Maarg Manthan · Topic 13.7

Top Ten Air Pollutant Gases and Control Measures

Environment & Ecology › Air, Marine, Noise and Other Pollution · Topic 13.7

There is no single official list of "top ten" air pollutant gases, but ten gases are repeatedly named in syllabi and by regulators: carbon monoxide (CO), carbon dioxide (CO2), sulphur dioxide (SO2), nitrogen oxides (NOx), ground-level ozone (O3), methane (CH4), ammonia (NH3), volatile organic compounds including benzene, hydrogen sulphide (H2S) and chlorofluorocarbons (CFCs). India's National Ambient Air Quality Standards (2009) set limits for SO2, NO2, CO, ozone, ammonia, benzene, lead, arsenic, nickel, benzo(a)pyrene and particulate matter. CO is a poisonous gas from incomplete combustion that binds haemoglobin; SO2 comes from burning coal and oil and causes acid rain; NOx from vehicles and power plants form smog and acid rain; ozone near the ground is a lung irritant formed from NOx and VOCs; methane and CO2 are greenhouse gases; CFCs destroy stratospheric ozone. Control measures include cleaner fuels, flue gas desulphurisation, catalytic converters, low-NOx burners, vapour recovery, better combustion, afforestation and strict standards.

5 min read · UPSC Prelims · Environment & Ecology · Air, Marine, Noise and Other Pollution

At a Glance

Poison gas Carbon monoxide
Acid gases SO2 and NOx
Ground-level gas Ozone
Climate gases CO2 and methane
Control device Catalytic converter, FGD
Exam link UPSC Prelims, Environment & Ecology: Air, Marine, Noise and Other Pollution

Where Does It Fit?

This topic follows the general account of air pollution and goes through the individual gases. It prepares for the next topic on smog, ozone depletion and acid rain, in which several of these gases play a role. Previous topic: Causes and Harmful Effects of Air Pollution. Next topic: Smog, Ozone Depletion and Acid Rain.

Which Gases Are the Key Air Pollutants?

Gas	Source, effect and control
Carbon monoxide (CO)	Incomplete burning in vehicles, stoves; binds haemoglobin as carboxyhaemoglobin and reduces oxygen delivery. Control: catalytic converters, better combustion, ventilation
Carbon dioxide (CO2)	Fossil fuel burning, cement, deforestation; main greenhouse gas. Control: renewables, efficiency, afforestation
Sulphur dioxide (SO2)	Coal and oil burning, smelting; lung irritant, acid rain. Control: low-sulphur fuel, flue gas desulphurisation
Nitrogen oxides (NOx)	High temperature combustion in vehicles and plants; lung damage, smog, acid rain. Control: low-NOx burners, catalytic converters, SCR
Ground-level ozone (O3)	Secondary pollutant from NOx and VOCs with sunlight; harms lungs and crops. Control: cut NOx and VOC emissions
Methane (CH4)	Livestock, paddy, landfills, gas leaks; greenhouse gas and ozone precursor. Control: better waste and livestock management, leak repair
Ammonia (NH3)	Fertiliser and animal waste; forms fine particles. Control: improved fertiliser use and manure handling

Which Other Gases Matter and What Are the Standards?

Volatile organic compounds and benzene: from vehicles, solvents and fuel evaporation; benzene is a carcinogen and has a NAAQS limit of 5 micrograms per cubic metre annually
Hydrogen sulphide (H2S): from sewage, oil refineries and paper mills; rotten egg smell and toxic at high levels
Chlorofluorocarbons (CFCs): from old refrigerators and aerosols; destroy stratospheric ozone and are also greenhouse gases
NAAQS 2009: 12 pollutants listed, with 8-hour or 24-hour and annual averages
SO2 standard: 50 micrograms per cubic metre annual and 80 for 24 hours
NO2 standard: 40 micrograms per cubic metre annual and 80 for 24 hours
CO standard: 2 milligrams per cubic metre for 8 hours and 4 for one hour
Ozone standard: 100 micrograms per cubic metre for 8 hours and 180 for one hour

What Else Should You Remember?

