Air quality can be influenced by 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
The weather can have a significant effect 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 called "secondary pollutants". However, a primary pollutant such as wood smoke in winter is often more localised and perhaps confined to areas sharing common nighttime 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.