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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.
Low wind speeds and limited vertical mixing on January 9 caused locally emitted pollutants to accumulate. High ambient concentrations of SO2 and NO2 were recorded at all three sites. At Mong Kok, the NO2 concentrations exceeded the 24-hour Air Quality Objective of 150 microgrammes per cubic metre.
The highest level of dust in the month was recorded on January 12. At Mong Kok, RSP levels exceeded the 24-hour Air Quality Objective value of 180 microgrammes per cubic metre.
The generally high dust levels were likely to be caused by the combined effects of local sources and northerly winds bringing in dust to the territory.
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, and by the atmospheric oxidation of nitric oxide (NO), also a product of combustion.
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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.
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