I have a question regarding the use of the ‘Subtract from…’ and ‘Correction’ options in Qdoas version 3.6.10. I wasn’t able to find detailed information about these features in the downloadable manual. Could you kindly clarify if these options are intended to help remove interference from strong absorbers when retrieving weak absorbers? If so, I would greatly appreciate any additional information on how they work and how to properly use them. Thank you very much for your time and assistance!
Hi, thank you for your question. These features are not properly documented at the moment, indeed.

“Subtract from”: This is mostly used when fitting 2 cross sections for the same molecule at 2different temperatures. By using “Subtract from XS(T0)”, you can replace XS(T1) by the difference Delta_XS = XS(T1)  XS(T0). This way, the total slant column density is obtained from the fit of XS(T0), and, assuming linear dependence of the cross section on temperature, the effective temperature can be obtained from the result of the subtraction. How well it works depends on the noise level and the amount of crosstalk between the difference Delta_XS and the original cross section.

Correction: these are corrections where extra cross sections (“pseudoabsorbers” if you will) are added to the fit to compensate for the fact that the optical depth a given molecule is not actually the product of a constant slant column density and a single wavelengthdependent cross section:

Slope / Pukite: here, we add 1 or 2 extra terms from the Taylor expansion of the slant column with respect to wavelength, following equation 13 of this 2010 paper by Pukite et al AMT  Extending differential optical absorption spectroscopy for limb measurements in the UV (where the example is given for ozone). “Slope” only adds the term proportional to lambda * sigma, and “Pukite” also adds the term sigma**2.

Molecular Ring: this correction takes into account the smoothing effect of inelastic (Raman) scattering on the absorption signature. The correction is described for a direct fitting approach in this 2014 paper by Lerot et al. https://doi.org/10.1002/2013JD020831 (section “Semiempirical Ring correction”). I can’t find a public reference for the DOAS approximation of this method used in QDOAS right now: it is described in the ATBD for the Sentinel5 total ozone column, but as far as I can see, that ATBD has not yet been published.
