The introduction of the CP-FTMW technique by Pate and co-workers has revolutionized the rotational spectroscopy field providing rapid acquisition of broadband spectra.Brown, G. G.; Dian, B. C.; Douglass, K. O.; Geyer, S. M.; Shipman, S. T.; Pate, B. H. Rev. Sci. Instrum. 2008, 79 (5), 053103he design of a newly constructed chirped-pulse Fourier transform microwave spectrometer CP-FTMW covering the range of 6-18 GHz will be presented. In particular, the chirped pulse (6-18 GHz, 4 μs) is generated by a fast-arbitrary waveform generator (AWG, Keysight M8195A 65 GSa/s). Free Induction Decays (FID) are detected and collected on a recent generation of a fast oscilloscope (Keysight DSOZ634A 160 GSa/s). The high speed of the oscilloscope allows to achieve a high spectral resolution (FWHM better than 40 kHz) by recording the FID during 80 μs. Up to three pulsed nozzles can be used simultaneously.Seifert, N. A.; Steber, A. L.; Neill, J. L.; Pérez, C.; Zaleski, D. P.; Pate, B. H.; Lesarri, A. Phys. Chem. Chem. Phys. 2013, 15 (27), 11468–11477he CP-FTMW spectrometer is currently used to study volatile organic molecules of atmospheric interest. The results of this work will be discussed in detail. Hervé Damart and Gauthier Dekyndt are gratefully acknowledged for their technical assistance. The present work was funded by the ANR Labex CaPPA, by the Regional Council Hauts-de-France, by the European Funds for Regional Economic Development, and by the CPER CLIMIBIO and CPER P4S.

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Footnotes:

Brown, G. G.; Dian, B. C.; Douglass, K. O.; Geyer, S. M.; Shipman, S. T.; Pate, B. H. Rev. Sci. Instrum. 2008, 79 (5), 053103T Seifert, N. A.; Steber, A. L.; Neill, J. L.; Pérez, C.; Zaleski, D. P.; Pate, B. H.; Lesarri, A. Phys. Chem. Chem. Phys. 2013, 15 (27), 11468–11477T

The water vapour self- and foreign-continuum are newly measured at room temperature in the high energy edge of the 1.25 cm⁻¹ window by using highly stable and sensitive cavity ring-down spectroscopy (CRDS). Self-continuum cross-sections, C_S, are derived between 8290 and 8620 cm⁻¹ at 29 selected spectral points by using pressure ramps (up to 15 Torr) of pure water vapour. Purely quadratic pressure dependence is obtained for the absorption coefficient at each measurement point. Although the spectral measurement points were chosen to minimize the contribution of resonance line absorption, the latter represents between 30 and 70 % of the measured absorption in the studied region. The self-continuum measurements are found consistent with a previous study of the low-frequency edge of the 1.25 cm⁻¹ window (Campargue et al. J Geophys Res Atmos 2016;121:13,180 – 13,203. doi:10.1002/2016JD025531). The frequency dependence of the retrieved C_S values shows an overall good agreement with the MT_CKD values. Nevertheless, an additional broad absorption feature is observed with a centre near 8455 cm⁻¹. It is tentatively interpreted as a possible impact of the uncertainties on the resonance line contribution on the derived C_S values or as possible evidence of a band of the bound dimers, (H₂O)₂ Foreign-continuum cross-sections, C_f, are derived for humidified nitrogen, humidified oxygen and humidified air between 8120 and 8500 cm⁻¹ by using pressure ramps (up to 750 Torr with 10000 ppm of H₂O) at 5 selected spectral points for each gas mixture. Although data treatment is in progress, the H₂O-air and H₂O-N₂ C_f cross-section values seem to be comparable while the H₂O-O₂ C_f value appears to be significantly smaller. A satisfactory agreement of the retrieved C_f for H₂O-air mixture with the MT_CKD model is demonstrated. To the best of our knowledge, it is the first H₂O-air, H₂O-N₂, H₂O-O₂ foreign-continuum study in this frequency range.

Large wildfires inject smoke and biomass burning products into the midlatitude stratosphere where they destroy ozone, which protects us from ultraviolet radiation. The infrared spectrometer on the Atmospheric Chemistry Experiment (ACE) satellite has measured the spectra of smoke particles from the Black Summer Australian fires in late 2019 /early 2020, demonstrating that they contain oxygenated organic functional groups and water adsorption on the surfaces. The injected smoke particles produce unexpected and extreme perturbations in stratospheric gases beyond any seen in the previous 15 years of measurements: increases in formaldehyde, chlorine nitrate, chlorine monoxide and hypochlorous acid, and decreases in ozone, nitrogen dioxide and hydrochloric acid. These perturbations in stratospheric composition have the potential to affect ozone chemistry in unexpected ways.

