The ACE (http://www.ace.uwaterloo.ca/) [1,2] satellite has been measuring atmospheric composition by solar occultation from low Earth orbit since 2004. The primary ACE instrument is a high-resolution (0.02 cm⁻¹) infrared Fourier transform spectrometer (ACE-FTS). ACE-FTS version 5.2 processing yields altitude profiles for the concentrations of 46 molecules as well as spectra of clouds and aerosols. After 20 years on orbit, ACE data are useful for climate change observations. Topics covered will include measurements of greenhouse gases, gases associated with the Montreal Protocol on Substances that Deplete the Ozone Layer, fire emissions [3] and volcanic eruptions. ACE-FTS measures infrared spectra of aerosols and clouds by removing gas phase features to leave “residual” spectra. ACE retrievals depend on laboratory measurements and the presentation will highlight desirable improvements to spectroscopic data. [1] P. F. Bernath, The Atmospheric Chemistry Experiment (ACE), J. Quant. Spectrosc. Rad. Transfer 186, 3-16 (2017). [2] P. F. Bernath, The Atmospheric Chemistry Experiment (ACE): Aerosol and Gas Analysis from Orbit, Trends in Analytical Chemistry 166, 117207 (2023). [3] P. Bernath, C. Boone and J. Crouse, Wildfire smoke destroys stratospheric ozone, Science 375, 1292-1295 (2022).

The comprehensive updates of hydrogen cyanide (HCN) and methyl cyanide (CH₃CN) line lists in the HITRAN database address current needs in both atmospheric monitoring and astrophysical research. In this work, we validated the MOMeNT-90 Mellau, G. C., et al. JQSRT, 270, 107666, 2021.       ^c Müller, H. S., et al. JMS, 378, 111449, 2021.ine list, which was proposed for updating the HCN line list in HITRAN. This line list utilizes a new, highly accurate potential energy surface (PES) and an ab initio dipole moment surface (DMS). It combines line intensities calculated through variational methods with line centers derived from experimental energy levels, where possible. Furthermore, the list includes broadening parameters. Extensive validations against experimental cross-sections were carried out, and further semi-empirical improvements have been introduced. Regarding CH₃CN, line parameters were initially included in HITRAN2008, with no updates since then. The predictions were based on a combined fit of rotational and ro-vibrational data involving states up to v₈=2, which were published by Müller Müller, H. S., et al. JMS, 312, 22-37, 2015.              ^d Gordon, I. E., et al. JQSRT, 277, 107949, 2021. The analysis took into account interactions between these states as well as interactions of v₈=2 with v₄=1, v₇=1, v₈=3 and the last three states with each other. This project constructs all available CH₃CN data into a HITRAN-formatted line list, validating it against existing measurements and calculations. The new update is based on a more recent study^c which includes accurate rotational and rovibrational data of v₄=1, its associated ν₈ hot band, and slight extensions of the lower state data. To complement the updates to HCN and CH₃CN line lists in the HITRAN database^d, a brief update to the software tool for calculating look-up tables of cross-sections of different molecules within the HITRAN database under a custom set of pressures, temperatures and volume mixing ratios - X-MASS - will be presented.

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Footnotes:

Mellau, G. C., et al. JQSRT, 270, 107666, 2021.       ^c Müller, H. S., et al. JMS, 378, 111449, 2021.l Müller, H. S., et al. JMS, 312, 22-37, 2015.              ^d Gordon, I. E., et al. JQSRT, 277, 107949, 2021..

