💨 Chapter 05 · Topic 01 · Environmental Pollution

Air Pollution — Pollutants, AQI, Smog & Acid Rain

Primary & secondary pollutants, PM2.5, PM10, AQI categories, photochemical smog, acid rain, sources and control measures — complete UPSC & PSC notes.

💨 What is Air Pollution?

Air pollution is the presence of harmful substances (solid, liquid, or gaseous) in the atmosphere in concentrations that are injurious to human health, animals, plants, and property. It is one of the leading environmental health risks globally.

📌 Key Fact: According to WHO, air pollution causes approximately 7 million premature deaths per year globally. India has 14 of the world’s 20 most polluted cities (IQAir 2023). Delhi is frequently ranked as the most polluted capital city in the world.

💨 Primary vs Secondary Pollutants

Type	Definition	Examples	Sources
Primary Pollutants	Directly emitted into the atmosphere from identifiable sources	CO, SO₂, NO₂, PM2.5, PM10, lead, hydrocarbons	Vehicles, industries, power plants, biomass burning
Secondary Pollutants	Not directly emitted; formed in the atmosphere by chemical reactions between primary pollutants	Ground-level ozone (O₃), PAN, smog, acid rain, secondary PM2.5	Formed from NOₓ + VOCs + sunlight; SO₂ + water vapour

⭐ Exam Tip: Ground-level ozone is a secondary pollutant (harmful). Stratospheric ozone is beneficial (protects from UV). This distinction is frequently tested. PAN (Peroxyacetyl Nitrate) is also a secondary pollutant — causes eye irritation and damages plants.

💨 Key Air Pollutants — Detailed

Particulate Matter (PM)

PM2.5 — particles with aerodynamic diameter <2.5 micrometres; most dangerous; can penetrate deep into lungs (alveoli) and enter the bloodstream; causes respiratory and cardiovascular diseases, cancer
PM10 — particles with diameter <10 micrometres; causes respiratory problems; filtered by nose and throat to some extent
Sources of PM: vehicle exhaust, construction dust, industrial emissions, crop stubble burning, road dust, coal combustion
Secondary PM2.5 — formed in atmosphere from SO₂, NOₓ, and ammonia reactions; major contributor to Delhi’s winter pollution

Carbon Monoxide (CO)

Colourless, odourless gas — makes it especially dangerous (no warning)
Produced by incomplete combustion of fossil fuels, biomass, and tobacco
Binds with haemoglobin to form carboxyhaemoglobin (COHb) — 200–250× greater affinity than oxygen
Reduces oxygen-carrying capacity of blood → oxygen deprivation in tissues
Symptoms: headache, dizziness, confusion; at high concentrations — death
Major source: vehicle exhaust (especially petrol engines)

Sulphur Dioxide (SO₂)

Produced by burning coal and oil (especially in thermal power plants) and smelting of metal ores
Causes acid rain when it reacts with water vapour → H₂SO₄ (sulphuric acid)
Respiratory irritant — causes bronchitis, asthma; damages lung tissue
Damages vegetation — bleaches leaves, reduces photosynthesis
India’s thermal power plants are the largest source of SO₂ emissions

Nitrogen Oxides (NOₓ)

Includes NO and NO₂; produced by high-temperature combustion (vehicles, power plants)
NO₂ is a reddish-brown gas with pungent smell; causes respiratory problems
Contributes to acid rain (forms HNO₃) and photochemical smog
Reacts with VOCs in sunlight to form ground-level ozone

Lead (Pb)

Previously a major pollutant from leaded petrol — India phased out leaded petrol in 2000
Still emitted from battery manufacturing, smelting, paint
Causes neurological damage, especially in children (affects brain development)
Bioaccumulates in bones and soft tissues

VOCs (Volatile Organic Compounds)

Organic chemicals that evaporate easily at room temperature
Sources: paints, solvents, vehicle exhaust, petroleum products, dry cleaning
React with NOₓ in sunlight to form ground-level ozone and photochemical smog
Some VOCs (benzene, formaldehyde) are carcinogenic

🌫️ Smog

Smog = smoke + fog. A mixture of air pollutants that reduces visibility and causes health problems. Two main types:

