Answer within 200 words : Give an account of secondary air pollutants .

Explanation

Secondary Air Pollutants
On the basis of the form in which it persists after the release, a pollutant may be of two types, primary pollutant and secondary pollutant.
Secondary pollutants are formed through reaction between primary pollutants with atmospheric constituents. These are more potent than the primary pollutants. Examples are tropospheric ozone, Peroxyacetyl Nitrate (PAN), nitrogen oxides, smog, etc.
(i) Tropospheric ozone Nitrogen dioxide (NO2) released from primary pollutants dissociates into nitric oxide (NO) and nascent oxygen (Öl under the influence of UV radiation. This nascent oxygen reacts with molecular oxygen forming ozone in the troposphere. Ozone, so formed is toxic and a corrosive greenhouse gas. It traps heat and adds to the greenhouse effect. It also participates in the formation of photochemical smog.
`underset"(Nitric oxide)"(2NO)+O_2rarrunderset"(Nitrogen dioxide)"(2NO_2)`
`NO_2underset"radiation"overset"UV"rarr2NO+[O]`
Peroxyacetyl Nitrate (PAN) It forms when nitrogen reacts with non-methane hydrocarbons. Acetaldehyde, methyl glyoxyl and various byproducts of oxidation of aromatic compounds are all precursors of PAN. The vehicular emission contains several primary pollutants. PAN is an important constituent of photochemical smog.
(iii) Photochemical smog It is formed by combination of smoke and fog formed during winters. Water vapour surround smoke, dust or soot particles forming secondary particles, which remain suspended in the air.
Classical smog is reducing in nature and different from oxidising photochemical smog. During warmer sunny days, oxides of nitrogen and sulphur and hydrocarbons from vehicular exhaust undergo a series of photochemical reactions producing many photochemical oxidants. These combine with tropospheric ozone forming a brownish hazy fume called photochemical smog. It irritates the eyes and lungs and causes extensive damage to vegetation and. rubber goods.