Hamburg 2009 – scientific programme

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UP: Fachverband Umweltphysik

UP 9: Methoden II

UP 9.5: Talk

Thursday, March 5, 2009, 17:36–17:48, VMP 8 R206

Implementing automated wavelength calibration and parameterization of the optical transfer function in DOAS data retrievals — •Thomas Lehmann, Christoph Kern, Leif Vogel, and Ulrich Platt — Institut fuer Umweltphysik, University of Heidelberg, Heidelberg, Germany

Differential Optical Absorption Spectroscopy (DOAS) is an universal technique to measure the concentrations of atmospheric trace gases. The quality of DOAS data retrievals depends mainly on the reproduction ("fitting") of measured trace gas optical densities with an appropriate model function. Exact knowledge of the wavelength calibration of the measured spectra and an accurate parameterization of the spectrometer's optical transfer function (OTF) are both critical in obtaining optimal evaluation results. There is a need to improve current DOAS retrieval algorithms for several reasons: To begin with, the in-situ calibration and determination of the OTF are often elaborate and inaccurate, as both may vary throughout the measurement. Standard DOAS retrievals typically assume an OTF constant in time and for all wavelengths, but the true OTF is usually a function of spectrometer temperature and wavelength. An advanced DOAS retrieval was developed which dynamically re-calibrates the measured spectra using available Fraunhofer lines and computes a wavelength-depending OTF. The retrieval is implemented as an enhanced Levenberg-Marquardt Fit inside the DOASIS evaluation software.