l0pt Figure Ammonia is the most abundant alkaline atmospheric gas. It is emitted by a range of anthropogenic sources, most notably through the use of nitrogen-based fertilizers in agriculture. Ammonia plays a major role in the formation of PM_2.5, which can significantly affect human health.K. E. Wyer et al., J. Environ. Manage. 2022, 323, 116285.t also contributes to visibility degradation and to the atmospheric deposition of nitrogen on sensitive ecosystems. The interpretation of satellite remote sensing measurements to determine the amounts of trace gases such as ammonia in the atmosphere requires accurate radiative transfer calculations. These in turn are heavily reliant on accurate spectroscopic line parameters, which are best derived from high quality laboratory measurements. The HITRAN 2020 database reports line parameters for ammonia that are derived from both theoretical calculations and laboratory measurements.I.E. Gordon et al., J. Quant. Spectrosc. Radiat. Transfer 2022, 277,107949.n this work, we report the measurement of new, high-resolution infrared spectra of pure and air-broadened ammonia. Using a multispectrum fitting approach, we determine new spectroscopic line parameters for the NH₃ 0100 00 0 s ← 0000 00 0 a and 0100 00 0 a ← 0000 00 0 s bands (using HITRAN notation), including the first reported values of self and foreign pressure-induced shifts.

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Footnotes:

K. E. Wyer et al., J. Environ. Manage. 2022, 323, 116285.I I.E. Gordon et al., J. Quant. Spectrosc. Radiat. Transfer 2022, 277,107949.I

Carbon dioxide isotopologues have been studied thoroughly in the troposphere. In the stratosphere, the minor isotopologues have anomalous abundances due to their exchange reactions with isotopically fractionated ozone, differentiating them from tropospheric CO₂[1]. This anomalous abundance continues into the mesosphere but with no experimental observations, just model predictions[2]. The Atmospheric Chemistry Experiment Fourier Transform Spectrometer (ACE-FTS) is recording infrared transmittance spectra of the Earth’s limb from low Earth orbit (solar occultation). These infrared spectra provide accurate measurements of global CO₂ isotopologue volume mixing ratios (VMRs) from the upper troposphere to the lower thermosphere. Data for the O¹³CO, OC¹⁷O and OC¹⁸O isotopologues will be presented, including seasonal altitude-latitude VMR distributions. The role of photolysis in the upper atmosphere for OC¹⁷O and OC¹⁸O fractionation has been analyzed and found to be “mass independent”.

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Thiemens MH, Jackson T, Mauersberger K, Schueler B, Morton J. Oxygen isotope fractionation in stratospheric CO2. Geophys. Res. Lett.1991; 18, 4, https://doi.org/10.1029/91GL00121

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Liang M, Blake GA, Lewis BR, Yung YL. Oxygen isotopic composition of carbon dioxide in the middle atmosphere. Proceedings of the National Academy of Sciences 2007; 104. https://doi.org/10.1073/pnas.0610009104.

Carbon dioxide (CO₂) is a strong absorber of infrared radiation. Its continuing increase in the Earth’s atmosphere is a strong driver of climate change. CO₂ has a range of absorption bands throughout the infrared that are measured in the Earth's atmosphere by satellite-borne atmospheric sounders. These bands are utilised in a range of applications, for example monitoring the concentration of CO₂ in the atmosphere, and determining atmospheric temperature profiles for numerical weather prediction. The motivation for this work is to provide experimental non-Voigt line parameters in order to improve the representation of CO₂ in atmospheric radiative transfer codes. It is recognised that the Voigt profile is inadequate in accurately modelling spectra measured at high signal-to-noise ratio. Measurements of pure and air-broadened CO₂ spectra at 296 K over a wide range of pressures were obtained using a Bruker IFS 125 HR spectrometer at the Physikalisch-Technische Bundesanstalt (PTB) in Braunschweig, Germany. The measurements of sample pressure, temperature, and pathlength are all SI-traceable. A multispectrum fitting analysis has been performed using the LabFit program to derive non-Voigt lineshape parameters for various bands of CO₂. Line positions and intensities have been constrained using quantum mechanical expressions. Comparisons between the derived parameters and those from the HITRAN2020 molecular spectroscopic database are made.

