Air pollution: sources, smog, acid rain and Delhi's winter crisis

Detail

1. What is air pollution?

• Air pollution means harmful things are added to the air in amounts large enough, and for long enough, to harm plants, animals and property.
• These things are dust, fumes, gas, fog, odour, smoke or vapour.

• Two things decide the harm: how much is added (quantity) and how long it stays (duration).

• Particulate matter (PM) means tiny solid or liquid bits floating in the air.

• PM10 bits are 10 micrometres across or smaller. They can enter the nose and throat.

• PM2.5 bits are 2.5 micrometres across or smaller. They go deep into the lungs and the blood.

• WHO Air Quality Guidelines 2021:

• PM2.5: an annual average of 5 µg/m³ and a 24-hour average of 15 µg/m³.
• PM10: an annual average of 15 µg/m³ and a 24-hour average of 45 µg/m³.

• NO₂: an annual average of 10 µg/m³ and a 24-hour average of 25 µg/m³ [9].

• Global scale (WHO, 2022):

• 99% of the world's people breathe air that is above WHO limits.
• Air pollution causes about 7 million preventable deaths each year.
• More than 6,000 cities in 117 countries now check their air quality [9].

• Fewer than 1% of monitored cities in low- and middle-income countries meet the PM guidelines. In high-income countries, 17% do [9].

• India has many of the world's most polluted cities on PM2.5. Most of them are in the Indo-Gangetic plain.

2. Sources of air pollution

Source	What it releases	Where it is seen in India
Burning fossil fuels (coal, oil, gas)	Oxides of sulphur (SOₓ) and nitrogen (NOₓ), CO₂, CO	Coal-based thermal power plants, cities
Mining	Dust, asbestos	Coal and iron ore belts of the Chota Nagpur plateau
Industry	SOₓ, NOₓ, hydrocarbons, lead, asbestos	Refineries, smelters, cement plants
Vehicles	NOₓ, CO, hydrocarbons, PM2.5 (and lead before leaded petrol was phased out)	Delhi, Mumbai, Kolkata, Bengaluru
Construction and road dust	PM10	Fast-growing cities
Burning biomass and waste	PM2.5, CO, black carbon	Crop fields in Punjab and Haryana, landfills, household chulhas
Brick kilns	PM, SO₂, black carbon	Edges of cities across the Indo-Gangetic plain

• Hydrocarbons are compounds of hydrogen and carbon from unburnt fuel. In sunlight they react with NOₓ and form ground-level ozone.
• CO (carbon monoxide) is a poison gas from incomplete burning. It stops blood from carrying oxygen.

3. Effects on health

• Air pollution causes diseases of three body systems:
• Respiratory system (breathing): asthma, bronchitis, lung cancer.
• Nervous system: lead and CO damage the brain and nerves.

• Circulatory system (heart and blood): heart attacks and stroke.

• WHO adds heart disease, stroke, COPD (chronic obstructive pulmonary disease), cancer and pneumonia to this list [9].

• It also damages crops, forests and buildings.

4. Smog

• Smog is smoke + fog: a smoky fog over cities caused by pollution. It is very harmful to health.
• How it forms:
• Smoke and particles from vehicles and factories mix with water droplets in cool, damp air.
• Each particle becomes a seed for fog. The fog turns thick and dark.

• If the wind is calm, the smog stays over the city for days.

• Two types:

Type	Cause	Season	Example
Classical (sulphurous) smog	Coal smoke (SO₂ + soot) + fog	Cold, damp winter	London, 1952; Delhi in winter
Photochemical smog	Sunlight acts on NOₓ + hydrocarbons, forming ozone	Warm, sunny days	Los Angeles; Indian cities in summer

5. Acid rain

• Acid rain is rain with a pH below about 5.6.
• pH is a scale of acidity from 0 to 14. Below 7 is acidic.

• Clean rain already has a pH of about 5.6. It dissolves a little CO₂ from the air and forms weak carbonic acid. Anything below 5.6 is "acid rain".

• How it forms:

• Coal, oil and vehicles release SO₂ and NOₓ.
• These gases mix with water vapour and oxygen in the air.

• They turn into sulphuric acid and nitric acid, which fall with rain, snow or fog.

• NCERT fact: in cities, the first rain after summer always has a lower pH than later rains.

• Through the dry summer, pollutants pile up in the air.
• The first shower washes them all out at once, so it is the most acidic.

• Later rains fall through cleaner air.

• Damage:

• Soils lose nutrients such as calcium and magnesium.
• Lakes turn acidic and fish die.
• Forests have their leaves damaged.

• Monuments made of marble and limestone wear away.

• Taj Mahal case (Agra, Uttar Pradesh):

• SO₂ from the Mathura refinery and from nearby foundries turned into acid. It began to yellow and pit the white marble ("marble cancer").
• The Supreme Court, in the M.C. Mehta case (1996), created the Taj Trapezium Zone (TTZ). This is a trapezium-shaped area around the Taj.
• Inside the TTZ, polluting industries had to switch to natural gas or move out.

6. Delhi's winter smog (November–January)

Step 1 — Local sources pile up

• Vehicles, road and construction dust, industry, power plants and waste burning release pollution over Delhi-NCR all year round.

Step 2 — Winter weather traps it

• Temperature inversion means a layer of cold air sits near the ground under a layer of warmer air.
• Normally, air gets cooler as it rises, so warm polluted air near the ground rises and spreads out.
• On long, clear winter nights the ground loses heat fast. The air touching it becomes cold and heavy.

