1.1 Composition of the Atmosphere

Describe the gases and other materials that make up the atmosphere.

No other known planet has an atmosphere that is as supportive of life as Earth’s. The atmosphere (or air) is the envelope of gases that surrounds the planet. If Earth were the size of a basketball, most of the atmosphere would be only about the thickness of a single piece of paper. As Figure 1.2 shows, the atmosphere’s thinness can be seen from the vantage point of space.

Figure 1.2

Earth’s atmosphere. The edge of Earth’s atmosphere can be seen in this photo taken by astronauts aboard the International Space Station. The thin blue shell seen here edge-on is all that separates life on Earth from the inhospitable conditions of space. Although it looks like the atmosphere ends abruptly in this photo, it fades gradually with altitude.
(NASA)

Gases in the Atmosphere

There are two groups of gases in the atmosphere: permanent gases and variable gases (or trace gases). Proportions of the permanent gases fluctuate only a little. Just over 99% of the atmosphere is composed of only two permanent gases: nitrogen (N2) and oxygen (O2). Table 1.1 shows their proportions in the atmosphere and lists other permanent gases that occur in minute amounts.

Table : TABLE 1.1 AT A GLANCE: Permanent Gases

PERMANENT GAS NAME

PERCENTAGE BY VOLUME

Nitrogen (N)

78.08

Oxygen (O)

20.95

Argon (Ar)

  0.93

Neon (Ne)

  0.0018

Helium (He)

  0.0005

Hydrogen (H)

  0.00005

Xenon (Xe)

  0.000009

Variable gases exist in extremely small quantities and do change in their proportions. There are many variable gases, but only those that absorb and emit thermal energy, called greenhouse gases, are included in this discussion. Table 1.2 lists six prominent greenhouse gases and their concentrations in the atmosphere.

greenhouse gas

A gas that can absorb and emit thermal energy.

greenhouse gases

Gases that can absorb and emit thermal energy.

Table : TABLE 1.2 AT A GLANCE: Variable Gases that Act as Greenhouse Gases

VARIABLE GAS NAME

PERCENTAGE BY VOLUME

PPM*

Water vapor (H2O)

        0 to 4

 

Carbon dioxide (CO2)

        0.0399

  399

Methane (CH4)

        0.00017

      1.7

Nitrous oxide (N2O)

        0.00003

      0.3

Ozone (O3)

        0.000004

      0.04

CFCs and HFCs**

        0.00000002

      0.0002

* ppm = parts per million. For example, 398 ppm carbon dioxide means that for every 1 million molecules of air, 398 are carbon dioxide molecules.

** Chlorofluorocarbons (CFCs) and hydrofluorocarbons (HFCs). CFCs and HFCs are manufactured by people and do not form naturally.

As we will see in Section 2.5, greenhouse gases create Earth’s natural greenhouse effect. They absorb and radiate heat in the atmosphere. Without greenhouse gases, Earth’s atmosphere would be very cold.

Gas Sources and Sinks

Scientists use the terms sources and sinks to describe how gases enter and exit the atmosphere or are changed into different gases. Like water that enters a kitchen sink from the faucet (the source) and exits down the drain (the sink), gases enter the atmosphere through a source, and they leave the atmosphere through a sink. Sinks can be either physical environments (such as a lake or wetland) or processes (such as a chemical reaction), or both. Gas sources can be both natural and anthropogenic (created or influenced by people). Table 1.3 details sources and sinks for the major gases in the atmosphere.

anthropogenic

Created or influenced by people.

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Table : TABLE 1.3 Sources and Sinks for Atmospheric Gases

GAS

SOURCE(S)

SINK(S)

Nitrogen (N2)

Decaying and burning organic matter, volcanic eruptions, weathering of rocks

Enters soil and water with rain and biological activity

Oxygen (O2)

Photosynthesis

Decomposition and weathering of rocks absorb O2 from the atmosphere

Water vapor (H2O)

Evaporation from oceans, plant photosynthesis, volcanic eruptions

Condensation and deposition convert H2O to liquid and solid (ice) states

Carbon dioxide (CO2)

Volcanic eruptions, decay of living matter, respiration, humans’ burning of coal, oil, and natural gas for energy

Plants, oceans, and chemical reactions with rocks absorb CO2

Methane (CH4)

Anaerobic (oxygen-free) bacterial decomposition

Breakdown by ultraviolet radiation from the Sun

Nitrous oxide (N2O)

Soil bacterial processes, human activities

Breakdown by ultraviolet radiation from the Sun

Ozone (O3)

Ultraviolet radiation, burning fossil fuels

Breakdown by ultraviolet radiation from the Sun

CFCs and HFCs

Anthropogenic

Breakdown by ultraviolet radiation from the Sun

Aerosols in the Atmosphere

Question 1.1

What are clouds made of?

Clouds are made of microscopic droplets of liquid water and ice crystals.

Aerosols are another important component of the atmosphere. Aerosols are microscopic solid or liquid particles suspended in the atmosphere. They are not gases because they are made up of matter in a solid or liquid, rather than a gaseous, state. Clouds are composed of aerosols because they are made up of cloud droplets and tiny ice crystals. Cloud droplets are microscopic drops of liquid water found in clouds.

aerosols

Microscopic solid or liquid particles suspended in the atmosphere.

cloud droplets

Microscopic drops of liquid water found in clouds.

Other common aerosols in the atmosphere include windblown dust, pollen, spores, bacteria, emissions from human activities such as farming and industrial pollutants, salt particles from the oceans, volcanic ash, and smoke.

Solid aerosols play an important role in the atmosphere as condensation nuclei in cloud formation processes. This topic is covered further in Section 3.5.