Absorption by Gaseous Molecules
The energy of a gaseous molecule can exist in various forms:
- Translational Energy: Energy due to translational motion of the centre of mass of the molecule. The average translational kinetic energy of a molecule is equal to kT/2 where k is the Boltzmann's constant and T is the absolute temperature of the gas.
- Rotational Energy: Energy due to rotation of the molecule about an axis through its centre of mass.
- Vibrational Energy: Energy due to vibration of the component atoms of a molecule about their equilibrium positions. This vibration is associated with stretching of chemical bonds between the atoms.
- Electronic Energy: Energy due to the energy states of the electrons of the molecule.
The last three forms are quantized, i.e. the energy can change only in discrete amount, known as the transitional energy. A photon of electromagnetic radiation can be absorbed by a molecule when its frequency matches one of the available transitional energies.
Absorption of ultraviolet (UV) in the atmosphere is chiefly due to electronic transitions of the atomic and molecular oxygen and nitrogen. Due to the ultraviolet absorption, some of the oxygen and nitrogen molecules in the upper atmosphere undergo photochemical dissociation to become atomic oxygen and nitrogen. These atoms play an important role in the absorption of solar ultraviolet radiation in the thermosphere. The photochemical dissociation of oxygen is also responsible for the formation of the ozone layer in the stratosphere.
Ozone in the stratosphere absorbs about 99% of the harmful solar UV radiation shorter than 320 nm. It is formed in three-body collisions of atomic oxygen (O) with molecular oxygen (O2) in the presence of a third atom or molecule. The ozone molecules also undergo photochemical dissociation to atomic O and molecular O2. When the formation and dissociation processes are in equilibrium, ozone exists at a constant concentration level. However, existence of certain atoms (such as atomic chlorine) will catalyse the dissociation of O3 back to O2 and the ozone concentration will decrease.
It has been observed by measurement from space platforms that the ozone layers are depleting over time, causing a small increase in solar ultraviolet radiation reaching the earth. In recent years, increasing use of the flurocarbon compounds in aerosol sprays and refrigerant results in the release of atomic chlorine into the upper atmosphere due to photochemical dissociation of the fluorocarbon compounds, contributing to the depletion of the ozone layers.
There is little absorption of the electromagnetic radiation in the visible part of the spectrum.
The absorption in the infrared (IR) region is mainly due to rotational and vibrational transitions of the molecules. The main atmospheric constituents responsible for infrared absorption are water vapour (H2O) and carbon dioxide (CO2) molecules. The water and carbon dioxide molecules have absorption bands centred at the wavelengths from near to long wave infrared (0.7 to 15 µm).
In the far infrared region, most of the radiation is absorbed by the atmosphere.
The atmosphere is practically transparent to the microwave radiation.
The Earth's Atmosphere
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