Question

Photochemical smog is caused by a light-mediated reaction between
(a) ${N}{O}_{2}$ and unsaturated hydrocarbons
(b) ${N}{O}_{3}$ and ${O}_{3}$
(c) ${S}{O}_{2}$ and unburnt hydrocarbons
(d) ${S}{O}_{2}$ and ${O}_{2}$

Answer 1

Hint: Photochemical smog often referred to as "summer smog". It is a type of smog produced when ultraviolet light from the sun reacts with organic compounds in the atmosphere. Smog is seen as a brown haze, and is most prominent throughout the morning and afternoon, especially in densely populated, warm cities.

Complete answer: The reaction of sunlight with nitrogen oxides and unstable organic compounds present in the atmosphere, which produce airborne particles and ground-level ozone is known as Photochemical smog. Photochemical smog depends on primary pollutants also because of the formation of secondary pollutants. An important secondary pollutant for photochemical smog is ozone, which is formed when hydrocarbons (HC) and nitrogen oxides ${N}{O}_{2}$ combine in the presence of sunlight.
These primary pollutants include nitrogen oxides and other volatile organic compounds. The relevant secondary pollutants include peroxyacetyl nitrates (PAN), tropospheric ozone, and aldehydes. In addition, when ${S}{O}_{2}$ and ${N}{O}_{2}$ are emitted they eventually are oxidized in the troposphere to nitric acid and sulfuric acid, which, when mixed with water, form the main components of acid rain. All of these harsh chemicals are usually highly reactive and oxidizing.

So, the correct answer is ‘${N}{O}_{2}$ and unsaturated hydrocarbons’.

Note: Photochemical smog is considered to be a problem of modern industrialization. It is present in modern cities, but it's more common in cities with sunny, warm, dry climates and an outsized number of automobiles. Because it travels with the wind, it can affect sparsely populated areas also.