Pollutants are nothing but any substance that may present in an atmosphere in such a concentration that it may harmful to surrounding, living organism or human being. Sampling and analysis of air pollutants is known as Air quality monitoring and it is major part of environmental monitoring. There are many advanced types of ambient air Sampling methods. we will learn here only which are most commonly used.
So the most important objective of any air sampling exercise is to obtain a genuine & representative sample. There are both gaseous & particulate pollutants whose concentration is find out in milligram per cubic meter of air.
Particulate sampling is done with help of high volume air sampler, Dust sampler. In these samplers, particles less than 10 micron are taken into consideration. Filter paper being used in that sampler.
Difference between weights of filter papers divided by quantity of air passed gives you particulate matter present in air.
For the Gaseous sampling, collection of pollutant gas is very important and it can be collected by below shown types of ambient air sampling methods.
Ambient Air Sampling Methods
There are many methods available for collection of Air pollutants from atmospheric Air i.e. Grab Sampling, absorption in liquids, adsorption on solids materials & Freeze-out sampling in process of Environmental Monitoring.
In grab sampling, air sample is collected by filling an evacuated flask. This is very old and traditional Ambient Air Sampling Method. Plastic bags have been used for grab sampling for storage of gas & then subjected to analysis of grabbed sample.
Grab sample may be taken using rigid wall containers made from glass or stainless steel. These containers first evacuated & then allow air to enter to fill the container. Alternatively, a container may be filled with water & then used as a collector simply by draining water which is replaced by filling air sample.
Absorption in Liquids
Absorption of gaseous pollutants in liquid medium is one of the most common method for collecting air samples. To bring out high degree (pollutant) gas-liquid contact, impingers & midget type’s devices are used. These devices can handle sample flow rates about 30 to 3 litres per minutes respectively. Particular absorbent-liquid (say 0.1 N NaOH) is filled inside Impinger. Flow is controlled with help of flow control devices. If done with sampling procedure, particular absorbent is then desorbed into lab for analysis & then concentration of (say, NOx) required gas is find out.
Majority of Environmental Consultancy firms prefer this absorption in liquids among ambient air sampling Methods because of there is very negligible loss of degree of quality as well as quantity of pollutants while carrying it to laboratory.
Adsorption on Solids
In this Ambient Air Sampling Method pollutant gas is absorbed on surface of solid. The air sample is passed though packed column containing finely divided adsorbent on whose surface pollutants are retained. Commonly used adsorbent is activated charcoal & silica gel. After sampling, sample gases are desorbed into lab for analysis. This may be accomplished by washing adsorbent with liquid solvent.
Freeze-out sampling contains series of cold traps which being used to condensate air pollutants from air. The traps are bought into laboratory, samples are removed, & analysis be means of mass spectrophotometry, Gas Chromatography, Spectrophotometry etc.
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Learn more about Environmental monitoring Methods
Also read Indoor Air Quality Methods