Air is one of the most important components of the environment from which all aerobic organisms take up oxygen. We breathe twenty four hours a day taking into our bodies about sixteen times more air than we ingest food and water added together.
This air goes straight to our lungs wherein it is in direct contact with blood supply across a thin membrane. Whatever is absorbed in the alveoli of our lungs goes straight to the heart from where it is quickly pumped to all parts of our body. This makes observation on air quality a very important aspect of environmental monitoring.
Since times immemorial man has been using atmosphere as a sink for smoke, waste gases, fumes and vapours. No doubt these gaseous materials react and interact with each other and are finally brought down as dry or wet precipitation to earth’s surface to be assimilated and eliminated.
Smaller quantities usually pose no problems but large in-puts before nature has time to dispose them off cause problems. Interaction in the atmosphere in between these gaseous pollutants and other constituents cause the formation of secondary pollutants which at times are more injurious than the primary ones.
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Some of the gaseous wastes we discharge in the atmosphere stay back in air (like CO2) rising concentrations of which has been causing global warming or reach the stratosphere (for example CFCs) where they damage the vital ozone shield. Particulate material introduced in the atmosphere tends to concentrate lower down and causes respiratory problems to those who breath this air. Atmospheric air is, therefore, sampled and monitored usually for the estimation of the following:
Undesirable gaseous constituents:
These constituents are introduced into the atmosphere by human activity as well as through natural processes. Important among these are Carbon monoxide, Oxides of nitrogen, Sulphur dioxide, Hydrogen sulphide, Hydrocarbons etc.
2. Particulate material or air borne solids in the atmosphere:
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These include particle of dust, organic debris, carbon particles, fly-ash, fibres of asbestos, silica, viable spores, pollens etc.
(1) Air Sampling:
In order to obtain a reasonably accurate idea of the quality of air at any locality representative samples of air is very important. This sample should be large enough so as to yield adequate amount of various pollutants in concentrations which fall within the range of analytical methods being used for the monitoring.
The volume of air sample or the total volume of air required for analyses decreases as the concentration of the pollutants rise. Also smaller samples are required if the method used for the analysis is very sensitive. It general a sample of about 10 cubic metres is taken which is considered sufficient for most of the routine analysis.
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The rate of sampling, e.g., the amount of air sampled per unit time is also very important in collecting a good and representative sample of air. A large volume of air sucked in by a powerful aspirator in a short time may yield misleading results as it could represent simply a chance variation. A sample collected over a long time at a slower rate is usually considered more reliable. For most of the practical purposes a sample collected over a period of 2.5-4.00 hours is considered to be the most reliable.