H1 Environmental Chemistry - Greenhouse effect
By: Chang won



What is greenhouse effect?


The Greenhouse effect s a natural process that aids in heating he Earth's surface and atmosphere. It results from the fact that certain stmosphere gases, such as water vapor, carbon dioxide and other trace ases, such as methane and nitrous oxide. They are able to change the energy balance of the planet by absorbing long wave radiation emitted from the Earth's surface. Furthermore, it takes a little over 8minutes for the Sun's energy to reach the Earth. The 70% of the Sun's energy that makes it to the Earth's surface is responsible for heating our planet and providing us with a comfortable atmosphere to live in. Without he greenhouse effect life on Earth ould probably not exist, moreover, the average temperature on Earth would be a nippy -18degc, rather than the present 15degc.

The_Greenhouse.jpg

  1. 1. Earth receives radiation from the sun.

  1. 2. The solar energy passes through the atomosheric gases unaffected.

  1. 3. The earth absorbs this energy and releases some IR radiation into the atmosphere.

4. This energy is absorbed by gases and then the energy is re-radiated as heat back towards the earth's surface.

  1. 5. This creates a warming known as greenhouse effect.


Figure 1: Greenhouse effect

Greenhouse gases
The greenhouse gases are involved in the human caused enhancement of the greenhouse effect. Carbon dioxide (CO2), methane (CH4), water vapor (H2O), nitrous oxide (N2O) and chlorofluorocarbons (CFCs) are the main greenhouse gases that capture outgoing IR (infrared) radiation from the earth, thereby warming the planet.

The table on the below show the list of main greenhouse gases. It will show us to understand easily by showing percentage of the contribution to increased global warming and number of greenhouse factor.
Greenhouse gases
Greenhouse factor
Overall contribution to increased global warming %
Source
Carbon dioxide (CO2)
1
50
Forest; Glassland; Oceans; Soils;
Soil cultivation; Fertilizers; Biomass burning;
Burning of fossil fuels.
Methane (CH4)
30
18
Organic decay; Forest fires; Volcanoes;
Burning fossil fuels; Deforestation; Land-use-change
Water vapor (H2O)
0.1
-
Seas, Lakes and Oceans
Nitrous oxide (N2O)
160
6
Refrigerators; Aerosol spray propellants;
Cleaning solvents
Chlorofluorocarbons (CFCs)
~20000
14
Refrigerants, pollutants, foaming agent, solvents
Table 1: Greenhouse gases and percentage of the contribution

In addition, the rising levels of methane are linked to the world's population growth. More people need more food, which has increased the levels of intensice farming. Similarly the levels of nitrous oxides are increasing because of the use of nitrogen based fertilizers.

Carbon dioxide
Before 1700, levels of carbon dioxide were about 280 ppm. Concentrations of carbon dioxide in the atmosphere are now about 390 ppm. This increase in carbon dioxide in the atmosphere is mainly due to activities associated with the Industrial Revolution. Emissions from the combustion of fossil fuels account for about 65% of the carbon dioxide added to the atmosphere. The remaining 35% is derived from deforestation and the conversion of prairie, woodland, and forested ecosystems primarily into less productive agricultural systems.

Methane
Since 1750, atmospheric concentrations of the greenhouse gas methane have increased more than 150%. The primary sources for the additional methane added to the atmosphere are rice cultivation, domestic grazing animals, deep seabed frozen methane clathrate thawing, termites, landfill out-gassing, oil and gas extraction, and coal mining.

Water vapor
With an increase in the Earth's temperature, water is being evapolated from lakes, riveers and oceans, and this means that there is an increased concentration of water vapor in the atmosphere; however, it has little effect as a greenhouse because it cannot absorbing longeer wavelength IR light.

Nitrous oxide
The average concentration of nitrous oxide is now increasing at a rate of 0.2 to 0.3% per year. The role of nitrous oxide in the enhancement of the greenhouse effect is minor relative to the other greenhouse gases already discussed. Nitrous oxide also contributes to the artificial fertilization of ecosystems.

Chlorofluorocarbons
Chloroflurocarbons are human-made chemicals that are highly persistent in the Earth's atmosphere. They are typically thousands of times more potent as greenhouse gases than carbon dioxide.


Greenhouse Gas Molecules (How do greenhouse gases absorb energy?)
carbon.gif

Figure 2: A carbon dioxide molecule vibrates when it absorbs heat
Certain gases in the atmosphere have the property of absorbing infrared radiation. Oxygen and nitrogen the major gases in the atmosphere do not have this property. The infrared radiation strikes a molecule such as carbon dioxide and causes the bonds to bend and vibrate, this is called the absorption of IR energy. The molecule gains kinetic energy by this absorption of IR radiation. This extra kinetic energy may then be transmitted to other molecules such as oxygen and nitrogen and causes a general heating of the atmosphere

The greenhouse gas molecules are shown the next series of figures along with the IR spectra and the bending and vibrations caused by absorbing the IR radiation. The arrows on the molecules indicate the direction of the bends and vibrations of the bonds. The IR spectra indicates the specific energies at certain wavelengths which are absorbed. Radiation that is 100% transmittance is not blocked but travels straight through the sample. The dips in the lines are caused by the absorption of energy, hence only 10% of the energy is transmitted. Below graphics are 'Carbon dioxide', 'Water', 'Methane' and 'Nitrous oxide'


irCO2.jpgirmethane.jpgirN2O.jpg
irwater.jpg
Figure 3: 4 main Greenhouse Gas Molecules
These gases absorb passing infrared photons. The energy of the photon is converted into an excited vibration state of the GHG . The energy cases the molecules to spin and vibrate faster, absorbing energy in the process. Then they slao down again, releasing the energy. There should be a bet change in the dipole movement of molecules when they vibrate.


