Why this chapter matters for UPSC: Air quality, air pollution, and the atmosphere are tested extensively in GS3 (environment) and GS2 (health policy). India faces a severe air pollution crisis — 66 of the world's 100 most polluted cities are in India (IQAir 2025 World Air Quality Report, March 2026). This connects to National Clean Air Programme, AQI, PM2.5, and health impacts.

PART 1 — Quick Reference Tables

Composition of Air

Component	Percentage	Role
Nitrogen (N₂)	~78%	Dilutes oxygen; prevents rapid burning; soil bacteria fix N₂
Oxygen (O₂)	~21%	Respiration; combustion
Argon (Ar)	~0.9%	Inert gas; no biological role
Carbon dioxide (CO₂)	~0.04%	Photosynthesis; greenhouse gas (rising due to emissions)
Water vapour	Variable (0–4%)	Weather; humidity; clouds
Other gases	Traces	Neon, helium, methane, ozone, etc.

Air Pollutants — Key Facts

Pollutant	Sources	Health Effects	Standard
PM2.5 (fine particles <2.5μm)	Vehicle exhaust, industry, crop burning, dust	Lung disease, cardiovascular disease, cancer; penetrates blood	WHO: 5 μg/m³ annual; India NAAQS: 40 μg/m³
PM10 (coarse particles <10μm)	Dust, construction, road dust	Respiratory disease	India NAAQS: 60 μg/m³
NO₂ (Nitrogen dioxide)	Vehicles, power plants	Respiratory irritant; forms smog and acid rain	—
SO₂ (Sulphur dioxide)	Coal burning, industry	Respiratory disease; acid rain	—
CO (Carbon monoxide)	Incomplete combustion, vehicles	Binds haemoglobin; reduces O₂ carrying capacity; deadly in enclosed spaces	—
Ozone (O₃ — ground level)	Vehicle/industrial emissions reacting with sunlight	Lung irritant; smog component	—
Lead (Pb)	Leaded fuel (phased out); some industries	Neurotoxin; especially harmful for children's brain development	Leaded petrol banned in India 2000

PART 2 — Detailed Notes

Why Air is Essential

Explainer

Functions of air:

Oxygen: Essential for aerobic respiration in all animals and plants; combustion
CO₂: Required for photosynthesis — plants need CO₂ to make food
Nitrogen: Inert buffer; Nitrogen cycle — bacteria convert N₂ to ammonia → plants use → returned to air by denitrifying bacteria
Water vapour: Forms clouds and rain; moderates temperature
Wind: Carries seeds and pollen for plant reproduction; drives wind energy turbines; evaporates water from oceans driving weather systems

India's Air Pollution Crisis

UPSC Connect

UPSC GS3 + GS2 — India's air quality emergency:

Scale of the problem:

Over 17 lakh (1.7 million) premature deaths in India attributable to air pollution in 2022 (Lancet Global Burden of Disease study — the old "7 lakh" figure is severely outdated)
66 of world's 100 most polluted cities in India (IQAir 2025 World Air Quality Report, released March 2026, covering 2025 data); India ranked 6th most polluted country (national PM2.5 avg: 48.9 µg/m³); Loni (Ghaziabad, UP) = most polluted city globally (112.5 µg/m³); Delhi (82.2 µg/m³) = world's most polluted capital
North Indian Plain (Delhi-NCR, UP, Bihar) suffers worst pollution — geography (bowl shape), cold winters (temperature inversion trapping pollutants), and crop burning

Sources of air pollution:

Vehicles: Largest source in cities; BS-VI emission norms implemented nationwide (April 2020)
Industry: Coal-fired thermal power plants; brick kilns; cement plants
Agriculture: Stubble (crop residue) burning in Punjab and Haryana after paddy harvest (October-November) → contributes 20–30% of Delhi's winter pollution
Construction dust
Domestic cooking: Biomass burning (wood, dung) in rural areas — a major source of indoor air pollution

Policy responses:

