RADIATION IS ENERGY transmitted by electromagnetic waves. Electromagnetic waves travel at the speed of light (when passing through a vacuum) and have a characteristic wavelength, X, which is inversely proportional to their frequency, v, by
where c is the speed of light. Electromagnetic radiation is conceptualized in contemporary theory both as a wave and as a stream of particles called photons (this dual approach is referred to as wave-particle duality). The energy of any photon, E, of radiation is inversely proportional to the wavelength by
where h is Planck’s constant. This relationship allows us to order electromagnetic waves from high energy/ short wavelength (for example, x-rays), to low energy/ long wavelength (for example, radio waves). The resulting progression is referred to as the electromagnetic spectrum (Figure 1). The visible region of the electromagnetic spectrum is bound by infrared (IR) radiation on the lower energy side of the visible region (around 1 pm to 1 mm in wavelength), and by UV radiation (UV) on the higher energy side (from 400 nm to 1 nm). Microwave radiation is slightly lower in energy than IR, with a wavelength of around 1 cm.
Figure 1: The electromagnetic spectrum
All objects both emit and absorb radiation. Although all objects emit radiation at all wavelengths, the frequency of maximum emission, Xmax, is proportional to the temperature of the object by Wien’s law
where a is a constant equal to 2897 pm K. This implies that hotter objects emit higher energy radiation, as would be expected from everyday experiences. From Wien’s law, the surface temperature of the sun can be calculated based on its emission peak at ~0.5 |im (green light) to be around 5800 K. The average temperature of the Earth’s surface is around 18 degrees C (290 K) which corresponds to a peak emission at around 10 |im, in the infrared to microwave region.
There are three basic modes of motion: translational (movement through space), rotational, and vibrational. These are important, because along with electronic energy, they are the ways in which gas molecules can store energy. Quantum theory dictates that energy levels are discrete, not continuous; this implies that molecules will only absorb discrete frequencies of radiation that correspond to the gap between a high and lower energy state. UV radiation corresponds to the gap in energy between electronic energy levels in a molecule. When a molecule absorbs UV radiation, it may be promoted to an electronically excited state. In the general, this will make the bonds holding the atoms together weaker and may help facilitate reactions or the breakup of molecules. For example, the reactions that complete the Chapman mechanism in the stratosphere:
The Chapman mechanism is chemically a null cycle, but it is important in the context of life, as it prevents most of the high-energy radiation from below 320 nm reaching the Earth’s surface. IR and microwave radiation, being lower in energy than UV, correspond to the gaps between rotational and vibrational energy levels, respectively. Quantum theory dictates that molecules interact with IR/microwave radiation only when two conditions (selection rules) are met:
1. The energy of the radiation corresponds to the energy gap between two of the discrete energy levels in the molecule and,
2. The resulting motion results in a change in the dipole moment (electron distribution) of the molecule.
This implies that atmospheric components that are symmetrical molecules, such as N2 and O2 that have an even electron distribution and cannot be rotated, bent, or stretched in such as way as to create one, do not absorb microwave and IR radiation. Molecules possessing a dipole, which can be altered by bending and stretching, absorb radiation. Atmospheric components that fill this requirement include CO2, CH4, H2O and CFC’s, for example, the asymmetric stretch of CO2. These gases are collectively known as greenhouse gases. Radiation from the sun is received at the Earth’s surface, mainly in the UV and visible region, with other frequencies cut out by the atmosphere and electromagnetic field.
