Atmospheric Composition

What is the atmosphere made of?

The atmosphere is composed of air: a mixture of discrete gases, but blended so thoroughly it behaves like a single gas. Air is naturally odorless, colorless, tasteless, and formless. Some components are more stable (constant gases), while others vary across space and/or time (variable gases).

Constant Gases

Constant Gases are the most stable components of air. These are gases that have little to no variation over time or space – hence the term ‘constant’. Three constant gases make up just under 100% of the atmosphere:

1. Nitrogen (N): ~78%
2. Oxygen (O₂): ~21%
3. Argon (AR): ~0.9%

Nitrogen and Oxygen are the most common gases making up the atmosphere. Other gases such as neon, helium, kryton, and xenon are also present, but in very small quantities.

The vast majority of Earth's air is made up of Nitrogen (78.08%) and Oxygen (20.95%).

Variable Gases

Variable Gases exist in small, variable amounts across time and space. In other words, you will find different amounts of these gases at different locations and at different times. These gases are very important for weather and life systems on Earth.

There are four main variable gases:

1. Water vapor (H₂O) is the source material for cloud formation and precipitation, thus playing a very important role in weather systems. Water vapor concentrations range from about 4% by volume in tropical areas to less than 1% in some deserts.
2. Carbon Dioxide (CO₂) occurs naturally in the atmosphere from volcanic eruptions, plant respiration (i.e. plants ‘breathing’), wildfires, and the exchange of dissolved CO₂ with oceans. CO₂ is vital for plant photosynthesis, and thus for the foundations of most food webs on Earth.
3. Methane (CH₄) occurs naturally in the atmosphere from the anaerobic (meaning low/no oxygen) decay of organic material in wetlands and bogs, fermentation in the digestive tracts of ruminant animals (i.e. cow farts), oceans, and termites. (Termites are responsible for at least 11% of atmospheric methane, and perhaps as high as 40% by some estimates!)
4. Ozone (O₃) is concentrated in the stratosphere, one of Earth’s atmospheric layers. It not only helps reduce incoming ultraviolet radiation, but also helps regulate surface temperatures on Earth. However, when present in the troposphere, ozone can be an irritant and damage plants.

These gases all have different properties and react with incoming solar energy (insolation) in different ways.