Photo chemical smog always contains

Question

UnlockIAS · Accepted Answer

Correct — A, Ozone. Photochemical smog is the oxidising, sunlight-driven haze that builds over traffic-heavy cities on warm, still, sunny days. Nitrogen oxides and volatile organic compounds pouring out of engine exhaust are broken up by ultraviolet light, and the free oxygen atoms released combine with molecular oxygen to make ozone, O3. Ozone is therefore not something anyone emits — it is a secondary pollutant manufactured in the air itself, and it is the invariable product of the reaction chain that defines this kind of smog. That is why air-quality agencies use ground-level ozone as the marker for photochemical smog: if the chemistry has happened, the ozone is there. Alongside it the same chain yields peroxyacetyl nitrate (PAN), aldehydes and nitrogen dioxide, the last of which gives the smog its brownish tint.

Why the other options are wrong — (b) Aluminium ion belongs to soil and water chemistry, not to gas-phase atmospheric chemistry. It is mobilised when acid deposition lowers the pH of soils and lakes, which is why it appears in discussions of acid rain damage to forests and fish. Nothing in the photochemical chain — nitrogen oxides, hydrocarbons, sunlight — produces or requires a metal cation.  (c) Phosphorus is the classic pollutant of the water cycle, not the air. Excess phosphorus from detergents, fertiliser runoff and sewage drives eutrophication and algal blooms in lakes and rivers. It has no role in the sunlight-driven reactions between nitrogen oxides and hydrocarbons that make photochemical smog.  (d) The most tempting wrong answer, because methane is a hydrocarbon and hydrocarbons do feed this chemistry. But methane is by far the least reactive of them — so unreactive that atmospheric chemists deliberately exclude it and work with 'non-methane volatile organic compounds' when studying smog formation. Methane is present in ambient air everywhere on earth, in smog and out of it, so it can neither characterise photochemical smog nor be produced by it. It matters as a greenhouse gas, not as a smog constituent.

Concept: Two entirely different phenomena are called smog. Classical or London smog is reducing: it is smoke plus sulphur dioxide trapped in cool, damp winter air over coal-burning cities, and the Great Smog of London in December 1952 is its defining episode. Photochemical or Los Angeles smog is oxidising: nitrogen oxides and volatile organic compounds from vehicle exhaust are driven by strong sunlight into a chain that produces ozone, peroxyacetyl nitrate, aldehydes and nitrogen dioxide. The distinction between primary and secondary pollutants is the heart of it — nitrogen oxides and hydrocarbons are emitted directly (primary), while ozone and PAN are built in the atmosphere afterwards (secondary).

State PCS 2022 Q72 — Environment & Ecology PYQ with Answer

State PCS 2022 Q72 — Environment & Ecology PYQ with Answer