Various characters are taken into account for settling of airborne pollen and spores on the substrates. The specific weight of pollen and spores and the velocity of fall are significant. It is known that the size, shape, volume, density, morphology and other properties together decide the air buoyancy, mode and rate of deposition of air-spora. Bacteria and viruses occasionally become dispersed along with pollen and spores. Air pollutants such as soot particles may act as rafts for pollen and spores and often pollen grains themselves function as carriers of airborne pollutants.
The air is a very important source for the dispersal and distribution of bioparticles. It is a viscous, mobile medium with a number of properties associated with the physical and chemical laws alongwith the processes of meteorology. The air is usually an unstable medium due to solar heating, temperature differences and friction, which form circulating air masses.
Several viable microorganisms such as bacteria and fungi have been reported from the stratosphere (Gregory 1973). Fulton (1966) had trapped Alternaría and Cladosporium at the altitudes up to 30,000 m. Agashe and Chatterjee (1987) had carried out an aeropalynological survey at different altitudes by using the aircraft sampling method. They reported the occurrence of several pollen grains including Parthenium hysterophorous, and several fungal spores at the attitude of 1,000 m in the atmosphere of Bangalore, India.
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