Composition of the atmosphere

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	0.0393	393
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
*For CO2, 393 parts per million means that out of every million air molecules, 393 are CO2 molecules.

Major Components of the Atmosphere

The Atmosphere is mostly composed of Nitrogen and Oxygen. These make up 99% of the volume of "dry air". In this context "dry air" refers to all gases, except water vapor.

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

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

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 35% since 1750. There is concern that this will strengthen the natural greenhouse effect leading to global warming.

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 of it 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