Spectroscopic parameters of methane from many different studies were gathered to improve the HITRAN database towards its 2024 version. After a validation process using high-resolution FTS and CRDS spectra, about 80,000 lines of the four most abundant isotopologues were replaced from the dyad to the triacontad regions. These changes amount to 51,000 transition wavenumbers, 18,000 line intensities, 33,000 pressure-broadening half-widths, and 3300 assignments. 44,000 new lines were added with 16,000 old lines removed, extending the database from 12,000 cm(-1) up to 14,000 cm(-1), and covering some gaps. A greater focus was brought on the pentad, octad, and tetradecad regions, targeted by several remote sensing instruments. In these regions, comparisons of spectral fits from multiple line lists were performed, taking only the parameters that provide best fit for each line. In the nu(3) band, in addition to replacing the previous values, speed-independent pressure broadening parameters of (CH4)-C-12 were gathered and used to fit Pade-approximant functions. These functions then replaced any outdated experimental data in nu(3), missing data in the new lines, as well as the values that were determined to be outside their physical boundaries. The CH3D broadening parameters were replaced in the same manner, for missing and low or high values, using a semi-empirical formula instead.

Pure water vapor measurements were conducted over a temperature range of 278.7-353.0 K and a wavenumber range of 700-2000 cm-1 using a commercial Bruker IFS 125HR Fourier-transform spectrometer with a multi-reflection cell. Line parameters and continuum contributions were derived from the same spectra. In the out-of-band region 700-1350 cm-1, the H2O self-continuum was determined at ambient temperature. The measured continuum shows good agreement with a narrow-band measurement at 1185 cm-1. In line with measurements that led to the development of MT_CKD 4.2, it was found that the self-continuum is substantially weaker than had been assumed for decades. Compared to MT_CKD 4.2, even lower values by as much as 17% were found below 1000 cm-1. Since the out-of-band continuum overlaps with the infrared atmospheric window, which is of high importance for the Earth's radiation budget, "clear-sky" radiative transfer calculations were performed using both MT_CKD 4.2 and the data of this study. The largest difference was found for tropical atmosphere, where this study yielded a 0.31 W m-2 higher outgoing longwave radiation. The self-continua in the in-band region around nu 2 were fitted using a model that combines bound and quasi-bound dimer spectra, enabling the determination of temperature-dependent equilibrium constants and corresponding formation enthalpies: Delta H296K,Db =-1202(32) cm-1 and Delta H296K,Dq =-748(168) cm-1. The sum of the equilibrium constants, KDb(T) + KDq(T), exceeds the value predicted by second virial coefficients, well beyond the uncertainty margin.

Fifteen transitions of the 20012-00001 band of 13CO2 in air have been recorded at 296 K using a cavity ring-down spectrometer linked to an optical frequency comb referenced to a GPS-disciplined Rb oscillator. In parallel, measurements of 12CO2 in air were made with a Fourier transform spectrometer for transitions belonging to the same 20012-00001 band, enabling comparison of the air-induced half-width and line shift coefficients of both isotopologues. For that, a multi-spectrum fit procedure is adopted with a Hartmann-Tran (HT) line profile. A decrease of about 0.4% is demonstrated for the 13CO2 air-broadening coefficients compared to the corresponding values of the main isotopologue. To the best of our knowledge, this observation is the first experimental evidence of an isotopic effect on the air-broadening coefficients in CO2 (which are generally assumed to be independent of the isotopologue in spectroscopic databases). On the theoretical side, a refinement of the simple model described in [Lamouroux et al. JQSRT 111;2010:2321] is proposed which enables the determination of the air-broadening coefficients for the 13CO2 isotopologue from the 12CO2 values. In addition, complex Robert-Bonamy-Ma (CRBM) calculations have been performed for 12CO2 and 13CO2 in collision with N2 or O2 for the 20012-00001 band with J '' values from 0 to 85. From these CRBM data, the 13CO2-air and 12CO2-air half-width and line shift coefficients were computed and the obtained isotopologue effect on these parameters was compared to the experimental data. A good average agreement is achieved for the CRBM calculations for the air-broadening coefficients while the simple model leads to slightly smaller isotopic effects. In contrast, for the air-pressure shift coefficients, a quite large disagreement is observed between the calculations and the experimental data.

