Earth's Atmosphere
1. Composition of the Atmosphere
Learning outcomes
- I can identify the major gases that make up Earth's atmosphere.
- I can compare the relative abundances of atmospheric gases.
- I can describe the roles of nitrogen, oxygen, and trace gases.
- I can explain how the atmosphere supports life on Earth.
- I can analyze changes in atmospheric composition over time.
Introduction
Earth's atmosphere is a thin blanket of gases that surrounds our planet. Although it extends hundreds of kilometres into space, most of its mass is concentrated within the lowest 15–20 km above Earth's surface. Without this protective layer, life as we know it could not exist.
The atmosphere provides the oxygen we breathe, the carbon dioxide plants need for photosynthesis, and the nitrogen required for the growth of living organisms. It also protects Earth from harmful ultraviolet (UV) radiation, burns up many incoming meteoroids, and helps regulate global temperatures by trapping some of the Sun's heat.
What Is the Atmosphere?
The atmosphere is the mixture of gases surrounding Earth that is held in place by gravity.
Although it may appear invisible, the atmosphere:
- has mass
- exerts pressure
- contains water vapour and tiny particles
- constantly moves through winds and weather systems
- changes naturally over time
The atmosphere is essential for maintaining conditions that allow plants, animals, and humans to survive.
Composition of Earth's Atmosphere
The atmosphere is made up primarily of four gases.
| Gas | Approximate Percentage |
|---|---|
| Nitrogen (N₂) | 78% |
| Oxygen (O₂) | 21% |
| Argon (Ar) | 0.93% |
| Carbon dioxide (CO₂). | 0.04% |
| Other trace gases | Less than 0.03% |
Although carbon dioxide makes up only a tiny fraction of the atmosphere, it plays an important role in Earth's climate.
The Major Atmospheric Gases
Nitrogen (N₂)
Nitrogen is the most abundant gas in Earth's atmosphere.
Properties
- Colourless and odourless
- Relatively unreactive
- Exists as molecules containing two nitrogen atoms (N₂)
Why Is Nitrogen Important?
Although most organisms cannot use nitrogen gas directly, it is essential because it is needed to produce:
- proteins
- DNA
- RNA
- chlorophyll in plants
Nitrogen must first be converted into usable compounds such as nitrates or ammonium through the nitrogen cycle.
Oxygen (O₂)
Oxygen is the second most abundant gas.
Roles of Oxygen
- Needed for respiration in most living organisms.
- Supports combustion (burning).
- Dissolves in lakes and oceans, allowing aquatic life to survive.
- Forms ozone (O₃) in the upper atmosphere, which protects Earth from harmful ultraviolet radiation.
Without oxygen, complex animal life would not exist.
Argon (Ar)
Argon is a noble gas.
It is:
- colourless
- odourless
- chemically unreactive
Because it rarely reacts with other substances, argon plays only a small role in biological or chemical processes.
Argon is commonly used in:
- welding
- fluorescent lighting
- insulating windows
Carbon Dioxide (CO₂)
Carbon dioxide makes up only about 0.04% of the atmosphere, yet it is one of the most important gases.
Roles of Carbon Dioxide
- Used by plants during photosynthesis.
- Part of the carbon cycle.
- Helps regulate Earth's temperature through the greenhouse effect.
- Produced during respiration and combustion.
Although carbon dioxide is essential, excessive amounts contribute to global climate change.
Trace Gases
Several other gases are present in very small amounts.
Examples include:
- Neon (Ne)
- Helium (He)
- Krypton (Kr)
- Xenon (Xe)
- Methane (CH₄)
- Ozone (O₃)
- Hydrogen (H₂)
Even though these gases are present in tiny concentrations, some have very important effects.
For example:
- methane is a powerful greenhouse gas,
- ozone protects life from UV radiation,
- water vapour strongly influences weather and climate.
Water Vapour
Unlike most atmospheric gases, water vapour changes constantly.
Its concentration varies from almost 0% in very cold, dry regions to about 4% in warm, humid tropical areas.
