To a certain extent nature's own air conditioning can keep the air clean. Wind mixes the gases and dilutes them, rain washes the dust and other substances to the ground, and plants absorb the carbon dioxide and replace it with oxygen. Air pollution arises when nature can no longer maintain the levels of certain substances below safe limits and they start having negative impacts.
Generally, because we can not see, taste or smell the air, we ignore it and take it for granted. We become concerned only when its quality is
reduced but by then, the efforts required to fix the problem are often complex and expensive.
The extent to which pollutants are emitted in a given area can impact on air quality elsewhere depends on a number of factors:
- topography - which means the form of the land - e.g. mountains, valleys, rivers and plains
- the chemical and physical properties of pollutants.
Simply measuring air pollution alone does not tell us very much. To obtain an understanding about why air quality can vary from day to day,
we must measure meteorological conditions as well, such things as:
- cloud cover
- air pressure conditions
Aspects of weather are very important as they have a significant affect on the concentration of air pollution that actually builds up in
the air in a region. For example:- a valley such as the one in which Launceston is built, pollution can build up on very still days. Areas
such as this are called "airsheds". At other times, fresh winds will clear away the pollution and the concentrations will be very low.
In the case of some pollutants the airshed may extend relatively large distances from the city centre. These include gases such as ozone
that are formed by chemical reactions in the atmosphere, they are call "secondary pollutants". However, a primary pollutant such as wood smoke in winter, is often more localised and perhaps confined to areas sharing common night time air flows.
The build-up of pollution in an airshed depends on many factors. For example, if there is circulation of polluted air through the airshed
over several days, levels of pollutants such as ozone may build up to very high levels, this often happens in Sydney and can be seen as smog.
See the Bureau of Meteorology's Education Area website for more information on temperature inversions, wind and other factors affecting the dispersal of air pollution.