Re: What is Precursor Organic Compounds (POC) that cause air pollutants.

POCs are made up of many different substances. They are sometimes also called VOCs or "volatile 
organic compounds". For a substance to be a POC, there are only two requirements -- it must have a 
carbon-hydrogen bond in its structure, and it must readily form either a vapour or a very fine aerosol.

The starting point for the type of air pollution where POCs get involved is the hydroxyl radical -- *OH
(This is not the same as a hydroxyl ion, which you encounter in water chemistry in alkaline solutions. It 
has one less electron, so that one of its electrons is unpaired. In effect it has half a dangling chemical 
bond, which it is anxious to complete. It is very reactive.)

A hydroxyl radical is totally relentless in its need to steal a hydrogen atom from somewhere to make 
water: HOH. Because an oxygen-hydrogen bond in water is a stronger bond than any carbon-hydrogen 
bond, hydroxyl radicals will steal a hydrogen atom from any organic compound. The product will be a 
reactive organic free radical, that will immediately add oxygen from the atmosphere to make an organic 
peroxy radical. And the whole complicated air pollution chapter unfolds from there.

Hydroxyl radicals are produced abundantly in any combustion reaction. Small numbers are always 
present in the natural environment anyway, but there are many more in urban or industrial settings. 
The source of the hydroxyl radicals is not too important because the complicated series of reactions 
that occur in polluted air on sunny days actually recycles them.

All petrochemical substances are organic. Many of them are gases, or volatile liquids, or readily form 
very fine mists or dusts. But on the whole, industries like the petrochemical industry have to have very 
good standards of containment for their products and waste -- for the sake of the health of their 
workers, and for keeping happy the government inspectors who watch their act very closely, quite apart 
from air pollution problems. The overwhelming source of atmospheric POCs in the typical urban 
environment is from vehicular exhaust.