Stratospheric sulfate aerosols play a crucial role in the physical and chemical processes in the Earth’s atmosphere. They have a strong impact on climate by absorbing and scattering both incoming and outgoing radiation. The Atmospheric Chemistry Experiment Fourier Transform Spectrometer is recording infrared transmittance spectra of the Earth's limb from low Earth orbit (solar occultation). These infrared spectra provide accurate measurements of sulfate aerosol composition¹, but have difficulty providing information on physical properties such as the particle size distribution. In contrast, optical extinction measurements, such as from the SAGE III/ISS instrument on the International Space Station, provide information on physical properties, but little data on composition. In situ measurements, made from aircraft, with a mass spectrometer and laser light scattering provide some information on composition and reliable information on physical properties². By combining the information from satellite observations and in situ measurements, a more complete characterization of stratospheric sulfate aerosols has been obtained.

#1#2 P. Bernath, C. Boone, A. Pastorek, D. Cameron, M. Lecours (2023). Satellite characterization of global stratospheric sulfate aerosols released by Tonga volcano. Journal of Quantitative Spectroscopy and Radiative Transfer 299, 108520 DOI:10.1016/j.jqsrt.2023.108520

#1#2 D. Murphy, K. Froyd, I. Bourgeois, C. Brock, A. Kupc, J. Peischl, G. Schill, C. Thompson, C. Williamson, P. Yu (2021). Radiative and chemical implications of the size and composition of aerosol particles in the existing or modified global stratosphere. Atmos. Chem. Phys. 21, 8915 DOI:10.5194/acp-21-8915-2021.

Large volcanic eruptions create an enhanced layer of sulfate aerosols in the stratosphere. These sulfuric acid droplets persist for many months, altering the climate and stratospheric chemistry. Sulfate aerosols scatter sunlight back to space, cooling the surface of the Earth and absorb outgoing thermal radiation, heating the stratosphere. The calculation of the climate impact of sulfate aerosols depends on their physical properties such as droplet size and chemical composition. These properties are not well known, and this uncertainty contributes to the errors in climate model predictions. Here we derive the first empirical formula that predicts the composition of stratospheric sulfate aerosols from volcanic eruptions from the air temperature and water vapor pressure. Measurements of atmospheric infrared transmittance of the Hunga Tonga-Hunga Ha’apai sulfate aerosol plume by the Atmospheric Chemistry Experiment (ACE) satellite were analyzed to determine composition (weight percent of sulfuric acid) and median particle radius. These data are supplemented by measurements of the Raikoke and Nabro eruptions. Our analysis allows the properties of volcanic aerosols in the stratosphere to be predicted reliably in atmospheric models. Reference to publication: Bernath, P.; Boone, C.; Pastorek, A.; Cameron, D.; Lecours, M. Satellite characterization of global stratospheric sulfate aerosols released by Tonga volcano. JQSRT, 299, 108520 (2023)

The Atmospheric Chemistry Experiment Fourier Transform Spectrometer (ACE-FTS) measures infrared transmittance spectra of the atmosphere from low Earth orbit using the Sun as a light source (solar occultation)Bernath, P.F. The Atmospheric Chemistry Experiment (ACE). J. Quant. Spectrosc. Radiat. Transf. 2017, 186, 3–16. These spectra are used to derive altitude abundance profiles of more than 40 molecules, and properties of clouds and aerosolsLecours, M.J.; Bernath, P.F.; Sorensen, J.J.; Boone, C.D.; Johnson, R.M.; LaBelle, K. Atlas of ACE spectra of clouds and aerosols, Journal of Quantitative Spectroscopy and Radiative Transfer, Volume 292, 2022, 108361 The Doppler shifts on atmospheric lines can be used to determine line-of-sight windsBoone, C.D.; Steffen, J.; Crouse, J.; Bernath, P.F. Line-of-Sight Winds and Doppler Effect Smearing in ACE-FTS Solar Occultation Measurements. Atmosphere 2021, 12, 680. Line-of-sight winds are a new data product for version 5.0 of ACE-FTS processing. These winds are being validated through comparison with independent winds observations from meteor radars and from the ICON-MIGHTI satellite instrument. Bernath, P.F. The Atmospheric Chemistry Experiment (ACE). J. Quant. Spectrosc. Radiat. Transf. 2017, 186, 3–16.. Lecours, M.J.; Bernath, P.F.; Sorensen, J.J.; Boone, C.D.; Johnson, R.M.; LaBelle, K. Atlas of ACE spectra of clouds and aerosols, Journal of Quantitative Spectroscopy and Radiative Transfer, Volume 292, 2022, 108361. Boone, C.D.; Steffen, J.; Crouse, J.; Bernath, P.F. Line-of-Sight Winds and Doppler Effect Smearing in ACE-FTS Solar Occultation Measurements. Atmosphere 2021, 12, 680..

