Chemistry of the Environment: Air Quality and Climate (0620)

Hello future environmental chemist! This chapter is all about the air we breathe and how human activity is changing the delicate balance of our planet's atmosphere. Don't worry if these issues seem big—we'll break down the chemistry behind air pollution, climate change, and how we can use science to find solutions. This topic connects directly to real-world impact, so let’s get started!

1. The Composition of Clean, Dry Air (Core)

Before we talk about pollution, we need to know what "normal" air looks like. Clean, dry air is mostly made up of just two elements, with a tiny mixture of others making up the rest.

  • Nitrogen (\(\text{N}_2\)): Approximately 78%. This gas is quite unreactive and acts mostly as a diluent.
  • Oxygen (\(\text{O}_2\)): Approximately 21%. Essential for combustion, respiration, and life!
  • Remainder (Approximately 1%): This tiny fraction contains mostly noble gases (like Argon) and carbon dioxide (\(\text{CO}_2\)).

Quick Tip: Notice how Nitrogen and Oxygen make up 99% of the air!

2. Air Pollutants: Sources and Adverse Effects (Core)

An air pollutant is a substance in the air that harms people, animals, plants, or materials. We need to know where these pollutants come from and the damage they cause.

2.1 Pollutants Leading to Global Warming (Climate Change)

These gases trap heat in the atmosphere, leading to an overall increase in global temperatures, known as global warming, which causes changes in weather patterns (climate change).

A. Carbon Dioxide (\(\text{CO}_2\))

  • Source: Complete combustion of carbon-containing fuels (e.g., burning coal, oil, or gas in power stations and vehicles).
  • Adverse Effect: Higher levels of \(\text{CO}_2\) lead to increased global warming and subsequent climate change.

B. Methane (\(\text{CH}_4\))

  • Source:
    • Decomposition of vegetation (e.g., in swampy areas or landfills).
    • Waste gases from digestion in animals (especially livestock like cows).
  • Adverse Effect: Higher levels of \(\text{CH}_4\) also lead to increased global warming. Methane is a far more effective greenhouse gas than \(\text{CO}_2\), even though it stays in the atmosphere for less time.
2.2 Pollutants Leading to Acid Rain and Smog

These gases dissolve in rainwater, lowering the pH and causing severe damage to environments and infrastructure.

C. Sulfur Dioxide (\(\text{SO}_2\))

  • Source: Combustion of fossil fuels (especially coal) which naturally contain sulfur compounds. When burned, the sulfur reacts with oxygen.
  • Adverse Effect: Acid rain (damages buildings, forests, and aquatic life).

D. Oxides of Nitrogen (\(\text{NO}_x\))

  • Source: Produced primarily in car engines and power stations, where the high temperatures cause unreactive nitrogen (\(\text{N}_2\)) and oxygen (\(\text{O}_2\)) from the air to react.
  • Adverse Effects: Acid rain, photochemical smog (a hazy mixture causing poor visibility), and respiratory problems (breathing difficulties).
2.3 Immediate Health Hazards

E. Carbon Monoxide (\(\text{CO}\))

  • Source: Incomplete combustion of carbon-containing fuels. This happens when there is insufficient oxygen present for the fuel to burn completely to \(\text{CO}_2\).
  • Adverse Effect: Highly toxic gas. It is dangerous because it is colourless and odourless and binds irreversibly to haemoglobin in the blood, preventing oxygen transport.

F. Particulates (Soot/Smoke)

  • Source: Also produced by incomplete combustion of carbon-containing fuels.
  • Adverse Effect: Increased risk of respiratory problems (like asthma) and cancer. They also darken buildings and reduce visibility.

KEY TAKEAWAY: Combustion is the main culprit! If combustion is complete, we get \(\text{CO}_2\). If it's incomplete (low oxygen), we get toxic \(\text{CO}\) and particulates. If the fuel is dirty (contains sulfur or is burnt at high heat), we get \(\text{SO}_2\) and \(\text{NO}_x\).

3. Strategies to Reduce Pollution (Core & Supplement)

We can use chemistry and technology to mitigate the damage caused by these pollutants.

