Question

Photochemical smog is formed in:
  (A) summer during day time
  (B) summer during morning time
  (C) winter during morning time
  (D) winter during day time

Answer 1

Hint: The photochemical smog is most common in sunny and dry cities. This is because the chemical reactions which involve oxides of nitrogen and other pollutants take place only in the presence of sunlight and warm climate.


Complete step by step solution:
 - Let's start with the idea of smog. In general, we can say that smog is formed as a result of the combination of fog and smoke. The classification of smog varies in accordance with the constituents present in it.
- In the question we are asked about the occurrence of photochemical smog (Los Angeles Smog). The formation of photochemical smog is mainly due to three major ingredients such as sunlight, nitrogen oxides and hydrocarbons. Or in other words this smog occurs as a result of the chemical reaction of sunlight, organic compounds and nitrogen oxides in the atmosphere.
- Due to the presence of these higher concentrations of oxidants in photochemical smog it has an oxidizing nature. This smog is characterized by brown, hazy fumes which irritates lungs and eyes along with other health issues. It also causes extensive damage to plant life and materials also.
- The major constituents of photochemical smog are ozone, Peroxyacetyl nitrate (PAN), Nitric acid and Nitrous acid.
- The process of formation of photochemical smog involves the following steps
  (i) In the presence of sunlight, the photodissociation of oxygen and nitrogen present in the nitrous oxide and nitrogen dioxide takes place.
  (ii) These oxygen atoms react with water and as a result, hydroxyl radicals are formed.
  (iii) The hydrocarbons are oxidized by hydroxyl radicals and hydrocarbon radicals are formed
  (iv) The oxidation of this hydrocarbons results in the formation of some aldehydes
  (v) These aldehydes oxidize to aldehyde peroxy acids and formaldehyde peroxides.
 From the above-mentioned mechanism of formation of photochemical smog, it's clear that sunlight and a warm climate is required for the formation of this smog.


Therefore, the answer is option (A). That is the photochemical smog is formed in summer during day time.


Note: Do not confuse the photochemical or Los Angeles smog with classical smog (reducing or London smog). The photochemical smog is formed from oxides of nitrogen and other pollutants and thus it would have an oxidizing nature whereas London smog is from the oxides of Sulphur and would have a reducing nature.