Selective atmospheric response to solar radiation

Radiant energy arriving from the Sun can be traced through all of the energy transfers and transformations that embrace hydroclimatology. As the photons penetrate the Earth's atmosphere, they are transmitted through the gaseous envelope, or they are reflected or absorbed by gases, particulates, clouds, and water droplets. The reflected photons constitute a major component of the Earth-atmosphere albedo. The photons absorbed by atmospheric gases are surprisingly small in number. The relatively transparent nature of the atmosphere to solar radiation is attributable to the molecular response of atmospheric gases to the stream of photons impinging on the atmosphere. A simplified account of thermal energy absorption and emission by the principal atmospheric gases is presented in this section and in Section 2.10.

2.9.1 Energy absorption by atmospheric gases

Atmospheric gases are viewed as isolated atoms and molecules that can absorb and emit energy at certain discrete energy states and can only undergo discrete changes between these states (Peixoto and Oort, 1992). Atoms absorb and emit energy in narrowly defined spectral lines and cannot contain extra energy. They must be at one of their energy levels which resonate with their structure. Each energy level of atoms comprising atmospheric gases is associated with a precisely defined energy amount. This results in an atmospheric absorption spectra consisting of many lines that correspond to electronic energy transitions characteristic of each particular atomic species (Peixoto and Oort, 1992). Energy absorption by a molecular gas occurs in bands consisting of a large number of closely spaced spectral lines.

For a photon to be absorbed, its energy must be transferred to the substance absorbing it either in the form of increased internal energy or as heat. The energy of the photon is absorbed if it corresponds to the difference between the energy of two allowable states of the molecule. A range of energies corresponds to each mode of energy storage in a molecule. An atmospheric gas molecule can store energy in rotational, translational, vibrational, or electronic forms.

2.9.2 Rotational energy transitions

Rotational energy is related to the spinning of the atoms in a molecule around a common axis perpendicular to the line joining the atoms. A convenient model for diatomic molecules with an electric dipole moment is a spring connecting two balls that are different. Changes in the rate of rotation of molecules require the smallest energy differences and correspond to the energy of photons with wavelengths shorter than about 1cm (Hartmann, 1994). However, molecules that are symmetric about their center of mass have no dipole moment and resist rotational transitions. Such atmospheric gas molecules include H2, O2, and N2, and those with more complex structures like CO2, and CH4. A CO2 molecule has a symmetric linear arrangement (Fig. 2.4), and the CH4 molecule has a spherically symmetric arrangement.

2.9.3 Translational energy transitions

Translational energy is kinetic energy associated with the movement of molecules from one location to another in the unconfined atmosphere.

Diagram Triatomic Molecule H2o

Fig. 2.4. Diagram showing molecular structure as the basis for a molecule's allowable energy transition. (a) A symmetric linear molecule, CO2, that lacks pure rotational transitions. (b) A bent triatomic molecule, H2O, that has pure rotation bands and vibration-rotation bands due to its permanent dipole moment.

Fig. 2.4. Diagram showing molecular structure as the basis for a molecule's allowable energy transition. (a) A symmetric linear molecule, CO2, that lacks pure rotational transitions. (b) A bent triatomic molecule, H2O, that has pure rotation bands and vibration-rotation bands due to its permanent dipole moment.

Rotational and vibrational are other kinetic energy forms, but they are quantized while translational energy is not quantized. Translational energy does not have stationary states in an unconfined spatial domain, but it is a component of the equilibrium energy state. The collision of atmospheric molecules can supply or remove energy to interactions between photons and gaseous molecules. This kinetic or translational energy of a molecule is generally small at terrestrial temperatures, but it is larger than rotational energies and smaller than the energy necessary for vibrational transitions.

2.9.4 Vibrational energy transitions

Vibrational energy is related to rapid variations of the interatomic distances within molecules. The forces of atomic attraction and repulsion are in balance for stable molecules at their appropriate interatomic distance. The absorption of a photon's energy causes an excited state of the binding forces between atoms. For a diatomic molecule, this can be thought of as functioning like a spring between two masses in which the spring expands and contracts. Vibrational energy levels of common atmospheric gases are separated by energies much higher than rotational levels. Therefore, vibrational transitions involve the absorption of shorter wavelength photons than for rotational transitions.

A molecule in an excited vibrational state will have rotational energy and engage in an energy transition that alters both the vibrational and rotational energy content of the molecule. Oscillations around this stable point can involve pure rotational transitions or vibrational-rotational transitions depending upon the molecular structure of the gas. A linear triatomic molecule, like CO2, responds like a diatomic molecule except its structure permits the molecule to have one bending mode and two stretching modes. Water vapor is a bent triatomic molecule that gives it a permanent dipole moment (see Fig. 2.4). This structure means the water vapor molecule has pure rotation bands in addition to vibration-rotation bands. While vibrational transitions require a photon with a wavelength less than 20 mm, the combination of vibrational and rotational transitions supports a large number of closely spaced photon frequencies representing an absorption band (Salby, 1996).

2.9.5 Electronic energy transitions

Electronic transitions occur when the outer electrons of an atom or molecule are promoted from their ground state to an excited state sufficient to break their molecular or electronic bonds. High-frequency, shortwave solar radiation has more energy per photon than low-frequency radiation and is capable of supporting the high energy requirements of electronic transitions. These transitions occur very fast, and the energies of the electronic states are quantized. Electronic transitions associated with photodissociation or electron excitation correspond to the largest energy differences and are supported by wavelengths shorter than 1 mm (Hartmann, 1994).

In a molecule, the electronic transition can be accompanied by rotational and vibrational transitions. Superposition of rotational and vibrational transitions on the electronic transition results in a combination of overlapping spectral lines appearing as a continuous spectral band (Salby, 1996).

2.9.6 Absorption lines and bands

Most of the photons with wavelengths shorter than 0.2 mm are absorbed in the upper atmosphere through electronic transitions involving photodissociation and ionization of nitrogen and oxygen (see Fig. 2.2). Photons with wavelengths between 0.2 and 0.3 mm are absorbed by electronic transitions with ozone in the stratosphere (Hartmann, 1994). Sunburn caused by radiation at 0.295 to 0.33 mm with a peak at 0.3075 mm demonstrates that some solar radiation at these wave lengths does penetrate to the Earth's surface. Spectral bands are particularly important to electronic energy transitions in the ultraviolet and visible wavelengths but are relatively unimportant for the far-infrared wavelengths (Peixoto and Oort, 1992).

The visible component of solar radiation occurs at wavelengths of 0.4 to 0.7 mm. Photons at these wavelengths are too energetic to be absorbed by vibrational-rotational transitions related to most of the gases in the atmosphere and not energetic enough to support electronic transitions associated with photodissociation. In the absence of transitions corresponding to the photons' energy, the photons have a good chance of passing through the atmosphere

2.10 Terrestrial radiation and the greenhouse effect 43

without absorption. The result is that the atmosphere is transparent to solar radiation in the visible wavelengths.

Photons of near-infrared solar radiation at wavelengths of 0.7 to 4.0 mm are slightly less energetic than visible solar radiation wavelengths, and they are weakly absorbed by vibrational-rotational transitions involving CO2, O3, O2, and H2O. These energy transitions account for most of the relatively small magnitude of solar radiation absorption by molecules in the Earth's troposphere (Hartmann, 1994). Consequently, the atmosphere is relatively transparent to solar radiation.

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