The spectrum can be considered in terms of wavelength regions characterized by the type of physical change that takes place in the absorbing or emitting molecule and by the type of instrumentation used in studying the spectrum.

1. Ultraviolet, 0.25 to 0.40 microns

The ultraviolet (uv) is the region of absorption by electronic transitions in molecules. Combined with the electronic changes are vibrational and rotational changes. In a few cases, especially for small molecules, there is characteristic structure in the ultraviolet bands. While the ultraviolet band systems of different molecules are likely to overlap, correlation techniques have been developed for distinguishing one band system in the presence of others. The larger molecules do not have uv fine structure at atmospheric pressure, and therefore the applicability of the uv in pollution monitoring is limited.

2. Visible, 0.40 to 0.70 microns

The visible region of the spectrum, by definition has very few molecular absorption bands. NO2 is the only colored gas which is a common pollutant.

3. Near Infrared, 0.70 to 2.50 microns

This is the spectral region where the overtones of the fundamental molecular vibration-rotation bands appear. The overtones are about 100 times weaker than the fundamentals; thus this region is not generally useful for molecular detection and analysis. A notable exception is in the study of the atmospheres of the planets, where large concentrations of absorbing gas are viewed over extremely long optical paths. Thus the overtone region has been the principal source of information on the atmospheres of Mars, Venus and Jupiter.

4. Middle Infrared, 2.50 to 25 microns

This is the spectral region where the strong fundamental vibration-rotation bands of molecules appear. Nearly every air pollutant will have a characteristic absorption band in this region. It is sometimes called the “fingerprint” region of the spectrum because the absorption bands in this region differ widely as to shape, location, and intensity distribution. Even two members of pairs of similar molecules, such as CO-NO and O3-SO2, have significantly different spectra.

5. Far Infrared, 25 to 500 microns

Rotational lines as well as some vibration-rotation bands appear in the far infrared. Unfortunately, very intense water vapor absorption blanks out the far infrared in atmospheric work.

6. Millimeter and Microwave

Molecular rotational lines appear in the microwave spectrum. Most large molecules have a microwave spectrum, and the region is very powerful in the study of molecular structure and other physical properties. In order for the fine structure of the spectrum to be resolvable, however, the molecular gas under study must be at very low pressure . At atmospheric pressure the spectra are so “smeared out” that distinguishing between molecules is impractical.

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