the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Impact of instrumental line shape characterisation on ozone monitoring by FTIR spectrometry
- 1Izaña Atmospheric Research Centre (IARC), State Meteorological Agency of Spain (AEMet), Santa Cruz de Tenerife, Spain
- 2Karlsruhe Institute of Technology (KIT), Karlsruhe, Germany
- 3TRAGSATEC, Madrid, Spain
- anow at: Employment Observatory of the Canary Islands (OBECAN), Santa Cruz de Tenerife, Spain
- 1Izaña Atmospheric Research Centre (IARC), State Meteorological Agency of Spain (AEMet), Santa Cruz de Tenerife, Spain
- 2Karlsruhe Institute of Technology (KIT), Karlsruhe, Germany
- 3TRAGSATEC, Madrid, Spain
- anow at: Employment Observatory of the Canary Islands (OBECAN), Santa Cruz de Tenerife, Spain
Abstract. Retrieving high-precision concentrations of atmospheric trace gases from FTIR (Fourier Transform Infrared) spectrometry requires a precise knowledge of the instrumental performance. In this context, this paper examines the impact on the ozone (O3) retrievals of several approaches used to characterise the Instrumental Line Shape (ILS) function of ground-based FTIR spectrometers within NDACC (Network for the Detection of Atmospheric Composition Change). The analysis has been carried out at the subtropical Izaña Observatory (IZO, Spain) by using the 20-year time series of the high-resolution FTIR solar absorption spectra acquired between 1999 and 2018. The theoretical quality assessment and the comparison to independent O3 observations available at IZO (Brewer O3 total columns and Electrochemical Concentration Cell, ECC, sondes) reveal consistent findings. The inclusion of a simultaneous retrieval of the ILS parameters in the O3 retrieval strategy allows, on the one hand, a rough instrumental characterisation to be obtained and, on the other hand, the precision of the FTIR O3 products to be slightly improved. The improvement is of special relevance above the lower stratosphere, where the cross-interference between the O3 vertical distribution and the instrumental performance is more significant. However, it has been found that the simultaneous ILS retrieval leads to a misinterpretation of the O3 variations on daily and seasonal scales. Therefore, in order to ensure the independence of the O3 retrievals and the instrumental response, the optimal approach to deal with the FTIR instrumental characterisation is found to be the continuous monitoring of the ILS function by means of independent observations, such as gas-cell measurements.
Omaira Elena García et al.
Status: final response (author comments only)
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RC1: 'Comment on amt-2022-44', Anonymous Referee #2, 23 Apr 2022
The work by Garcia et al. focus on the impact of the instrumenta line shape of the high resolution FTIR spectrometer on the retrieval of vertical ozone profiles. The authors evaluate ver systematically more than 8 retrieval steregies to use y estimate the instrumental lineshape and evaluate these strategies using the long term measurements of FTIR spectra together with measurements from Brewer and the frequent ozone soundings. These collocated measurements at the Izaña observatorio are an unique posibility to evaluate the different retrieval strategies with respect to the instrumenta line shape and to study the sensitivity of the ozone profile retrieval on the estimation of the ILS.
The optimization of retrieval strategies for vertical gas profiles and especially of ozone by a very focused study is going to improve this remote sensing method towards a more exact measuremet method which is an important step for measurements in the NDACC network. The work fits perfectly in the scope of AMT, the manuscript is well written and clear presented.
In my opinion the manuscript is ready and paper should be published as is.
Just some few specific comments:
I would recomend to include a headline like "relative difference" in table 1,3,4 andit would be helpfull to see in addition to the infromation with respect to the brewer column measurements, a relative diference between the retrieval sterategies, maybe with respect to the favourite retrieval sterategies of the authors. This would ilustrate clear how sensible the ozone profile depend on the retrieval sterategy with respect to the ILS.
I would recomend to include a little more basic information about the modulation eficiency and the phase error in the begining of the section 3.1, so that the reader get the important information which and how the ILS depends on both parameters without consulting the cited papers.
It is very interessting that the authors recomend to fit the pase error at ZPD, but use the modulation efficiemncy from the cell measurements. It would be very nice if the authors might try to give a possible physical explanation, why a fit of the "pase error" at ZPD from an individual spectrum imporves the retrieval? Does it depend on alignment the temperature of the beamsplitter the phasecorrection duringt the calculation of the spectrum?
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RC2: 'Comment on amt-2022-44 “Impact of instrumental line shape characterisation on ozone monitoring by FTIR spectrometry” by Omaira E. García et al.', Anonymous Referee #1, 03 May 2022
Comment on amt-2022-44 “Impact of instrumental line shape characterisation on ozone monitoring by FTIR spectrometry” by Omaira E. García et al.
General comments
The IRWG of NDACC is an international network bringing together more than 20 observational FTIR sites over the world since the 1990s. In comparison to other observational methods used for the investigation of gaseous composition of the atmosphere, FTIR spectrometry provides a unique advantage of simultaneous measurements of the total columns (or even profiles) of a number of climatically active gases. The significant efforts are being made by the IRWG community to develop the unified retrieval strategies for deriving total columns/profiles of atmospheric species including O3. The main target of these efforts is verifying and harmonizing the results obtained by different groups operating FTIR sites. To get reliable information on O3 trends in the stratosphere which are currently nearly zero, it is necessary to provide the FTIR products of high accuracy and precision. Achieving this goal requires knowledge of the parameters characterizing the alignment of FTIR spectrometer (instrumental line shape function, ILS) and correct accounting of ILS in the retrieval procedure. Paper by García et al. is devoted to the detailed study of the influence of several ILS approaches (used in the procedure of inverse problem solving) on the O3 retrieval results (focusing the stratosphere). FTIR instruments having different alignment status are considered.
The manuscript corresponds to the AMT main subject areas and can be recommended for publication (minor revision is required) after a few points are addressed (please, see specific comments section).
Specific comments
Results presented in Appendix B deserve to be moved to the main text of the paper as a separate section. But the final decision is up to the authors.
Authors tested several approaches of ILS for the getting best retrieval results on O3. Is there a “universal” recipe for the processing FTIR observations (for example, archive spectra) in the absence of information on instrument alignment (ILS function)? Is it correct that in the case of the ideal ILS function should be used for overall spectra processing? Is it possible to create a homogeneous O3 row by stitching separate O3 time series obtained as a result of processing FTIR spectra using different ILS approaches? Analysing such a complex time series can be an additional challenge to reveal long-term O3 trends close to zero.
It is not quite clear whether the AVKs (averaging kernels) were taken into account when comparing the O3 results obtained by the FTIR and Brewer techniques?
Omaira Elena García et al.
Omaira Elena García et al.
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