Previously we produced precision measurements of H₂O transitions broadened by O₂, which has improved intermolecular potential of the collisional pair, H₂O-O₂, to a sub-percent level [See Gamache et al. Gamache et al. 2023 https://doi.org/10.1080/00268976.2023.2281592. Now it has turned out that the modeling precision of Air-broadened spectroscopy of H₂O is limited by the accuracy of the intermolecular potential of the H₂O-N₂, which can be semi-empirically determined from the measurements of line shape parameters. Thus we have extended our laboratory measurements to H₂O transitions broadened by N₂. For this, we have obtained three N₂-mixture spectra in the ν₂ band at room temperature using the same high-resolution Fourier transform spectrometer (Bruker IFS-125HR) at the Jet Propulsion Laboratory, as previously used for the O₂-broadened H₂O study. For the sake of the best consistency, we have also used the same high-precision spectrum fitting package, Labfit, which adopts non-linear least squares curve fitting algorithm based on a Voigt line shape profile. We have retrieved their line widths and pressure-shifts by fitting all the three N₂-broadened H₂O spectra simultaneously, but while holding their line intensities to the HITRAN values. Results from this work have been combined to derive Air-broadened widths and pressure-shifts, which are compared to be lower than the HITRAN values by about 2 - 5%, depending transitions. Using these new measurements, the H₂O-N₂ intermolecular potential for the Complex Robert-Bonamy-Ma calculations is being developed, which leads to precision modeling of Air-broadened line shape parameters for the H₂O transitions in the entire infrared region. In this talk, we present and discuss the retrieval methodology and the improvement on the modeling of the line shape parameters expected for the rovibrational transitions of H₂O in the infrared.Government support acknowledged.html:<hr /><h3>Footnotes: Gamache et al. 2023 https://doi.org/10.1080/00268976.2023.2281592] Government support acknowledged.

In this work, accurate measurements on the line intensity of the HO₂ ν₃ transitions near 9 μm have been demonstrated. In the experiment, HO₂ is generated by flash photolysis of the flowing gas mixture of Cl₂/CH₃OH/O₂. Upon photolysis, Cl atom is produced and reacts with CH₃OH to form HCl and CH₂OH. CH₂OH further reacts with O₂ to generate HO₂ and HCHO. By employing synchronized two-color time-resolved dual-comb spectroscopy near 3 and 9 μm, high-resolution spectra of both HCl and HO₂ radical can be obtained simultaneously to further derive the absolute line intensity of HO₂ transitions. Moreover, the high-resolution spectral measurements of HO₂ from 1100 to 1150 cm⁻¹ with a spectral resolution of  ∼  0.0005 cm⁻¹ are carried out by using spectrally interleaved, comb-mode-resolved dual-comb spectroscopy.

The hydroxyl radical (OH) is one of the most important species in physical chemistry, atmospheric science, and astrophysics. In both field observations and laboratory studies, quantitative determination of the OH radical is essential to decipher the complex oxidation chain reactions as well as to evaluate the oxygen abundance in the stars. Herein, we report direct measurements of line strengths of the OH ν₁ transitions via simultaneous determination of H₂O₂ and OH in the 248-nm photolysis of H₂O₂ reaction system using synchronized two-color time-resolved dual-comb spectroscopy.P.-L. Luo and I-Y. Chen, Anal. Chem. 94, 5752 (2022).igh-resolution spectra of H₂O₂ and OH were simultaneously measured at a time resolution of tens of μs for studying the photodepletion of H₂O₂, the formation of OH radicals, the reaction between OH and H₂O₂, and then further to determine the absolute line strength of the OH ν₁ transitions. By fully analyzing the high-resolution time-resolved dual-comb spectra, accurate measurements of the line strength of ten OH transitions near 3378, 3408, 3422, 3465, and 3484 cm⁻¹ were achieved with a small uncertainty of less than 10%.C.-W. Chang, I-Y. Chen, C. Fittschen, and P.-L. Luo, J. Chem. Phys. 159, 184203 (2023).html:<hr /><h3>Footnotes: P.-L. Luo and I-Y. Chen, Anal. Chem. 94, 5752 (2022).H C.-W. Chang, I-Y. Chen, C. Fittschen, and P.-L. Luo, J. Chem. Phys. 159, 184203 (2023).

