Choose the correct statements about photochemical smog:

(A) It is a mixture of smoke and fog

(B) London smog was a classical example of photochemical smog

(C) NOₓ, VOC, SO₂ are essential in the gradient of photochemical smog

(D) Photochemical smog is formed maximally when the Sun is overhead during the summer season

(E) Aldehyde, ozone are important constituents of photochemical smog
Choose the correct answer:

Correct option is C

Introduction

• Photochemical smog is a type of air pollution that results from the reaction of sunlight with pollutants like nitrogen oxides (NOₓ) and volatile organic compounds (VOCs).
• The key points about photochemical smog are:
  
• Nitrogen oxides (NOₓ) and volatile organic compounds (VOCs) are key precursors for the formation of photochemical smog. However, SO₂ is not a primary component.
  
• Photochemical smog forms under intense sunlight, especially during the summer when the sun is overhead. The heat and UV radiation promote the reactions between pollutants.
• Ozone (O₃) and aldehydes are primary constituents of photochemical smog, produced from the chemical reactions of NOₓ and VOCs under sunlight.

Information Booster:

• Photochemical smog forms maximally when the Sun is overhead (midday) during summer, because high-intensity solar UV radiation drives the photolysis of NO₂ and subsequent ozone-forming reactions; this is why it peaks on hot, sunny afternoons.
• The important constituents (secondary pollutants) of photochemical smog include ozone (O₃), peroxyacetyl nitrate (PAN), aldehydes (e.g., formaldehyde, acrolein) and NO₂ all formed photochemically.
• Photochemical smog (also called oxidising smog, "Los Angeles smog" or "summer smog") is a brownish, hazy air pollution that forms when oxides of nitrogen (NOₓ) and volatile organic compounds (VOCs/hydrocarbons) chiefly emitted from automobile exhaust  react in the presence of intense sunlight (ultraviolet radiation). This sequence of photochemical reactions produces secondary pollutants such as ozone, PAN and aldehydes. Evaluating each statement:

Additional Knowledge

• "A mixture of smoke and fog" defines classical (reducing/sulphurous) smog, from which the very word smog (smoke + fog) originates. Photochemical smog does not depend on smoke and fog; it is generated by sunlight-driven reactions and typically forms in warm, dry, sunny conditions, not foggy ones.
• The London smog (1952) was the textbook example of classical/reducing smog, caused by coal combustion releasing smoke and SO₂ under cold, foggy, stagnant winter conditions — not photochemical smog.
• The essential precursors of photochemical smog are NOₓ and VOCs together with sunlight. SO₂ is the hallmark of classical reducing smog, not photochemical smog, so its inclusion as "essential" makes this statement wrong.