Following the eruption of the Raikoke volcano in 2019, infrared spectra from the Atmospheric Chemistry Experiment satellite¹ were used to evaluate the composition of stratospheric aerosols in the Northern Hemisphere. The layer of aerosols observed after the eruption ranged from 9 to 20 km in altitude and persisted in the stratosphere for several months. This layer was composed nearly entirely of sulfate aerosols, droplets of a mixture of sulfuric acid and water. To determine the aerosol composition, the spectra were modeled using extinction values calculated with Mie scattering code and sulfuric acid optical constants. Contrary to previous reports, there is no evidence of stratospheric smoke being present in the Arctic region. ¹P. F. Bernath. The Atmospheric Chemistry Experiment (ACE). JQSRT 2017;186:3-16. https://doi.org/10.1016/j.jqsrt.2016.04.006.

Clouds and aerosols play a vital role in the Earth’s climate. Detecting polar mesospheric clouds, polar stratospheric clouds and aerosols is useful for monitoring climate change and atmospheric chemistry. ACE satellite data¹ is used to provide an infrared spectral atlas of polar mesospheric clouds, three types of polar stratospheric clouds (nitric acid trihydrate, sulfuric/nitric acid ternary solutions, and ice), cirrus clouds, smoke from fires, and sulfate aerosols. Nearly all example spectra have been modeled with either Mie scattering or T-matrix codes using the appropriate optical constants. ¹P. F. Bernath. The Atmospheric Chemistry Experiment (ACE). JQSRT 2017;186:3-16. https://doi.org/10.1016/j.jqsrt.2016.04.006.

Ozonolysis of alkenes is an important oxidation pathway of alkenes in the troposphere because it is involved in the production of organic aerosol and OH radicals. The mechanism of ozonolysis of alkenes involves the formation of a primary ozonide (POZ), which then decomposes into a carbonyl and a high-energy carbonyl oxide (Criegee intermediate). Criegee intermediates are produced with a broad internal energy distribution. High energy Criegee intermediates decompose into atmospherically important compounds (e.g. vinoxy, OH radical). Stabilized Criegee intermediates (sCIs) undergo reactions to produce secondary ozonides and organic aerosols. Cavity ring-down spectroscopy (CRDS) was utilized in combination with chemical titration with sulfur dioxide (SO₂) to quantify sCIs. The reaction is carried out under various flow and low-pressure conditions. Reference cross-sections of products and reactants are fitted with spectral features to obtain product number densities. The yields of sCIs were measured at different low pressures and the nascent yields were determined by extrapolation to zero pressure. Endocyclic alkenes (cyclopentene and cyclohexene) show no sCI production at the pressures studied. However, acyclic alkenes show pressure-dependent sCI yields. Formaldehyde oxide (CH₂OO) from the alkenes studied (propene, 1-butene and isoprene) has a high nascent yield due to its relatively high energy barrier for dissociation. Cis-2-butene produces higher nascent sCI than trans-2-butene, possibly due to different syn- and anti-CI branching ratios, or different POZ conformations. There is an indication that alkenes larger than 2,3-dimethyl-2-butene would have higher nascent sCI yields. The information on low-pressure yields from the current studies can be used as a benchmark for theoretical calculations.

Methane sulfonamide (CH₃S(=O)₂NH₂, MSAM) is an important trace compound detected for the first time in ambient air over the Red Sea and the Gulf of Aden.Edtbauer, A. et al. Atmos. Chem. Phys. 2020, 20, 6081.^,Berasategui, M. et al. Atmos. Chem. Phys. 2020, 20, 2695.he average mixing ratios of this compound were found to be in the range of 20 – 50 ppt with a maximum value of 60 ppt.^b,c The energetics and rate coefficients for its reactions with Cl radical and in presence of atmospheric oxygen (^3O_2) to form various products have not been reported. In the present work, we investigated the atmospheric oxidation mechanism and energetics of the reaction of MSAM with Cl radicals using high level quantum chemistry calculations. The MSAM + Cl radical reaction mainly proceeds by H−abstraction paths. Abstraction of H−atom from the methyl group of MSAM by Cl radical to form CH_2S(=O)_2NH_2 radical + HCl products was found to be dominant compared to other possible paths. The barrier height for this reaction was found to be 4.8 kcal mol^-1 above the energy of the starting reactants at the CCSD(T)/aug−cc−pV(T+d)Z//M06−2X/aug−cc−pV(T+d)Z level. The rate coefficients were calculated for all possible H−atom abstraction paths associated with the MSAM + Cl radical using canonical variational transition state theory (CVT) with a small curvature tunneling (SCT) approximation in the temperatures between 200 and 300 K. The rate coefficient data, atmospheric lifetime of MSAM, branching ratios and thermodynamic parameters associated with the MSAM + Cl radical reaction are discussed. In addition, the atmospheric fate of the major product (i.e., the CH_2S(=O)_2NH_2 radical) with respect to its interaction with ^3O_2 to form the RO_2 radical adduct (R = −CH_2S(=O)_2NH_2) using the same level of theory was also investigated. The formed RO_2 radical adduct proceeds through various multichannel pathways in the presence of HO_2 radical to form several greenhouse gases and environmental pollutants including SO_2, CO_2, CO, HC(O)OH and HNO_3 Berasategui, M. et al. Atmos. Chem. Phys. 2020, 20, 2695.T

