What are different methods employed in air sampling? Explain

the four main methods of air sampling. The methods are: 1. Gravity Sedimentation Methods 2. Inertial Methods 3. Filtration 4. Precipitation.

1. Gravity Sedimentation Methods:

a. Sedimentation from still air:

Alvarez and Castro (1952) constructed a simple box for the study of airborne fungi, has two hinged slides and a covered tray at the bottom for inserting a microscopic slide or petridish. During air sampling the hinged slides are raised horizontally and wind is allowed to blow through the box.

The slides are then closed and the entrapped spores are sedimented under gravity. Here sampling is discontinuous and a small volume of air being sampled at a time. An improved form of this model was later described by Ogden (1974).

b. Sedimentation from wind:

The method of examining air from deposits on a freely exposed, horizontal surface, such as microscopic slides was used by earlier scientists like Pasteur, Pouchet and even can be traced back to van Leeuwenhock but it fails to examine air quantitatively.

The different methods are

i) The gravity slide was used by Blackley (1873), Durham (1946), Hyde and Williams (1944), Gregory (1961), Tauber (1974). Some of the common samples are Tauber trap, Durham Sampler, Individual Pollen Collector etc. These are cheap, simple and operate continuously.

But they fail to estimate the airspora quantitatively and gives a distorted picture of airspora, because they preferentially select the larger particles. To correct this distortion Scheppegrell (1922) tried to calculate the volumetric concentration using a formula based on particle diameter which was later corrected by Cocke (1937) to particle radius. But inspite of these defects it is widely used by aerobiologists and has contributed much knowledge to the air spora.

ii) Frankland and Hart (1887) introduced “Gravity Petridish” in which the petridish containing sterile nutrient media were exposed in open air for 1-10 minutes, incubated and after development of colonies these are identified, counted and subcultured for further study.

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