ABSTRACT
The description of atmosphere monitoring using methods of electromagnetic radiation absorption in the Terahertz (THz) region is the main topic of this chapter. The absorption in THz region is due mainly to intense rotational transitions of the water molecules. THz radiation is the electromagnetic spectrum with the frequency defined from 0.1 to 10 THz (1 THz=1012 Hz). This spectral range corresponds to a wavelength between 30 μm and 3 mm. Terahertz frequencies lay between the operation ranges of classical microwave and infrared spectroscopy and thus cannot be effectively covered by any of these techniques. Significant progress in the development of THz spectroscopy has been achieved only in the last 30 years, when suitable for use sources and detectors of THz radiation have been developed.
However, there still exist large difficulties in developing appropriate technology for water vapor monitoring in THz range. Water vapor absorption lines are extremely ubiquitous and result in relatively short atmospheric propagation paths through most of the THz range. In addition, as the relative humidity increased, the windows, which are present on the THz range, got narrower and shallower. As a result, now we do not have yet an adequate model that could describe the atmospheric propagation THz radiation in different kinds of climatic conditions. Therefore, to develop hygrometers, working in THz region, we need detailed studies aimed at (1) the construction of radiative transfer algorithm, (2) the collection of accurate spectral parameters, that is, to collect the spectroscopic fingerprinting of atmospheric molecules for THz atmospheric monitoring, and (3) the standardization of measurement procedures with a purpose to improve both the signal-to-noise ratio, and the restoration of the original signal from the observed signal by the process of deconvolution.
