Several experimental studies have shown that 1,4-hydroxycarbonyls can undergo sequential reactions involving cyclization followed by dehydration to form dihydrofurans.Atkinson, R. et al. Atmos. Environ. 2008, 42, 5859; Ranney, A. P.; Ziemann, P. J. J. Phys. Chem. A 2016, 120, 2561.s dihydrofurans contain a double bond, they are highly reactive towards OH, O₃, and NO₃ in the atmosphere. In this work, we investigate the energetics and kinetics of the cyclization and dehydration reaction steps associated with 4-hydroxybutanal (4-OH-BL), a prototypical 1,4-hydroxycarbonyl molecule using ab initio calculations. The cyclization step transforms 4-OH-BL into 2-hydroxytetrahydrofuran (2-OH-THF), which can subsequently undergo dehydration to form 2,3-dihydrofuran. Since the barriers associated with the cyclization and dehydration steps for 4-OH-BL are respectively 34.8 and 63.0 kcal/mol in the absence of any catalyst, both reaction steps are not feasible under atmospheric conditions. However, the presence of a suitable catalyst can significantly reduce the reaction barriers. Therefore, we investigate the effect of a single molecule of H₂O, HO₂ radical, HC(O)OH, HNO₃, and H₂SO₄ as catalysts on the reaction. We find that H₂SO₄ lowers the reaction barriers the greatest, with the barrier for the cyclization step being reduced to -13.1 kcal/mol and that for the dehydration step going down to 9.2 kcal/mol, below their respective separated starting reactants. Interestingly, our rate calculations shows that HNO₃ provides the fastest rate due the combined effects of larger atmospheric concentration and reduced barrier. Thus, our study suggests that with acid catalysis the cyclization reaction step can readily occur for 1,4-hydroxycarbonyls in the gas phase. The 2-OH-THF products, once formed, likely undergo reaction with OH radicals in the atmosphere because the dehydration step involves a large barrier even with acid catalysis. The reaction pathways and rate constant for this reaction in the presence of molecular oxygen (³O₂) were also investigated using computational chemistry over the 200-300K temperature range. The main products found from the 2-OH-THF + OH/³O₂ reactions are succinaldehyde + HO₂ and 2,3-dihydro-2-furanol + HO₂.

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Footnotes:

Atkinson, R. et al. Atmos. Environ. 2008, 42, 5859; Ranney, A. P.; Ziemann, P. J. J. Phys. Chem. A 2016, 120, 2561.A

New particle formation is a process in which trace vapors in the atmosphere cluster and grow into larger particles. Recently, we have employed a multi-trap gas-phase ion preparation technique and cryogenic ion vibrational predissociation (CIVP) spectroscopy to identify common structural motifs for the uptake of water into cationic clusters relevant to NPF. Here we further extend this exploration using a multi-trap gas phase variable temperature mass spectrometry (VTMS) technique in which we probe the water uptake of clusters as a function of temperature to determine the associated thermodynamic parameters. We then seek to correlate these water uptake parameters to structural motifs present in the dry clusters.

New particle formation (NPF) plays a crucial role in the formation of secondary aerosol particles, which in turn actively influence the climate radiative forcing balance. The magnitude of this impact is currently unclear, which limits the accuracy of climate models and their predictions. In order to minimize this radiative forcing uncertainty due to aerosol particles, information regarding surface structure and growth mechanisms of these precursor NPF clusters is necessary. Using a combination of mass spectrometry and infrared spectroscopy, these clusters containing sulfuric acid, ammonia, and water can be mass selected and irradiated with infrared light as a means to gain insight into bonding and structure of the smallest atmospherically relevant clusters. Quantum chemical calculations also provide important supplementary information regarding potential isomer stability at varying cluster sizes. However, even at the smallest cluster sizes there are several isomers that contribute to the experimental spectrum and there are even more potential isomer arrangements to explore computationally. As cluster size increases, computational expense increases exponentially, and water further complicates the computational process. Combining these experimental and computational efforts allows key structural features impacting cluster growth and stability to be identified for a subset of simple clusters. Cationic clusters containing two ammonium and one bisulfate as well as cationic clusters containing four ammonium and three bisulfate molecules are the focus of the study, as well as their singly hydrated counterparts. By understanding these small cluster binding motifs, growth models can be developed and applied to larger atmospherically relevant clusters, and ultimately help improve the accuracy of climate models.

