Section 1.4:
Composition of the Atmosphere

Learning Objective

List the major gases composing Earth’s atmosphere and identify the components that are most important meteorologically.

Section Content

Mini-Lecture Video - Composition of the Atmosphere (Click to watch the video)

Sometimes the term air is used as if it were a specific gas, but it is not. Rather, air is a mixture of many discrete gases, each with its own physical properties, in which varying quantities of tiny solid and liquid particles are suspended. The composition of air is not constant; it varies from time to time and from place to place (Box 1.1). If the water vapor, dust, and other variable components were removed from the atmosphere, we would find that its makeup is very stable up to an altitude of about 80 kilometers (50 miles).

Box 1.1

Origin and Evolution of Earth’s Atmosphere

The air we breathe is a stable mixture of mainly nitrogen and oxygen along with small amounts of other gases, including argon, carbon dioxide, and water vapor. However, our planet’s original atmosphere 4.6 billion years ago was substantially different.

Earth’s Primitive Atmosphere

Early in Earth’s formation, the planet’s atmosphere likely consisted of gases most common in the early solar system: hydrogen, helium, methane, ammonia, carbon dioxide, and water vapor. The lightest of these gases, hydrogen and helium, escaped into space because Earth’s gravity was too weak to hold them. Most of the remaining gases were probably largely scattered into space by strong solar winds (vast streams of particles) from the young active Sun.

Earth’s first enduring atmosphere was generated by a process called outgassing, through which gases trapped in the planet’s interior are released. Outgassing from hundreds of active volcanoes remains an important planetary function worldwide (Figure 1.A). However, early in Earth’s history, when the planet’s interior experienced massive heating and fluid like motion, the gas output must have been immense. Our understanding of modern volcanic eruptions indicates that Earth’s early atmosphere probably consisted of mostly water vapor, carbon dioxide, and sulfur dioxide, with minor amounts of other gases and minimal nitrogen.

Figure 1.A

Earth’s first enduring atmosphere was formed by a process called outgassing, which continues today, from hundreds of active volcanoes worldwide.

Video - The Influence of Volcanic Ash

Equally important, molecular oxygen (O2) was not present in Earth’s atmosphere in appreciable amounts for at least the first 2 billion years of Earth history. Molecular oxygen is often called “free oxygen” because it consists of oxygen atoms that are not bound to other elements, such as hydrogen (in water molecules, H2O) or carbon (in carbon dioxide, CO2).

Oxygen in the Atmosphere

As Earth’s surface cooled, water vapor condensed to form clouds, and torrential rains began to fill low-lying areas that eventually became the oceans. In those oceans, nearly 3.5 billion years ago, primitive bacteria known as cyanobacteria (once called blue-green algae) developed the ability to carry out photosynthesis and began to release oxygen into the water. Photosynthesis is the production of energy-rich molecules of sugar from molecules of carbon dioxide (CO2) and water (H2O), using sunlight as the energy source. The sugars (glucose and other sugars) generated by photosynthesis are used in metabolic processes by living things, and the by-product of photosynthesis is molecular oxygen.

Initially, the newly released oxygen was readily consumed by chemical reactions with other atoms and molecules (particularly iron) in the ocean. Once the available iron satisfied its need for oxygen and as the number of oxygen-generating organisms increased, oxygen molecules began to build up in the atmosphere. Chemical analyses of rocks suggest that a significant amount of oxygen appeared in the atmosphere as early as 2.3 billion years ago. During the following billion years, oxygen levels in the atmosphere probably fluctuated but remained below current levels. Then, roughly 550 million years ago, the level of free oxygen in the atmosphere began to increase once again. The availability of abundant oxygen in the atmosphere contributed to the proliferation of aerobic life-forms (oxygen-consuming organisms).

Another significant benefit of this “oxygen explosion” is that oxygen molecules (O2) readily absorb ultraviolet radiation and rearrange themselves to form ozone (O3). Today, ozone is concentrated above the surface in a layer called the stratosphere, where it absorbs much of the Sun’s ultraviolet radiation that strikes the upper atmosphere. For the first time, Earth’s surface was protected from this type of solar radiation, which is particularly harmful to DNA. Marine organisms had always been shielded from ultraviolet radiation by the oceans, but the development of the atmosphere’s protective ozone layer made the continents more hospitable.

Nonvariable Components

As you can see in Figure 1.15, two gases—nitrogen and oxygen—make up about 99 percent of the volume of clean, dry air. Although these gases are the most plentiful components of the atmosphere and are of great significance to life on Earth, they are of little or no importance in affecting weather phenomena. The remaining 1 percent of dry air is mostly the inert gas argon (0.93 percent) plus tiny quantities of other gases listed in Figure 1.15.

Figure 1.15
Composition of the atmosphere

Proportional volume of gases composing dry air. Nitrogen and oxygen obviously dominate.

