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 show 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 an important contributor for climate simulation and ozone retrievals.

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 timescale of about one month. Sulfate aerosols increase the Earth's albedo and cool the surface.

After almost 17 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.4.0 (soon to be 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. See http://www.ace.uwaterloo.ca for more information.

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.

In support of the precision atmospheric remote sensing (e.g., OCO-2, GOSAT missions), the collision-induced absorptions (CIA) of O2-O2, O2-Air, and O2-H2O have been measured in the O2(A) band region centered at 760 nm. For this, a newly developed 1 m pathlength high-pressure cell (rated up to 150 bars at an operating temperature of 315 Celcius) was configured to a Fourier transform spectrometer, Bruker 125HR, at the Jet Propulsion Laboratory, along with a super-luminant Laser-Driven Light Source (LDLS). A series of spectra of pure O2 and dry air were obtained at various pressures up to 116 bars at room temperature. For the O2-H2O CIA measurement, we collected the spectra at elevated temperatures, 500 K, to secure sufficiently high pressure of water vapor. The CIA of the O2 A-band was derived from multiple spectra in two steps; (i) First, their monomer absorptions have been simulated by using a speed-dependent Voigt line shape profile at the experimental conditions of individual observed spectra. The line mixing effects have been taken into account through both the Rosenkranz first-order approximation and the full-line mixing matrix operation, respectively. (ii) The simulated monomer absorption contribution has been subtracted from their corresponding observed spectra. The remaining absorption component was interpreted as the CIA component in the region. The results will be presented for O2-O2 and O2-Air, respectively, at the room temperatures along with the comparison with the existing data sets and discussion. The O2(A) band CIA by hot water will also be presented at 500 K for the first time.

The addition reactions of iodine atom with propene in solid para-hydrogen (p-H₂) matrices were investigated with infrared (IR) absorption spectroscopy. Mixtures of propene and I₂ seeded in p-H₂ were co-deposited at 3.3 K for 7 to 8 hours, followed by irradiations with ultraviolet lights at various wavelengths to study the reaction of I atoms with propene. Quantum chemical calculations were carried out at the B3LYP/aug-cc-pVTZ-pp level in order to determine the relative energies, vibrational wavenumbers and IR intensities of 1-iodopropyl and 2-iodopropyl radicals. The transitions belong to I₂-propene complex, 1-iodopropyl radical, and the anti conformer of 1,2-diiodopropane were recorded and assigned. The assignments were based on expected reaction, the vibrational wavenumbers and IR intensities from theoretical calculations, and secondary photolysis behavior. The observation of 1-iodopropyl radical, the isomer with the least energy, indicates that the addition of iodine atom occurs at the terminal carbon atom. The role of the p-H₂ matrices, the difference between this reaction and the previously reported Cl + propene reaction will be discussed.

Infrared spectra of the CO₂-N₂ dimer are observed in the 4.2 micron region using a tunable infrared optical parametric oscillator to probe a pulsed slit jet supersonic expansion. Previous results for the fundamental band are extended to higher values of K_a. A combination band involving the lowest in-plane intermolecular bending mode is observed. This yields a value of 21.4 cm⁻¹, and represents the first experimental determination of an intermolecular mode for CO₂-N₂. This intermolecular frequency is at odds with the value of 45.9 cm⁻¹ obtained from a recent 4D intermolecular potential generated at CCSD(T)-F12/augmented correlation consistent triple zeta (aug-ccpVTZ) level.S. Nasri, Y. Ajili, N.-E. Jaidane, Y.N. Kalugina, P. Halvick, T. Stoecklin, M. Hochlaf, J. chem. Phys. 142 (2015) 174301.^,M. Lara−Moreno, T. Stoecklin, P. Halvick, M. Hochlaf, Phys. Chem. Chem. Phys. 21 (2019) 3550.n addition, two weak bands near 2337 cm^-1 are assigned to the CO_2 hot band transition (v_1, v_2^l_2, v_3) = (01^11) (01^10). They yield a value of 2.307 cm^-1 for the splitting of the degenerate CO_2 _2 bend into in−plane and out−of−plane components due to the presence of the nearby N_2 M. Lara-Moreno, T. Stoecklin, P. Halvick, M. Hochlaf, Phys. Chem. Chem. Phys. 21 (2019) 3550.I