We present accurate transition intensities for the R(0) to R(10) manifolds of 12CH4 in the 2v3-band tetradecad region based on independent cavity ring-down spectroscopy and Fourier Transform spectroscopy measurements from three laboratories. Two sets of experiments involved multi-spectrum fits to low-pressure spectra acquired at Doppler-limited conditions which greatly reduced complications from effects caused by collisional broadening and line mixing. In a third experiment, cavity ring-down spectroscopy measurements of lineshapes made at elevated pressures were used constrain residual collisional broadening effects in the low-pressure spectra, thereby improving measurement precision. Intensities are reported for manifolds and specified multiplets comprising blended transitions, as well as for individual transitions. Summation of the manifold and multiplet intensities agree at the permille level, whereas highly blended individual transitions agree at the several permille level. For measurements of methane, the intensities reported here will enable more accurate spectroscopic determinations of amount-of-substance, and will support progress in remote sensing missions and future refinements of its other spectroscopic parameters.

Fourier-transform measurements of pure CO2 spectra in the 1.6 µm region covering bands from the ground state to the 30011, 30012, 30013, and 30014 states at ambient temperature and 212 K with pressures up to 1 atm have been recorded. The measured spectra have been multispectrum-fitted with the Hartmann-Tran model with first order line-mixing taken into account. In the fits, a linear decrease of the line intensities with increasing pressure was introduced. From the fitted baselines, the self-continuum, which complements the contribution of the local lines within +/- 25 cm−1, was determined for the two strongest bands, 30012-00001 and 30013-00001. The depleted intensity was found to be transferred to the continuum for both temperatures. The intensity in the continuum at 1 atm was about 1% of the total band intensity for ambient temperature and about 3% at 212 K. For both temperatures, the average depleted intensity/continuum area was found in excellent agreement with values estimated from the second virial coefficient. From these results, a new spectroscopic database with pure CO2 line profile parameters was produced, with systematic line intensity uncertainties well below 0.1%.Retrieval simulations for the CO2 1.6 µm and 2 µm bands measurements, planned with the future CO2M mission, similar to the OCO mission, were carried out to assess the impact of intensity depletion and continuum. The impact of the continuum was larger than that of the depletion and larger for the 2 µm band than for the 1.6 µm band, with a substantial column error of 2% for the 2 µm band when omitting both, depletion and continuum.The findings regarding pressure-dependent intensity and continuum may be applicable to other in-band continua showing similarity to the monomer band in shape and position. The new results may increase the knowledge on the physical nature of continua.

The HITRAN database is a compilation of molecular spectroscopic parameters. It was established in the early 1970s and is used by various computer codes to predict and simulate the transmission and emission of light in gaseous media (with an emphasis on terrestrial and planetary atmospheres). The HITRAN compilation is composed of five major components: the line-by-line spectroscopic parameters required for high-resolution radiative-transfer codes, experimental infrared absorption cross-sections (for molecules where it is not yet feasible for representation in a line-by-line form), collision-induced absorption data, aerosol indices of refraction, and general tables (including partition sums) that apply globally to the data. This paper describes the contents of the 2020 quadrennial edition of HITRAN. The HITRAN2020 edition takes advantage of recent experimental and theoretical data that were meticulously validated, in particular, against laboratory and atmospheric spectra. The new edition replaces the previous HITRAN edition of 2016 (including its updates during the intervening years). All five components of HITRAN have undergone major updates. In particular, the extent of the updates in the HITRAN2020 edition range from updating a few lines of specific molecules to complete replacements of the lists, and also the introduction of additional isotopologues and new (to HITRAN) molecules: SO, CH3F, GeH4, CS2, CH3I and NF3. Many new vibrational bands were added, extending the spectral coverage and completeness of the line lists. Also, the accuracy of the parameters for major atmospheric absorbers has been increased substantially, often featuring sub-percent uncertainties. Broadening parameters associated with the ambient pressure of water vapor were introduced to HITRAN for the first time and are now available for several molecules. The HITRAN2020 edition continues to take advantage of the relational structure and efficient interface available at www.hitran.org and the HITRAN Application Programming Interface (HAPI). The functionality of both tools has been extended for the new edition. (C) 2021 The Author(s). Published by Elsevier Ltd.

