http://www.ces.fau.edu/nasa/mod ule-2/how-greenhouse-effect- works.php This figure shows the blackbody spectra of Earth and sun. The incoming radiation from the sun is much more intense (Y-axis) than that of outgoing radiation from the Earth because the energy emitted from a blackbody is proportionate to its temperature to the fourth (σT4) – i.e. the sun emits a far greater amount of energy than the Earth. Incoming solar radiation is shortwave (X-axis, wavelength in microns) and in the wavelength range of ultraviolet and visible radiation (shown as the rainbow spectrum of colors). Outgoing Earth’s radiation is long wave and and is in the range of infrared radiation (shown in red). Below the blackbody spectra, molecules in the atmosphere, known as greenhouse gases, interfere with incoming and outgoing radiation. For instance, ozone (O3) in the stratosphere absorbs some of incoming radiation and is known as the ozone layer. That said, greenhouse gases (N2O, O3, CO2, and H2O) mainly interfere with outgoing radiation. Let’s talk about the molecular motion of these greenhouse gases to understand the greenhouse effect. Molecular Motions and the Greenhouse Gases H2O and CO2 2349cm-1 667cm-1 Here are the physical causes (molecular motion) of the greenhouse effect. But first… it may be a bit chunky, so sit back, take a deep breath! Gas molecules can absorb or emit radiation in the infrared range in two different ways. One way is by changing the rate at which the molecules rotate. The theory of quantum mechanics describes the behavior of matter on a microscopic scale – that is, the size of molecules and smaller. According to this theory, molecules can rotate only at certain discrete frequencies as if vibrations of a piano string in that they tend to be at specific “ringing” frequencies. (The rotation frequency is the number of revolutions that a molecule completes per second.) The molecule can absorb incident wave (energy), if this incident wave has just the right frequency. This frequency of the radiation that can be absorbed or emitted depends on the molecule’s structure. The H2O molecule is constructed in such a manner that it absorbs infrared radiation of wavelengths of about 12 micrometers and longer. This interaction gives rise to a very strong absorption feature in Earth’s atmosphere called the H2O rotation band. As shown in the previous slide, virtually 100 % of infrared radiation longer than 12 micrometers is absorbed with a combination of CO2 and H2O. (By the way, the H2O rotation band extends all the way into the microwave region of the electromagnetic spectrum, i.e. above a wavelength of 1000 micrometer, which is why a microwave oven is able to heat up anything that contains water.) Molecular Motions and the Greenhouse Gases H2O and CO2 2349cm-1 667cm-1 The second way in which molecules can absorb or emit infrared radiation is by changing the amplitude at which they vibrate. Molecules ...