FT spectroscopy in support of atmospheric spectroscopic databases: Recent advances at DLR

High resolution Fourier Transform spectroscopy of atmospheric trace gas has been carried out at DLR since 1990. The primary focus is on supplying well characterized uncertainties. The core instrument is the commercial high-resolution Bruker IFS 125 HR Fourier-transform spectrometer operating from 10 to 40000 cm-1. The laboratory infrastructure has been continuously improved over the last 30 years, especially the absorption cells. A 22 cm absorption path 200-350 K cell features two windows pairs allowing quasi-simultaneous measurement from FIR to UV. A 200 m absorption path multireflection cell operates in the temperature range 200-350 K with high temperature homogeneity. Line fitting software was steadily improved, resulting in a multispectrum fitting tool with several line shape models including the Hartmann-Tran profile. The instrumental line shape of the Fourier-transform spectrometer is adopted from the LINEFIT software by Frank Hase, IMK, which is also used by the TCCON community. Recent results are a spectroscopic database of the O3 fundamentals and temperature dependent UV absorption cross sections for O3, together solving the 4% discrepancy between UV and MIR atmospheric O3 columns, a comprehensive H2O spectroscopic database in the range 1850-4300 cm-1, a new method to obtain H2O continua, H2O foreign- and self-continua in the 3 µm region, and a CO2 database in the range 6000-7000 cm-1 with absolute intensity uncertainties <0.15%. Most of these results have become part of the HITRAN20 database

Fourier-transform intensity measurements with 0.1% accuracy

Remote sensing of atmospheric trace gases with climate relevance requires spectroscopic data with better than 0.1% absolute intensity accuracy. This requirement poses a big challenge, especially to Fourier-transform spectroscopy (FTS). The alternative CRDS method has the advantage of a higher sensitivity compared to FTS. CRDS is also independent from cavity length and there is no instrumental line shape. In contrast, the FTS technique has the major advantages of a broadband coverage and the entire spectral range is measured simultaneously. Both techniques are thus complementary. The demanding metrological requirements for multireflection cell absorption path determination as well as for pressure and temperature measurement to achieve 0.1% absolute intensity accuracies will be addressed. Furthermore, the instrumental line shape characterization, the treatment of offset errors, and multipassing will be discussed. For this high accuracy application, a sophisticated multispectrum fitting software is essential. Finally, highly accurate results require several measurements under different conditions to validate the overall uncertainties. As examples CO 3-0 and CO2 1.6 µm intensity measurements will be presented

Pressure-Dependent Line Intensity and Continuum Absorption for Pure Co2: Experimental Results

Fourier-transform measurements of pure CO2 in the 1.6 µm region covering bands from ground state to 30011, 30012, 30013, and 30014 states at ambient temperature and 212 K with pressures up to 1 bar have been recorded. Line parameters have been retrieved by multispectrum fitting. An intensity depletion parameter quantifying linear intensity dependence on pressure was introduced and fitted. From the fitted baseline polynomials the self continuum was determined for the 30012 and 30013 bands. The depleted intensity was found to be transferred to the continuum for both temperatures, thus the band intensity is conserved. The intensity in the continuum at 1 atm was about 1% of the total band intensities for ambient temperature and about 3% at 212 K. For both temperatures the depleted intensity/continuum area was found in excellent agreement with values calculated from the second virial coefficient. The experimental work is accompanied by rCMDS calculations. The results presented here have significant impact on CO2 retrieval from atmospheric measurements. For OCO/CO2M-type observations it was calculated that in case of the 2 µm band retrieved CO2 columns are too large by about 3% when omitting depletion and continuum. A new spectroscopic database was produced. Systematic line intensity uncertainties are well below 0.1%

Measurements of intensities and self-broadening line shape parameters of H2O absorption lines in the spectral range 700-2000 cm-1

