Abstract

Abstract. The analysis of high spectral resolution Fourier Transform infrared (FTIR) solar absorption spectra is an important issue in remote sensing. If this is done carefully, one can obtain information, not only about the total column abundances, but also about the vertical distribution of various constituents in the atmosphere. This work introduces the application of the information operator approach for extracting vertical profile information from ground-based FTIR measurements. The algorithm is implemented and tested within the well-known retrieval code SFIT2, adapting the optimal estimation method such as to take into account only the significant contributions to the solution. In particular, we demonstrate the feasibility of the method in an application to ground-based FTIR spectra taken in the framework of the Network for the Detection of Atmospheric Composition Change (NDACC) at Ile de La Réunion (21° S, 55° E). A thorough comparison is made between the original optimal estimation method, Tikhonov regularization and this alternative retrieval algorithm, regarding information content, retrieval robustness and corresponding full error budget evaluation for the target species ozone (O3), nitrous oxide (N2O), methane (CH4), and carbon monoxide (CO). It is shown that the information operator approach performs well and in most cases yields both a better accuracy and stability than the optimal estimation method. Additionally, the information operator approach has the advantage of being less sensitive to the choice of a priori information than the optimal estimation method and Tikhonov regularization. On the other hand, in general the Tikhonov regularization results seem to be slightly better than the optimal estimation method and information operator approach results when it comes to error budgets and column stability.

Highlights

  • Since 2002 the Belgian Institute for Space Aeronomy (BIRA-IASB) has been responsible for measurements of high-resolution ground-based Fourier Transform infrared (FTIR) solar absorption spectra at the Observatoire de Physique de l’Atmosphere de La Reunion (OPAR)

  • First note that the retrieval of vertical profiles from FTIR data is an underconstrained problem, because of the following reasons: (1) a profile is a continuous function of altitude, whereas an FTIR spectrometer provides measurements only at a discrete number of wavelengths; and (2) there are components in the actual profile which do not contribute to the measurements and, cannot be determined from them

  • In this paper we have shown the application of the information operator approach (IOA) to the retrieval of the vertical distribution of atmospheric constituents from groundbased high spectral resolution FTIR solar absorption measurements

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Summary

Introduction

Since 2002 the Belgian Institute for Space Aeronomy (BIRA-IASB) has been responsible for measurements of high-resolution ground-based FTIR solar absorption spectra at the Observatoire de Physique de l’Atmosphere de La Reunion (OPAR). This station is located at 21◦ S, 55◦ E, in the Indian Ocean, East of Madagascar, at the edge between the southern tropics and subtropics and it is coordinated by the Laboratoire de l’Atmosphere et des Cyclones (LACy) of the Universitede La Reunion. 3 shows the retrieval results and error budget evaluations for the target species obtained from the above mentioned FTIR spectra, when applying the OEM, IOA and TR.

General description of the information operator approach
Forward model
Inverse model
Adapted retrieval method
October 2 October
Information content and sensitivity
Tikhonov regularization
Application of the IOA to ground-based FTIR data
Specifications of the FTIR measurements
Retrieval strategy and spectral fits
Choice of eigenvalues and eigenvectors to be used
Vertical profiles and column amounts
Influence of a priori information
Theoretical study
Findings
Conclusions
Full Text
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