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Sulphur Dioxide
Most fossil fuels such
as coal, diesel and heavy oils naturally contain sulphur in
varying amounts. Less polluting, "cleaner" fuels which
contain lower levels of sulphur are typically more costly.
When sulphur containing fuels are
burned sulphur is oxidised to form sulphur dioxide.
Sulphur dioxide is also emitted
directly by chemical reactions used in some industrial
processes.
In Durban, 91.4% of sulphur dioxide
pollution originates from industrial processes, mainly from
coal burning. Motor vehicles contribute only 6.9%, mainly
from diesel driven engines.
Click to view graphs showing
contributions by different activities.
Oxides of Nitrogen
These are also produced
when fuels are burned. During high temperature combustion,
nitrogen in the air reacts with oxygen to produce oxides of
nitrogen (NOx). Most of the NOx created during this process
is nitric oxide (NO).
Nitrogen dioxide (NO2) is
formed as NO combines with oxygen in the air (oxidation). This
NO2 is the predecessor of gaseous
nitric acid, and is the oxide of nitrogen which has the
biggest health impact.
In Durban, motor vehicles are the
main source of NOx emissions (74.7%), with an almost equal
contribution from diesel and petrol driven vehicles.
Industrial fuel burning processes account for approximately
20.6% of all NOx emissions in the city.
Click here to view graphs showing
contributions by different activities.
Nitrogen oxides are key precursors
needed to create photochemical smog. Photochemical smog is a
complex mixture of primary and secondary pollutants that
forms when nitrogen oxides and some volatile organic
compounds react under the influence of sunlight. The
resultant photochemical compounds can be harmful to
organisms and degrade visibility.
As vehicle traffic increases in the morning, NOx levels
rise and begin reacting with volatile organic compounds, in
the presence of sunlight, to yield photochemical
smog.
Volatile Organic Compounds
- These are unburned hydrocarbons
emitted mainly from motor vehicle exhausts. They are also
emitted from solvent evaporation processes such as vehicle
manufacture, the coatings industry and general solvent
storage, distribution and use.
- In Durban, motor vehicles are responsible for 70.2% of
VOC emissions, with 27.3% arising from industrial processes.
- Click here to view graphs showing
contributions by different activities.
- VOC's are one of the main reactive compounds in the
production of photochemical smog.
Fine Particulate Matter
- Particulate matter is the term
given to the fine particles of solid material in the air.
- The majority of anthropogenic
(human-made) particulates are usually fine and more easily
inhaled.
- In the DMA, most (69.4%) of the
anthropogenic particulate matter is emitted from industrial
combustion process emissions of fine ash and deposits.
Particulate matter discharged from petrol and diesel driven
engines account for 26.8% of the total. To a lesser extent
soot is discharged from fireplaces and wood stoves.
- These proportional contribution
estimates have not considered natural windborne dust from
roadways, fields and construction sites which commonly
contribute significantly (55%) to the total particulate
matter emissions.
- Click here to view graphs showing
contributions by different activities.
- Photochemical reactions in the air can also form fine
particulate matter which impairs visibility.
Carbon Monoxide
- Carbon monoxide arises
primarily from incomplete or inefficient combustion of
carbon in fuels.
- Oxygen concentration, flame
temperature, fuel residence time and combustion chamber
turbulence are all important variables that affect the
exhaust concentrations of CO.
- In the DMA, fuel combustion in
petrol driven motor vehicles is the principle source of CO
emissions.
- Click here to view graphs
showing contributions by different activities
Ozone (ground level, not stratospheric)
Ground level ozone is
not generally emitted directly in significant quantities. It
is formed as a secondary pollutant from photochemical
reactions in the air between certain reactive volatile
organic compounds and nitrogen oxide precursors.
The major emitters of these two
precursors are, therefore, the main contributors to ground
level ozone concentrations.
The main sources of volatile
organic compounds in the DMA are motor vehicle exhausts,
and, petrol and solvent storage, distribution and use. Motor
vehicles, and to a lesser degree industrial fuel combustion,
are the main sources of nitrogen oxides.
Click here to view a graph
showing the typical daily cycle of Ozone in the
air.
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