Photochemical Smog

Photochemical Smog

When atmosphere is loaded with large quantities of automobile exhausts, during warm sunny days with gentle winds and low level inversion, then the exausts gases are trapped by the inversion layers with stagnant air masses and simultaneously exposed to intense sunlight. Then a number of photochemical reactions involving NO2 , hydrocarbons and other organic compound and free radicals take place leading to the formation of ozone, peroxides and other photochemical oxidants in the atmosphere. This gives rises to the phenomenon of photochemical smog which is characterized by the formation of aerosols that reduced visibility, generation of brown hazy fomes that irritate eyes and lungs.

Generally, the smog which is formed by photochemical reaction is known as photochemical smog.

Smoke + Fog >> Smog

The main components of photochemical smog are unsaturated hydrocarbons, NOX, CO and some S compound.

Formation of photochemical smog/PAN

The mechanism of smog formation includes the following steps-

1. Reactive hydrocarbons (which have C=C groups) from automobile exhaust interact with ozone to form Hydrocarbon free radical (RCH2).
2. RCH2 rapidly reacts with oxygen to form another free radical RCH2O2.
3. RCH2O2. rapidly reacts with NO to produce NO2 and free radical RCH2O.
4. This new free radical next interacts with oxygen to yield a stable aldehyde RCHO and hydroperoxy radical HO2.
5. Then HO2. reacts with NO and form NO2 and free radical HO.
6. HO. Is extremely reactive and rapidly reacts with a stable hydrocarbon RCH3 to yield H2O and regenerate the hydrocarbon free radical RCH2., there by completing cycle.

This goes on and on as a chain reactions, one complete cycle yields, two molecules of NO2, one molecule of aldehyde RCHO and regenerate the free radical RCH2 to start all over again, very soon there is rapidly build up of smog product.

7. the aldehyde RCHO may initiate another root by interaction with the HO. Radical, leading to the formation of an acyl radical RC=O which then reacts with oxygen gives peroxyacyl radical (RCOOO). At last this active radical reacts with NO2 and gives peroxy acyl nitrite (PAN).



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