Reliable spectroscopic information, including that provided in the HITRAN databaseGordon, I. E., et al. JQSRT 277 (2022): 107949. is essential when interpreting data from high-resolution remote sensing spectrometers that monitor the concentrations of gases in the terrestrial atmosphere. High spectral resolution molecular absorption calculations over a wide spectral range and diverse parameter space using line-by-line models are often considered too slow to be used in operational retrieval algorithms. This is further slowed when using advanced line-shapes and line-mixing parametrizations that are available in HITRAN for many molecules (e.g. Hashemi et al.Hashemi, R., et al. JQSRT 271 (2021): 107735., which are necessary for accuracy. As an alternative, retrieval codes often rely on massive sets of pre-calculated tables of absorption cross-sections for target molecules that cover a representative set of environmental conditions. For some missions, such as the NASA Orbiting Carbon Observatory (OCO-2/3)Crisp, D., et al. Atmospheric Measurement Techniques 10.1 (2017): 59-81. molecular absorption coefficients are calculated off-line for a range of pressures, temperatures, and H₂O volume mixing ratios and stored in “ABSCO” lookup tablesPayne, V.H., et al. JQSRT 255 (2020): 107217. We present a Python tool (X-MASS) that allows the massive set of ABSCO tables to be calculated using the HAPI software packageKochanov, R.V., et al. JQSRT 177 (2016): 15-30.ith complete utilization of the parameters’ accuracy in HITRAN, including sophisticated line shapes. The outputs will be made available in the convenient HDF5 or NetCDF formats given user-defined wavenumber step, set of pressures, temperatures, and diluent gas contents. X-MASS will be an open-source library where users can specify parameters for their applications. In addition, a set of pre-calculated ABSCO tables covering spectral range of molecules at a finer grid and resolution will be provided on the HITRAN website. This work will facilitate the timely integration of state-of-the-art spectroscopic data into atmospheric radiative transfer codes.

---

Footnotes:

Gordon, I. E., et al. JQSRT 277 (2022): 107949., Hashemi, R., et al. JQSRT 271 (2021): 107735.) Crisp, D., et al. Atmospheric Measurement Techniques 10.1 (2017): 59-81., Payne, V.H., et al. JQSRT 255 (2020): 107217.. Kochanov, R.V., et al. JQSRT 177 (2016): 15-30.w

Global measurements of greenhouse gas sources and sinks require accurate molecular absorption models. These absorption models are frequently developed using laboratory gas cells in known concentrated conditions, and applied to satellite measurements over different unknown non-uniform gas conditions. Dual-comb spectroscopy (DCS) can bridge these measurements by measuring the same absorption features in relatively known, uniform atmospheric conditions. To this end, we present open-path DCS measurements at the Mauna Loa Observatory in Hawaii, co-located with the longstanding calibrated point measurements of CO₂, CH₄, and meteorology at the Mauna Loa site. Nocturnal measurements at this alpine site are typically unaffected by local sources and sinks, providing the most controlled conditions for comparison of open-path measurements and molecular absorption models with reference-gas-calibrated point sensors. We compare laboratory- and ab initio-derived CO₂ absorption models near 1600 nm in the 30012 and 30013 bands. Our fits using HITRAN2020 match the point sensors to 0.4%, although we also observe wavelength-dependence in the retrieved mole fraction.

Isoprene (C₅H₈) is an important hydrocarbon that is naturally produced by plants and is the most abundant nonmethane hydrocarbon in the atmosphere. Isoprene plays an important role in the formation of secondary organic aerosol particles and the production of ozone in the troposphere. Isoprene is also one of the simplest conjugated organic molecules, exhibiting interesting confromational properties with respect to rotation about the central C-C single bond. To better understand the spectral and structural properties of isoprene, we have measured the high-resolution rovibrational spectrum of the ν₂₆ band of isoprene near 992 cm⁻¹. The isoprene was cooled in a supersonic expansion to better resolve and analyze the rotational structure of this band and measured with a quantum cascade laser (QCL)-based spectrometer. This work complements previous spectra our group has obtained of room-temperature isoprene. We will present our analysis of the rovibrational spectrum and the insights the cold spectrum provides in the context of the room-temperature spectrum, as well as a comparison to anharmonic quantum calculations previously done in our group.

