Composition of the atmosphere

The Earth's atmosphere is mostly composed of a mixture of gases with very tiny quantities of aerosols, which are solid or liquid particles suspended in the air. You should understand that most of the gases in the atmosphere are nitrogen and oxygen. These will be referred to as the major components of the atmosphere. You should also realize that more than half of Earth's atmosphere is composed of nitrogen (roughly 78%) with comparatively less oxygen (about 21% of the atmosphere). The remaining gases in Earth's atmosphere are called trace gases because these gases make up a very small percentage of the total. By far the most abundant of these trace gases is argon (close to 1% of the total). Even though a small percentage of the total atmosphere, there are hundreds of trace gases in Earth's atmosphere, and some of them are absolutely essential for life as we know it. Some of the trace gases are listed in the table below. Notice in the pie chart that argon, carbon dioxide, and all the other trace gases, except water vapor, make up a very small slice. In fact the concentrations on the right side of the table are given in units of parts per million (ppm), where each ppm means one out of every 1 million molecules. For example, the current concentration of carbon dioxide is about 400 ppm, which means that for every 1,000,000 gas molecules only 400 of them are carbon dioxide.

The table below breaks up gases into permanent and variable categories as well. This is to indicate that the concentrations variable gases change with time. The most variable of these is water vapor, which is the gas form of water (literally molecules of H₂O moving around with the rest of the gases in the atmosphere). Water vapor only averages about 0.4% of the total gases, however, it varies quite a lot depending on time and place, and can be over 2% of the total under warm, humid conditions. Obviously in places where water vapor is high, the percentage of all other gases must go down some. This is indicated in the pie chart below. You should be familar with some of the features of the important trace gases described in the text below.

Composition of the Atmosphere near the Earth's Surface Permanent Gases Variable Gases Gas Name Chemical Formula Percent (by Volume) Dry Air Gas (and Particles) Symbol Percent (by Volume) Parts per Million (ppm)* Nitrogen N2 78.08 Water Vapor H2O 0 to 4   Oxygen O2 20.95 Carbon Dioxide CO2 0.0400 400 Argon Ar 0.93 Methane CH4 0.00017 1.7 Neon Ne 0.0018 Nitrous Oxide N2O 0.00003 0.3 Helium He 0.0005 Ozone O3 0.000004 0.04 Hydrogen H2 0.00005 Particles (dust, soot, etc.)   0.00001 0.01-0.15 Xenon Xe 0.000009 Chlorofluorocarbons (CFCs)   0.00000002 0.0002

Pie chart showing percentage concentatrations of gases in Earth's atmosphere. Water vapor is shown as a slice that can be up to 2% of the total. The concentration of water vapor is highly variable and ranges from near 0% to over 2%. Averaged throughout the entire atmosphere, water vapor makes up about 0.4% of the total.

Major Components of the Atmosphere

The Atmosphere has two main components: nitrogen(78%) and oxygen(21%). These make up 99% of the volume of "dry air". In this context "dry air" refers to all gases, except water vapor. Remember that even under the most humid conditions on Earth, water vapor is at most 2% of the atmosphere. Thus, if you were an alien studying the planet earth, you would report that Earth's atmosphere is mostly nitrogen and oxygen. The text below mentions the main ways that nitrogen and oxygen gas are removed from the atmosphere and enter the atmosphere as part of chemical cycles.

Nitrogen:

Removed from atmosphere by biological processes that involve soil bacteria. Returned to the atmosphere through the decaying of plant and animal matter.

Oxygen:

Removed from atmosphere by when organic matter decays, combines with other substances, or is taken in during breathing. Is added to the atmosphere through photosynthesis by plants.

Some Important Trace Components of the Atmosphere

Trace gases by definition are scarce in Earth's atmosphere. Yet several of these trace gases are essential for the life that has developed on Earth.

Water Vapor:

• The gas phase of water. Water vapor is literally individual molecules of H₂O that are part of the collection of gases in the atmosphere.
• Varies greatly from place to place, and from time to time. It averages only about 0.4% of the atmosphere, but varies from as much as 4% in the humid tropics to near 0% in cold polar regions.
• Enters the atmosphere through evaporation of liquid water.
• Water vapor condenses into liquid and solid cloud particles that grow in size and fall to earth as precipitation
• Redistributes heat energy on earth and is important to the formation of storms. This is because large quantities of energy are involved in phase changes:
• Evaporation (liquid to gas) energy is absorbed from environment
• Condensation (gas to liquid) energy is released to the environment
• Is a strong greenhouse gas that warms the earth's surface and its atmosphere. In fact water vapor is the most important greenhouse gas on Earth in that it contributes most to the atmospheric greenhouse effect.

Carbon Dioxide:

• Second most important greenhouse gas on Earth.
• Enters the atmosphere through the decay of vegetation, volcanic eruptions, respiration, burning of fossil fuels, and from deforestation. It is removed from the atmosphere by photosynthesis, and the oceans.
• Concentration has been increasing due to human activities, mainly buring fossil fuels and deforestation. The amount of carbon dioxide has increased over 42% since 1750, from 280 ppm to 400 ppm.
• There is concern that this will strengthen the natural greenhouse effect leading to global warming, sea level rise, and other potentially harmful climate changes.

Methane:

• Another greenhouse gas that is increasing due to human activity. There is concern that the increasing amount of methane will also contribute to human caused global warming.
• Since 1750, methane concentrations have increased by more than 150% mainly due to human activity.
• The main sources are the breakdown of plant material in rice paddies, domestic grazing animals (biological reactions in their stomach), biological activities of termites.

Nitrous Oxide:

• Another important greenhouse gas. that is increasing due to human activity. There is concern that the increasing amount of nitrous oxide will also contribute to human caused global warming.
• Since 1750, nitrous oxide conntrations have increased by more than 20% mainly due to human activity.
• Forms in the soil by bacterial processes and is destroy by ultraviolet light from the sun.

Ozone:

• Most ozone is found in the stratosphere where it forms the ozone layer (~20 - 30 km above the ground surface). The ozone layer protects plants, animals, and humans from the sun's harmful ultraviolet radiation by absorbing the radiation.
• Very little is found naturally near the ground where it is a toxic pollutant. Sometimes dangerously high concentrations develop near large cities in a process called photochemical smog

Aerosols:

Aerosols are tiny solid or liquid particles that are suspended in the air. Most aerosols are microscopic and too small to see individually without a microscope. Aerosols include things like dust, pollen, smoke, and even cloud droplets. When there are high concentrations of aerosols in the air, they do affect the propagation of light, and thus affect visibility. Examples are the visible thick smoke that comes off fires and normal clouds.

• Important for climate naturally and through human activities that release aerosols into the atmosphere
• Affect passage of solar radiation through the atmosphere
• Influence cloud formation
• Natural and manmade aerosols can affect human health
• "Particulate" air pollution