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Transboundary pollution in its technical sense covers pollution that
moves between the different layers of the atmosphere. A good example here is
ozone, where it is a very useful molecule in the stratosphere blocking high
energy UV from affecting us in the troposphere but a serious polluter affecting
human health at high concentrations in the troposphere. The protective
stratospheric ozone layer is being damaged by transboundary
movement of CFC’s from the troposphere which then react to convert ozone back
to oxygen.
A commoner use of the term
transboundary is to describe pollution that travels
great distances in the troposphere, carried by strong winds and thermal
currents. We will cover examples of both.
The main issues of transboundary pollution affecting cities are ozone episodes
and PM10 episodes often involving particulate sulphate.
Ozone episodes occur
where large regions are affected by precursors from outside, locally generated
precursors and transboundary stratospheric ozone
which together with stable conditions and bright sunlight stimulate the
production of tropospheric ozone giving rise to a
high ozone episode and global warming.
Acid rain is another transboundary pollutant formerly dispersed by high chimneys
affecting far distant regions. Sulphuric and sulphurous acids are oxidised to
the sulphate form and can be transported as fine particulates across 1000’s of
kilometres within the troposphere.
Particulates from
forest fires, volcanic eruptions and large industrial sources may also be
transported many hundreds of kilometres from their source and add a high
background to locally produced pollution. Volcanoes can ‘push’ material out of
the troposphere i.e. HCl gas, but particulates are
normally too heavy.
The main atmospheric transboundary pollutants affecting cities are global warming
molecules such as CO2, hydrocarbons, CFC’s etc. Many cities have grown up where
rivers meet or flow into deltas or estuaries and are, therefore, threatened by
global warming sea level rises. Cities need to include global warming molecules
in their air quality strategies and need to work together to reduce emissions
on a regional basis.
Many cities have now joined
climate protection alliances and are developing their own climate protection
strategies on the basis that everyone doing a little will add up to a great
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