National Clean Air Programme (NCAP, 2019): Target revised to 40% reduction in PM10 by 2026 in 131 non-attainment cities (base year 2017); only 23 of 131 cities achieved the target as of Jan 2026 (CREA); deadline effectively being missed
National Air Quality Index (AQI): 6 colour-coded categories (Good, Satisfactory, Moderate, Poor, Very Poor, Severe); 8 pollutants measured
GRAP (Graded Response Action Plan): Emergency measures in Delhi-NCR activated when AQI crosses thresholds (construction bans, odd-even vehicle scheme, school closures)
BS-VI fuel and vehicles: Dramatically reduced PM, NOₓ, SO₂ emissions from vehicles
PUSA Bio-decomposer: IARI-developed microbial solution that decomposes stubble in 15–20 days (alternative to burning); distributed to Punjab/Haryana farmers
PM Ujjwala Yojana (PMUY): Free LPG connections to BPL families — ~10.33 crore connections (March 2025); 25 lakh additional approved FY2025-26; reduces biomass/kerosene cooking → reduces indoor air pollution + household PM2.5 exposure

Nitrogen Cycle — Ecological Importance

Explainer

Nitrogen cycle: Atmosphere is 78% nitrogen but most organisms cannot use N₂ directly. Nitrogen must be "fixed" (converted to usable forms):

Nitrogen fixation: Bacteria (Rhizobium in legume roots; free-living Azotobacter) convert N₂ → NH₃ (ammonia)
Nitrification: Bacteria convert NH₃ → NO₂⁻ → NO₃⁻ (nitrates — usable by plants)
Assimilation: Plants absorb nitrates → make proteins; animals eat plants → animal proteins
Ammonification: Dead organisms + urine → bacteria convert organic N → NH₃
Denitrification: Bacteria convert NO₃⁻ → N₂ → returned to atmosphere

UPSC relevance:

Urea fertiliser: Industrial nitrogen fixation (Haber-Bosch process) — India is the world's largest consumer of urea
Urea subsidy: India spends ~₹1.5 lakh crore on fertiliser subsidies annually; PM Pranam scheme promotes reduced chemical fertiliser use
Nano Urea: IFFCO's liquid nano urea (approved 2021) — reduces urea requirement by 50% through direct foliar application
Soil microbiome: Rhizobium in legume root nodules = natural nitrogen fixation; reducing synthetic fertiliser need

[Additional] 15a. Atmospheric Layers — The Vertical Structure of Air

The chapter discusses air composition but does not explain that the atmosphere has distinct layers with different properties. These layers are directly tested in UPSC Prelims geography and science questions.

Key Term

Five layers of the atmosphere (from Earth's surface upward):

Layer	Altitude	Temperature Trend	Key Features
Troposphere	0–12 km (avg); 8 km at poles, 16 km at equator	Decreases with altitude (~6.5°C per km)	Contains ~75% of atmosphere's mass and virtually all water vapour; all weather occurs here; ends at "tropopause"
Stratosphere	12–50 km	Increases with altitude (ozone absorbs UV)	Contains the ozone layer (20–35 km); aircraft cruise here to avoid turbulence; calm, dry air
Mesosphere	50–85 km	Decreases with altitude (coldest layer)	Meteors burn up here ("shooting stars"); temperature can reach −90°C
Thermosphere	85–700 km	Very high temperature (but low density — thin air)	Aurora Borealis/Australis occur here (charged particles hit air molecules); International Space Station (ISS) orbits at ~400 km within this layer
Exosphere	Above ~700 km	Merges with space	Extremely thin; satellites in geosynchronous orbit (~36,000 km) are here; gradually merges into outer space

Key distinction tested in UPSC:

Troposphere: Where weather happens; temperature decreases with altitude
Stratosphere: Where ozone layer is; temperature increases with altitude (because ozone absorbs UV — a heating mechanism)
This temperature "inversion" in the stratosphere is why the troposphere and stratosphere don't mix — it acts like a lid on weather systems

[Additional] 15b. Ozone Layer — Protection, Depletion, and Recovery

Stratospheric ozone (O₃) in the ozone layer (20–35 km altitude) absorbs 97–99% of the Sun's harmful ultraviolet (UV-B and UV-C) radiation before it reaches Earth's surface.

Without the ozone layer:

Skin cancer rates would increase dramatically
Cataracts and other eye damage would increase
Phytoplankton (base of ocean food chains) would be devastated — marine ecosystem collapse
Crop yields would fall due to UV damage to plant DNA

Key Term

Ozone depletion — the mechanism:

CFCs (Chlorofluorocarbons): Used in refrigerators, air conditioners, aerosol spray cans, and foam blowing agents. In the troposphere, CFCs are chemically stable — harmless. However, in the stratosphere, UV breaks CFC molecules apart, releasing chlorine (Cl) atoms.

