Why Burning Crop Residue Releases Four Distinct Pollutants

Every autumn, satellites over north India light up with thousands of small fires. Each one releases more than just smoke — a mix of at least four distinct pollutants, all at once.

Preparing to burn crop residue in a harvested rice field in Punjab, before the wheat season. Photo by Neil Palmer (CIAT), via Wikimedia Commons. Licensed CC BY-SA 2.0.

📑 Contents

1Must Know
2Good to Know
3Test Yourself
4Great to Know
5Previous Year Questions

One Fire, Four Pollutants

EnvEco0094

35M

Tonnes of crop waste burned each year across north India

Distinct pollutants released: CO, methane, ozone, and SO₂

Sep–Nov

The peak burning window, between paddy harvest and wheat sowing

Indirect

Ozone isn’t emitted directly — it forms afterward, in sunlight

The pattern: incomplete combustion is the real driver here. A cooler, oxygen-starved fire releases far more of these pollutants than a complete, well-oxygenated burn would.

🏛️ Must Know

Why Farmers Burn Crop Residue

The Short Window Farmers in Punjab, Haryana, and western Uttar Pradesh have only a few weeks between harvesting paddy and sowing wheat. Burning the leftover straw is the fastest, cheapest way to clear the field.
The Scale Roughly 35 million tonnes of crop waste are burned this way every year across north India. Punjab alone generates around 18 million tonnes of paddy straw annually.
Four Pollutants Released This burning releases carbon monoxide, methane, ozone, and sulphur dioxide into the atmosphere, all documented consequences of the same fires.

Good to Know

📘 Good to Know

Where Each Pollutant Actually Comes From

Carbon Monoxide Carbon monoxide forms specifically from incomplete combustion. When a fire doesn’t get enough oxygen, or burns at a lower temperature, it produces far more carbon monoxide than a complete burn would.
Methane Methane is also a product of incomplete combustion. Smouldering, oxygen-starved biomass fires release more of it than fast, well-oxygenated ones.
Sulphur Dioxide Crop residue itself contains some sulphur, though usually far less than coal or furnace oil. Burning it still releases this sulphur as sulphur dioxide gas.
Ozone, Indirectly Ozone isn’t released directly by the fire itself. Instead, precursor gases like nitrogen oxides and volatile organic compounds react in sunlight afterward, forming ground-level ozone downwind.

Delhi’s Annual Smog Season

Downwind Impact Smoke from these fires drifts southeast on seasonal winds, adding heavily to Delhi’s already poor air quality every October and November.
One Contributor Among Several Vehicle emissions, construction dust, and industrial pollution all add to Delhi’s air quality problem too. Crop burning sharply worsens what is already a chronic, year-round issue.

📝 Previous Year Questions

UPSC CSP 2019 — Gases Released by Burning Crop or Biomass Residue

UPSC 2019 Carbon monoxide, methane, ozone, and sulphur dioxide are all released into the atmosphere by burning crop or biomass residue. See UPSC CSP 2019 GS Paper I, Q35. View this question.

Test Yourself

Great to Know

🌟 Great to Know

Alternatives to Burning

Happy Seeder Machines like the Happy Seeder can sow wheat directly into standing paddy stubble, without needing to burn it first. Government subsidies have pushed adoption, though uptake still lags the scale of the problem.
Straw as a Resource Some of this straw is now used to generate biomass power, compressed into biogas, or turned into cattle fodder and packaging material, instead of being burned in the field.
A Recurring Policy Challenge Balancing farmers’ narrow sowing window against public health costs downwind remains one of north India’s most persistent environmental policy problems, revisited every winter.
See Also This is one of five related combustion-pollutant PYQ facts compared side by side in EnvEco0099 — Common Air Pollutants from Fuel and Waste Burning.