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Solar FTIR Spectrometry - Tutorial

(by Ralf Sussmann)

Using a high-resolution infrared spectrometer (FTIR) with the sun as radiation source it is possible to obtain:

 

Retrieved Substances

This information is obtained via molecular absorption features within the terrestrial solar infrared spectrum, the so-called finger prints:

Absorption Spectra

Solar absorption spectra are measured using a Fourier-Transform Infrared Spectrometer (FTIR):

Michelson Interferometer

A high-resolution FTIR can spectrally resolve individual vibration–rotation lines of the absorption bands:

Molecular Absorption

The vertical column density of an atmospheric trace species (unit: molecules/cm2) can then be derived from the absorption depth and the area of a single measured vibration-rotation line. 

In addition, the shape of the absorption line of an atmospheric trace species contains information on the altitude profile of its concentration (e.g., volume mixing ratio as a function of altitude). The reason for this is the (linear) pressure broadening of the lines, which implies an altitude dependency of the line width:

Forward Calculation of Solar Absorption

Unfortunately, only the calculation of a spectral line shape (vector y) into a vertical concentration profile (vector x) is straight forward possible (forward calculation y = F(x)). The inverse problem x = I(y) is physically ill posed. In this situation one possible way to the solution is via the Bayesian approach:

Bayesian Approach

The retrieved concentration profile xret is a weighted mean from information from the actual spectral measurement and the a priori information (or regularization) used in the inversion process. The relative contribution of spectral and a priori information depends on various factors (e.g. solar zenith angel and noise of the spectral measurement). Therefore, the retrieved profile has to be characterized via a so-called averaging kernel matrix A. It provides a linear relation between the true profile at the moment of observation xtrue, the retrieved profile xret and the a priori profile xa:

A Priori Ensemble and Averaging Kernel Matrix

The colored functions are the rows of A which are called averaging kernels characterizing the retrieval at a certain altitude. Their vertical half-width is a measure for the achieved altitude resolution, their area is a measure for the sensitivity at the nominal altitude of the kernel.