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 World Air Quality Report 2025). 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:

  • ~7 lakh premature deaths annually in India due to air pollution (Lancet study)
  • 66 of world's 100 most polluted cities in India (IQAir World Air Quality Report 2025, covering 2024 data); India ranked 6th most polluted country (national PM2.5 avg: 48.9 µg/m³); Loni (Ghaziabad, UP) = most polluted city; Delhi (99.6 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 20–30% reduction in PM2.5 and PM10 in 131 non-attainment cities by 2024 (revised to 40% by 2026)
  • 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: Free LPG connections to BPL families → 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):

  1. Nitrogen fixation: Bacteria (Rhizobium in legume roots; free-living Azotobacter) convert N₂ → NH₃ (ammonia)
  2. Nitrification: Bacteria convert NH₃ → NO₂⁻ → NO₃⁻ (nitrates — usable by plants)
  3. Assimilation: Plants absorb nitrates → make proteins; animals eat plants → animal proteins
  4. Ammonification: Dead organisms + urine → bacteria convert organic N → NH₃
  5. 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

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

Previous Year Questions

Prelims:

  1. The largest component of air by volume is:
    (a) Oxygen
    (b) Nitrogen
    (c) Carbon dioxide
    (d) Argon

  2. PM2.5 refers to particulate matter with diameter less than:
    (a) 10 micrometres
    (b) 2.5 micrometres
    (c) 1 micrometre
    (d) 25 micrometres

  3. The National Clean Air Programme (NCAP) was launched in:
    (a) 2014
    (b) 2016
    (c) 2019
    (d) 2022

  4. 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:

  1. 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)