AP Syllabus focus:
‘Earth’s atmosphere contains several major gases, each present at its own relative abundance.’
Earth’s atmosphere is a mixture of gases with fairly stable proportions for the most abundant components. Knowing which gases dominate, and which occur in tiny amounts, helps explain many basic environmental patterns.
Major atmospheric gases (by volume)
“Dry air” as a reference mixture
Scientists often describe atmospheric composition using dry air (air with water vapor removed) because water vapor varies widely by place and time. In dry air, the dominant gases are:
Diagram showing the relative abundances of major atmospheric gases in dry air: nitrogen (~78%), oxygen (~21%), argon (~0.93%), and carbon dioxide (~400 ppm, ~0.04%). It visually emphasizes the steep drop from percent-level gases to trace-level gases reported in ppm. Source
Nitrogen (N₂): ~78%
Oxygen (O₂): ~21%
Argon (Ar): ~0.93%
Carbon dioxide (CO₂): ~0.04% (about 420 ppm, and changing over time)
These proportions are typically reported as percent by volume, because gases mix and behave as fluids.
Pie chart of the gaseous composition of dry air with labeled percentages (approximately 78% N₂, 21% O₂, ~0.93% Ar, and a very small CO₂ slice). This format is useful for quickly comparing relative abundance and seeing why CO₂ is often discussed in ppm despite being present in the atmosphere. Source
Water vapor: abundant but highly variable
Water vapor (H₂O) is a major atmospheric gas in real (non-dry) air, but its abundance is not constant:
Approximately 0–4% by volume in the lower atmosphere
Higher in warm, humid air; lower in cold, dry air
Because it changes so much, water vapor is usually listed separately from dry-air percentages.
Trace gases and relative abundance
Most gases besides N₂, O₂, Ar, CO₂, and H₂O are present as trace gases (very small concentrations). Even at low abundance, they can matter environmentally, but their defining feature in this topic is simply that they are scarce compared with the major components.
Common trace gases include:
Neon (Ne), helium (He), methane (CH₄), krypton (Kr), hydrogen (H₂), ozone (O₃) (highly variable), and others
A key skill is interpreting how “small” a gas concentration is by using common concentration units.
Parts per million (ppm): A concentration unit meaning 1 part of a gas per 1,000,000 parts of air (by volume for gases); used for very low-abundance atmospheric gases like CO₂.
Because ppm values are much smaller than percents, they help compare trace gases without using many decimals.
How to interpret “relative abundance”
Relative abundance is comparative, not just a list
“Relative abundance” means comparing amounts across gases in the mixture:
N₂ is the most abundant gas, so it dominates atmospheric composition.
O₂ is the second most abundant, enabling aerobic respiration and many oxidation reactions.
Ar is far less abundant than N₂ and O₂ but still makes up nearly 1% of dry air.
CO₂ is a tiny fraction of air compared with N₂ and O₂, so it is reported in ppm or hundredths of a percent.
What stays fairly constant vs what varies
At broad spatial scales, the atmosphere is well mixed for the major dry-air gases, so N₂, O₂, and Ar remain relatively stable in percentage terms. In contrast:
Water vapor varies strongly with temperature and humidity.
Several trace gases can be patchy or change over time, so their measured abundance depends on location, season, and emissions.
Why the “major gases” list looks the way it does
The atmosphere’s composition reflects both the planet’s history and ongoing processes, but for AP Environmental Science this subtopic emphasizes recognizing the major gases and their typical proportions. Students should be able to:
Identify which gases are most abundant in air
State approximate percentages for N₂, O₂, Ar, and CO₂ (dry air)
Explain why water vapor is treated as a variable component
Recognise that trace gases occur at very low concentrations, often described in ppm
FAQ
They commonly use gas chromatography and mass spectrometry on collected air samples.
Remote sensing can also infer concentrations by analysing absorbed/emitted radiation.
Most argon is $^{40}$Ar produced by radioactive decay of potassium in rocks.
It is chemically inert, so it is not easily removed by reactions.
A mixing ratio expresses the proportion of a gas in a mixture (often by volume for gases).
ppm is a mixing ratio scaled to “per million” for convenience.
Divide ppm by 10,000 to get percent.
For example, 420 ppm $\approx$ 0.042%.
Evaporation rates depend strongly on temperature and available surface water.
Cold air holds less moisture, so humidity and water vapour concentration are typically lower.
Practice Questions
State the two most abundant gases in Earth’s atmosphere and give their approximate percentages (by volume, dry air). (2 marks)
Nitrogen (N) ~78% (1)
Oxygen (O) ~21% (1)
Describe the relative abundance of the major gases in Earth’s atmosphere. Include approximate values for N, O, Ar and CO, and explain why water vapour is often treated separately from “dry air”. (5 marks)
N is the most abundant at about 78% (1)
O is about 21% (1)
Argon is about 0.93% (accept ~1%) (1)
CO is about 0.04% (accept ~400–450 ppm) and is much lower than the others (1)
Water vapour varies greatly (e.g. ~0–4%) with place/time, so “dry air” excludes it to give consistent percentages (1)
