Earth's Atmosphere: Early vs. Current Composition

Our current atmosphere is a thin, gaseous envelope composed mainly of nitrogen and oxygen with small amounts of other gases (e.g., water vapor, carbon dioxide, methane, argon, chlorofluorocarbons, etc.)

Earth's first atmosphere (4.6 billion years ago) probably consisted of hydrogen and helium and to a lesser extent methane and ammonia.

It is widely believed that this atmosphere escaped into space and was replaced by a second atmosphere formed from volcanic gases. Scientists believe that this second atmosphere was composed of gases similar to those released today from volcanoes: mainly water vapor (80%), carbon dioxide (CO2) (10%), and nitrogen (5%). There are also other trace gases.

Our current concentrations of these three gases is very different. Water vapor ranges between 0-4% by volume, CO2 is at 0.038% by volume and nitrogen is at 78%. Therefore, many processes have taken place over time to turn this early atmosphere into the one we breathe today.

Historically, as the earth cooled, the large amount of atmospheric water vapor led to the formation of clouds and the production of rain. Early rainfall events evaporated in the air or turned to steam after hitting the hot surface. As the earth cooled below the boiling point of water, the water could finally fill low-lying areas to form lakes and the oceans. Water in the atmosphere also can undergo a process called photodissociation, whereby energetic ultraviolet radiation from the sun breaks apart the water vapor molecules into a hydrogen molecule and an oxygen atom.

H2O --> H2 + O

CO2 also decreased in concentration: dissolved in the oceans and is also stored as carbonate rocks (limestone). What then remained in the atmosphere was nitrogen.

The major source of oxygen is green plants. Plants use carbon dioxide in the presence of sunlight and release oxygen (as a waste product). This is the process of photosynthesis.

Earth's current Atmosphere: Nitrogen (78%), Oxygen (21%), and Argon (0.98%)

Permanent Gases vs. Variable Gases

* Permanent Gases: found in the same proportion in the atmosphere over space and time.

- Nitrogen, Oxygen, Argon, Neon, Helium, Methane, Krypton, and Hydrogen

* Variable Gases: vary in amount from place to place and over time.

1 - Water vapor (H2O): a greenhouse gas

Water vapor is the most important gas and is necessary for weather (clouds, precipitation) and latent heat exchange

Varies from 0-4%

Generally largest % near the equator and smallest % near the poles. This is being very general. There are, of course, exceptions.

*Example to think about: What is the variation in water vapor content here in Phoenix between May and August? What about the difference in water vapor content in May between Phoenix and Miami, Florida? These examples demonstrate the variability in water vapor content over time and space.

2 - Carbon dioxide (CO2): a greenhouse gas

Strongly absorbs infrared radiation emitted by Earth.

3 - Ozone (O3): Occurs in two locations

Surface: ingredient in photochemical smog, a pollutant

Stratosphere (upper air): found between 20-50 kilometers. Beneficial since it absorbs ultraviolet radiation

4- Others: Carbon Monoxide (CO), Sulfur oxides (SOx), Nitrogen oxides (NOx), Volatile organic compounds, etc. (the 'x' in sulfur dioxide and nitrogen dioxide represent a number such as a 1 or 2, e.g., SO2 = sulfur dioxide)


Vertical Structure of the Atmosphere

Earth's atmosphere can be divided into layers based upon several different variables such as temperature, composition, and function.


Based on temperature: One of the most common ways is to examine the distribution of temperature with height and rates of temperature change.

Troposphere: means 'to turn' - it is the mixed layer





Important to know...

*Make sure you know why the thickness of the troposphere (or the height of the tropopause) changes over time and from place to place. This is an important concept and it will come up again later in the semester. Make sure you understand the relationship between atmospheric height (thickness) and temperature.

*Make sure you understand how air pressure and density change as we go up into the atmosphere. Remember, gases have weight and take up space. These gases are easily compressed (packed together) and have the greatest compression closest to the Earth's surface.

We will see these terms again later in the semester.


Based on composition: the atmosphere can be divided into two major categories: the homosphere (surface to 80 km) and the heterosphere (80 km upward).


Based on function: there are two zones: the Ionosphere and the Ozonosphere

Ionosphere: Outer layer that is electrically charged due to the absorption of very energetic short wavelength radiation from the sun (cosmic, gamma, x-ray, and shortest UV). This layer helps or sometimes hinders radio communications.

Ozonosphere: found in the stratosphere and it is the region of the ozone layer. The ozone molecules absorb ultraviolet radiation (all UVC and 90% of UVB) from the sun and prevent these dangerous energetic wavelengths from reaching life at the earth's surface.



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