Variable Components

Many of the gases and particles that make up air vary significantly from time to time and place to place. Important examples include carbon dioxide, water vapor, aerosols, and ozone. Although usually present in small percentages, they can significantly affect weather and climate.

Carbon Dioxide

Carbon dioxide, a gas present in only minute amounts (0.0400 percent, or 400 parts per million [ppm]), is nevertheless an important constituent of air. Carbon dioxide is of great interest to meteorologists because it is an efficient absorber of energy and thus influences the heating of the atmosphere. Although the proportion of carbon dioxide in the atmosphere is relatively uniform from place to place and at different heights in the atmosphere, its percentage has been rising steadily for more than a century. Figure 1.16 is a graph that shows the growth in atmospheric CO2 since 1958. Much of this rise is attributed to the burning of ever-increasing quantities of fossil fuels, such as coal and oil. Some of this additional carbon dioxide is absorbed by the ocean or is used by plants, but more than 40 percent remains in the air. Estimates project that by sometime in the second half of the twenty-first century, atmospheric carbon dioxide will be twice as high as pre-industrial levels.

Figure 1.16
Monthly CO2 concentrations

Atmospheric CO2 has been measured at Mauna Loa Observatory, Hawaii, since 1958. There has been a consistent increase since monitoring began.

Lecture Tutorial - The Mauna Loa Carbon Dioxide Record (Click to watch the video)

Most atmospheric scientists agree that increased carbon dioxide concentrations have contributed to a warming of Earth’s atmosphere over the past several decades and will continue to do so in the decades to come. The magnitude of such temperature changes is uncertain and depends partly on the quantities of CO2 contributed by human activities in the years ahead. The role of carbon dioxide in the atmosphere and its possible effects on climate are examined in more detail in Chapters 2 and 14.

Water Vapor

You are probably familiar with the term humidity from watching weather reports on TV. Humidity refers to the amount of water vapor in the air. As you will learn in Chapter 4, there are several ways to express humidity. The amount of water vapor in the air varies considerably, from practically none to up to about 4 percent by volume. Why is such a small fraction of the atmosphere so significant? The fact that water vapor is the source of all clouds and precipitation would be enough to explain its importance. However, water vapor has other roles. Like carbon dioxide, water vapor absorbs heat given off by Earth as well as some solar energy. It is therefore important when we examine the heating of the atmosphere and the movement of energy on Earth.

When water changes from one state to another (see Figure 4.3), it absorbs or releases heat. This energy is termed latent heat, which means “hidden heat.” As we shall see in later chapters, water vapor in the atmosphere transports this latent heat from one region to another, and it is the energy source that helps drive many storms.


The movements of the atmosphere are sufficient to keep a large quantity of solid and liquid particles suspended within it. These tiny solid and liquid particles are collectively called aerosols. Although visible dust sometimes obscures the sky, these relatively large particles are too heavy to stay in the air very long. However, many particles are microscopic and remain suspended for considerable periods of time. They may originate from many sources, both natural and human made, and include sea salts from breaking waves, fine soil blown into the air, smoke and soot from fires, pollen and microorganisms lifted by the wind, ash and dust from volcanic eruptions, and more (Figure 1.17).

Figure 1.17

A. The satellite image shows two examples of aerosols. First, a large dust storm is blowing across northeastern China toward the Korean Peninsula. Second, a dense haze toward the south (bottom center) is human-generated air pollution. B. As the photo on the right shows, dust in the air can cause sunsets to be especially colorful.

Aerosols are most numerous in the lower atmosphere near their primary source, Earth’s surface. Nevertheless, the upper atmosphere is not free of them: Some particles are carried to great heights by rising currents of air, while others are contributed by meteoroids that disintegrate as they pass through the atmosphere.

From a meteorological standpoint, these tiny, often invisible particles are important. First, many act as surfaces on which water vapor may condense, a critical function in the formation of clouds and fog. Second, aerosols can absorb or reflect incoming solar radiation. Thus, when an air pollution episode is occurring or when ash fills the sky following a volcanic eruption, the amount of sunlight reaching Earth’s surface can be measurably reduced. Finally, aerosols contribute to an optical phenomenon we have all observed—the varied hues of red and orange at sunrise and sunset. The photo on the right in Figure 1.17 illustrates this phenomenon.


Another important component of the atmosphere is ozone. It is a form of oxygen that contains three oxygen atoms in each molecule (O3), unlike the oxygen we breathe, which has two atoms per molecule (O2). There is very little ozone in the atmosphere; overall, it accounts for just 3 out of every 10 million molecules. Moreover, its distribution is not uniform. It is concentrated in a layer called the stratosphere, between 10 and 50 kilometers (6 and 31 miles) above the Earth’s surface.

