO2), nitrogen dioxide (NO2), total suspended particulates (TSP) which comprise all sizes of dust particles and the respirable fraction of the dust (RSP). All these pollutants can affect respiratory health in sufficient concentration.

High concentrations of dust were recorded at all stations on November 7. At Mong Kok, the TSP level exceeded the 24-hour Air Quality Objective limit of 260 ug/m3.

As the airflow on November 5, 6 and 7 was generally from the north and northeast, the high dust levels might be caused by dust from these directions combined with local emissions.

On November 14 and 15, pollutants were flushed from the territory by strong winds, resulting in lower concentrations of gaseous pollutants.

The gases and particles described originated from various sources. SO2 is mostly produced when fuels that contain sulphur are burned. NO2 is formed during combustion by the combination of nitrogen and oxygen.

However the majority of it is formed in the atmosphere by the oxidation

of nitric oxide (NO), also a product of combustion.

Vehicle exhaust is an important source of NO and NO2 in terms of impact on local air quality. It is also a major source of airborne particulate matter, especially the smaller respirable particles.

Diesel-engined vehicles such as taxis, public light buses, passenger coaches, franchised buses and light and heavy goods vehicles are the greatest contributor of particulate matter. Other sources include industry, furnaces and boilers, construction activities, the sea and the soil.