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The main precursors of winter smog are:

HPPSC · 2022 · GS1
Question:
The main precursors of winter smog are:

Options

(1) N2O and hydrocarbons
(2) NOx and hydrocarbons
(3) SO2 and hydrocarbons
(4) SO and ozone
Answer: (3) SO2 and hydrocarbons

Explanation:
Winter smog (also called London-type smog or reducing smog) is primarily formed from sulphur dioxide (SO2) emitted by burning of coal and other fossil fuels, combined with particulate matter, hydrocarbons, and fog. SO2 reacts with moisture and oxygen to form sulphates and sulphuric acid droplets in the fog — creating the characteristic thick, yellowish-grey smog. This is distinct from summer/photochemical smog (Los Angeles type) which involves NOx and hydrocarbons reacting with sunlight.

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