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 show no sCI production at the pressures studied. However, acyclic alkenes show pressure-dependent sCI yields. Formaldehyde oxide (CH₂OO) from the alkenes studied 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.

Greenhouse gases were monitored on an open-path over kilometer length with calibrated wavelength modulation spectroscopy (WMS). A movable platform was developed capable of detecting atmospheric variations of the concentrations of methane and carbon dioxide. Two single-mode continuous commercial Distributed Feedback Diode Lasers (DFB-DL) with a fiber tail were employed as the laser sources with the wavelengths  1654 nm and  1602 nm (corresponding to the absorption features of methane and carbon dioxide, respectively) and the output powers of 10 mW each. These near-IR wavelengths give advantages of weaker interference of water absorption and readily available emitting and receiving equipment developed for applications at telecommunication wavelengths. The intensity modulation caused by turbulence was also observed. We realized an automated continuous monitoring of methane for more than 30 hours [1]. Similar measurements of carbon dioxide were performed for about 10 hours. The results show that the detection limits of methane and carbon dioxide are  2 ppb and  20 ppm with integration times of 60 seconds and 20 seconds, respectively. The developed sensor is a promising tool for monitoring of greenhouse gases. This work was supported by the Robert A Welch Foundation (grant No A1546) and a T3 grant from Texas A and M University and Open Fund of State Key Laboratory of Applied Optics (SKLA02020001A12). [1] J. Xia, Feng Zhu, Sasa Zhang, Alexandre Kolomenskii, Jian Dong, Kunihiro Okada, James Strohaber, Hans. A. Schuessler, Probing greenhouse gases in turbulent atmosphere by long-range open-path wavelength modulation spectroscopy, Optics and Lasers in Engineering, 117, 21-28 (2019). [2] Jinbao Xia: State Key Laboratory of Applied Optics, Changchun Institute of Optics, Fine Mechanics and Physics, Chinese Academy of Sciences Changchun, 130033, China

For the first time, THz spectroscopy was used for a direct monitoring of molecular species in the Dunkirk atmospheric simulation chamber called CHARME for Chamber for the Atmospheric Reactivity and the Metrology of the EnvironmentMeng, L.; Coeur, C.; Fayad, L.; Houzel, N.; Genevray, P.; Bouzidi, H.; Tomas, A.; Chen, W., J. Atmos. Environ., 240, 1117740, 2020. Within this chamber, we have developed a specially designed Chernin multi-pass cell "MULTICHARME" allowing to adjust a pathlength from 120 m. (24 paths) to 280 m. (56 paths) coupled to a submillimeter wavelength radiation. We demonstrate the capability to detect and quantify greenhouse gases at trace levels by probing their rotational transitions close to the Doppler broadening limit. Significant absorbances of 400 ppm residual N₂O traces at 577.58 GHz and 200 ppm O₃ traces have been measured with a path-length adjusted to 200 meters.Cuisset, A.; Hindle, F.; Mouret, G.; Bocquet, R.; Bruckhuisen, J.; Decker, J.; Pienkina, A.; Bray, C.; Fertein, É.; Boudon, V., Appl. Sci., 11, 1229, 2021.ight now, the accessible detection levels for both compounds are limited to tens of ppm. We are presently developing a model of the baseline in order to correct the strong variations caused by multiple interfering stationary waves in the Chernin cell. The first results obtained in CHARME coupling a THz source to the MultiCHARME Chernin cell opens new possibilities especially for the monitoring of stratospheric reaction processes at low-pressure. In particular, the versatility of the submillimeter electronic sources will allow to perform time-resolved quantitative spectroscopies of reactants, oxidants and products involved in targeted reactions occurring in the high altitude atmospheric layers.

