Ruprecht-Karls-Universität Heidelberg

Carbon Cycle Group IUP Heidelberg CO2

In-situ FTIR Spectrometer

The in-situ Fourier Transform-InfraRed (FTIR) spectrometer uses molecular absorption in the infrared to analyse the amount of various greenhouse gases in the sample. The in-situ FT-IR analyzer used in Heidelberg was developed by the University of Wollongong, Australia, and operates routinely in Heidelberg since January 2011. It measures the trace gases CO2, 13CO2, CH4, N2O and CO simultaneously in the ambient Heidelberg air. The in-situ FTIR works continuously on a three minute time basis. In the following paragraph a brief description of the measurement principle is given. Infrared light is emitted by a thermal source and directed into an absorption white cell. The cell contains a volume of 3.5 liters which can be filled with a dried gas sample, for example air sample, and it has an optical pathway of 24 m. While passing the cell, the intensity of the beam is lowered at certain wavenumbers as described by Beer-Lambert law. This is due to the absorption of the incoming photons by the gas molecules. The resulting interferogram shows the intensity as function of optical path length difference of the interferometer. By Fourier transformation of the interferogram a spectrum is achieved. In the spectrum the resulting intensity is lowered at the wave numbers, where the molecule-specific absorption has occurred. In order to rule out optical artifacts specific to the instrument set-up, a background spectrum without gas sample is taken. The transmittance spectrum can finally be obtained by calculating the ratio of the sample spectrum and the background spectrum (see figure). With that the concentration of absorbent constituents in the gaseous sample can be calculated. The retrieved concentration can afterwards be converted to the wanted mole fraction by correcting for sample pressure and temperature which are simultaneously measured in the cell.

Normalized transmittance spectrum showing the individual absorptions of a dried ambient air sample (Konrad, 2011).

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