1.jpg

Figure 4
Water vapor absorbs most wavelengths of the infrared radiation emitted by the Earth's , trapping it as heat. At some wavelength, however, the absorption is weak or close to zero allowing infrared radiation to escape into space. Other greenhouse gases absorb infrared radiation at these wavelengths and reduce the amount of heat lost into space.

Trends in Major Greenhouse Gases

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Figure 5: Trends in Major Greenhouse Gases
Trends for greenhouse gases: Carbon dioxide (CO2) and nitrous oxide (NOx) concentrations in the atmosphere are still increasing. For the other major greenhouse gases, the steady upward trend has been broken.

Annual Greenhouse Gas Emissions by sector
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Figure 6: Annual Greenhouse Gas Emissions by sector
The lower part of the picture shows the sources individually for the gases carbon dioxide, methane and nitrous oxide, respectively.


Intensity of the Sun's radiation
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Figure 7: Way to measuring Earth's Albedo
The intensity of the Sun's radiation incident on a planet is also known as the solar constant, S.
This is the average solar power per m2 arriving at the top of the Earth's atmosphere. It's value is 1.37kW/m2. This value indicates that 1.37 kilojoules of solar energy reach each square matre of the top of the Earth's atmosphere every second.
What is the Albedo?
Albedo is a specific term defined as the percentage of solar energy reflected back by a surface. The value ranges from 0 (dark) to 1 (bright). The average albedo of the Earth is 30%.
Factor's determine a planet's albedo
  • Human activities such as forest clearance and farming affect the level of albedo.
  • Thickness of clouds; thicker and low latitude cloud reflect back the solar radiation, thus reduce the Earth's surface temperature.
  • The color of an object also affect a planet's albedo. For example, the ocean can absorb more sunlight than the land does.

Atmosheric effects of greenhouse gases
The enhanced greenhouse effect has both beneficial and detrimental effects.
Positive
Negative
  • Lower heating bills
  • Longer growing season
  • Air conditioning usage is higher and thus more CFCs needed
  • Changes in biodiversity
  • Thermal expansion of the oceans
  • Melting
Table 2: Positive and Negative affects on the atmosphere of increasing amounts of Greenhouse Gases

The Average Global Temperature
The greenhouse effect causes the atmosphere to trap more heat energy at the Earth's surface and within the atmosphere by absorbing and reemitting long wave energy. Of the long wave energy emitted back to space, 90% is intercepted and absorbed by greenhouse gases. Without the greenhouse effect the Earth's average global temperature would be -18degc, rather than the present 15degc. In the last few centuries, the activities of humans have directly or indirectly caused the concentration of the major greenhouse gases to increase. Scientists predict that this increase may enhance the greenhouse effect making the planet warmer. Some experts estimate that the Earth's average global temperature has already increased by 0.3 to 0.6degc, since the beginning of this century, because of this enhancement. Predictions of future climates indicate that by the middle of the next century the Earth's global temperature may be 1 to 3degc higher than today.

Bibliography
  • Figure 1: N.A. (n.d). Greenhouse effect. Retrieved: April 8 2011, from The White House:
http://clinton4.nara.gov/Initiatives/Climate/greenhouse.html
  • Figure 2~4&7: from my own ppt (lost bibliography)...
  • Figure 5 & 6: N.A. (23 August 2009). Trends and Sector. Retrieved: April 9 2011, from Schools-Wikipedia:
http://schools-wikipedia.org/wp/g/Greenhouse_gas.htm
  • Table 1: Cartin Brown Mike Ford (2009). Greenhouse gases and percentage of the contribution. Retrieved on April 9, 2011, from Pearson Baccalaureate, Chemistry Developed specifically for the IB Diploma.
  • Dr. Michael Pidwirny. (05/07/2009). Greenhouse gases. Retrieved: April 9 2011, from PhysicalGeography:
http://www.physicalgeography.net/fundamentals/7h.html
  • N.A. (January 1, 2011) Greenhouse gases in Chemistry. Retrieved: April 10 2011, from The Encyclopedia of Earth:
http://www.eoearth.org/article/Greenhouse_gas?topic=49554
  • Anthony Watts (17 October 2007). Earth's albedo. Retrieved on April 11, 2011, from WattsUp With That?:
http://wattsupwiththat.com/2007/10/17/earths-albedo-tells-a-interesting-story/
  • N.A. (n.d). Greenhouse gas molecules. Retrieved: April 10 2011, from elmhurst:
http://www.elmhurst.edu/~chm/vchembook/globalwarmA5.html
  • N.A. (n.d). The greenhouse effect. From Environmental chemistry. (Table 2)
















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