Q. Photochemical smog is a resultant of the reaction among (UPSC Prelims 2013)
Answer:
NO
2, O
3 and peroxyacetyl nitrate in the presence of sunlight
Notes: The correct answer is
[A] NO2, O3 and peroxyacetyl nitrate in the presence of sunlight. Photochemical smog, also known as "Los Angeles smog," is a type of air pollution derived from vehicular and industrial emissions that react in the atmosphere with sunlight.
- Formation Process (Statement A is Correct): Photochemical smog forms when primary pollutants—specifically Nitrogen Oxides (NOx) and Volatile Organic Compounds (VOCs)—react under the influence of Ultraviolet (UV) radiation. This complex chemical reaction produces secondary pollutants:
- Ozone (O3): Formed at the ground level (troposphere), it is a major constituent of smog.
- Peroxyacetyl Nitrate (PAN): A powerful eye irritant and respiratory toxin produced by the reaction of hydrocarbons and nitrogen oxides.
- Nitrogen Dioxide (NO2): Gives the smog its characteristic brownish-yellow haze.
- The Role of Sunlight: Unlike classical smog (London smog), which occurs in cool, humid climates and involves sulfur dioxide, photochemical smog requires intense sunlight and high temperatures. This is why it is most prominent during peak daylight hours in urban areas.
- Incorrect Components (Statements B, C, and D):
- Carbon Monoxide (CO): While CO is a pollutant found in urban air, it is not a primary reactant in the specific chain reaction that defines photochemical smog.
- Low Temperature (Statement C): Photochemical smog thrives in warm/hot weather; "low temperature" is a characteristic of classical (Sulfurous) smog.
- Evening (Statement D): Concentrations of photochemical smog typically peak in the early afternoon when sunlight is strongest, rather than in the evening.