In the last decade high resolution FT-IR spectrometers have become increasingly important for remote sensing of the stratosphere. While the number of observations from space and stratospheric balloons is still small there is already a large number of ground based instruments measuring ozone and a variety of important trace gases. Most of the ground based instruments work in solar occultation (transmittance measurement) within the Network for Detection of Stratospheric Change whereas instruments measuring thermal emission of radiation are rare. For source photon noise limited observation the sensitivity of an emission measurement is better if the optical band width is larger than about 1-10 cm-1. Furthermore, emission measurements have a larger versatility since they are independent from the time of day.

This paper outlines the major updates of the line-shape parameters that were performed for the nitrous oxide (N2O) and carbon monoxide (CO) molecules listed in the HITRAN2020 database. We reviewed the collected measurements for the air- and self-broadened N2O and CO spectra to determine proper values for the spectroscopic parameters. Careful comparisons of broadening parameters using the Voigt and speed-dependent Voigt line-shape profiles were performed among various published results for both N2O and CO. Selected data allowed for developing semi-empirical models, which were used to extrapolate/interpolate existing data to update broadening parameters of all the lines of these molecules in the HITRAN database. In addition to the line broadening parameters (and their temperature dependences), the pressure shift values were revised for N2O and CO broadened by air and self for all the bands. The air and self speed-dependence of the broadening parameter for these two molecules were added for every transition as well. Furthermore, we determined the first-order line-mixing parameters using the Exponential Power Gap (EPG) scaling law. These new parameters are now available at HITRANonline.

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). (C) 2021 Elsevier Ltd. All rights reserved.

Methane (CH4) spectra in the nu(3) band near 3.3 m were measured for 0, 50, 150, 240, 320, and 400 Torr pressure of added hydrogen. The spectra were recorded using a high resolution Fourier transform spectrometer. The CH4 spectra were measured at 5 different tem peratures from room temperature up to similar to 1100 K. A multi-spectrum non-linear least squares fit method was used to determine the line parameters at each temperature. Voigt lineshape functions were used to determine the broadening and shifting of methane lines in the P and R branches. The temperature dependence of exponent parameters for the line width and the linear frequency shift coefficients were determined from a fit for temperatures ranging from 296 to 1098 K. The temperature dependence of the retrieved line broadening parameters was observed to follow the Double Power Law (DPL) proposed by Gamache and Vispoel. The temperature dependence of the pressure shift coefficients follows a linear trend for the first three temperatures. The pressure broadening parameters decrease with increasing temperature, and the pressure shift parameters increase with increasing temperature, especially for the first three temperatures. Finally, the dependence of pressure broadening (gamma(0)) and shift (delta(0)) parameters on the rotational quantum number (J) was studied. The pressure broadening coefficients decrease about 20-30% for temperatures up to 894 K and about 40% for 1098 K, with increasing J quantum number. A complete set of fitted parameters including line position (sigma), line intensity (S), pressure broadening (gamma(0) ), shifting (delta(0)), and their corresponding fitting errors are provided in supplementary tables. (C) 2021 Elsevier Ltd. All rights reserved.

This paper describes the updates of the line positions and intensities for the carbon dioxide transitions in the 2020 edition of the HITRAN spectroscopic database. The new line list for all 12 naturally abundant isotopologues of carbon dioxide replaces the previous one from the HITRAN2016 edition. This update is primarily motivated by several issues related to deficient HITRAN2016 line positions and intensities that have been identified from laboratory and atmospheric spectra. Critical validation tests for the spectro-scopic data were carried out to find problems caused by inaccuracies in CO2 line parameters. New sources of data were selected for the bands that were deemed problematic in the HITRAN2016 edition. Extra care was taken to retain the consistency in the data sources within the bands. The comparisons with the ex -isting theoretical and semi-empirical databases (including ExoMol, NASA Ames, and CDSD-296) and with available experimental works were carried out. The HITRAN2020 database has been extended by includ-ing additional CO2 bands above 8000 cm(-1), and magnetic dipole lines of CO2 were introduced in HITRAN for the first time by including the nu(2)+nu(3) band in the 3.3-mu m region. Although the main topic of this article is line positions and intensities, for consistency a recent algorithm for the line-shape parameters proposed in Hashemi et al. JQSRT (2020) was reapplied (after minor revisions) to the line list. (C) 2021 Elsevier Ltd. All rights reserved.

