Absorption of solar radiation by water vapor in the near-UV region is a poorly-understood but important issue in atmospheric science. To better understand water vapor near-UV absorption, we constructed a cavity ring-down spectrometer with a bandwidth comparable to those of field UV spectrometers and determined water vapor absorption cross-sections at 1 nm increments in the 290-350 nm region. We also measured water vapor absorption cross-sections at 0.05 nm intervals surrounding major absorption bands. We provide field evidence to support laboratory water vapor near-UV absorption measurements. Field UV residual spectra not only exhibited increased attenuation at higher atmospheric water vapor loadings but also showed structures suggested by the laboratory water vapor absorption spectrum. Spaceborne UV radiance spectra have spectral structures resembling the differential cross-section spectrum constructed from the laboratory wavelength-dependent water vapor absorption cross-sections. We incorporated water vapor absorption cross-section data into a radiative transfer model and obtained estimated energy budget of such absorption for the standard US atmosphere and for the tropics. We conclude that water vapor near-UV absorption is a significant contributor for climate simulation and ozone retrievals.

After almost 18 years in low-Earth orbit, the ACE satellite is making near-real time measurements of numerous trace gases, thin clouds, aerosols and temperature by solar occultation. The primary instrument is a high-resolution (0.02 cm⁻¹) infrared Fourier transform spectrometer (FTS) operating in the 750-4400 cm⁻¹ region, which provides data for the vertical distribution of trace gases. Our current version of ACE-FTS processing, v.4.1, retrieves an unprecedented 44 molecules (H₂O, O₃, N₂O, NO, NO₂, HNO₃, N₂O₅, H₂O₂, HO₂NO₂, O₂, N₂, SO₂, HCl, HF, ClO, ClONO₂, CFC-11, CFC-12, CFC-113, COF₂, COCl₂, COFCl, CF₄, SF₆, CH₃Cl, CCl₄, HCFC-22, HCFC-141b, HCFC-142b, HFC-134a, HFC-23, CO, CH₄, CH₃OH, H₂CO, HCOOH, C₂H₂, C₂H₆, OCS, HCN, CH₃C(O)CH₃, CH₃CN, PAN (CH₃C(O)OONO₂), high and low altitude CO₂ as well as 24 additional isotopologues. ACE monitors the Montreal Protocol on substances that deplete the ozone layer, and all of the main greenhouse gases, including CO₂. Altitude-latitude distributions and trends in atmospheric abundance will be presented for a subset of the ACE molecules including new research retrievals for HOCl. See http://www.ace.uwaterloo.ca for more information.

The version 4.0 dataset from the Atmospheric Chemistry Experiment – Fourier Transform Spectrometer (ACE-FTS) on SCISAT, released in March of 2019, has sulfur dioxide (SO₂) volume mixing ratio (VMR) profiles as a routine data product. From this dataset, global SO₂ distributions between the altitudes of 10.5 km and 23.5 km are analyzed. The global distribution of all SO₂ VMR data by altitude is divided into 30^° and 5^° latitude zones. Seasonality of global SO₂ distribution is explored. Volcanic SO₂ plumes are isolated in the dataset and compared with aerosol extinction data from the ACE-FTS Imager. SO₂ is converted to sulfate aerosols on a timescale of about one month. Sulfate aerosols increase the Earth's albedo and cool the surface.

The accuracy of atmospheric trace gas retrievals depends directly on the accuracy of the molecular absorption model used within the retrieval algorithm. For remote sensing of well-mixed gases, such as carbon dioxide (CO₂), where the atmospheric variability is small compared to the background, the quality of the molecular absorption model is key. Recent updates to oxygen (O₂) absorption coefficients (ABSCO) for the 0.76 μm A-band and the water vapor (H₂O) continuum model within the 1.6 μm and 2.06 μm CO₂ bands used within the Orbiting Carbon Observatory (OCO-2 and OCO-3) algorithm are described here. Updates in the O₂ A-band involve the inclusion of new laboratory measurements within multispectrum fits to improve relative consistency between oxygen line shapes and collision-induced absorption (CIA). The H₂O continuum model has been updated to MT_CKD v3.2, which has benefited from information from a range of laboratory studies relative to the model utilized in the previous ABSCO version. Impacts of these spectroscopy updates have been evaluated against ground-based atmospheric spectra from the Total Carbon Column Observing Network (TCCON) and within the framework of the OCO-2 algorithm, using OCO-2 soundings covering a range of atmospheric and surface conditions. The updated absorption coefficients (ABSCO version 5.1) are found to offer improved fitting residuals and reduced biases in retrieved surface pressure relative to the previous version (ABSCO v5.0) used within B8 and B9 of the OCO-2 retrieval algorithm and have been adopted for the OCO B10 Level 2 algorithm.

The pure rotational spectrum of 2-chloroethyl radical, CH₂ClCH₂, has been observed for the first time using a cavity-based Fourier-transform microwave spectrometer in the frequency region of 10 GHz – 34 GHz. The radical was generated through electric discharge by applying a 1 kV voltage to the precursor molecule of either 1, 2-dichloroethane, CH₂ClCH₂Cl, or 1-chloro,2-iodoethane, CH₂ClCH₂I, where the latter gave 2-3 times stronger signal. Nine rotational transitions, both a-type and b-type, have been measured with resolved fine and hyperfine components for both ³⁵Cl and ³⁷Cl isotopic species in the ground vibrational state. The spectrum is highly congested due to the interactions of the electron spin, nuclear spin of chlorine atom, and nuclear spins of four hydrogen atoms. To aid in our analysis of the spectrum, we performed CCSD(T) calculation on the geometry optimization of the radical and a single point calculation at MP2 level to obtain the fine and hyperfine constants. We will present and discuss the corresponding assignments of features in the spectrum.

