CH₄ is a potent greenhouse gas with a 100-year Global Warming Potential more than thirty times larger than CO₂ if carbon-climate feedbacks are considered. In urban areas such as Los Angeles, anthropogenic methane emissions are poorly characterized because of the large diversity of sources: landfills, sewage treatment plants, agriculture, leaks in the natural gas distribution system, cattle and dairy farms, thermogenic emissions from oil fields and seeps. The California Laboratory for Atmospheric Remote Sensing (CLARS), operated by the Jet Propulsion Laboratory, is a mountaintop facility overlooking most of the Los Angeles basin, equipped with JPL-built Fourier transform spectrometers for measurements of the slant column abundances of several greenhouse gases including methane with high spatial and temporal resolution. This presentation will cover several topics including the design features of the two FTS instruments, spectroscopic issues associated with the retrieval of slant column abundances, and uncertainty analysis. One FTS has been in continuous operation since 2011, providing sufficient data to identify several CH₄ emission hot spots in the LA basin. On October 23, 2015, a well pipe suffered a failure in a natural gas storage facility in Aliso Canyon, northwest of downtown Los Angeles resulting in a massive CH₄ plume transported by winds throughout the LA basin. The CLARS FTS captured the plume propagation throughout the 4-month duration of the leak. We will show how the emission ratio method may be employed to derive a lower bound to the CH₄ emission rate from the leaking well without the use of complex atmospheric transport models. The CLARS measurement system provides a small-scale example of the data that would be acquired by an imaging FTS on a geostationary space platform.copyright 2016, California Institute of Technology. Government sponsorship acknowledged.html:<hr /><h3>Footnotes: copyright 2016, California Institute of Technology. Government sponsorship acknowledged.

ACE (also known as SCISAT) is making a comprehensive set of simultaneous measurements of numerous trace gases, thin clouds, aerosols and temperature by solar occultation from a satellite in low earth orbit. A high inclination orbit gives ACE coverage of tropical, mid-latitudes and polar regions. 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 the vertical distribution of trace gases, and the meteorological variables of temperature and pressure. Aerosols and clouds are being monitored through the extinction of solar radiation using two filtered imagers as well as by their infrared spectra. Although now in its thirteenth year, the ACE-FTS is still operating nominally. A short introduction and overview of the ACE mission will be presented (see http://www.ace.uwaterloo.ca for more information). This talk will focus on ACE observations of the CO, CH₄ and N₂O isotopologues, and comparisons with chemical transport models.

Measurements of radiocarbon (¹⁴C) provide a unique platform in order to determine the age of a material or alternatively for source attribution between biogenic and petrochemical sources. We describe a cavity ring-down spectrometer which uses an infrared quantum cascade laser to probe the fundamental of ¹⁴CO₂. This instrument offers one of the highest sensitivities which has been reported in the mid-infrared and has fully automated spectral scanning for continuous data acquisition. Despite the ultra-low abundance of ¹⁴CO₂ in the atmosphere (1.2 pmol/mol relative to ¹²CO₂) we have been able to rapidly record the ¹⁴CO₂ transition and determine the origin of carbon dioxide samples. Our experimental approach and future improvements to the instrument will be discussed as well as selected measurement targets.

In this work, we demonstrated a mid-IR (3.3 μm) cw cavity ringdown spectrometer capable of measuring atmospheric ethane abundance and ethane to methane ratio. This technique can measure atmospheric ethane concentration as low as 70 ppb. Since ethane is a tracer for thermogenic methane emissions, this technique could be used to identify sources of atmospheric methane. We have demonstrated the capability of this instrument by measuring the atmospheric ethane composition and ethane to methane ratio in ambient air in Pasadena, California.

In this work, we demonstrated a spectroscopic method of measuring abundances of doubly-substituted methane isotopologues (¹³CH₃D, ¹²CH₂D2). In this method, we use a frequency stabilized cavity ringdown spectroscopy (FS-CRDS) technique to measure ∆¹²CH₂D2 in naturally abundant methane to sub 0.1% level within one hour of average. Compare to traditional isotope-ratio mass spectrometer, which requires more than 24 hours of average to achieve comparable precision, this method provides a fast way of measuring clumped isotopologue abundance optically without destroying samples.