3.1 Strategies to Combat Climate Change (CO₂ and CH₄)
  • Planting Trees: Trees absorb \(\text{CO}_2\) for photosynthesis, reducing the amount in the atmosphere.
  • Reduce Livestock Farming: Since digestion in animals produces methane (\(\text{CH}_4\)), reducing this practice helps lower methane emissions.
  • Decreasing Fossil Fuel Use: Using less coal, oil, and gas means less \(\text{CO}_2\) is released.
  • Increasing Renewable Energy: Switching to hydrogen, wind, and solar power, which do not produce greenhouse gases.
3.2 Strategies to Combat Acid Rain (\(\text{SO}_2\) and \(\text{NO}_x\))

A. Reducing Sulfur Dioxide (\(\text{SO}_2\))

  1. Use Low-Sulfur Fuels: Choose fuels that naturally contain less sulfur.
  2. Flue Gas Desulfurisation: This is used in large power stations.
    • Sulfur dioxide is removed from the smoke ('flue gas') before it leaves the chimney.
    • This involves passing the gas through a spray of water mixed with calcium oxide (\(\text{CaO}\)) (lime) or calcium carbonate.
    • The \(\text{CaO}\) (a basic oxide) reacts with the acidic \(\text{SO}_2\) to form a harmless salt (calcium sulfite).

B. Reducing Oxides of Nitrogen (\(\text{NO}_x\)) and Carbon Monoxide (\(\text{CO}\))

This is done using a device called a catalytic converter, fitted into the exhaust systems of modern cars.

How it works:

The converter contains expensive transition metals (like Platinum and Rhodium) which act as catalysts. They speed up the reaction that converts harmful gases into harmless ones.

  • Harmful gases enter: Carbon Monoxide (\(\text{CO}\)) and Nitric Oxide (\(\text{NO}\)).
  • Catalysts facilitate the reaction:
    \(2\text{CO} + 2\text{NO} \rightarrow 2\text{CO}_2 + \text{N}_2\) (Supplement)
  • Harmless gases exit: Carbon Dioxide (\(\text{CO}_2\)) and Nitrogen (\(\text{N}_2\)).
  • (Did you know? Even though \(\text{CO}_2\) is still produced, it is much less harmful than the highly toxic \(\text{CO}\) or the smog-forming \(\text{NO}\).)

KEY TAKEAWAY: Acid gases (\(\text{SO}_2\), \(\text{NO}_x\)) can be neutralised or converted back into harmless gases (\(\text{N}_2\)) using catalysts or basic chemicals like calcium oxide.

4. Photosynthesis: Nature's Carbon Solution (Core & Supplement)

Photosynthesis is the key natural process that manages atmospheric carbon dioxide levels.

4.1 The Process and Equation

Photosynthesis is the reaction between carbon dioxide and water to produce glucose (sugar) and oxygen. This reaction requires chlorophyll (the green pigment in plants) and energy from light.

Word Equation (Core):
Carbon dioxide + water \(\rightarrow\) glucose + oxygen

Symbol Equation (Supplement):
\(6\text{CO}_2 + 6\text{H}_2\text{O} \rightarrow \text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2\)

4.2 Importance

Photosynthesis is vital because it removes \(\text{CO}_2\) from the atmosphere (helping to regulate climate) and produces the oxygen necessary for respiration.

5. Supplement Focus: Explaining the Greenhouse Effect

This section is crucial for students aiming for the extended curriculum (Grades A*-C). We need to explain how gases like \(\text{CO}_2\) and \(\text{CH}_4\) actually cause warming.

Think of the Earth as wearing a warm blanket. Greenhouse gases are the fibres of that blanket.

The greenhouse effect is a natural process, but human activity has strengthened it too much.

  1. The Earth absorbs energy (radiation) from the Sun.
  2. The Earth heats up and re-emits this energy back out as thermal energy (heat, also called infrared radiation).
  3. Greenhouse gas molecules (\(\text{CO}_2\), \(\text{CH}_4\)) in the atmosphere are special: they are able to absorb and reflect this outgoing thermal energy.
  4. They then re-emit this thermal energy in all directions, including back down towards the Earth's surface.
  5. This absorption and re-emission action means less thermal energy escapes into space, effectively reducing thermal energy loss to space.
  6. The trapped energy causes the atmosphere and the surface of the Earth to warm up, leading to global warming.

Common Mistake to Avoid: Greenhouse gases do not trap the sun's incoming light energy. They absorb the outgoing thermal energy (heat) radiated back from the Earth’s surface.

KEY TAKEAWAY (Chapter Summary): Air pollution impacts life through toxicity (\(\text{CO}\)), respiratory issues (particulates, \(\text{NO}_x\)), acid rain (\(\text{SO}_2\), \(\text{NO}_x\)), and long-term climate change (\(\text{CO}_2\), \(\text{CH}_4\)). Solutions involve cleaning up emissions (catalytic converters, desulfurisation) and shifting energy sources.