Physically realistic lineshape models that precisely describe molecular absorption spectra are critical to accurate spectroscopic intensity and amount-of-substance determination in both laboratory and remote sensing applications. We recently employed cavity ring-down spectroscopy to demonstrate a pressure-dependent broadening of the molecular lineshape that depletes intensity from the core of the line and redistributes it to its far wings leading to super-Lorentzian shapes [1]. This phenomenon is not modeled in the IUPAC-recommended Hartmann-Tran Profile [2], which was derived based on the impact approximation in which collision events are assumed to be instantaneous. However, molecular dynamics simulations show that the observed intensity depletion is due to the finite duration of collisions between the absorber and collision partner. Notably, typical collision time scales are orders of magnitude shorter than the characteristic time between collisions- the latter dominating the width of homogenously broadened lines. From Fourier analysis, the resulting spectral widths from these two mechanisms scale inversely with the respective time scales. Here we present high-precision cavity ring-down spectroscopy line intensity measurements of several transitions in the (3-0) band of N₂-broadened ¹²C¹⁶O exhibiting trends consistent with theoretical predictions. This effect is manifest as a pressure-dependent depletion in the experimental intensity obtained by integrating over only the core region of the absorption feature and its near wings. This mechanism is expected to have a significant effect on the accuracy of molecular sensing across atmospherically relevant species. Here we discuss its physical basis and how it influences far-wing line shapes. We also consider how this effect may be included in remote sensing retrievals and line-by-line molecular databases. [1] Z.D. Reed et al Phys. Rev. Lett. 130, 143001, 2023 [2] J. Tennyson et al Pure and Applied Chemistry 86, 12, 2014

Molecular oxygen (O₂) has a well-known and uniform molar fraction within Earth’s atmosphere. Because of this, the near-infrared O₂ molecular absorption bands, centered at 0.76 μm and 1.27 μm, are commonly used in remote sensing (TCCON, COCCON) and satellite missions (GOSAT, OCO-2/3, SCIAMACHY) to measure atmospheric air mass. For these missions, physics-based spectroscopic models and experimentally and theoretically determined line-by-line parameters are used to predict the temperature- and pressure-dependence of the absorption cross-section as a function of wave number, pressure, temperature, and water vapor concentration. The accuracy of atmospheric retrievals requires the spectroscopic model to describe all relevant physics. The Hartmann-Tran line profile is recommended for high-resolution spectroscopy and can be reduced to the well-known Voigt profile [1]. This profile assumes that collisions occur instantaneously. However, when this assumption is not satisfied, the finite duration of collisions leads to a redistribution of the line intensity from the line cores to the far wings, an effect that increases with pressure [2-4]. Failure to account for this redistribution leads to an apparent depletion in the core line intensity as a function of pressure, such that retrieved concentration would yield a bias if determined from absorption cross-section based on observations acquired at substantially different pressures. This work reports cavity ring-down spectroscopy measurements of line intensities and pressure-dependent intensity redistribution in the 0.76 μm O₂ band. The core intensity depletion magnitude and rotational quantum number dependence are compared to those calculated by renormalized classical molecular dynamic simulations. Additionally, we discuss how including the previously unaccounted-for physics might affect satellite and remote sensing retrievals. [1] Tennyson, J., et al., Pure and Applied Chemistry, 2014. 86(12): p. 1931-1943. [2] Reed, Z.D., et al., Phys Rev Lett, 2023. 130(14): p. 143001. [3] Tran, H., et al., J Chem Phys, 2023. 158(18). [4] Tran, H., et al., Phys Chem Chem Phys, 2023. 25(15): p. 10343-10352.