One chlorine atom destroys up to 100,000 ozone molecules via a catalytic chain reaction:

Cl + O₃ → ClO + O₂ (ozone destroyed)
ClO + O → Cl + O₂ (chlorine recycled → destroys more ozone)
The chlorine atom is regenerated — it keeps destroying ozone without being consumed

Ozone hole: First detected in 1985 over Antarctica by British Antarctic Survey scientists (Farman, Gardiner & Shanklin). The hole forms each Antarctic spring (September–October) as polar vortex conditions and extreme cold accelerate CFC-driven ozone destruction. As of 2024, the ozone hole is slowly healing — 2024 was among the smaller holes since recovery began in 1992 (NASA) — but full recovery expected by ~2070.

UPSC Connect

[Additional] Montreal Protocol and India — GS3 (International Agreements/Environment):

Montreal Protocol on Substances that Deplete the Ozone Layer (1987):

Adopted: September 16, 1987 (World Ozone Day is marked on September 16 each year)
Parties: 197 — the first and only UN treaty with universal ratification (all UN member states plus Palestine and the Holy See)
Outcome: Phased out 99% of ozone-depleting substances (CFCs, halons, carbon tetrachloride, etc.); ozone layer recovering

India and Montreal Protocol:

India ratified: June 1992 (as an Article 5 developing country — given longer phase-out timelines)
India phased out Class I ODS (CFCs, halons, carbon tetrachloride) by January 1, 2010 — 5 years ahead of the developing-country deadline

Kigali Amendment (2016):

Problem: HFCs (hydrofluorocarbons) were introduced to replace CFCs — they don't deplete ozone, but they are extremely potent greenhouse gases (HFC-134a is 1,430 times more potent than CO₂ over 100 years)
Kigali Amendment to Montreal Protocol (adopted in Rwanda, 2016): Phases down HFCs
India ratified the Kigali Amendment in 2021 (Cabinet approved ratification)
India's HFC phasedown schedule (as developing country): 10% by 2032 → 20% by 2037 → 30% by 2042 → 85% by 2047
Alternatives: HFOs (hydrofluoroolefins) — low global warming potential refrigerants

Natural greenhouse effect vs ozone:

Tropospheric ozone (ground level) = air pollutant (smog component; harmful to lungs)
Stratospheric ozone (ozone layer) = protective shield against UV
Same molecule (O₃); entirely different role depending on altitude — a key UPSC trap question

Greenhouse effect (related mechanism):

Short-wave solar radiation passes through atmosphere → warms Earth's surface → Earth re-emits long-wave infrared radiation (heat)
Greenhouse gases (CO₂, CH₄, water vapour H₂O, N₂O, ozone) absorb this outgoing heat and re-emit it back toward Earth
Natural greenhouse effect: Warms Earth by ~33°C — essential for life (without it, Earth's average temperature would be −18°C instead of +15°C)
Enhanced greenhouse effect: Burning fossil fuels increases atmospheric CO₂ → more heat trapped → global warming/climate change

Exam Strategy

Prelims traps:

Largest component of air: Nitrogen (78%) — NOT oxygen (21%); common confusion
PM2.5 = particles less than 2.5 micrometres — more dangerous than PM10 because they penetrate deeper into lungs and bloodstream
CO (carbon monoxide) = deadly, colourless, odourless; from incomplete combustion; NOT CO₂
Ground-level ozone (O₃) = air pollutant (harmful smog component); Stratospheric ozone = protective (blocks UV) — these are two different things at different altitudes
BS-VI norms implemented from April 2020 — biggest improvement in India's fuel quality in decades

Practice Questions

Prelims:

The largest component of air by volume is:
(a) Oxygen
(b) Nitrogen
(c) Carbon dioxide
(d) Argon
PM2.5 refers to particulate matter with diameter less than:
(a) 10 micrometres
(b) 2.5 micrometres
(c) 1 micrometre
(d) 25 micrometres
The National Clean Air Programme (NCAP) was launched in:
(a) 2014
(b) 2016
(c) 2019
(d) 2022
Rhizobium bacteria, which fix atmospheric nitrogen, are found in:
(a) Soil around all plants
(b) Root nodules of leguminous plants
(c) Leaf surfaces
(d) Water bodies

Mains:

Air pollution has emerged as India's largest environmental health crisis. Critically examine the causes and evaluate the effectiveness of India's policy response. (GS3, 15 marks)

Air Around Us