l0pt Figure We present the first (preliminary) investigation of the ν₁ band (OH stretching mode) of Nitric acid (HNO₃) centered at 3551.766 cm⁻¹ using high resolution Fourier transform spectra. These spectra were recorded in the 2.5 μm to 3.23 μm spectral regions on the spectrometer located on the AILES beamline of the SOLEIL synchrotron. Because of the large value of the Doppler linewidth (about 0.003 cm⁻¹) in the 2.8 μm region at 220 K or 296 K), the analysis was very complex and often uncertain and dubious. Furthermore, the ν₁ band is severely affected by numerous perturbations. Among these ones, unexpected line splittings were observed during all the analyses. Finally we have generated a preliminary list of "reasonable" line positions and intensities for the ν₁ band and of the ν₁+ν₉-ν₉ bands and ν₁+ν₇-ν₇ hot bands.

Methylfurans (MF) are methylated aromatic heterocyclic volatile organic compounds (VOCs) and primary or secondary pollutants in the atmosphere due to their capability to form atmospheric particles such as secondary organic aerosols (SOAs).¹ MFs are produced by cracking biomass such as wood combustion and the pyrolysis of biomass, lignin and cellulose.² Therefore there is a fundamental interest to monitor these molecules in the gas phase. The high resolution spectroscopic studies of methylated furan compounds, except 2-MF³, are generally limited to pure rotational spectroscopy in the ground state. This might be explained by the difficulties arisen from the internal rotation with a medium barrier and the complexity of the vibrationally excited state rotational spectra. As Finneran et al. for 2-MF, we faced the same difficulties for 3-MF to treat the first torsional state (ν_t=1) using a local approach (XIAM⁴) and therefore the global treatment, including all torsional levels given by the BELGI code⁵, was used. This gave us access to the V₆ term characterising the anharmonicity of the potential, together with some higher order perturbation and coupling terms. Carrying out a BELGI global fit of ν_t=0 and ν_t=1 states using our new assignment for 3-MF and the assigned transitions of Finneran et al. for 2-MF enabled us to compare the molecular parameters of these two isomers.

O₂ is important for spectroscopic applications in the IR, Visible and UV regions. In this work eight lowest electronic states were studied using the CASSCF and MRCI methods and the AV5Z basis sets with the D₂h point group symmetry, namely X ³Σ⁻_g, A ³Σ⁺_u, A′ ³∆_u, a ¹∆_g, b ¹Σ⁺_g, c ¹Σ⁻_u (bound), C ³Π_g, d ¹Π_g (unbound). Potential energy curves (PECs) for 8 electronic states and spin-orbit coupling, electronic angular moment and transition quadrupole moment curves for the five states X ³Σ⁻_g, a ¹∆_g, b ¹Σ⁺_g, d ¹Π_g and C ³Π_g, were computed and used to predict rovibronic spectra and lifetimes of O₂. Our aim is to construct an accurate ro-vibronic molecular line list for O₂. This will require an empirical refinement of the ab initio curves and will be considered in our future work.

New particle formation (NPF) comprises a substantial part of secondary aerosol particle formation in the atmosphere, and these particles play an important role in the radiative forcing balance governing climate change. Significant uncertainties in current global climate models persist in part due to the uncertainty surrounding NPF growth mechanisms. Establishing the surface structure and growth mechanisms of early-stage NPF clusters is necessary to develop accurate descriptions of particle formation and growth rates that can be included in climate models. Clusters containing ammonium, bisulfate, and water have previously been studied via mass spectrometry coupled with infrared spectroscopy as well as via quantum chemical calculations which provided structural and bonding information as well as potential isomer stability. Here we focus on an emerging class of clusters made of ammonium and iodate, which may be important for particle formation in coastal and polar regions. Cationic clusters containing zero, one, or two ammonia, iodic acid, and diiodide pentoxide molecules are the focus of this study. Ammonia appears to stabilize the clusters and promote the formation of larger iodine oxides with presumably lower vapor pressure, which would be expected to lead to higher stability and faster growth. Halogen bonding competes with hydrogen bonding in determining the minimum energy structures of these clusters. These studies are key benchmarks for computational efforts to model these clusters for their inclusion in larger-scale modeling efforts.