Carboxyhaemoglobin: haemoglobin bound to CO, which cannot carry oxygen
CO poisoning: headache, dizziness and death in closed spaces with poor ventilation
Greenhouse gases: CO2, methane, nitrous oxide, HFCs and others that trap heat
Global warming potential: methane is far more potent than CO2 per tonne over 20 years
Acid gases: SO2 and NOx dissolve in water to form sulphuric and nitric acids
Flue gas desulphurisation: uses limestone slurry to remove SO2 from power plant emissions; India’s thermal plants were told to install it in phases
Selective catalytic reduction: uses ammonia or urea to convert NOx to nitrogen and water, as with AdBlue in diesel vehicles
Three-way catalytic converter: converts CO, hydrocarbons and NOx into CO2, water and nitrogen in petrol vehicles
Low-NOx burner: reduces peak flame temperature and NOx formation
Electrostatic precipitator: removes particles from flue gases
Scrubber: uses liquid to absorb gases
Vapour recovery: captures petrol vapour at pumps
Ozone layer and ozone smog: stratospheric ozone protects, ground-level ozone harms
Benzene: component of petrol and a cause of leukaemia
Hydrogen cyanide and phosgene: highly toxic industrial gases
Ammonia and secondary particles: reacts with SO2 and NOx to form ammonium sulphate and nitrate particles
Methane sources: wetlands, ruminants, rice fields, coal mines, oil and gas
Nitrous oxide: a long-lived greenhouse gas from fertiliser and combustion
Emission standards: set by the Ministry of Environment for industries under the Environment Protection Rules
Green belts: trees trap particles and absorb some gases
Fuel switching: from coal to gas or renewables
Air quality standards: NAAQS 2009 and a draft revision are used
Link to previous: see Causes and Harmful Effects of Air Pollution
Link to next: see Smog, Ozone Depletion and Acid Rain
Link to climate: see Climate Change Basics and Causes macro
Caution: no official top ten list exists, and standards are revised from time to time
Exam hint: learn each gas with source, effect and control, and the NAAQS numbers for SO2, NO2, CO and ozone

Why Does It Matter for the Exam?

Poison gas: CO
Acid rain gases: SO2, NOx
Secondary gas: Ozone
Current affairs link: FGD deadlines for plants
Common errors: calling ground-level ozone beneficial

As an analytical point, most harmful gases come from the same few activities, so one fix such as cleaner fuel or a better combustion technology can cut several pollutants at once. Standards matter only if monitoring and enforcement keep up.

Why Is This Topic Important?

Source Burning
Harm Lungs, climate
Fix Cleaner fuel, devices

What Came Next?

The next topic covers smog, ozone depletion and acid rain. Next topic: Smog, Ozone Depletion and Acid Rain.

Key People and Terms

NOx Nitrogen oxides
FGD Flue gas desulphurisation
SCR Selective catalytic reduction
VOC Volatile organic compound
CFC Chlorofluorocarbon
NAAQS National ambient air quality standards

Exam Corner

Points to Remember

CO binds haemoglobin.
SO2 and NOx cause acid rain.
Ozone is secondary.
FGD removes SO2.
Catalytic converter cuts CO, HC, NOx.

Do Not Confuse With

Ground-level ozone and stratospheric ozone Harmful pollutant versus protective layer
CO and CO2 Poison gas versus greenhouse gas
FGD and SCR SO2 removal versus NOx removal

Memory Hook

CO poisons, SO2 and NOx make acid, ozone forms in sun, methane and CO2 warm.

Mains Angle

Questions ask for source, effect and control of gases. These points help.

Gases Ten
Control Devices, fuels
Standards NAAQS

A Question You May Face

An original practice question, not a past paper question.

“Discuss the major air pollutant gases, their sources and effects, and the technologies used to control them.”

How to Answer

Gases List.
Sources Burning.
Effects Health, climate.
Control Devices.

GS Relevance

Prelims: gases, sources, standards, controls. GS Paper 3: air pollution control technologies.

Frequently Asked Questions

Why is carbon monoxide dangerous?

Carbon monoxide is a colourless, odourless gas produced by incomplete burning of fuel. When inhaled, it binds to haemoglobin in the blood about 200 times more strongly than oxygen, forming carboxyhaemoglobin, which reduces the oxygen reaching the body's tissues. In closed spaces with poor ventilation it can cause headache, dizziness, unconsciousness and death.

How do sulphur dioxide and nitrogen oxides cause acid rain?

Sulphur dioxide from burning coal and oil and nitrogen oxides from vehicles and power plants rise into the atmosphere and react with oxygen and water to form sulphuric and nitric acids. These fall as acid rain, snow or dry deposition, harming forests, lakes, soil, crops and buildings. They also irritate the lungs.

How is ground-level ozone formed?

Ground-level ozone is a secondary pollutant. It forms when nitrogen oxides and volatile organic compounds from vehicles, industry and solvents react in the presence of sunlight. It is the main component of photochemical smog, irritates the lungs, worsens asthma and reduces crop yields, unlike stratospheric ozone, which protects life from ultraviolet radiation.

What is flue gas desulphurisation?

Flue gas desulphurisation is a technology that removes sulphur dioxide from the exhaust gases of coal-fired power plants by passing them through a wet scrubber with limestone slurry, converting the SO2 to gypsum. The Environment Ministry set emission norms in 2015 that require thermal power plants in India to install it, with deadlines that have been extended.

What is a catalytic converter?

A catalytic converter is a device in the exhaust system of petrol vehicles that uses catalysts such as platinum, palladium and rhodium to convert harmful gases, namely carbon monoxide, unburnt hydrocarbons and nitrogen oxides, into carbon dioxide, water vapour and nitrogen. It requires unleaded fuel because lead poisons the catalyst.

PYQ Practice — Statement Analysis

1 Carbon monoxide binds to haemoglobin.

True

Forming carboxyhaemoglobin.

2 Ground-level ozone is a primary pollutant.

False

It is secondary.

3 FGD is used to remove sulphur dioxide.