The Atmosphere » Atmosphere Composition: Gases

What you'll learn this session

Study time: 30 minutes

The main gases that make up Earth's atmosphere and their proportions
The importance of each atmospheric gas for life on Earth
How human activities are changing atmospheric composition
The difference between the troposphere and stratosphere
The significance of the ozone layer and greenhouse gases

Introduction to Earth's Atmosphere

The atmosphere is the thin layer of gases that surrounds our planet. It's like a protective blanket that shields us from harmful radiation, regulates temperature and provides the gases we need to breathe. Without our atmosphere, life as we know it wouldn't exist on Earth!

Key Definitions:

Atmosphere: The layer of gases surrounding Earth, held in place by gravity.
Atmospheric composition: The mixture of gases that make up the atmosphere.
Troposphere: The lowest layer of the atmosphere where weather occurs (0-12km altitude).
Stratosphere: The second layer of the atmosphere (12-50km) containing the ozone layer.

Composition of the Atmosphere

Our atmosphere is made up of a mixture of gases, with some being much more abundant than others. Let's look at the main components:

★ Major Atmospheric Gases

Nitrogen (N₂): 78% - The most abundant gas in our atmosphere

Oxygen (O₂): 21% - Essential for respiration in animals and humans

Argon (Ar): 0.93% - An inert (non-reactive) noble gas

Carbon Dioxide (CO₂): 0.04% (400ppm) - A greenhouse gas vital for plant photosynthesis

⊕ Trace Gases

These gases make up less than 1% of the atmosphere but are still important:

Water vapour (H₂O) - Variable amount (0-4%)
Methane (CH₄) - 1.8 ppm
Nitrous oxide (N₂O) - 0.3 ppm
Ozone (O₃) - Variable, concentrated in the stratosphere

The Role of Each Atmospheric Gas

Nitrogen (N₂)

Nitrogen is the most abundant gas in our atmosphere at 78%. Despite being all around us, most organisms can't use nitrogen directly from the air. It must be "fixed" (converted) into compounds like nitrates before plants can use it.

Dilutes oxygen, preventing rapid combustion
Essential nutrient for plants (after being converted by bacteria)
Used in the production of proteins and DNA in all living things

Oxygen (O₂)

Making up about 21% of the atmosphere, oxygen is vital for aerobic respiration in animals and humans. It's also highly reactive and supports combustion (burning).

Produced by plants during photosynthesis
Essential for cellular respiration in animals
Supports combustion of fuels

Interestingly, oxygen levels weren't always this high. About 3.5 billion years ago, there was almost no oxygen in Earth's atmosphere. It took billions of years of photosynthesis by cyanobacteria and plants to reach today's levels.

Carbon Dioxide (CO₂)

Though only making up about 0.04% (400 parts per million) of the atmosphere, carbon dioxide plays crucial roles:

Essential for photosynthesis in plants
Acts as a greenhouse gas, helping to warm the planet
Part of the carbon cycle that moves carbon between the atmosphere, oceans, soil and living things

CO₂ levels have increased by over 45% since the Industrial Revolution (from about 280ppm to over 410ppm today), primarily due to human activities like burning fossil fuels and deforestation.

♦ Water Vapour

Variable amount (0-4%)

Forms clouds and precipitation
Most powerful greenhouse gas
Essential for the water cycle

♦ Methane

About 1.8 parts per million

Powerful greenhouse gas (28× stronger than CO₂)
Released from wetlands, livestock, rice paddies
Released during fossil fuel extraction

♦ Ozone

Concentrated in the stratosphere

Absorbs harmful UV radiation
Protects life on Earth from DNA damage
At ground level, it's a pollutant

Layers of the Atmosphere

The atmosphere isn't uniform from the ground up. It's divided into several layers, each with different properties:

↓ Troposphere

The layer we live in (0-12km altitude)

Contains 75% of atmospheric mass
Temperature decreases with height (~6.5°C per km)
Where all weather occurs
Contains most water vapour and clouds

↑ Stratosphere

Above the troposphere (12-50km altitude)

Contains the ozone layer
Temperature increases with height
Very dry with few clouds
Where commercial aircraft typically fly

The Ozone Layer

The ozone layer is a region in the stratosphere where ozone (O₃) is concentrated. It's our planet's natural sunscreen!

Ozone Layer Protection

The ozone layer absorbs 97-99% of the sun's high-frequency ultraviolet (UV) radiation. Without it, life on land would suffer from:

Increased skin cancer rates
More eye cataracts
Damaged immune systems
Reduced crop yields
Disruption to marine ecosystems

The Ozone Hole

In the 1980s, scientists discovered that the ozone layer was thinning, especially over Antarctica. This "ozone hole" was caused by chemicals called chlorofluorocarbons (CFCs) that were used in refrigerators, air conditioners and aerosol sprays.

When CFCs reach the stratosphere, ultraviolet light breaks them down, releasing chlorine atoms that destroy ozone molecules. One chlorine atom can destroy over 100,000 ozone molecules!

Case Study: The Montreal Protocol

In 1987, countries around the world signed the Montreal Protocol, agreeing to phase out ozone-depleting substances. It's one of the most successful international environmental agreements ever:

CFC production ended in developed countries by 1996
The ozone layer is slowly recovering
Full recovery is expected by 2050-2070
Shows how global cooperation can solve environmental problems

Greenhouse Gases and Climate Change

Some atmospheric gases trap heat from the sun, creating a natural "greenhouse effect" that keeps Earth warm enough for life. The main greenhouse gases are:

Water vapour (H₂O) - The most abundant greenhouse gas
Carbon dioxide (CO₂) - Levels increasing due to human activities
Methane (CH₄) - 28 times more powerful than CO₂ at trapping heat
Nitrous oxide (N₂O) - 265 times more powerful than CO₂

Human activities are increasing the concentration of greenhouse gases, enhancing the greenhouse effect and causing global warming:

! Human Sources of Greenhouse Gases

Burning fossil fuels (coal, oil, gas) for electricity, transport and industry
Deforestation - reduces CO₂ absorption and releases stored carbon
Agriculture - livestock produce methane; fertilisers release nitrous oxide
Industrial processes - cement production, chemical manufacturing

✓ Reducing Greenhouse Gas Emissions

Switching to renewable energy (solar, wind, hydro)
Improving energy efficiency in buildings and transport
Protecting and restoring forests
Developing sustainable farming practices
Reducing waste and consumption

Air Quality and Pollution

Besides the main atmospheric gases, our air can contain pollutants that harm human health and the environment:

Particulate matter (PM) - Tiny particles from vehicles, industry and fires
Nitrogen oxides (NO_x) - From vehicle exhausts and power plants
Sulphur dioxide (SO₂) - From burning coal and oil
Ground-level ozone - Forms when pollutants react in sunlight
Carbon monoxide (CO) - From incomplete combustion of fuels

Air Pollution Facts

The World Health Organization estimates that:

9 out of 10 people breathe air with high levels of pollutants
Air pollution causes about 7 million premature deaths annually
91% of these deaths occur in low and middle-income countries
Air pollution is linked to heart disease, stroke, lung cancer and respiratory infections

Summary: Why Atmospheric Composition Matters

The composition of our atmosphere is crucial for life on Earth. It provides the oxygen we breathe, protects us from harmful radiation, regulates temperature and supports the water cycle. Human activities are changing this composition, with potentially serious consequences for our climate and health.

Understanding atmospheric gases helps us appreciate the delicate balance that sustains life on our planet and the importance of protecting our atmosphere for future generations.

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