Selective absorption and emission of atmospheric gases

1. Energy level of atoms or molecules

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Quantum jump: transition between different energy levels

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2. Different energy form of a molecule or atom

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What Happens to Incoming Solar Radiation

a. Rotational energy

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Rotational energy transition can happenas long as a photon’s wavelength is shorter than 1 cm, usually associated with microwave wavelength.

b. Vibrational energy

Polar molecule haspermanent dipole

Non-polar molecule doesnot have permanent dipole.

Vibrational energy level transition requires a photon's wavelength shorter than 20 micrometer, usually in the infrared band.

Vibration and rotation sometimes combine together to form vibration-rotation mode, the transition between vibration-rotation modes alsoinvolves certain frequencies.

c. Photodissociation

Solar ultraviolet photon

For photodissociation to occur, the wavelength of a photon must be in the ultraviolet band. To dissociate Oxygen the wavelength of radiation must be shorter than 200 nm.

Ozone is a loosely bonded molecule. To dissociate a Ozone molecule, the frequency of a photon can be as low as 300 nm.

d. Electronic excitation

1st Shell 2nd Shell

e. Photoionization

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Electrons may be excited from one shell to another shell by a photon with a sufficiently high energy level. The wavelength is usually shorter than 1 micrometer.

Photoelectron

To photoionize a molecule requires the radiation with a wavelength shorter than 100 nm.

Electronic excitationPhotoionization

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overlap

Almost all solar radiations shorter than ultraviolet are used up in the upper layer for photoionization, electronic excitation, and molecule dissociation. Since most of solar energy is in the visible band, they have nothing to do with molecule vibration and rotation transition, so solar radiation can reach Earth's surface almost without any attenuation. On the other hand, terrestrial radiation in the infrared band, which is involved with atmospheric molecule vibration and rotation transitions, can be absorbed by the atmosphere to cause greenhouse effect.

Highly un-reactive greenhouse gases containing bonds of fluorine-carbon or fluorine-sulfur, such asPerfluorocarbons (CF4,C2F6, C3F8) and Sulfur Hexafluoride (SF6). These trace gases have strongabsorption lines right in the atmospheric window.

Greenhouse effect: shortwave solar radiation is nearly transparent to the atmosphere, but longwave terrestrial radiation is trapped by greenhouse gases, causing the increase of surface temperature.

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σ: emissivity

Effects of atmospheric convection

If the Earth system were in a radiative equilibrium only, it would not be in a dynamic equilibrium because the air near surface will warm up by contacting with hot surface, thus, convection will happen. The situation is further complicated by the phase change of water.

Difference between convection and advection

Heat Budget of Earth’s Atmosphere