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How Greenhouse Gases Trap Thermal Energy Thermal energy (heat) absorption from greenhouse gasses (GHG) depend mostly on its shape and the wavelength of the Electro-Magnetic (E-M) spectrum it is excited by. Photons, packets of energy in the form of waves, hit the molecules with a specific wavelength and frequency, which has differing amounts of energy, exciting the molecules and making them “wiggle”. In essence, a passing E-M wave converts its energy to kinetic motion of the GHG molecule. This stops the energy from radiating back out into space. The Earth receives the Sun’s energy in the form of light and other E-M waves, such as ultraviolet and infrared. These waves emitted range in wavelengths from 200 nm – 300nm, or frequencies of 1012 Hz to 10 16 Hz. (Image 1: Sun’s Spectrum). We can only see the visible (white) light, however, after the Earth absorbs the visible light it emits infrared energy back out toward space. Infrared energy has longer wavelengths, therefore lie in the frequency range of different GHG’s giving each molecule a different global warming potential (GWP). (Image 2: global warming potentials) Each greenhouse gas has a unique range of frequencies or wavelengths it absorbs, creating a window of energy capturing ability. Some molecules, such as methane have smaller windows with few frequencies, others have larger windows of absorption opportunity like water vapor and carbon dioxide with larger frequency ranges. (Image 3: absorption ability of different GHG’s) It is carbon dioxides extensive frequency absorption capacity that makes it the most talked about GHG. Image 1: Source - http://ds9.ssl.berkeley.edu/LWS_GEMS/2/images_2/ems510.jpg

Image 2: Source - http://priceoncarbon.org/wp-content/uploads/2015/01/All-GHG-Arent-Alike.jpg

When E-M photons interact with a GHG molecule it converts this to energy of motion of kinetic energy. It is the molecule wiggling according to the frequency of the given wavelength. The shape and complexity of the GHG is what helps determine the GWP because the more positions and ways it can bend or wiggle the greater the strength of relative absorption. For example, N2, O2, and Ar are not considered Greenhouse gasses because of their simplicity and inability to create much motion. Ar is a simple atom, whereas nitrogen and oxygen generally come as diatomic molecules (the exception being ozone); they can wiggle in and out in 1 direction. Whereas CO2, a linear 3 atom molecule can have its center atom (C) go back and forth, or the outside atoms (O) go in and out. Then there is the up and down motion of the central carbon atom; this creates many options for movement. H2O, a bent molecule, has greater options for movement, its central atom (O) has the ability to go up and down as well. (Image 4: options of CO2 motion) Therefore, GWP values increase with the complexities of the molecule (as well as the lifetime expectancy of the molecule). Imagine the many possible movements of complex molecules such as the CFC’s similar to CCl2F2. (Image: gif of a Chlorofluorocarbon)

Image 4: Source -American Chemical Society ACS Climate Science Toolkit Thompson Higher Education)

Thermal energy and thus temperature can change with the addition of energy. Not just within each molecule, but within a given area as well. As each molecule gets excited and is wiggling, more interactions and collisions occur. “Collisions between these energized molecules and others transfer energy among all molecules, which increase the average thermal energy and hence raises the temperature” (Lienhard, John).

Image 5: source - http://www2.ess.ucla.edu/~schauble/MoleculeHTML/CCl2F2_html/CCl2F2_nu5.gif

Resources Bell, Jerry A. et al. “What Are the Properties of Greenhouse Gases.” American Chemical Society: ACS Climate Science Toolkit, American Chemical Society, 20013, www.acs.org/content/acs/en/climatescience/greenhousegases/properties.html. Frazier, Reid. “Why Methane Is Such a Potent Greenhouse Gas.” The Allegheny Front, 8 July 2016, www.alleghenyfront.org/why-methane-is-such-a-potent-greenhouse-gas/. kachina. “Energy from the Sun.” Http://ds9.Ssl.berkeley.edu/LWS_GEMS/2/images_2/ems510.Jpg, U Cal Berkley, 2002, ds9.ssl.berkeley.edu/LWS_GEMS/2/images_2/ems510.jpg. Leinhard, John H. “Carbon Dioxide.” No. 1776: Carbon Dioxide, Engines of Our Ingenuity, 2003, www.uh.edu/engines/epi1776.htm. Russell, Randy. “The Greenhouse Effect & Greenhouse Gases.” The Greenhouse Effect & Greenhouse Gases - Windows to the Universe, National Earth Science Teachers Association, 1 June 2007, www.windows2universe.org/?page=%2Fearth%2Fclimate%2Fgreenhouse_effect_gases.html. “Chloroflorocarbon .” Http://www2.Ess.ucla.edu/~Schauble/MoleculeHTML/CCl2F2_html/CCl2F2_nu5.Gif, U Cal Los Angeles, 29 May 2009, www2.ess.ucla.edu/~schauble/MoleculeHTML/CCl2F2_html/CCl2F2_nu5.gif.