Method for on-site determination of the instrument line shape of mobile remote sensing Fourier transform spectrometers

Abstract

The method of passive remote sensing by Fourier transform spectroscopy allows the retrieval of column densities or concentrations of molecules in gas plumes such as exhaust gas plumes of aircraft or vapor plumes emitted after chemical accidents. State- of-the-art retrieval algorithms require two models: a radiative transfer model and an instrument model, the instrument line shape (ILS). The instrument line shape of real Fourier transform spectrometers (FTS) differs significantly from the instrument line shape of an ideal FTS, in particular if the instrument is optimized for high signal-to-noise ratio, which is achieved by interferometer designs with high optical throughput (etendue). The real instrument line shape may be modeled by convolution of the instrument line shape of the ideal FTS with an inherent instrument line shape describing the deviations. In this work, the inherent instrument line shape is modeled by a function which is dependent on a small number of parameters. In order to determine these parameters automatically, a new method has been developed. Spectra of a well-known gas in a gas cell are measured. The measured spectrum is approximated using a least squares fit with a model that contains the parameters of the instrument line shape. The fitting procedure is performed automatically. The instrument line shape model, the experimental setup of the method for the determination of the instrument line shape, and results of measurements using the instrument line shape are presented. In addition to the analysis of spectra with the ILS determined by the new method, analysis results obtained with an ideal instrument line shape are presented to demonstrate the negative effect of an inaccurate instrument line shape on the retrieved column density.