Question

Which of the following are essentially required for photochemical smog formation in ambient atmosphere?

• A. Smoke, water vapour and low temperature (> 25 degrees)
• B. NO_x, SO₂ and high temperature (> 25 degrees)
• C. NO_x, VOCs and high temperature (> 25 degrees)
• D. Smoke, NO_x and low temperature (< 25 degrees)

Answer 1

The Correct Option is C

Photochemical smog is a type of air pollution that results from the reaction of sunlight with pollutants such as nitrogen oxides (NO_x) and volatile organic compounds (VOCs) in the atmosphere. Here are the essential components: 

• Nitrogen Oxides (NO_x): Emitted from vehicle exhausts, power plants, and industrial emissions, NO_x plays a crucial role in the formation of photochemical smog by reacting with VOCs in the presence of sunlight.
• Volatile Organic Compounds (VOCs): These are hydrocarbon compounds that can vaporize into the air. They react with NO_x under sunlight to form ozone, a major component of photochemical smog.
• High Temperature (> 25 degrees): High temperatures accelerate the chemical reactions involved in smog formation. Warmer temperatures increase the evaporation of VOCs and enhance the photochemical reactions producing smog.

The given options for photochemical smog formation include:

• Smoke, water vapour and low temperature (> 25 degrees)
• NO_x, SO₂, and high temperature (> 25 degrees)
• NO_x, VOCs, and high temperature (> 25 degrees)
• Smoke, NO_x, and low temperature (< 25 degrees)

Among these, NO_x, VOCs, and high temperature (> 25 degrees) are the critical factors for photochemical smog formation, making this the correct answer.

Answer 2

The essential requirements for photochemical smog formation in ambient atmosphere are NOx, VOCs and high temperature (>25°C).

Additional Context:

• Key components:
  • NOx (Nitrogen Oxides): From vehicle emissions and combustion processes
  • VOCs (Volatile Organic Compounds): From fuels, solvents, and industrial emissions
  • Sunlight (UV radiation): Drives photochemical reactions
  • High temperature: Accelerates reaction kinetics (>25°C ideal)
• Formation process:
  • NOx + VOCs + sunlight → Ozone (O₃) + PAN (Peroxyacyl nitrates)
  • Secondary pollutants cause eye irritation and respiratory problems
• Other options analysis:
  • Smoke + water vapor + low temp: Describes London-type smog (industrial smog)
  • NOx + SO₂: SO₂ contributes to acid rain, not photochemical smog
  • Smoke + NOx + low temp: Incomplete combination missing VOCs and sunlight
• Characteristic features:
  • Brownish haze (unlike gray industrial smog)
  • Peaks in afternoon due to sunlight intensity
  • Common in cities like Los Angeles, Mexico City
• Control measures:
  • Catalytic converters in vehicles (reduces NOx)
  • VOC emission controls in industries
  • Use of alternative fuels

Correct Answer: (3) NOx, VOCs and high temperature (>25°C).