Air quality and climate

Section: Chemistry | Syllabus: Cambridge IGCSE Combined Science (0653)

Composition of Clean, Dry Air

Gas	Approximate percentage
Nitrogen, N₂	78%
Oxygen, O₂	21%
Noble gases (mainly argon) and carbon dioxide, CO₂	remainder (~1%)

Pollutant	Source
Carbon dioxide, CO₂	complete combustion of carbon-containing fuels
Carbon monoxide, CO, and particulates	incomplete combustion of carbon-containing fuels
Methane, CH₄	decomposition of vegetation, and waste gases from digestion in animals
Oxides of nitrogen	car engines
Sulfur dioxide, SO₂	combustion of fossil fuels which contain sulfur compounds

Question: which equation describes the formation of two atmospheric pollutants that are not present in clean air - one producing only CO₂ and H₂O, one producing CO₂, CO and H₂O, one producing CO₂, C and H₂O, or one producing only CO, C and H₂O?
- Answer: the equation producing only CO, C and H₂O (no CO₂ at all). CO₂ is already a normal part of clean air, so it doesn't count as a pollutant "not present in clean air" - only CO (carbon monoxide) and C (carbon/soot particulates) qualify, and this is the only option that produces both without also producing CO₂.

❌ Treating carbon dioxide as if it isn't naturally present in clean air
- ✓ CO₂ already makes up about 0.04% of clean, dry air - the problem is the increased level caused by burning fuels, not its mere presence

Pollutant	Adverse effect
Carbon dioxide	increased global warming, leading to climate change
Carbon monoxide	toxic gas
Particulates	increased risk of respiratory problems and cancer
Methane	increased global warming, leading to climate change
Oxides of nitrogen	acid rain and respiratory problems
Sulfur dioxide	acid rain

The greenhouse gases carbon dioxide and methane cause global warming through the absorption, reflection and emission of thermal energy:

Thermal energy is emitted from the Earth's surface (as it re-radiates the energy it absorbed from the Sun).
Greenhouse gases in the atmosphere absorb this thermal energy.
Greenhouse gases then emit thermal energy back towards the Earth.
The result is reduced thermal energy loss to space - energy that would otherwise escape is instead sent back down, so the Earth warms up.

Question (based on a real exam diagram): a diagram shows "emission of thermal energy from the Earth" → X → Y → Z, with a feedback arrow from Z back to X. Identify X, Y and Z.
- Answer: X = absorption of thermal energy by greenhouse gases; Y = emission of thermal energy to the Earth; Z = reduced thermal energy loss to space.

❌ Thinking greenhouse gases cause warming by emitting thermal energy into space, or that the result is increased energy loss to space
- ✓ it's the opposite - greenhouse gases absorb outgoing thermal energy and emit it back towards the Earth, so less energy escapes to space; that trapped energy is what causes warming

Planting trees (they absorb CO₂ during photosynthesis)
Reduction in livestock farming (livestock digestion is a major source of methane)
Decreasing use of fossil fuels
Increasing use of renewable energy, e.g. wind and solar power

Question (based on a real exam question): which row shows one way to increase and one way to decrease the amount of carbon dioxide in the air - options included "wind generation of electricity" / "using solar power", and "using diesel vehicles" / "planting trees"?
- Answer: using diesel vehicles (increases CO₂ - burning diesel is combustion of a fossil fuel) / planting trees (decreases CO₂ - absorbed during photosynthesis). The wind/solar row is wrong because both of those methods decrease CO₂ - neither increases it.

Reducing emissions of sulfur dioxide by using low-sulfur fuels.

❌ Picking a strategy such as "wind power" or "solar power" as something that increases CO₂
- ✓ renewable energy sources like wind and solar don't involve combustion, so they only ever decrease (or have no effect on) CO₂ emissions - they can never be the "increases" side of a comparison

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