Photochemical smog is a resultant of the reaction among

Answer: (a) NO2, O3 and peroxyacetyl nitrate in the presence of sunlight

Photochemical smog forms when sunlight drives reactions between nitrogen oxides (such as NO2) and volatile organic compounds. This produces ozone and peroxyacetyl nitrate (PAN), along with other oxidants. Option (a) is the only one that names NO2, O3 and PAN together with sunlight.

• ✓ (a) Sunlight acts on nitrogen oxides and hydrocarbons (VOCs) to give ozone (O3) and PAN. NO2, O3 and PAN in the presence of sunlight therefore describe photochemical smog.
• ✗ (b) Carbon monoxide and ordinary O2 are not the key ingredients. The smog-forming precursors are nitrogen oxides and VOCs, which react under sunlight to give ozone and PAN.
• ✗ (c) CO, CO2 and NO2 at low temperature do not describe the process. Photochemical smog needs sunlight and is typically a problem of hot, sunny days.
• ✗ (d) The reaction is photochemical, so it needs sunlight. It builds up during the day, not in the evening.

Remember · Photochemical smog = NOx + VOCs + sunlight, giving ground-level ozone and PAN. It forms on sunny days and is an oxidising smog, unlike sulphurous (classical) smog.

Question: UPSC Civil Services (Preliminary) 2013, General Studies paper. Explanation: Minimalist IAS, checked 25 Oct 2026 (how we verify). · Report an error