The H2O self- and foreign- in-band continua in the region 3400-3900 cm(-1) were experimentally determined for 296 and 353 K from multispectrum fitting results of line parameters using the Hartmann-Tran line profile (HTP) and Rosenkranz line mixing. The continua were extracted from the baselines which were determined in the microwindow-based multispectrum fits. Continua were then obtained by simultaneous fitting of all baselines from measurements containing continuum information. The self-continuum at 296 K was determined from self-broadened measurements and agrees with that determined from air broadened measurements. The overall shape and strength of the new self-continuum agrees with the CAVIAR results between 3600 and 3800 cm(-1) but differences exceed the stated uncertainties at higher and lower wavenumbers. Moreover, the new self-continuum is much smoother, has no gaps and is obtained with a high resolution of 2.4 cm(-1). The self-continuum was fitted as sum of modeled bound and quasi-bound dimer spectra. From rotational constants, the bound dimer parallel and perpendicular band shapes of the near prolate symmetric top molecule were calculated and used as kernels to fit fundamental wavenumbers, relative band intensities and partitioning of parallel and perpendicular band type, while the integral of the band intensities of the four fundamentals was fixed to published experimental/theoretical data. A dimerization constant for the bound dimer of K-Db = 0.026(2) atm(-1) and the quasi-bound dimer of K-Dq = 0.044(5) atm(-1) was derived from the fits. The foreign-continuum has no gaps, a spectral resolution of 6-16 cm(-1), and is about 40% smaller than the MT_CKD3.2 continuum model. It has a distinctly different shape showing a pronounced P-Q-R branch structure. The foreign-continuum shape is narrower than the monomer band shape which is also true for the MT_CKD3.2 continuum model. The CAVIAR foreign-continuum is much noisier but on average is in good agreement with the new measurements. (C) 2020 Elsevier Ltd. All rights reserved.

Abstract. We evaluate different sets of high-resolution ozone absorption cross-sectiondata for use in atmospheric ozone profile measurements in the Hartley andHuggins bands with a particular focus on BDM 1995 (Daumont et al. 1992; Brion et al., 1993; Malicet et al., 1995),currently used in our retrievals, and a new laboratory dataset byBirk and Wagner (2018) (BW). The BDM cross-section data have beenrecommended to use for retrieval of ozone profiles using spaceborne nadir-viewing backscattered ultraviolet (BUV) measurements since its improvedperformance was demonstrated against other cross-sections including Bass andPaur (1985) (BP) and those of Serdyuchenko et al. (2014) and Gorshelev etal. (2014) (SER) by the “Absorption Cross-Sections of Ozone” (ACSO) activity.The BW laboratory data were recently measured within the framework of theEuropean Space Agency (ESA) project SEOM-IAS (Scientific Exploitation of Operational Missions –Improved Atmospheric Spectroscopy Databases) to provide an advancedabsorption cross-section database. The BW cross-sections are made frommeasurements at more temperatures and in a wider temperature range than BDM,especially for low temperatures. Relative differences of cross-sectionsbetween BW and BDM range from ∼2 % at shorter UVwavelengths to ∼5 % at longer UV wavelengths at warmtemperatures. Furthermore, these differences dynamically increase by up to±40 % at cold temperatures due to no BDM measurements having beenmade below 218 K. We evaluate the impact of using different cross-sectionson ozone profile retrievals from Ozone Monitoring Instrument (OMI)measurements. Correspondingly, this impact leads to significant differencesin individual ozone retrievals by up to 50 % in the tropopause where thecoldest atmospheric temperatures are observed. Bottom atmospheric layersillustrate the significant change of the retrieved ozone values, withdifferences of 20 % in low latitudes, which is not the case in highlatitudes because the ozone retrievals are mainly controlled by a prioriozone information in high latitudes due to less photon penetration down tothe lower troposphere. Validation with ozonesonde observations demonstratesthat BW and BDM retrievals show altitude-dependent bias oscillations ofsimilar magnitude relative to ozonesonde measurements, much smaller thanthose of both BP and SER retrievals. However, compared to BDM, BW retrievalsshow significant reduction in standard deviation, by up to 15 %,especially at the coldest atmospheric temperatures. Such improvement isachieved mainly by the better characterization of the temperature dependenceof ozone absorption.