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Footnotes:

Meng, L.; Coeur, C.; Fayad, L.; Houzel, N.; Genevray, P.; Bouzidi, H.; Tomas, A.; Chen, W., J. Atmos. Environ., 240, 1117740, 2020.. Cuisset, A.; Hindle, F.; Mouret, G.; Bocquet, R.; Bruckhuisen, J.; Decker, J.; Pienkina, A.; Bray, C.; Fertein, É.; Boudon, V., Appl. Sci., 11, 1229, 2021.R

We present measurements of molecular transition frequencies based on the comb-locked cavity ring-down spectroscopy technique [1], reporting vacuum transition frequencies of Doppler-broadened ¹²C¹⁶O₂ in the 1.6 μm region for the (30012) ← (00001) and (30013) ← (00001) bands with an average combined standard uncertainty of  1 kHz. A global multi-state model was fit to these data and literature values to provide spectroscopic parameters and a best-case fit precision of 4 kHz. We identified and assigned an interaction between the (30012) and (33301) states which was manifest as an observed Fermi resonance in the (30012) ← (00001) band. This interaction was accounted for in the global fit, significantly reducing uncertainties in the spectroscopic parameters of the (30012) state. We find excellent agreement with literature values, including better than 1 kHz RMS deviation with a recent saturation cavity ring-down study of the (30013) ← (00001) band [2]. Agreement is excellent with CDSD 2019 [3] and HITRAN 2016 [4] for the (30013) ← (00001) band, but much poorer for the (30012) ← (00001) band. [1] Z. D. Reed, D. A. Long, H. Fleurbaey, and J. T. Hodges, Optica 7, 1209-1220 (2020). [2] H. Wu, et al., Phys. Chem. Chem. Phys. 22, 2841-2848 (2020). [3] S. A. Tashkun, et al., J. Quant. Spectros. Rad. Trans. 228, 124-131 (2019). [4] I. E. Gordon, et al. J. Quant. Spectros. Rad. Trans. 203, 3-69 (2017).

We used two previously described [1,2] cavity ring-down spectroscopy systems to accurately measure line intensities in the following three ¹²C¹⁶O₂ rovibrational bands near 1.6 μm: (30012) ← (00001), (30013) ← (00001), and (30014) ← (00001). These bands are commonly used in remote sensing applications, including the Total Carbon Column Observing Network (TCCON) [3]. We estimate relative combined standard uncertainties for these band intensities of less than 0.1% and obtain percent-level deviations in the measured intensities relative to those in the literature and several spectroscopic databases. However, we find 0.1% level agreement with the (30013) and (30014) band intensities given in the HITRAN 2016 [4] database, which were calculated using ab initio dipole moment surfaces. Incorporation of the resulting line intensities into TCCON retrievals leads to significantly reduced biases in the (30012) and (30013) bands. These results indicate that refinements of spectroscopic databases are required to meet increasingly stringent remote sensing uncertainty targets. [1] Lin, H. et. al. J. Quant. Spectrosc. Radiat. Transfer, 161, 11-20. [2] Truong, G. W. et. al. (2013) Nat. Photonics, 7(7), 532-534. [3] Wunch, D. et. al. Philos. Trans. Royal Soc. A, 369(1943), 2087-211 [4] Gordon, I. E., et al. (2017), J. Quant. Spectrosc. Radiat. Transfer, 203, 3-69.

The fundamentals of XH stretching modes typically lie in the 2,500 – 3,700 cm⁻¹ (2.7 - 4.0 μm) range. Continuous-wave optical parametric oscillators (CW OPOs) enable high-resolution spectroscopy in this region. A Doppler-free saturated absorption spectroscopy apparatus with a high-power, widely tunable CW OPO as the light source has been built in our lab for high-resolution, high-precision spectroscopy of gas-phase molecules. We have obtained the Doppler-free spectrum of the Q branch of the ν₃ band of CH₄ in a proof-of-principle experiment. Preliminary results from the experiment will be reported in this talk. A cavity-enhanced two-photon spectroscopy setup using the same CW OPO is under construction.