Furfural, C₄H₃OCHO, is a volatile organic compound produced from biomass burning. Its oxidation may lead to secondary organic aerosol formation, making necessary to provide accurate spectroscopic data to evaluate its impact. Two conformations exist in the gas phase, with the oxygen atoms oriented in a more stable trans configuration than the cis one. Durig et al.T. S. Little, J. Qiu, J. R. Durig, Spectrochimica Acta, 45A, 789-794 (1989)xploited far-infrared (IR) data to determine relative stabilities and the conformational barrier height. Motiyenko et al.R. A. Motiyenko, E. A. Alekseev, S. F. Dyubko, F. J. Lovas, J. Mol. Spectr., 240, 93-101 (2006), R. A. Motiyenko, E. A. Alekseev, S. F. Dyubko, J. Mol. Spectr., 244, 9-12 (2007)ealized extensive microwave spectroscopic studies providing molecular parameters in the ground state (GS) and low-lying vibrational states up to 400 cm⁻¹. No splitting due to the internal rotation of the CHO top has been observed in these states. We report here a high resolution study of furfural on a wide infrared range combining the Jet-AILES set-up, a continuous supersonic jet on the IR AILES beamline at SOLEIL, and a long path cell, both coupled to a Fourier Transform Spectrometer and the SPIRALES set-up at MONARIS, a pulsed supersonic jet coupled to mid-IR quantum cascade lasersP. Asselin, J. Bruckhuisen, A. Roucou, M. Goubet, M-A. Martin-Drumel, A. Jabri, Y. Belkhodja, P. Soulard, R. Georges, A. Cuisset, J. Chem. Phys. 151, 194302 (2019), S. Chawananon, O. Pirali, M. Goubet, P. Asselin, J. Chem. Phys. 157, 064301 (2022) In the fingerprint region, 7 bands of trans- and 2 bands of cis-furfural were recorded at low temperature. About 15000 lines from GS and 7(2) excited states (ES) of trans(cis-)furfural, respectively, were fitted within experimental accuracy. In the far-IR range, global fits including GS and ES rotational and ES rovibrational transitions enable to refine molecular parameters derived from the microwave study and to determine precise vibrational band centers.

---

Footnotes:

T. S. Little, J. Qiu, J. R. Durig, Spectrochimica Acta, 45A, 789-794 (1989)e R. A. Motiyenko, E. A. Alekseev, S. F. Dyubko, F. J. Lovas, J. Mol. Spectr., 240, 93-101 (2006), R. A. Motiyenko, E. A. Alekseev, S. F. Dyubko, J. Mol. Spectr., 244, 9-12 (2007)r P. Asselin, J. Bruckhuisen, A. Roucou, M. Goubet, M-A. Martin-Drumel, A. Jabri, Y. Belkhodja, P. Soulard, R. Georges, A. Cuisset, J. Chem. Phys. 151, 194302 (2019), S. Chawananon, O. Pirali, M. Goubet, P. Asselin, J. Chem. Phys. 157, 064301 (2022).

Rapid reactions of organic acids with Criegee intermediates provide efficient gas phase removal process and are proposed to trigger the formation of atmospheric aerosols. The gas phase reaction between the simplest Criegee intermediate, CH₂OO, and acetic acid has been investigated by pulsed discharge nozzle Fourier transform microwave spectroscopy (PDN-FTMW). Two low lying conformers of hydroperoxymethyl acetate (HOOCH₂OCOCH₃, HPMA),which serves as the dominate nascent product from this reaction, was observed in the discharged plasma of a CH₂I₂/O₂/acetic acid gas mixture. Due to the three-fold methyl internal rotation and the low barrier height of the hindered methyl rotation, most of the observed pure rotational transitions in 13-21 GHz show large splitting corresponding to the A/E components. The relative abundance of the two observed hydroperoxymethyl acetate isomers is in agreement with the calculated relative energy. Also, the dehydrated product of HPMA, formic acid anhydride (OHCOCOCH₃), was observed in this work.

