Photochemical smog is a resultant of the reaction among
Answer & explanation
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.
Sources
- WHO – Types of pollutants (ozone and smog) ↗ “Ground-level ozone (O 3 ) is a major component of smog. It is formed from photochemical reactions with pollutants such volatile organic compounds”
- US EPA – Ground-level Ozone Basics ↗ “This happens when pollutants emitted by cars, power plants, industrial boilers, refineries, chemical plants, and other sources chemically react in the presence of sunlight.”
- US EPA – Integrated Science Assessment for Ozone (2022 trends report) ↗ “Tropospheric O3 and other oxidants, such as peroxyacetyl nitrate (PAN) and hydrogen peroxide, form in polluted areas by atmospheric reactions involving two main classes of precursor pollutants”
Question: UPSC Civil Services (Preliminary) 2013, General Studies Paper I. Explanation: Minimalist IAS, checked 25 Oct 2026 (how we verify). ·