This note provides a citable reference to a database reconciliation effort for ozone pure rotational intensities. The permanent dipole moment and theoretical adjustments for centrifugal distortion are utilized in a predictive calculation that is compared to experimental measurements. This prediction has been available in the JPL spectral line catalog since 2005, but has been at odds with the HITRAN database, which was originally based on an atlas provided by Flaud (HITRAN 1992) and subsequently scaled by 4% (HITRAN 2004). Using a modified partition function and isotopic abundance factor, the JPL 2005 prediction has now been utilized to create a self-consistent entry as an interim update to the HITRAN2016 database. Ramifications of the intensity change for users of HITRAN versions 2004-2012 are discussed. New pure rotational intensity measurements were carried out supporting the validity of the JPL 2005 database. (C) 2019 Elsevier Ltd. All rights reserved.

The planned space-borne differential LIDAR instrument MERLIN uses two spectral points in the 1.64 um wavelength region to derive weighted CH4 columns. The differential spectroscopic error, i.e. column change by spectroscopic error due to different atmospheric profiles or different ground altitudes below 2 km, should be below 0.623 ppb (0.033%). Within the German-funded SMERLIN project, retrieval simulations are carried out to specify the error requirements for individual line parameters and for systematic error sources in the laboratory measurement and analysis. Cavity ring-down spectroscopy (CRDS) measurements of CH4 carried out at NIST are utilized to derive requirements. The HITRAN spectroscopic database would be too far from reality to be used for this task, especially since important line parameters as speed dependence and line mixing are missing. Excellent laboratory measurements are advantageous for deriving requirements especially since realistic fit residuals are available. It will be shown that the non-noise fit residuals of the multispectrum fitting of the laboratory spectra, even when they are below the 0.1% level, are the largest error source and that broadening of CH4 by H2O must be considered. The current status of the spectroscopic database on CH4 will be reviewed and the requirements for future measurements shown. Furthermore, the modification needs for the level 2 processor to fulfill requirements based on the findings of the retrieval study will be discussed.

On 13 October 2017, the Tropospheric Monitoring Instrument (TROPOMI) was launched on the Copernicus Sentinel-5 Precursor satellite in a sun-synchronous orbit. One of the mission's operational data products is the total column concentration of carbon monoxide (CO), which was released to the public in July 2018. The current TROPOMI CO processing uses the HITRAN 2008 spectroscopic data with updated water vapor spectroscopy and produces a CO data product compliant with the mission requirement of 10 % precision and 15 % accuracy for single soundings. Comparison with ground-based CO observations of the Total Carbon Column Observing Network (TCCON) show systematic differences of about 6.2 ppb and single-orbit observations are superimposed by a significant striping pattern along the flight path exceeding 5 ppb. In this study, we discuss possible improvements of the CO data product. We found that the molecular spectroscopic data used in the retrieval plays a key role for the data quality where the use of the Scientific Exploitation of Operational Missions – Improved Atmospheric Spectroscopy Databases (SEOM-IAS) and the HITRAN 2012 and 2016 releases reduce the bias between TROPOMI and TCCON due to improved CH4 spectroscopy. SEOM-IAS achieves the best spectral fit quality (root-mean-square, rms, differences between the simulated and measured spectrum) of 1.5×10-10 mol s−1 m−2 nm−1 sr−1 and reduces the bias between TROPOMI and TCCON to 3.4 ppb, while HITRAN 2012 and HITRAN 2016 decrease the bias even further below 1 ppb. HITRAN 2012 shows the worst fit quality (rms = 2.5×10-10 mol s−1 m−2 nm−1 sr−1) of the tested cross sections and furthermore introduces an artificial bias of about -1.5×1017 molec cm−2 between TROPOMI CO and the CAMS-IFS model in the Tropics caused by the H2O spectroscopic data. Moreover, analyzing 1 year of TROPOMI CO observations, we identified increased striping patterns by about 16 % percent from November 2017 to November 2018. For that, we defined a measure γ, quantifying the relative pixel-to-pixel variation in CO in the cross-track and along-track directions. To mitigate this effect, we discuss two destriping methods applied to the CO data a posteriori. A destriping mask calculated per orbit by median filtering of the data in the cross-track direction significantly reduced the stripe pattern from γ=2.1 to γ=1.6. However, the destriping can be further improved, achieving γ=1.2 by deploying a Fourier analysis and filtering of the data, which not only corrects for stripe patterns in the cross-track direction but also accounts for the variability of stripes along the flight path.