• The warmer layer above acts like a lid, so the pollution cannot rise.

• Mixing height is the height up to which air near the ground mixes freely.

• In winter it falls to a few hundred metres, so the same pollution is packed into a much smaller volume of air.

• Calm winds mean little sideways movement, so the pollution is not blown away.

• Fog and high humidity help particles grow, and smog forms.

Step 3 — Outside smoke arrives

• North-westerly winds blow from Punjab and Haryana down the Indo-Gangetic plain towards Delhi.

• They carry smoke from paddy stubble burning (October–November).

• Festival firecrackers (Diwali, October–November) add a sharp spike.

Step 4 — Geography makes it worse

• Delhi is in a land-locked plain. There is no sea breeze to clean the air.
• The Himalaya to the north acts like a wall. Polluted air collects over the plain like water in a bowl.

7. Stubble burning

• Stubble is the short stalks of the paddy plant left in the field after harvest.
• Who burns it, where and when: farmers in Punjab, Haryana and western Uttar Pradesh, in October–November.
• Why they burn it:
• Combine harvesters (machines that cut and thresh in one go) leave tall stubble behind.
• The 2009 law on late transplanting (the Punjab Preservation of Subsoil Water Act, 2009) delays paddy transplanting to save groundwater. So the harvest also comes late.

• Farmers then have only 2–3 weeks before sowing wheat (rabi). Burning is the fastest and cheapest way to clear the field.

• Recent trend (official satellite counts):

• 2024: Punjab had 9,655 incidents and Haryana 1,118 [7].
• 2025: Punjab had 5,114 incidents. That is 53% lower than 2024 and 93% lower than 2021 [6].

• 2025: Haryana's count was 53% lower than 2024 and 91% lower than 2021 [6].

• Remedies:

Type	Method
In-situ (managed in the field)	Happy Seeder (sows wheat straight through the stubble), Super Straw Management System (Super SMS) fitted on combines, rotavators, mulchers
Biological	Bio-decomposer sprays (the PUSA decomposer from IARI), which rot the straw into manure
Ex-situ (used outside the field)	Straw used as fuel in thermal power plants (co-firing with pellets), in biomass plants, for bio-CNG and as packaging
Long-term	Crop diversification away from paddy to maize, pulses and cotton

• Machine support: by October 2023, subsidised crop residue machines had reached Punjab (1,17,672), Haryana (80,071) and UP-NCR (7,986) [8].
• Monitoring: IARI (Indian Agricultural Research Institute, New Delhi) counts farm fires every day from satellite images. It uses a standard protocol.

8. Policy response

• National Clean Air Programme (NCAP):
• Launched by MoEFCC in January 2019 for 131 cities in 24 States/UTs [2].
• These are non-attainment cities (cities that failed the national air standards for five years) and million-plus cities [2].
• Target: cut PM10 by up to 40%, or meet the National Ambient Air Quality Standards (NAAQS), by 2025-26 [3].
• 82 cities under NCAP have annual targets of a 3–15% PM10 cut [4].
• 49 cities funded by the XV Finance Commission air quality grant must cut PM10 by 15% a year [4].

• Progress (FY 2025-26):

  • 108 cities had lower PM10 than in 2017-18 [5].
  • 72 cut it by more than 20%, and 26 by more than 40% [5].
  • 14 cities met the NAAQS [5].

• Commission for Air Quality Management in NCR and Adjoining Areas (CAQM):

• Set up by an Act of Parliament in 2021 [5].
• It coordinates action on stubble burning in Punjab, Haryana, Rajasthan and UP [6].

Prelims Hooks

• Acid rain threshold: pH below about 5.6. The acids come from SOₓ and NOₓ, not from CO₂.
• In cities, the first rain after summer has the lowest pH.
• Taj Trapezium Zone: Supreme Court, M.C. Mehta case, 1996. The polluter was the Mathura refinery.
• Temperature inversion means cold air near the ground under warm air. It happens on calm, clear winter nights and traps smog.
• Delhi's stubble smoke arrives on north-westerly winds from Punjab–Haryana, along the Indo-Gangetic plain.
• NCAP: launched 2019, covers 131 cities, target of up to a 40% PM10 cut by 2025-26 [2][3].
• WHO 2021 PM2.5 annual guideline: 5 µg/m³ (the old guideline was 10) [9].
• Happy Seeder = in-situ management. Straw pellets in power plants = ex-situ management.
• CAQM was created by statute in 2021 [5].
• Photochemical smog needs sunlight, NOₓ and hydrocarbons. Classical smog comes from coal smoke and fog.

Mains Points

• Delhi's smog is caused by geography as much as by emissions.
• Winter inversion, low mixing height, calm winds, a land-locked basin and the Himalayan barrier combine with seasonal stubble and firecracker smoke.

• So city-only action is not enough. Airshed-level coordination across states, through CAQM, is needed (GS-I/III).

• Stubble burning is a policy problem in farming.

• Paddy is driven by MSP, the 2009 groundwater law shortened the gap between harvests, and combines leave stubble.

• Machines and ex-situ markets cut fires by about 90% between 2021 and 2025 [6]. Lasting change still needs crop diversification away from paddy in north-west India.

• NCAP's record is mixed.

• It targets PM10 (dust) rather than the more harmful PM2.5.
• Only 14 cities met the NAAQS by 2025-26 [5].

• Its funding is linked to performance [4].

• Acid rain and heritage.

• The Taj Trapezium Zone shows how courts can control land use for the environment. The precautionary and polluter-pays principles are applied through zoning.