MEV 013: Unit 08 - Chemistry of Air Pollution-I

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UNIT 8: CHEMISTRY OF AIR POLLUTION – I

8.0 Introduction

Air pollution refers to the presence of harmful or excessive quantities of substances in the atmosphere, resulting from natural and human-made sources. Chemical pollutants such as carbon monoxide, carbon dioxide, oxides of nitrogen, sulphur dioxide, ozone, and acid rain are major contributors to environmental degradation and public health issues. This unit focuses on the chemistry, sources, effects, and control of these pollutants.

8.1 Objectives

By the end of this unit, you should be able to:

  • Describe the sources and harmful effects of key air pollutants.
  • Understand the chemical reactions involved in atmospheric pollution.
  • Explain control measures for different pollutants.
  • Evaluate the environmental impact of carbon monoxide, carbon dioxide, nitrogen oxides, sulphur dioxide, ozone, and acid rain.

8.2 Carbon Monoxide (CO)

Carbon monoxide is a colorless, odorless, and tasteless gas formed primarily by incomplete combustion of fossil fuels.

Sources:

  • Vehicle exhaust
  • Industrial processes
  • Residential heating
  • Biomass burning

8.2.1 Harmful Effects of Carbon Monoxide

  • Binds with hemoglobin to form carboxyhemoglobin, reducing oxygen transport in blood.
  • Causes headaches, dizziness, fatigue, and even death at high concentrations.
  • Particularly dangerous in enclosed environments.

8.2.2 Control of Carbon Monoxide Emissions

  • Use of catalytic converters in vehicles.
  • Promoting electric or hybrid vehicles.
  • Improvement in fuel quality and combustion efficiency.
  • Urban traffic management and promoting public transport.

8.2.3 Sinks of Carbon Monoxide

  • Reaction with hydroxyl radicals (OH•) in the atmosphere:
    CO + OH• → CO₂ + H•
  • Uptake by soils and oceans.
  • Photochemical reactions converting CO into CO₂.

8.3 Carbon Dioxide (CO₂)

A natural constituent of the atmosphere and a major greenhouse gas. While it plays a vital role in Earth's carbon cycle, excessive anthropogenic emissions cause global warming.

Sources:

  • Fossil fuel combustion
  • Deforestation
  • Cement production
  • Respiration

8.3.1 Global Warming

Excess CO₂ traps heat in the atmosphere, causing a rise in global temperatures.

Mechanism:

  • Solar radiation passes through atmosphere.
  • Earth emits infrared radiation.
  • CO₂ absorbs and re-emits infrared radiation back to Earth.

8.3.2 Effects of Global Warming

  • Rising sea levels
  • Melting of glaciers and polar ice
  • Extreme weather events
  • Ocean acidification
  • Loss of biodiversity

8.4 Oxides of Nitrogen (NOₓ)

Oxides of nitrogen, mainly NO and NO₂, are produced during high-temperature combustion processes.

Sources:

  • Automobiles and power plants
  • Lightning
  • Biomass burning
  • Agricultural activities (e.g., fertilizers)

8.4.1 Harmful Effects of Nitrogen Oxides

  • Eye and respiratory tract irritation
  • Precursor to ground-level ozone and smog
  • Contributes to acid rain formation
  • Reduces visibility (formation of NO₂ haze)

8.4.2 Control of Nitrogen Oxides

  • Use of low-NOx burners and catalytic converters
  • Selective catalytic and non-catalytic reduction (SCR, SNCR)
  • Limiting combustion temperature
  • Regulations on vehicle emissions

8.5 Sulphur Dioxide (SO₂)

SO₂ is a toxic gas formed primarily by the combustion of sulphur-containing fuels.

Sources:

  • Coal-fired power plants
  • Oil refineries
  • Volcanic eruptions
  • Industrial processes

8.5.1 Sulphur Dioxide Reactions in the Atmosphere

  • SO₂ + OH• → HOSO₂•
  • HOSO₂• + O₂ → HO₂• + SO₃
  • SO₃ + H₂O → H₂SO₄ (Sulfuric acid formation)

These reactions contribute to acid rain.

8.5.2 Effects of Sulphur Dioxide

  • Respiratory and cardiovascular problems
  • Corrosion of buildings and monuments
  • Damage to vegetation
  • Acidification of soil and water bodies

8.5.3 Removal of Sulphur Dioxide

  • Flue-gas desulfurization (scrubbers using limestone or lime)
  • Fuel desulfurization before combustion
  • Switching to cleaner energy sources (natural gas, renewables)

8.6 Ozone (O₃)

Ozone in the stratosphere protects against harmful UV radiation, while tropospheric (ground-level) ozone is a harmful pollutant.

Sources (Tropospheric):

  • Secondary pollutant formed by photochemical reactions:
    NO₂ + sunlight → NO + O
    O + O₂ → O₃

8.6.1 Harmful Effects of Ozone

  • Causes chest pain, coughing, and throat irritation
  • Reduces lung function and aggravates asthma
  • Damages crops and ecosystems
  • Component of photochemical smog

8.7 Acid Rain

Definition: Precipitation with a pH lower than 5.6, caused by atmospheric reactions of SO₂ and NOₓ forming sulfuric and nitric acids.

8.7.1 Harmful Effects of Acid Rain

  • Acidifies lakes and streams, harming aquatic life
  • Leaches nutrients from the soil
  • Damages leaves, stunts plant growth
  • Corrodes metals, stone, and building surfaces
  • Reduces forest productivity and weakens trees

Control Measures:

  • Limiting SO₂ and NOₓ emissions
  • Use of scrubbers and catalytic converters
  • International agreements like the Clean Air Act and regional acid rain programs

8.8 Let’s Sum Up

In this unit, we examined major air pollutants, including carbon monoxide, carbon dioxide, oxides of nitrogen, sulphur dioxide, ozone, and acid rain. Their sources, harmful effects, chemical behavior, and control measures were discussed. Understanding these pollutants is vital for environmental protection, public health, and policy planning.

8.9 Glossary

  • Carbon Monoxide (CO)-Toxic gas from incomplete combustion, binds with hemoglobin.
  • Carbon Dioxide (CO₂)-Greenhouse gas contributing to global warming.
  • Nitrogen Oxides (NOₓ)-Group of gases that contribute to smog and acid rain.
  • Sulphur Dioxide (SO₂)-Gas that causes respiratory issues and acid rain.
  • Ozone (O₃)-Beneficial in stratosphere; harmful pollutant at ground level.
  • Acid Rain-Rainfall with acidic pH due to SO₂ and NOₓ in atmosphere.
  • Catalytic Converter-Device in cars that reduces emission of pollutants.

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