Principle Sources of Air Pollution
Air Quality (air pollutant concentrations) is the result of the interaction of pollutant emissions, chemical and physical transformations, dispersion of pollutants and pollution sinks. There are natural and „anthropogenic? (the result of human activities) sources of pollution. Dispersion of pollutants is greatly influenced by meteorology; the oceans and surfaces of plants, earth?s surface or buildings act as 'sinks'. All pollutants undergo dispersion, chemical transformation and deposition in the lowest layer of the atmosphere
- the troposphere. The troposphere extends to an altitude of about 16 to 18 km over the tropics, reducing to about 10 km over the poles and contains about 80% of the total air mass. All weather phenomena occur in this layer. Mixing between the troposphere and higher levels of the atmosphere (stratosphere and above) is negligible, therefore the dispersion of pollutants occurs almost exclusively within the troposphere.
The sources of air pollution may be classified as stationary point sources (generally industrial in origin), diffuse or area sources and mobile sources (mainly cars and trucks). The stationary industrial sources are usually classified by process type or sub-type. Thus an oil refining plant also includes large industrial boilers as a sub-type. Small and medium scale plants such as garment or food processing plants may include industrial boilers, a common source of air pollution. The quality and type of fuel used for energy production are important determinants of the air pollution potential of a plant. Each type of plant or activity generally emits more than one pollutant, and the pollutant emission rate depends on the fuel type and quality, the design of the plant (and whether fitted with air pollution control devices or not), and the activity rate or output of the plant.
Air quality monitoring
An air quality monitoring system essentially measures ambient air concentrations at a number of fixed locations, for example across a city or within a region. In principle, the function of a monitoring station is to compare the measured values against a standard or a guideline and to take action if the measured values exceed the standard or guideline. (Unfortunately, in the absence of a regulated management system, too frequently no action is taken even if guidelines are exceeded.)
Continuous monitors are instruments capable of measuring pollutant concentrations (for example, SO2, NO2, CO, PM) continuously and more or less instantaneously (in reality, over very short periods of time). The „instantaneous? values are not in themselves useful for assessing air quality. Thus these values may be averaged over time periods of 10 or 15 minutes, one hour, 3 hours, 8 hours, 24 hours or longer periods. The time-averaged values (time weighted averages) may be compared with air quality standards or guidelines, or may be used to estimate the potential health impacts of the air pollutant concentrations.
Instruments capable of measuring air pollutant concentrations continuously are comparatively expensive, and have to be housed in a protected and controlled environment, usually at a fixed site or in a mobile station or caravan. Thus a city with a monitoring system would have a limited number of monitoring sites, each measuring a limited number of pollutants. For example, the City of Cape Town has the following monitoring network:
The choice of locations for monitoring sites should consider: proximity to major pollution sources or suspected areas of high concentration, areas with high population density and an area remote from local pollution sources to assess „background? pollution levels.
Cape Town publishes monthly reports of the results obtained at its monitoring stations.
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