In this altitude range, oxygen molecules (O2) are split into single atoms of oxygen (O) when they absorb ultraviolet radiation emitted by the Sun. Ozone is then created when a single atom of oxygen (O) and a molecule of oxygen (O2) collide. This must happen in the presence of a third, neutral molecule that acts as a catalyst by allowing the reaction to take place without itself being consumed in the process. Ozone is concentrated in the 10- to 50-kilometer height range because a crucial balance exists there: The ultraviolet radiation from the Sun is sufficient to produce single atoms of oxygen, and enough gas molecules are present to bring about the required collisions.

The presence of this ozone layer in our atmosphere is essential to those of us who are land dwellers. The reason is that ozone absorbs much of the potentially harmful ultraviolet (UV) radiation from the Sun. If ozone did not filter a great deal of the ultraviolet radiation, land areas on our planet would be uninhabitable for most life as we know it. Thus, anything that reduces the amount of ozone in the atmosphere could affect the well-being of life on Earth. Just such a problem is described in Box 1.2.

Box 1.2

Ozone Depletion: A Global Issue

Although stratospheric ozone is concentrated high above Earth’s surface, it is vulnerable to human activities. Manufactured chemicals break up ozone molecules in the stratosphere, weakening our shield against UV rays. Measurements over the past three decades confirm that ozone depletion is occurring worldwide and is especially pronounced above Earth’s poles. Figure 1.B shows this effect over the South Pole.

Figure 1.B
Antarctic ozone hole

The two satellite images show ozone distribution in the Southern Hemisphere on the days in September 1979 and 2016 when the ozone hole was largest. The purple and blue colors are where there is the least ozone, and the yellows and reds are where there is more ozone.

Tutorial Video - The Ozone Hole (Click to watch the video)

Video - The Ozone Hole (Click to watch the video)

Over the past 80 years, people have unintentionally placed the ozone layer in jeopardy by polluting the atmosphere. The most significant of the offending chemicals are known as chlorofluorocarbons (CFCs). Developed in the 1930s, CFCs were used as coolants for air-conditioning and refrigeration equipment, cleaning solvents, and propellants for aerosol sprays.

Because CFCs are practically inert (not chemically active) in the lower atmosphere, some of these gases gradually make their way up to the ozone layer, where sunlight separates the CFCs into their constituent atoms. The release of a single chlorine atom, which acts as a catalyst, can be responsible for destroying thousands of ozone molecules.

Because ozone filters out most of the UV radiation from the Sun, a decrease in atmospheric ozone permits more of these harmful wavelengths to reach Earth’s surface. UV radiation’s most serious threat to human health is an increased risk of skin cancer. Increased UV radiation can also impair the human immune system and promote cataracts, a clouding of the eye lens that reduces vision and may cause blindness if not treated.

In response to this problem, an international agreement known as the Montreal Protocol was developed in 1987 to eliminate the production and use of CFCs. More than 190 nations eventually ratified the treaty. Although relatively strong action has been taken, CFC levels in the atmosphere will not drop rapidly. Once CFC molecules are in the atmosphere, they can take many years to reach the ozone layer, and once there, they can remain active for decades. This does not promise a near-term reprieve for the ozone layer. Nevertheless, the Montreal Protocol represents a positive international response to solve this global problem.

You might have wondered . . . 

Isn’t ozone some sort of pollutant?

Although the naturally occurring ozone in the stratosphere is critical to life on Earth, it is considered a pollutant when produced at ground level because it can damage vegetation and harm human health. Ozone is a major component in a noxious mixture of gases and particles called photochemical smog formed from pollutants emitted by motor vehicles and industries.

Section Glossary

Section Summary

Section Study Questions

Try to answer the following questions on your own, then click the question to see the correct answer.

What are the two major components of clean, dry air? What proportion does each represent?

The two major components of clean, dry air are nitrogen (78 percent) and oxygen (20.9 percent).

Why is carbon dioxide an important component of Earth’s atmosphere? Why are water vapor and aerosols important atmospheric constituents?

Carbon dioxide is an efficient absorber of energy emitted by Earth and thus influences the heating of the atmosphere. Water vapor is the source of all clouds and precipitation, also exerts a strong influence upon energy transfer through the atmosphere, and finally plays an important role in transferring heat from one place to another because of the heat absorbed and released during changes of state (termed latent heat). Latent heat also provides some of the energy to drive storms. Aerosols can act as surfaces upon which water vapor condenses, influence the amount of sunlight reaching the lower atmosphere by intercepting and reflecting some incoming solar energy, and contribute to optical phenomena such as an orange or red sunset.

What is ozone? Why is ozone important to life on Earth?

Ozone is a form of oxygen that combines three oxygen atoms into each molecule (O3). Ozone is very important to life on Earth because it absorbs damaging ultraviolet radiation from the Sun. If ozone were not present, our planet would be uninhabitable for most life as we know it.