The properties of our planet's atmosphere are derived from models which are mostly based on remote-sensing data. A detailed knowledge of spectra of various chemical compounds making up the terrestrial atmosphere is therefore crucial for understanding the various ongoing kinetic and dynamic processes. As these compounds are not isolated, intermolecular interactions involving most abundant atmospheric constituents - N₂ ( ∼  78%) and O₂ ( ∼  21%) - may affect the shapes of the spectra under consideration. In this context, oxygen spectra are of particular interest. O₂ is a prominent example of a diatomic molecule with a ³Σ ground-electronic term, and fine-structure resolved transitions involving this term have a wide range of applicability - from monitoring the plant vegetation status on the Earth's surface, to the quantification of the pollutant concentration in the atmosphere [J. Quant. Spectrosc. Radiat. Transf. 186, 118 (2017)]. Here, we consider the effect of O₂-N₂ collisions on the shape of the oxygen spectral lines from the theoretical point of view. Utilizing the exact close-coupling approach, we calculate the relevant scattering amplitudes based on the ab initio intermolecular potential energy surfaces (PESs). We account for the non-zero spin of the X³Σ⁻_g term of O₂ by performing a unitary transformation on the spin-free S-matrix, which allows us to compute the line shape parameters for the fine-structure resolved transitions in O₂ immersed in the bath of N₂. Having successfully tested our methodology against the experimental data in our previous study of the 118 GHz fine-structure transition in O₂(X³Σ⁻_g) [J. Chem. Phys. 155, 124307 (2021)], we apply it to the problem of N₂-perturbed lines in the oxygen A-band (i.e., electronic transition b¹Σ⁺_g← X³Σ⁻_g). This problem is more challenging as it requires the use of two PESs in the scattering calculations. The PESs used in this study were constructed automatically using the AUTOSURF code [J. Chem. Inf. Model. 59, 262 (2018)].

It has been long recognized that the spectroscopy of hydrogen halides (such as HF, HCl, HBr, and HI) provides a valuable tool, with atmospheric and astrophysical, as well as industrial applications [J. Quant. Spectrosc. Radiat. Transf. 130, 284 (2013)]. As far as the terrestrial-atmospheric studies are concerned, two halide species - HF and HCl - may be considered the most important of the bunch. Being the main reservoirs of fluorine and chlorine, HF and HCl are used as pollutant trace gases in the determination of the anthropogenic chlorofluorocarbons' concentration in the Earth's atmosphere. Since these molecular species are of the pollutant-type, their atmospheric spectra must neccesseriliy be considered as perturbed by intermolecular interactions involving the main constituents of our planet's atmosphere - N₂ and O₂. Indeed, the process of ozone depletion is mainly induced by Cl and ClO, which result from the photodissociation of HCl. Therefore, to be applicable in atmospheric science, the spectra of the terrestrial hydrogen halides must be considered in the presence of the surrounding molecular bath of N₂ and O₂. Here, we take a fully ab initio approach to the theoretical determination of atmospheric spectra of hydrogen halides, starting from the O₂-induced line shape perturbations. Taking the state-of-the-art potential energy surfaces (PESs) as our starting point, we perform close-coupling calculations of HF-O₂ and HCl-O₂ quantum scattering. We take a novel approach to the construction of the energy-level basis used in the close-coupling computations, and we study the convergence of the calculated cross sections against the size of the employed basis in full detail. Further, we calculate the temperature-dependent line shape parameters of the O₂-perturbed rotational R(0) line in HCl. The PESs used in this study were constructed automatically using the AUTOSURF code [J. Chem. Inf. Model. 59, 262 (2018)].

Ab initio SCF MRD-CI electronic structure calculations are carried out in C_s symmetry for the three lowest ²A′ and ²A" states of NO₂. The four lowest-lying species correspond, respectively, to X²A₁(1²A′), A²B₂(2²A′), B²B₁(1²A") and C²A₂(2²A") in C_2v symmetry. A cc-pVQZ basis set augmented with s, p, and d Rydberg functions is employed together with an extensive treatment of electron correlation. One-dimensional (1D) potential energy curves (PECs), associated to the equilibrium geometrical parameters of the X²A₁ ground state (GS), were computed for the four aforementioned doublet states located in the near-IR and visible regions. These 1D cuts are related loosely to the bending, symmetric stretch and asymmetric stretch vibrational modes of the GS. The PECs are then employed to discuss the NIR-Vis absorption/emission spectroscopy of NO₂ and the strong conical intersection (CI) between the ground and first excited A²B₂ states. In spite of being a small triatomic made up of 2nd-row atoms only, NO₂, a relatively ßimple" molecule turns out to be an important testing ground for the study of a number of fundamental molecular processes such as the dynamics of intramolecular energy redistribution which involves the coupling of electronic and nuclear motions and entails the breakdown of the adiabatic BO approximation (e.g., the <̃span class="roman">A-<̃span class="roman">X conical intersection, Renner-Teller interactions, electronic-to-vibrational (E-V) energy transfer, relaxation of vibrational energy), and for studies of the dynamics of photodissociation/predissociation and photoionization/autoionization. The NO₂ PECs reported in this work may be instrumental to visualize some of the above processes.