Detecting Altitudes

Fundamentally, gases absorb energy at very specific wavelengths. In the lower atmosphere, at higher pressure, molecules collide with each other and slightly alter the wavelength of absorption (this is called pressure broadening).

The width of the broadening is directly proportional to pressure, and the amount of broadening at 1000 millibars of pressure ranges from 0.04 to 0.15 cm^-1 for molecules of concern. At 100 millibars, the line is one tenth as broad.

The figure below shows a part of the ozone spectrum at two resolutions, one that is fine enough to see pressure broadening, and one that is not.

(Click to view the full size image)

We can determine the pressure-dependent broadening and other factors (strength of absorption, temperature dependence) to get altitude information, if we measure with enough spectral resolution (equal to or smaller than the width of the broadening).

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TES is a high-resolution, infrared, imaging Fourier-transform spectrometer with spectral coverage of 650 to 2250 cm^-1 at a spectral resolution of 0.1 cm^-1 (low resolution) or 0.025 cm^-1 (high resolution), offering line-width-limited discrimination of essentially all radiatively active molecular species in Earth's lower atmosphere.