SUPPLEMENTAL LECTURE MATERIALS...
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
- bottom layer where our weather takes place.
- it contains the major concentrations of water vapor and dust.
- temperatures generally decrease with height in this layer. The average rate of temperature decrease is called the environmental lapse rate and is 6.4°C per kilometer.
- there are locations within this layer where the temperature actually increases as we go up. These are called temperature inversions. These are stable layers and pollution can be trapped underneath inversions. See figure 3-16.
- the average height of this layer (troposphere) ranges between about 8-18 km. Height changes depending on temperature - higher when/where warmer and lower when/where colder.
- Above this layer is the tropopause, a boundary that separates the troposphere from the stratosphere. Since the tropopause is the top of the troposphere, its height also changes as the height or thickness of the troposphere changes. Again, higher when/where warmer, etc.
- temperatures are constant (isothermal) from the bottom to roughly 20 km then
- temperatures increase in the stratosphere due to the absorption of UV radiation by ozone (~20-50 km). Greatest concentration of ozone is found between ~20-30 km. (Note: the symbol ~ means approximately).
- at the top of this layer is the stratopause, a boundary that separates the stratosphere from the mesosphere
- temperatures decrease in this layer
- layer is from 50-80 km
- at the top of this layer is the mesopause, a boundary that separates the mesosphere from the thermosphere. Coldest portion of the atmosphere, -90ºC
- very little air mass so pressure is very low
- begins at 80 km, most consider that this layer has no well defined upper limit. To define an upper limit, researchers say that the location is the height at which the lightest gases fly off into space rather than stay with earth's atmosphere due to gravitational attraction. Therefore, we can call the upper limit for this layer the thermopause and its height varys significantly depending on solar activity (height: 250-550 km or 155 miles to 340 miles).
- layer accounts for a minute fraction of atmosphere's mass
- temperatures rise in this layer to 1200ºC (2200ºF) or more - this is considered the hottest layer
- because the air is so thin, temperature has little meaning
- thermosphere merges into this layer which then blends into outer space
- no true boundary between outer space and our atmosphere
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.
- Density is defined as mass per unit volume and decreases rapidly with increasing altitude above Earth.
- Atmospheric (air) pressure can be thought of as the weight of the atmosphere above a given point. Closer to the surface, there is more weight from the air above so there will be higher pressure. Pressure also decreases with height above the surface.
- Average sea level pressure measures 1013.25 millibars (mb) or 29.92 inches of Mercury (in. Hg).
- The term "lapse rate" is defined as the change in temperature with a change in height.
- Temperature inversion is defined as a region where the temperature increases with height. This can be an entire layer (stratosphere/thermosphere) or occur in part of a layer (like in the troposphere)
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).
- surface to 80 km
- gases are well mixed in this layer - composition remains uniform throughout the layer with the exception of ozone, water vapor, low level pollutants.
- 80 km upward to outer space
- gases are not evenly mixed - they are found in layers based upon their weight with the heaviest gases (oxygen and nitrogen) near the bottom of the layer and the lightest gases (hydrogen and helium) near the top.
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.