Which of the following is a key ingredient in the formation of photochemical…

Concept — what photochemical smog is: Photochemical smog is an oxidising haze that no source emits directly. Combustion first releases primary pollutants — oxides of nitrogen together with volatile organic compounds — and the ultraviolet component of sunlight then drives them into secondary pollutants such as ozone, peroxyacetyl nitrate and aldehydes. The key ingredient is therefore the primary pollutant that absorbs sunlight and starts that chain.

Application — the photochemical chain, step by step:

1. NO2 + sunlight (near-UV) → NO + O — nitrogen dioxide absorbs a near-ultraviolet photon and splits, releasing nitric oxide and a free oxygen atom.

2. O + O2 → O3 — that free oxygen atom attaches to molecular oxygen, building up ozone at ground level.

3. Volatile organic compounds in the exhaust form peroxy radicals that regenerate nitrogen dioxide from nitric oxide, so the cycle keeps turning and ozone accumulates instead of being consumed.

4. Further reaction of the acetyl peroxy (peroxyacetyl) radical with nitrogen dioxide yields peroxyacetyl nitrate (PAN), the eye-irritant characteristic of this haze.

Cross-check — how the other species offered behave:

• Carbon dioxide (CO2): chemically long-lived in the lower atmosphere, with infrared absorption as its characteristic effect; the near-ultraviolet light that reaches the troposphere does not photodissociate it.

• Carbon monoxide (CO): an incomplete-combustion pollutant that is removed slowly by hydroxyl radicals and is toxic through carboxyhaemoglobin formation, rather than being the species that absorbs sunlight to open the cycle.

• Sulphur oxides (SOx): oxidised in moist air to sulphuric acid aerosol, giving acid rain and the classical London-type sulphurous winter haze, which forms in cold, damp, sunless conditions — the opposite meteorology to the sunlit chain above.

Answer: Nitrogen oxides (NOx) — they supply the light-absorbing species that both starts and sustains the photochemical cycle, which is why this haze peaks on warm, sunny afternoons in traffic-dense cities such as Los Angeles or Delhi.