Feature	Classical / London Smog	Photochemical / LA Smog
Main pollutants	SO₂ + smoke + fog	NOₓ + VOCs + sunlight → O₃ + PAN
Type	Reducing smog	Oxidising smog
Conditions	Cold, humid, winter mornings	Warm, sunny, low humidity
Colour	Grey/black	Brownish/yellowish
Health effects	Respiratory problems, bronchitis	Eye irritation, chest pain, asthma
Famous incident	London Great Smog (1952) — 4,000+ deaths	Los Angeles smog (1940s onwards)
India relevance	Delhi winter smog has elements of both	Photochemical smog in Indian cities

📌 PAN (Peroxyacetyl Nitrate): A secondary pollutant formed in photochemical smog. It is a powerful eye irritant and damages plants by bleaching leaves. It is also a greenhouse gas. PAN is more toxic than ozone at low concentrations.

🌧️ Acid Rain

Caused by SO₂ and NOₓ reacting with water vapour in the atmosphere to form H₂SO₄ (sulphuric acid) and HNO₃ (nitric acid)
Normal rain pH = 5.6 (slightly acidic due to dissolved CO₂); acid rain pH < 5.6
Can travel hundreds of kilometres from the source — a transboundary pollution problem

Effects of Acid Rain

Target	Effect
Forests	Damages leaves, leaches nutrients from soil, kills trees (especially conifers)
Lakes & rivers	Acidifies water bodies; kills fish and aquatic life; “dead lakes”
Buildings & monuments	Marble cancer — dissolves CaCO₃ in marble; damages Taj Mahal, Colosseum
Crops	Reduces crop yield; damages leaves
Soil	Leaches essential nutrients (Ca, Mg); releases toxic aluminium
Human health	Indirect — through contaminated water and food

⭐ Taj Mahal & Acid Rain: The Taj Mahal in Agra is being damaged by acid rain and air pollution from nearby industries (especially Mathura refinery). The Supreme Court has set up a Taj Trapezium Zone (TTZ) — a 10,400 sq km area around the Taj where polluting industries are restricted. The M.C. Mehta vs Union of India case (1996) led to relocation of industries from the TTZ.

📊 Air Quality Index (AQI)

AQI is a standardised index used to communicate air quality to the public. India’s AQI is based on 8 pollutants: PM2.5, PM10, NO₂, SO₂, CO, O₃, NH₃, and Pb.

AQI Range	Category	Colour	Health Impact
0–50	Good	Green	Minimal impact
51–100	Satisfactory	Light Green	Minor breathing discomfort for sensitive people
101–200	Moderate	Yellow	Breathing discomfort for asthma patients, elderly, children
201–300	Poor	Orange	Breathing discomfort for most people on prolonged exposure
301–400	Very Poor	Red	Respiratory illness on prolonged exposure
401–500	Severe	Dark Red/Maroon	Affects healthy people; serious risk for sensitive groups; emergency conditions

📌 CPCB & AQI Monitoring: CPCB (Central Pollution Control Board) monitors AQI at 240+ stations across India through the CAAQMS (Continuous Ambient Air Quality Monitoring Stations) network. Real-time data is available on the SAMEER app and AQI India website.

🏭 Sources of Air Pollution in India

Source	Key Pollutants	Contribution
Vehicles (transport)	CO, NOₓ, PM, VOCs, lead	Major source in urban areas; ~30% of PM2.5 in Delhi
Thermal power plants	SO₂, NOₓ, PM, mercury	Largest source of SO₂ in India
Industries	SO₂, NOₓ, PM, heavy metals, VOCs	Varies by industry type
Crop stubble burning	PM2.5, CO, black carbon	Major cause of Delhi’s winter pollution (Oct–Nov)
Construction	PM10, PM2.5 (dust)	Significant in rapidly urbanising cities
Biomass burning	PM2.5, CO, black carbon	Cooking, heating in rural areas
Waste burning	Dioxins, furans, PM, CO	Open burning of municipal solid waste

🌡️ Temperature Inversion

Normally, temperature decreases with altitude — warm air rises, carrying pollutants upward and dispersing them
Temperature inversion — a layer of warm air traps cold air (and pollutants) near the ground
Prevents vertical mixing of air → pollutants accumulate near the surface
Common in winter mornings in Delhi and other cities — worsens smog episodes
Also called thermal inversion or atmospheric inversion

