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https://doi.org/10.5194/amt-2017-168
© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 3.0 License.
Research article
03 Jul 2017
Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Atmospheric Measurement Techniques (AMT).
The tilt-effect in DOAS observations
Johannes Lampel1,a, Yang Wang1,2, Andreas Hilboll3,4, Steffen Beirle1, Holger Sihler1,3, Janis Puķīte1, Ulrich Platt1,5, and Thomas Wagner1 1Max Planck Institute for Chemistry, Mainz, Germany
2Anhui Institute of Optics and Fine Mechanics, Chinese Academy Sciences, Hefei, China
3Institute of Environmental Physics (IUP), University of Bremen, Bremen, Germany
4Center for Marine Environmental Sciences (MARUM), University of Bremen, Bremen, Germany
5Institute of Environmental Physics, University of Heidelberg, Germany
anow at: Institute of Environmental Physics, University of Heidelberg, Germany
Abstract. Experience of differential atmospheric absorption spectroscopy (DOAS) shows that a spectral shift between measurement and reference spectrum is frequently required in order to achieve optimal fit results, while the straight forward calculation of the optical density proves inferior. The shift is often attributed to temporal instabilities of the instrument, but implicitly solved the problem of the tilt-effect discussed/explained in this paper.

Spectral positions of Fraunhofer and molecular absorption lines are systematically shifted for different measurement geometries due to an overall slope – or tilt – of the intensity spectrum. The phenomenon has become known as tilt effect for limb satellite observations, where it is corrected for in a first order approximation, whereas the remaining community is less aware of its cause and consequences.

It is caused by the measurement process because atmospheric absorption and convolution in the spectrometer do not commute. Highly resolved spectral structures in the spectrum first will be modified by absorption and scattering processes in the atmosphere. before they are recorded with a spectrometer, which convolves it with a specific instrument function. In the DOAS spectral evaluation process, however, the polynomial (or other function used for this purpose) accounting for broadband absorption is applied after the convolution is performed.

In this paper, we derive that changing the order of the two modifications of the spectra leads to different results. Assuming typical geometries for the observations of scattered sunlight and a spectral resolution of 0.6 nm, this effect can be interpreted as a spectral shift of up to 1.5 pm, which is confirmed in actual the analysis of the ground based measurements of scattered sunlight as well as in numerical radiative transfer simulations. If no spectral shift is allowed by the fitting routine, residual structures of up to 2.5 × 10−3 peak-to-peak are observed. Thus, this effect needs to be considered for DOAS application aiming at RMS of the residual of 10−3 and below.


Citation: Lampel, J., Wang, Y., Hilboll, A., Beirle, S., Sihler, H., Puķīte, J., Platt, U., and Wagner, T.: The tilt-effect in DOAS observations, Atmos. Meas. Tech. Discuss., https://doi.org/10.5194/amt-2017-168, in review, 2017.
Johannes Lampel et al.
Johannes Lampel et al.
Johannes Lampel et al.

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Short summary
Experience of differential atmospheric absorption spectroscopy (DOAS) shows that a spectral shift between measurement and reference spectrum is frequently required in order to achieve optimal fit results. The shift is often attributed to temporal instabilities of the instrument, but implicitly solved the problem of the tilt-effect discussed/explained in this paper. The tilt effect results from the finite resolution of the measurements and amounts to 2 pm for the example dataset.
Experience of differential atmospheric absorption spectroscopy (DOAS) shows that a spectral...
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