A Bruker IFS 125HR Fourier transform interferometer in combination with a multireflection cell was used to measure pure water vapor transmission spectra in the range 700-2000 cm-1. A total of 17 spectra with absorption path lengths between 14 m and 161 m and sample gas pressures from 0.01 to 20 mbar were measured at room temperature. The recorded spectra were corrected for detector non-linearity, thermal self-radiation and for deviations from an ideal instrumental line shape. A multispectrum-fitting approach was used, applying a quadratic speed-dependent Voigt line model which was extended to account for Rosenkranz line mixing. Experimentally determined line shape parameters are compared to the HITRAN database. Line intensities are also compared to values from two ab initio calculations [1][2]. The overall agreement with both is better than 0.5% for most lines. [1] L. Lodi, J. Tennyson, O. L. Polyansky, The journal of chemical physics, 2011, 135(3), 034113 [2] E. K. Conway, I. E. Gordon, A. A. Kyuberis, O. L. Polyansky, J. Tennyson, N. F. Zobov, Journal of Qualitative Spectroscopy and Radiative Transfer, 2020, 241, 10671

New method to establish frequency calibration of FTS with multireflection cells

In laboratory FT spectroscopy in the infrared region, instrumental line shape (ILS) determination and the associated frequency calibration is necessary for accurate measurements. For high pressure measurements, both can be obtained from a previous measurement of a calibration gas at low pressure, preferably under otherwise similar conditions. However, especially in combination with a multireflection cell, a subsequent change in pressure causes an alteration in the adjustment of multireflection cell due to mechanical deformations and change of refractive index. The beam path and profile are thus shifted, which especially comes into effect for high absorption paths. An unavoidable correction of the adjustment then usually causes a deviation from the original ILS and thus from frequency calibration. ILS contributions are distributed over the infrared beam in the spectrometer optics and therefore the adjustment can influence the ILS. Especially the detector and detector window exhibit a strong angular dependence of reflectivity, thus influencing the ILS. The known method of using a small cell with a calibration gas in series with the multireflection cell fails when narrow band filters are applied for obtaining good signal to noise ratios and there is no calibration gas available with strong lines in the spectral region of interest. Here, a method is presented by which the re-adjustment of the instrument can be performed in a way to restore the previous ILS and frequency calibration for large absorption paths and pressure changes of about 1 bar. Explained is the detailed restoring procedure as well as exemplary results using a multireflection cell of 0.8 m base length set to an absorption path of more than 100 m, where a secondary reference cell was used to validate the method

Measurements of H2O line parameters and self-continuum in the spectral range 700 - 2000 cm-1 (v2 + atmospheric window)

A Bruker IFS 125 HR Fourier transform interferometer in combination with a multireflection cell was used to measure pure water transmission spectra in the spectral range 700 - 2000 cm-1. Various spectra with absorption path lengths between 14 and 161 m, sample gas pressures from 0.003 to 20 mbar and temperatures between 278 and 350 K were recorded. The recorded spectra were corrected for detector non-linearity, thermal self-radiation and for deviations from an ideal instrumental line shape. A micro-window-based multispectrum-fitting-approach was used, applying a quadratic speed-dependent Voigt model which was extended to account for line mixing. Line positions, line intensities, self-shifts, self-broadening widths, their speed-dependence, temperature exponents, temperature shifts and in some cases line mixing were adjusted for fitting the corrected spectra. Continuum information was extracted from baselines, which were fitted simultaneously with monomer lines during the multispectrum fits. Continua were then obtained by a combined fit of all measurements containing significant continuum information. The self-continuum for room temperature was derived for the entire spectral range covered and is compared to measurements from other groups. The good quality of the self-continuum and the visibility of high-resolution features in the v2 in-band region allows for a comparison with dimer spectra. Results are presented for different temperatures

High accuracy CO2 Fourier transform measurements in the range 6000-7000 cm-1

A Bruker IFS 125HR Fourier-transform spectrometer has been used to measure pure carbon dioxide transmittance spectra in the spectral range 6000-7000 cm-1, including the bands 30011-00001, 30012-00001, 30013-00001, 30014-00001, and 00031-00001. A total of 10 measurements with absorption path lengths between 14.6 and 59.4 m and sample pressures from 3 to 80 mbar were performed at 295 K. A multispectrum fitting approach was used applying the Hartmann-Tran profile extended to account for line mixing in the Rosenkranz first order perturbation approximation. Line positions, self-shifts, intensities, self-broadened widths, their speed dependence and in some cases line mixing were adjusted for fitting the measurements. A rigorous error analysis has been performed. The primary goal of this work was to investigate whether Fourier transform spectroscopy can deliver line intensity accuracy down to the 0.1% level. In this work, a combined systematic standard uncertainty of 0.15% has been achieved, further improvements down to the 0.1% level are feasible. The achieved level of combined standard uncertainty has, to our knowledge, not been reached by Fourier-transform spectroscopy before. Comparisons with most recent work are presented for line positions, line intensities, self-broadening and its speed dependence, and self-shifts. Measurement and line parameter databases are provided on Zenodo (doi:10.5281/zenodo.4525272)