🛡️ Air Pollution Control Measures

Regulatory Measures

Air (Prevention and Control of Pollution) Act 1981 — sets air quality standards; empowers CPCB and SPCBs
Environment Protection Act 1986 — umbrella legislation; sets emission standards for industries
BS-VI (Bharat Stage VI) emission norms — implemented from April 2020; equivalent to Euro-6; drastically reduced vehicle emissions
National Ambient Air Quality Standards (NAAQS) — set by CPCB; define permissible concentrations of pollutants

Government Programmes

National Clean Air Programme (NCAP) 2019 — target 40% reduction in PM2.5 and PM10 by 2026; covers 131 non-attainment cities
FAME India (Faster Adoption and Manufacturing of Electric Vehicles) — promotes EVs to reduce vehicle emissions
PM UJJWALA Yojana — LPG connections to BPL households; reduces indoor air pollution from biomass cooking
PUSA Bio-Decomposer — microbial solution to decompose crop stubble in-situ; reduces stubble burning
Graded Response Action Plan (GRAP) — emergency measures for Delhi-NCR based on AQI levels

Technical Measures

Electrostatic Precipitators (ESP) — remove particulate matter from industrial flue gases
Scrubbers — remove SO₂ and other gases from industrial emissions using water/chemical spray
Catalytic converters — in vehicles; convert CO, NOₓ, and hydrocarbons to CO₂, N₂, and water
Flue Gas Desulphurisation (FGD) — removes SO₂ from power plant emissions
Transition to renewable energy — reduces dependence on coal; reduces SO₂ and PM emissions

⭐ BS-VI Fuel: India leapfrogged from BS-IV to BS-VI (skipping BS-V) in April 2020. BS-VI fuel has sulphur content of only 10 ppm (compared to 50 ppm in BS-IV). This significantly reduces SO₂, PM, and NOₓ emissions from vehicles. India is one of the few countries to implement such stringent emission norms.

🌍 Indoor Air Pollution

Indoor air can be 2–5× more polluted than outdoor air
Sources: cooking on biomass/coal stoves, tobacco smoke, building materials (asbestos, formaldehyde), cleaning products, radon gas from soil
Key pollutants: CO, PM2.5, VOCs, radon, asbestos fibres
Causes ~4 million deaths per year globally (WHO)
Major problem in rural India — women and children most affected by cooking smoke
PM UJJWALA Yojana — provides LPG to BPL households; reduces indoor air pollution

✅ Revision Checklist — Air Pollution

✅ Primary pollutants = directly emitted (CO, SO₂, NO₂, PM, lead, VOCs)
✅ Secondary pollutants = formed in atmosphere (ozone, PAN, smog, acid rain, secondary PM2.5)
✅ PM2.5 = <2.5 microns = most dangerous = enters bloodstream
✅ CO = colourless + odourless = forms carboxyhaemoglobin = reduces O₂ carrying capacity
✅ SO₂ = from coal burning = causes acid rain + respiratory problems
✅ NOₓ = from high-temperature combustion = acid rain + photochemical smog
✅ London smog = SO₂ + fog = reducing type = cold + humid
✅ Photochemical smog = NOₓ + VOCs + sunlight = oxidising type = warm + sunny
✅ PAN = secondary pollutant = eye irritant = damages plants
✅ Acid rain = pH < 5.6 = H₂SO₄ + HNO₃ = marble cancer = damages Taj Mahal
✅ Normal rain pH = 5.6; acid rain pH < 5.6
✅ AQI 0–50 = Good; 101–200 = Moderate; 301–400 = Very Poor; 401–500 = Severe
✅ India’s AQI based on 8 pollutants including PM2.5, PM10, SO₂, NO₂, CO, O₃
✅ Temperature inversion = warm air traps cold air + pollutants near ground = worsens smog
✅ NCAP 2019 = 40% reduction in PM by 2026 = 131 non-attainment cities
✅ BS-VI = April 2020 = 10 ppm sulphur = leapfrogged from BS-IV
✅ GRAP = Graded Response Action Plan = emergency measures for Delhi-NCR
✅ Catalytic converter = converts CO + NOₓ + hydrocarbons in vehicles
✅ ESP = Electrostatic Precipitator = removes PM from industrial emissions
✅ Taj Trapezium Zone (TTZ) = 10,400 sq km = restricts industries near Taj Mahal