(a) BOD: Biochemical oxygen demand is a measure
of the dissolved oxygen that would be needed by the microorganisms to oxidise these compounds. BOD, therefore, is a measure of the total contamination caused by compounds which can be oxidised in the presence of micro-organisms.
(b) Photochemical smog : It is formed as a result of photochemical reaction between Nitrogen oxide arid hydrocarbons in the presence of sunlight.
It is also called Los Angeles smog.
It is formed around mid day in summer months. It does not involve any smoke or fog.
It is oxidising in nature.
Ozone hole : Ozone layer is present in stratosphere which protects earth from harmful U.V. radiations of sun. The CFCs and oxide of nitrogen poses danger to ozone layer and becomes cause for its depletion.