H2O Self-Continuum Measurements in the Spectral Range 700 - 2000 cm-1 (v2+ Window)

A Bruker IFS 125 HR Fourier transform interferometer in combination with a multireflection cell was used to measure pure water transmission spectra in the spectral range 700 - 2000 cm-1. Spectra with absorption path lengths between 14 and 161 m, sample gas pressures from 0.003 to 20 mbar and temperatures between 278 and 350 K were recorded. The recorded spectra were corrected for detector non-linearity and thermal self-radiation and the deviations from an ideal instrumental line shape were taken into account. A micro-window-based multispectrum-fitting approach was used, applying a quadratic speed-dependent Voigt model (+ line mixing) to adjust spectral line parameters. Continuum information was extracted from baselines, which were fitted simultaneously with monomer lines during the multispectrum fits. Continua were then obtained from the baselines by a combined fit of all measurements containing significant continuum information. The self-continuum for room temperature was derived for the entire spectral range covered and is compared to measurements conducted by other groups. In-band continua were determined for different temperatures and the contribution of bound dimer was investigated

Meeting the Requirements for High Quality Measurements of the Water Vapor Continuum in the Spectral Region of 700 - 2000 cm-1

The water vapor continuum in the spectroscopic window around 1000 cm-1 makes an important contribution to the radiative energy budget of the Earth1. Furthermore, it is important for remote sensing, especially in case of Nadir sounding. Currently, only few spectroscopic reference data for conditions close to the Earth’s atmosphere exist in this spectral region. Also, the understanding of the physical nature of the water vapor continuum is not complete. To make a significant contribution to this field, water vapor continuum measurements in a broad region of 700 - 2000 cm-1 shall be conducted using high resolution FTS and a multireflection cell, which allows an absorption path of up to 160 m. Hereby it is possible to cover the spectral range of the strong v2 water band absorption together with an atmospheric window, and to investigate the foreign and the self-continuum of water. The line parameters of water will also be determined since they are needed to distinguish between continuum and local line contributions. Series with different pressures as well as temperatures down to -40 °C and up to 80 °C are planned. We present requirements for the proposed high quality water vapor measurements and show which efforts have been made to ensure and monitor baseline stability as well as to solve specific problems of the spectral range under investigation: For continuum measurements, an accurate reference measurement is of high importance as the baseline of an actual water vapor measurement cannot be distinguished from the continuum. Therefore, the quality of continuum measurements is particularly limited by the baseline stability and reproducibility. The baseline stability might for example be disturbed by influences on the instrument’s adjustment due to alignment changes in the multi reflection cell, caused by the different conditions between sample and reference measurements. Especially in the spectral region of interest, further matters like the non-linear behavior of the detector or the thermal self-radiation of the instrument arise and need to be handled

Pressure Dependent Line Intensity and Continuum Absorption for Pure Co2: Predictions by Requantized Molecular Dynamics Simulations

We investigate the pressure dependence of the line intensities retrieved from fits, using accurate isolated line shapes, of pure CO2 spectra predicted using classical molecular dynamics simulations (rCMDS). For that, rCMDS have been carried out, at 296 K and 215 K, for pressure from 0.5 to 1 atm. An accurate ab initio intermolecular potential has been used to represent the CO2-CO2 interactions. The usual quadratic speed dependent hard collision model, taking into account the first-order line-mixing, is used to fit the simulated spectra. The results show that the retrieved line intensities linearly decrease with increasing pressure and that this decrease is larger at low temperature, consistently with the experimental results 1 . Quantitatively, at 1 atm and room temperature (resp. 215 K), our predictions show that the retrieved intensity can be up to 2% (resp. 4%) smaller than the intensity obtained at zero pressure. The effect decreases with increasing rotational quantum number. From the spectra fit baselines, a continuum absorption proportional with the square of pressure, is deduced. Our analysis shows that the observed pressure dependent intensity and the continuum are mostly due to incomplete or ongoing collisions, which govern the dipole auto-correlation function at very short times. These collisions transfer a fraction of the intensity from the core region of the line to a broad and weak continuum, and thus reduce its area