Question

What is the primary and secondary precursor of photochemical smog?
A) Nitrogen dioxide and hydrocarbons
B) Hydrocarbons and peroxyacetyl nitrate
C) Peroxyacetyl nitrate and Oxygen
D) $C{O_2}$ and ozone

Answer 1

We have to know that photochemical smog is a mixture of pollutants that are formed when nitrogen oxides and volatile organic compounds (VOCs) react to sunlight, creating a brown haze above cities. It tends to occur more often in summer, because that is when we have the most sunlight.

Complete answer:
As we know, there are two major pollutants of photochemical smog which can be known as primary pollutants and secondary pollutants. The two major primary pollutants, nitrogen oxides and VOCs, combine to change in sunlight in a series of chemical reactions, outlined below, to create what are known as secondary pollutants. In Secondary pollutants, the secondary pollutant that causes the most concern is the ozone that forms at ground level. While ozone is produced naturally in the upper atmosphere, it is a dangerous substance when found at ground level. Many other hazardous substances are also formed, such as peroxyacetyl nitrate (PAN).
Photochemical smog occurs in warm and dry climates by the action of sunlight on the oxides of nitrogen.$N{O_2}$ and ${O_3}$ are strong oxidizing agents. They react with hydrocarbons that are not burnt to produce formaldehyde, acrolein, and PAN i.e. peroxyacetyl nitrate. These are the secondary compounds of photochemical smog.

Note:
We have to remember that in nature, bushfires, lightning and the microbial processes that occur in soil generate nitrogen oxides. VOCs are produced from the evaporation of naturally-occurring compounds, such as terpenes, which are the hydrocarbons in oils that make them burn. Eucalypts have also been found to release significant amounts of these compounds.