Water vapour is responsible for:
- cloud formation
- rain
- snow
- storms
- humidity
It is also the most abundant natural greenhouse gas.
How the Atmosphere Supports Life
Earth's atmosphere performs many essential functions.
It Provides Essential Gases
Living organisms depend on atmospheric gases.
| Gas | Importance |
|---|---|
| Oxygen | Respiration |
| Carbon dioxide. | Photosynthesis |
| Nitrogen | Growth through the nitrogen cycle |
| Water vapour | Weather and freshwater supply |
It Regulates Temperature
Certain gases trap some outgoing infrared radiation.
This greenhouse effect keeps Earth's average temperature around 15°C.
Without the natural greenhouse effect, Earth's average temperature would be about –18°C, making the planet far less suitable for life.
It Protects Earth
The atmosphere also protects us by:
- absorbing harmful ultraviolet radiation
- burning up many meteoroids before they reach the surface
- reducing extreme temperature differences between day and night
Changes in Atmospheric Composition Over Time
Earth's atmosphere has changed dramatically over billions of years.
Early Earth (about 4.5 billion years ago)
Scientists believe the early atmosphere contained:
- large amounts of carbon dioxide
- water vapour
- methane
- ammonia
There was very little oxygen.
Volcanic eruptions released many of these gases into the atmosphere.
The Rise of Oxygen
Around 2.4 billion years ago, photosynthetic microorganisms called cyanobacteria began producing oxygen.
Over millions of years:
- oxygen levels increased,
- carbon dioxide levels decreased,
- complex life became possible.
This event is known as the Great Oxygenation Event.
Modern Changes
Today, human activities are changing atmospheric composition.
Major causes include:
- burning fossil fuels
- deforestation
- industrial processes
- agriculture
These activities have increased concentrations of greenhouse gases such as:
- carbon dioxide
- methane
- nitrous oxide
These changes contribute to global climate change.
Worked Example
A student analyses a sample of dry air and finds:
- 78% nitrogen
- 21% oxygen
- 1% other gases
Question
Which gas is most abundant?
Solution
Compare the percentages.
Nitrogen = 78%
Oxygen = 21%
Other gases = 1%
Since 78% is the highest value, nitrogen is the most abundant gas.
Real-World Connection
Scientists continuously monitor atmospheric composition using satellites, weather balloons, and ground-based monitoring stations. Measuring changes in gases such as carbon dioxide, methane, and ozone helps researchers understand air quality, predict climate trends, and develop policies to reduce pollution. These measurements also help governments issue health warnings during periods of poor air quality caused by wildfire smoke or industrial emissions.
Did You Know?
Although oxygen is essential for life today, it was once considered a toxic gas for many early microorganisms. When oxygen levels began to rise during the Great Oxygenation Event about 2.4 billion years ago, many oxygen-intolerant organisms became extinct, while others evolved to use oxygen for respiration. This transformation paved the way for the evolution of complex plants and animals.
Key Terms
- Atmosphere — the layer of gases surrounding Earth.
- Nitrogen (N₂) — the most abundant atmospheric gas.
- Oxygen (O₂) — the gas required for respiration and combustion.
- Argon (Ar) — an unreactive noble gas found in the atmosphere.
- Carbon dioxide (CO₂) — a greenhouse gas used in photosynthesis.
- Trace gases — gases present in very small amounts but often with important environmental roles.
- Water vapour — gaseous water that influences weather and climate.
- Greenhouse effect — the natural warming of Earth caused by certain atmospheric gases trapping heat.
Key Takeaways
- Earth's atmosphere is composed mainly of 78% nitrogen and 21% oxygen, with small amounts of argon, carbon dioxide, and other trace gases.
- Nitrogen supports life through the nitrogen cycle, while oxygen is essential for respiration and combustion.
- Carbon dioxide and water vapour are vital greenhouse gases that help regulate Earth's temperature.
- The atmosphere protects Earth from harmful radiation, supports weather systems, and provides the gases needed for life.
- Atmospheric composition has changed significantly throughout Earth's history and continues to change today due to both natural processes and human activities.