r0pt Figure Silicon tetrafluoride (SiF₄) should be a normal trace component of volcanic gases. However, a better knowledge of spectroscopic parameters is needed for this molecule in order to derive accurate concentrations. As explained last year, we undertook an extensive high-resolution study of its infrared absorption bands, for the there isotopologues in natural abundance: ²⁸SiF₄ (92.23 %), ²⁹SiF₄ (4.67 %) and ³⁰SiF₄ (3.10 %). We present here an update of this study. It includes a new global fit with consistent parameter sets for the ground and excited states (the Figure on the right presents the ν₄ bending fundamental region). In particular, all existing rotational line data have been included. The 2ν₄ band of ²⁸SiF₄ could also be analyzed in detail. A first rough estimates of the dipole moment derivative for the ν₃ band has been performed, leading to to an integrated band intensity which is consistent with literature values, around 680 km/mol. The isotopic dependance of band centers and Coriolis parameters has been studied, thanks to the formula presented in talk P4363.

Vegetation is a major source of emission of Biogenic Volatile Organic Compounds (BVOCs), which play an important role in atmospheric chemistry. Apart from methane and isoprene, monoterpenes (C₁₀H₁₆) are emitted in significant amounts by plants. Among these, α-pinene, β-pinene, and limonene are the predominant speciesP. S. Monks et al., Atmospheric Composition Change – Global and Regional Air Quality. Atmos. Environ. 2009, 43 (33), 5268–5350. Recently, limonene and limonene oxide have been studied extensively with Fourier transform microwave spectroscopy (FTMW)J.R. Moreno et al., Conformational Relaxation of S-(+)-Carvone and R-(+)-Limonene Studied by Microwave Fourier Transform Spectroscopy and Quantum Chemical Calculations. Struct. Chem. 2013, 24 (4), 1163–1170.^, D. Loru et al., Conformational Flexibility of Limonene Oxide Studied By Microwave Spectroscopy. ChemPhysChem 2017, 18 (3), 274–280..N.M. Donahue et al., Secondary Organic Aerosol from Limona Ketone: Insights into Terpene Ozonolysis via Synthesis of Key Intermediates. Phys. Chem. Chem. Phys. 2007, 9 (23), 2991–2998..M. Tudorie et al., Magnetic Hyperfine Coupling of a Methyl Group Undergoing Internal Rotation: A Case Study of Methyl Formate. J. Chem. Phys. 2011, 134 (7), 074314..

The ~B - ~X laser-induced fluorescence (LIF) and dispersed fluorescence (DF) spectra of the atmospherically important β-monofluoro ethoxy (MFEO), β,β-difluoro ethoxy (DFEO), and β,β,β-trifluoro ethoxy (TFEO) radicals were recorded with vibronic resolution under jet-cooled conditions. To simulate the spectra, Franck-Condon factors were obtained from quantum chemical computations carried out at the CAM-B3LYP/6-311++G(d,p) level of theory. The simulations reproduce well both the LIF and DF spectra. Both conformers (G and T) of MFEO and one (G) of the two conformers of DFEO contribute to the LIF spectrum. A comparison between the experimental and calculated spectra confirms the expected long-range field effects of the CH_XF_3−X group on electronic transition energies and bond strengths, especially in the excited electronic (~B) state. Although TFEO has only one conformer, its LIF spectrum is highly congested, which is attributed to the interaction between CO stretch and the -CF₃ internal rotation.

Isoprene (C₅H₈) is an important molecular target in trace gas sensing due to its presence in Earth's atmosphere and human breath. Isoprene is the most abundant of the so-called biogenic volatile organic compounds (BVOCs) emitted naturally into the atmosphere by plants, and so plays an important role in the chemistry of the troposphere. Isoprene is also one of the most abundant hydrocarbons present in human breath, and there is interest in measuring isoprene in breath as a way to perform noninvasive monitoring of patients. In both of these settings, isoprene is present as a trace gas at a concentration of parts per billion (ppb), making its detection quite challenging. We have used a quantum cascade laser-based infrared spectrometer to perform sensitive spectroscopy of isoprene down to the ppb level. We have used the strong Q-branch of the ν₂₆ band of isoprene near 992 cm⁻¹to monitor its concentration and achieved a minimum noise-equivalent concentration for our spectrometer of 3 ppb at an optimal averaging time of 25 s. We have also demonstrated the potential real-world applications of our approach by directly measuring the isoprene concentration in a breath sample from a volunteer.

The Great Oxygenation Event, the introduction of O₂ into the Earth’s atmosphere approximately 2.5 billion years ago, is a critical stage in the development of life on Earth. The exact timing of this event is thought to be correlated with the disappearance of sulfur isotope anomalies, called "Sulfur Mass Independent Fractionation" (S-MIF), in the rock record. However, the mechanism for the generation of S-MIF in a reducing atmosphere is still unknown. This talk explores the B-X system of S₂ where the short-lifetime B state is extensively perturbed by a long-lifetime B” state. We employ a master equation model that calculates rotationally and electronically inelastic collisional transfer rates between the B and B” states. For weakly perturbed B/B” level crossings (matrix element less than 1 cm⁻¹), these collisional transfer processes can generate significant isotope effects, where one isotopologue has a larger enhancement of excited state population than another. We discuss the effects of mass-dependent vibrational level shifts and nuclear permutation symmetry on this isotopic fractionation, and propose a possible mechanism for the S-MIF pattern observed in the rock record.