Remedy 1: Regular cell measurements and ILS retrievals are to be performed in a consistent manner
TRL6
The retrieval of vertical profile information for target gases from ground-based high-spectral-resolution FTIR solar absorption spectra is based on the analysis of the observed shape(s) of the absorption line(s) of the target species in the recorded spectra. Since the observed shape is a convolution of the intrinsic absorption line shape with the instrument line shape (ILS), the analysis must account for the ILS. Therefore, the ILS must be known highly accurately. To this end, a cell filled with a known gas concentration at a known temperature and pressure is put into the FTIR instrument and a spectrum of the cell gas is taken. The cell spectrum allows the retrieval of the ILS using optimal estimation as described by Rodgers (2000), and such a retrieved ILS comes with its uncertainty. The uncertainty on the retrieved ILS is a combination of the smoothing uncertainty, the noise, the forward model parameters, etc. This uncertainty will propagate into the total uncertainty budget of the retrieved target gas’ profile and total abundance.
In summary, one can state that the cell measurement serves as a calibration of the target gas retrieval but that this calibration method is itself indebted with some uncertainty that must be accounted for in the total uncertainty budget of the retrieval result, which is the target gas vertical profile and total abundance.
All missions/instruments that use ground-based FTIR data for validation
Progress has been made within GAIA-CLIM, to identify the contribution of the ILS uncertainty to the total uncertainty budget and to make it better traceable and better characterised. The uncertainty propagation routines that were developed during QA4ECV & GAIA-CLIM are such that the integration of the ILS uncertainty propagation is a straightforward extension. However, the harmonization between the different retrieval software packages is not complete yet, and the implementation at all FTIR stations should still be done consistently.
This gap should be considered at the same time as G2.18 as it is a contributing component to the broader uncertainty characterisation.
- Hase F., Improved instrumental line shape monitoring for the ground-based, high-resolution FTIR spectrometers of the Network for the Detection of Atmospheric Composition Change, Atmos. Meas. Tech., 5, 603–610, doi:10.5194/amt-5-603-2012, 2012.
- Rodgers, C. D., Inverse Methods for Atmospheric Sounding: Theory and Practice, Ser. Atmos. Oceanic Planet. Phys., Vol. 2, 1st ed., World Sci., Hackensack, N. J., 2000.
For the retrieval of information about the vertical distribution of target species from FTIR spectra, it is important to know the FTIR instrument line shape (ILS). Therefore, regular cell measurements are carried out to characterize the ILS of the FTIR spectrometers. However, these cell measurements have their own uncertainties since these are obtained using optimal estimation: an ILS retrieval comes along with an uncertainty and an averaging kernel. In particular the averaging kernel for an ILS retrieval is often not adequately considered (Hase, 2012). Inaccurate knowledge of the ILS mainly affects the retrieved vertical profile (e.g. for water vapour and ozone profile retrievals). The uncertainty on the ILS leads to larger uncertainties on the retrieved column-averaged concentrations of CH4 and CO2 (XCH4, XCO2). In other words, the uncertainties on the ILS retrieved from cell measurements will propagate to the total uncertainty budget of the retrieved species. Although the technical know-how is present within the NDACC IR working group, the actual implementation of the ILS uncertainty characterisation and propagation is not complete. In particular further harmonization between the different FTIR retrieval software packages is required.