Photochemical smog is a unique type of air pollution which is caused by reactions between sunlight and pollutants like hydrocarbons and nitrogen dioxide. Although photochemical smog is often invisible, it can be extremely harmful, leading to irritations of the respiratory tract and eyes. In regions of the world with high concentrations of photochemical smog, elevated rates of death and respiratory illnesses have been observed.

Smog itself is simply airborne pollution which may obscure vision and cause various health conditions. It is caused by small particles of material which become concentrated in the air for a variety of reasons. Commonly, smog is caused by an inversion, in which cool air presses down on a column of warm air, forcing the air to remain stationary.

Reactions for photochemical smog-

1. NO + O2 ---> NO2 + u.v. ---> O + NO:- In this first reaction, we start with Nitric Oxide (NO), which we already know is emitted from various combustion processes. It combines with oxygen in the atmosphere to form nitrogen dioxide ( NO2 ), which has a characteristic brown color that should be familiar to anyone who has lived in a smoggy region. When the u.v. rays of sunlight strike the NO2, it breaks off a single oxygen radical (O) that triggers many subsequent reactions of photochemical smog.

2. O + O2 ---> O3:- In this second reaction, we see how the single oxygen radical helps form ozone ( O3 ).
   A variety of molecules can act as catalysts for this reaction.

3. O3 + NO ---> O2 + NO2:- This third reaction is called a scavenging reaction, and it happens normally in the evening.
   Because it converts ozone to O2, the net result is a drop in the ozone concentration in the evenings.

4. RC + O ---> RCO + O2 ---> RCO3:-
   The fourth reaction shifts our attention to the hydrocarbons (represented here as RC).
   When combined with the oxygen free radical, it forms RCO, which represents a variety of aldehydes and ketones. Some of these constiutents can combine with oxygen to form peroxide readicals ( RCO3 ).

5. O2 + RCO3 ---> O3 + RCO2:- The fifth reaction demonstrates the importance of these peroxide radicals ( RCO3 ) -- it enhances the formation of ozone.

6. NO + RCO3 ---> NO2 + RCO2:- The last reaction shows a more subtle role of the peroxide radicals -- by enhancing the formation of nitrogen dioxide, we know that the nitrogen dioxide will go on to form more ozone.