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.
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
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
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.