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 D

Explanation:
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:
1. Statement (a) – Incorrect: Photochemical smog is not simply a mixture of smoke and fog; it involves complex chemical reactions under sunlight. The mixture of smoke and fog is referred to as classical or London smog.
2. Statement (b) – Incorrect: London smog (also known as classical smog) is caused by the burning of coal and the presence of sulfur dioxide (SO₂). It is not an example of photochemical smog. Photochemical smog is more associated with modern urban environments and high vehicle emissions.
3. Statement (c) – Correct:Nitrogen oxides (NOₓ) and volatile organic compounds (VOCs) are key precursors for the formation of photochemical smog. However, SO₂ is not a primary component.
4. Statement (d) – Correct: 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.
5. Statement (E) – Correct:Ozone (O₃) and aldehydes are primary constituents of photochemical smog, produced from the chemical reactions of NOₓ and VOCs under sunlight.
Information Booster:
1. Photochemical Smog Composition:
· Primary Pollutants: NOₓ, VOCs (from vehicle exhaust, industrial emissions).
· Secondary Pollutants: Ozone (O₃), peroxyacyl nitrates (PANs), and aldehydes.
2. Conditions Favoring Photochemical Smog:
· High temperature.
· Intense sunlight.
· Stagnant air conditions.
3. Differences Between Photochemical Smog and Classical Smog:
· Photochemical Smog: Forms in warm, sunny conditions; key pollutants include NOₓ and ozone.
· Classical Smog: Forms in cool, damp conditions; key pollutants include SO₂ and particulate matter.
4. Health Effects:
· Eye irritation.
· Respiratory problems (e.g., asthma).
· Damage to crops and vegetation.
Additional Knowledge:
· Option (a): Refers to classical smog, caused by burning coal or biomass, forming a mixture of smoke and fog.
· Option (b): London smog (classical smog) occurred due to coal burning and sulfur emissions during the industrial era.
· Option (c): NOₓ and VOCs are critical for photochemical reactions.
· Option (d): High temperatures and strong sunlight catalyze the formation of photochemical smog.
· Option (E): Aldehydes and ozone are byproducts of the reactions between NOₓ and VOCs under UV light.