The near-IR spectrum, from 5000-8960 cm⁻¹, of isotopically pure CH₃D was taken at temperatures of 294, 400, 500, 600, 700, 800, and 900K with a high resolution Fourier Transform machine at Old Dominion University. The spectra where analyzed to give the wavenumbers, integrated line intensities, and lower state term values (using lines observed in at least 3 different spectra). For the 294 K spectrum 12080 lines with S between 3.6x10⁻²² and 1x10⁻²⁷ cm (not corrected for CH₃D natural abundance) were determined for this spectral interval. A theoretical spectrum of CH₃D has also been calculated at the Univ. of Reins, with >400,000 transitions predicted between 5000-6300 cm⁻¹ with S values between 2.1x10⁻²² and 1 x 10⁻²⁷ cm at 294K. Comparison of the predictions with 175 J” = 0 and 1 transitions previously assigned by the ETH group¹ shows that for 130 of these the absolute difference between the observed and predicted line wavenumbers is less than 0.1 cm⁻¹ and for all but one transition the absolute difference is less than 1 cm⁻¹. In this project, we are combining the temperature dependence of the line intensities, combination differences, and comparisons of line positions and strengths with the theoretical spectrum to extend the assignments of CH₃D lines in this spectral region. Selected assignments will be confirmed by IR-IR double resonance measurements at the University of Virginia. Ultimately, we hope to give a global analysis of CH₃D spectrum using a global effective Hamiltonian model. 1. Ulenikov, O.N. et al., Molecular Physics 108, 1209-1240 (2010)

The Near-IR Spectrum of CH₄ is dense with many overlapping bands that perturb each other by vibrational and ro-vibrational interactions. Assignments of the individual lines are needed in order to simulate the spectrum as a function of pressure and temperature, as needed in the search for CH₄ in extrasolar planets. Both the group at the University College, London¹ and that at the University of Reins² have produced theoretical spectra that allows simulation up to the high temperatures expected on "Hot Jupiters’’. The accuracy of these theoretical spectra need to be further tested. Because CH₄ is a light spherical top, assignment of its perturbed spectra is a formable challenge as none of the lines allowed in the rigid rotor approximation have ground vibrational state combination differences. We are using IR-IR double resonance to observe modulation in the strength of near-IR absorption caused by a modulation of a 3 μm OPO beam that is tuned to a particular transition in the C-H stretching fundamental of CH₄. This produces V-type double resonance transitions (which share the lower state with the pump transition), which provides firm assignments for lines normally observed in absorption in the near-IR. We also observe sequential double resonance which reveals transitions that have a known rotational level of the ν₃ fundamental as the lower state and reaches final states in the 9000 cm⁻¹ spectral region. These are states of A, E, F₁ vibrational symmetries which are forbidden in transitions from the ground vibrational state. These 3 level double resonance transitions are Doppler Free and have a linewidth of  ∼ 10 MHz due to a combination of near-IR laser jitter and power broadening of the mid-IR transition. We also observed many 4-level double resonance transitions that we have tentatively assigned as arising from the ν₄ fundamental level. These are distinguished from the 3-level double resonance transitions by they being Doppler broadened and having a large phase shift relative to the intensity modulation. 1. S.N. Yurchenko, PNAS 111 9379-83 (2014); 2. M. Rey, JQSRT 18, 207-220 (2015), PCCP 18, 176-189 (2016)

We report new assignments of vibration-rotation line positions of methane (¹²CH₄) in the so-called Dyad (ν₂/ν₄) region (1000 - 1500 cm⁻¹), and the resulting update of the vibration-rotation effective model of methane, previously reported by Nikitin et al. [A.V. Nikitin et al. PCCP, 15, (2013), 10071], up to and including the Tetradecad. High resolution (0.01 cm⁻¹) emission spectra of methane have been recorded up to about 1400 K using the high-enthalpy source developed at IPR associated with the Fourier transform spectrometer of the SOLEIL synchrotron facility (AILES beamline). Analysis of these spectra allowed extending rotational assignments in the well-known cold band (Dyad−GS) and related hot bands in the Pentad−Dyad system (3000 cm⁻¹) up to J_max=30 and 29, respectively. In addition, 8512 new transitions belonging to the Octad−Pentad (up to J=28) and Tetradecad−Octad (up to J=21) hot band systems were successfully identified. As a result, the MeCaSDa database of methane was significantly improved. The line positions assigned in this work, together with the information available in the literature, were fitted using 1096 effective parameters with a dimensionless standard deviation σ = 2.09. The root mean square deviations d_RMS are 3.60 ×10⁻³ cm⁻¹ for Dyad−GS cold band, 4.47 ×10⁻³ cm⁻¹ for the Pentad−Dyad, 5.43 ×10⁻³ cm⁻¹ for the Octad−Pentad and 4.70 ×10⁻³ cm⁻¹ for the Tetradecad−Octad hot bands. The resulting new line list will contribute to improve opacity and radiative transfer models for hot atmospheres, such as those of hot-Jupiter type exoplanets.

