SEC G3 Combined Science Chemistry component K326/K328
C12: Maintaining Air Quality
Connect air composition, pollutant sources and effects, the carbon cycle, greenhouse gases, and consequences.
Reviewed for the 2027 cohort on 19 July 2026. The official syllabus remains authoritative for assessable wording and paper details.
Core notes
Air quality connects atmospheric composition to pollutant sources, health and environmental effects, carbon cycling, and greenhouse-gas consequences.
Air and pollutant sources
Dry air is approximately 78% nitrogen and 21% oxygen, with the remainder mainly noble gases, especially argon, plus carbon dioxide.
Common pollutants include carbon monoxide, methane, nitrogen oxides, ozone, sulfur dioxide, and unburned hydrocarbons. Carbon monoxide comes from incomplete combustion, nitrogen oxides from lightning and engines, and sulfur dioxide from volcanoes and fossil-fuel combustion.
Health and environmental effects
Carbon monoxide is toxic because it interferes with oxygen transport. Nitrogen dioxide and sulfur dioxide can irritate the respiratory system and react in the atmosphere to form acid rain, which damages buildings and can also affect health.
Link each effect to the relevant pollutant rather than listing all effects after all pollutants with no causal connection.
Carbon cycling and greenhouse gases
Photosynthesis removes carbon dioxide from air, while respiration and combustion return it. These opposing processes help regulate atmospheric carbon dioxide within the carbon cycle.
Carbon dioxide and methane are greenhouse gases. Carbon dioxide sources include fossil-fuel combustion and deforestation; methane sources include livestock, decay in oxygen-poor conditions, and fuel systems. Higher levels of these greenhouse gases can contribute to global warming, more extreme weather, and polar-ice melting.
Formulae and relationships
This chapter is assessed mainly through models, field patterns and explanations. Build the causal chain before adding any calculation.
Worked examples
Example 1: Explain how incomplete combustion creates a health risk.
- Incomplete combustion can produce carbon monoxide.
- Carbon monoxide is toxic and reduces effective oxygen transport.
- Tissues can then receive insufficient oxygen.
Answer: Incomplete combustion can expose people to carbon monoxide and impair oxygen delivery.
Chapter checkpoint
Use these three moves to organise the topic before attempting a mixed or practical question.
- Explain Air and pollutant sources with the named terms, evidence, and causal links kept distinct.
- Use Health and environmental effects to interpret the evidence given and justify each conclusion.
- Apply Carbon cycling and greenhouse gases to a new example, then check the conclusion against the information given.
Official outcome coverage
K326 C12: 6 mapped outcomes, references C12(a), C12(b), C12(c), C12(d), C12(e), C12(f). Check the official K326 syllabus.
K328 C12: 6 mapped outcomes, references C12(a), C12(b), C12(c), C12(d), C12(e), C12(f). Check the official K328 syllabus.
The outcome wording is not reproduced here. The relevant official syllabus remains authoritative for exact assessable scope.
Practical and data connection
Interpret pollutant, combustion or carbon-cycle data by identifying the source, measured change and environmental or health consequence. Separate correlation from a supported causal mechanism and keep the dry-air composition benchmark explicit.
Exam traps and retrieval check
Avoid these traps
- Including water vapour in the stated dry-air percentages.
- Assigning sulfur dioxide mainly to incomplete combustion.
- Saying all carbon dioxide released by biofuel is automatically cancelled.
Check from memory
What is the largest component of dry air?
Nitrogen.
Which named pollutants contribute to acid rain?
Nitrogen dioxide and sulfur dioxide.
Which process removes atmospheric carbon dioxide?
Photosynthesis.
Official Combined Science scope
This shared Combined Chemistry owner serves both K326 and K328. Dry-air composition, prescribed pollutants and sources, selected effects, the carbon cycle, greenhouse gases, and global-warming consequences.