Which one of the following is NOT a component of photochemical smog?

Correct — C, Polyaromatic hydrocarbons. Photochemical smog is the oxidising haze that forms over sunny, traffic-heavy cities when nitrogen oxides and volatile organic compounds react under strong sunlight. Its signature products are ozone, peroxyacetyl nitrate and various aldehydes, with nitric oxide and nitrogen dioxide cycling through as the drivers. Polyaromatic hydrocarbons are a product of incomplete combustion — they come out of coal fires, diesel exhaust and burning tobacco, and are not made by the photochemical reactions themselves.

• (a)Ozone — Ground-level ozone is the defining component. It is produced when nitrogen dioxide is split by sunlight and the freed oxygen atom joins an oxygen molecule.
• (b)Peroxyacetyl nitrate — Peroxyacetyl nitrate is the second signature of this smog, an eye irritant with no source other than these photochemical reactions.
• (d)Nitric oxide — Nitric oxide is the starting material. Vehicle engines emit it, sunlight converts it to nitrogen dioxide, and the whole cycle turns on that pair.

Two kinds of smog are worth telling apart. The classical London variety is reducing, forms in cold damp air, and is built on sulphur dioxide and soot from coal. The Los Angeles variety is oxidising, needs strong sunlight, and is built on nitrogen oxides and hydrocarbons from vehicles. The photochemical kind is the one that gives ozone and peroxyacetyl nitrate.

A temperature inversion is usually what allows either smog to build up, because a warm layer sitting above cooler surface air traps pollutants near the ground. Delhi's winter episodes have features of both, which is why the city's air quality debate covers vehicles, stubble burning and thermal power at the same time. Polyaromatic hydrocarbons remain a serious health hazard in their own right, several being carcinogenic; the point of this item is only that they are combustion products rather than photochemical products.

• Photochemical smog requires nitrogen oxides, volatile organic compounds and strong sunlight.
• Its characteristic constituents are ozone, peroxyacetyl nitrate, nitrogen oxides and aldehydes.
• Classical or London smog is reducing and is built on sulphur dioxide and particulate carbon from coal.
• Polyaromatic hydrocarbons arise from incomplete combustion of organic matter and are not products of the photochemical cycle.
• A temperature inversion traps pollutants close to the ground and is what lets either kind of smog accumulate.

• Treating ozone as always beneficial; it protects in the stratosphere and harms at ground level.
• Assuming every carbon-based urban pollutant belongs to photochemical smog.
• Overlooking that nitric oxide is a reactant here, which makes it a component of the mixture rather than an outsider.

A composition item. Learn the short list of photochemical products and every option resolves at once.

• practice — not a real PYQ

  Which one of the following is essential for the formation of photochemical smog?

  • (a)Cold and damp air
  • (b)Strong sunlight
  • (c)High sulphur dioxide concentration
  • (d)Heavy rainfall

  Answer(b) Strong sunlight — the reactions that make ozone and peroxyacetyl nitrate are driven by the photolysis of nitrogen dioxide.

• practice — not a real PYQ

  Peroxyacetyl nitrate, a powerful eye irritant, is associated with

  • (a)acid rain
  • (b)photochemical smog
  • (c)the ozone hole
  • (d)the greenhouse effect

  Answer(b) photochemical smog — it has no source other than the sunlight-driven reactions of nitrogen oxides with hydrocarbons.