Consider the following: Which of the above are released into atmosphere due to the burning of crop/biomass residue?

Answer: (d) 1, 2, 3 and 4

Burning residue releases all four (ozone forms from its gases).

Statement by statement

1. ✓ YES Incomplete burning makes carbon monoxide.

2. ✓ YES Smouldering residue gives off methane.

3. ✓ YES Its NOₓ and hydrocarbons form ozone in sunlight (secondary).

4. ✓ YES Straw contains some sulphur, released as SO₂.

The idea

Straw burning in a field is slow, smoky burning with too little air. Complete burning gives carbon dioxide and water. Incomplete burning gives carbon monoxide, methane, soot and many other gases. Some pollutants then form later in the air from this smoke: these are secondary pollutants, and ozone is one.

Primary (come straight from the fire): carbon monoxide, methane, sulphur dioxide, nitrogen oxides, soot → Sunlight acts on nitrogen oxides and hydrocarbons → Secondary: ozone forms

The trap

Ozone is not emitted directly, so it is dropped. UPSC counted it as released through the smoke’s chemistry.

Also know

In Punjab and Haryana, farmers have only two to three weeks between harvesting paddy and sowing wheat, so burning is the quickest way to clear the field. Alternatives: the Happy Seeder, which sows wheat through the stubble, and the Pusa bio-decomposer. Burning also destroys soil nutrients and organic matter. The Commission for Air Quality Management (2021) oversees the Delhi region.