Carbon tetrafluoride is a powerful greenhouse gas, mainly of anthropogenic origin. Its absorption spectrum is, however, still badly modeled, especially for hot bands in the strongly absorbing ν₃ region. To overcome this problem, we have undertaken a systematic study of all the lower rovibrational transitions of this molecule. In particular, new far-infrared spectra recorded at the SOLEIL Synchrotron facility give access to bands implying the "forbidden" modes ν₁ and ν₂ which have only been investigated previously thanks to stimulated Raman spectroscopyV. Boudon, D. Bermejo, R. Z. Martínez, J. Raman Spectrosc. 44, 731?738 (2013). that is with a lower accuracy and much less data. Combined with the previous analyses performed in our groupV. Boudon, J. Mitchell, A. Domanskaya, C. Maul, R; Georges, A. Benidar, W. G. Harter, Mol. Phys. 109, 17-18 (2011). we thus report here a new global fit of line positions of CF₄ by considering several transitions altogether: ν₂, 2ν₂−ν₂, ν₄, 2ν₄, ν₃ and ν₃−2ν₂. This gives a consistent set of molecular parameters that will be of great help for the analysis of hot bands like ν₃+ν₂−ν₂. A second separate global fit including the ν₁, ν₁−ν₄ and 2ν₁−ν₁ bands will also be presented.

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Footnotes:

V. Boudon, D. Bermejo, R. Z. Martínez, J. Raman Spectrosc. 44, 731?738 (2013)., V. Boudon, J. Mitchell, A. Domanskaya, C. Maul, R; Georges, A. Benidar, W. G. Harter, Mol. Phys. 109, 17-18 (2011).,

Since the signing of the Montreal protocol, long-lived chlorofluorocarbons have been banned due to their high ozone depleting potential. In order to minimise the effect of such molecules, hydrofluorocarbons (HFCs) were synthesized as replacement molecules to be used as refrigerants and foam blowing agents. HFC-134a, or 1,1,1,2-tetrafluoroethane, is one of these molecules. Although HFCs do not cause ozone depletion, they are typically strong absorbers within the 10 micron atmospheric window, which lead to high global warming potentials. A high resolution FT-IR analysis of the ν₈ band (near 665 cm⁻¹) of HFC-134a has been performed to help understand the intermode coupling between the ν₈ vibrational state and unobserved dark states.

To model correctly the SF₆ atmospheric absorption requires the knowledge of the spectroscopic parameters of all states involved in the numerous hot bands in the 10,5μm atmospheric transparency window. However, due to their overlapping, a direct analysis of the hot bands near the 10,5μm absorption of SF₆ in the atmospheric window is not possible. It is necessary to use another strategy, gathering information in the far and mid infrared regions on initial and final states to compute the relevant total absorption. In this talk, we present new results from the analysis of spectra recorded at the AILES beamline at the SOLEIL Synchrotron facility. For these measurements, we used a IFS125HR interferometer combined with the synchrotron radiation in the 100-3200 cm⁻¹range, coupled to a cryogenic multiple pass cell F. Kwabia Tchana, F. Willaert, X. Landsheere, J. M. Flaud, L. Lago, M. Chapuis, P. Roy, L. Manceron. A new, low temperature long-pass cell for mid-IR to THz Spectroscopy and Synchrotron Radiation Use. Rev. Sci. Inst. 84, 093101, (2013) The optical path length was varied from 45 to 141m with measuring temperatures between 223 and 153+/-5 K. The new information obtained on ν₂+ν₄-ν₅, 2ν₅-ν₆ and ν₃+ν₆-ν₄ allowed to derive improved parameters for ν₅, 2ν₅ and ν₃+ν₆. In turn, they are used to model the more important ν₃+ν₅-ν₅ and ν₃+ν₆-ν₆ hot band contributions. By including these new parameters in the XTDS model C. Wenger, V. Boudon, M. Rotger, M. Sanzharov, and J.-P. Champion,"XTDS and SPVIEW: Graphical tools for Analysis and Simulation of High Resolution Molecular Spectra", J. Mol. Spectrosc. 251, 102 (2008) we substantially improved the SF₆ parameters used to model the atmosphere.

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Footnotes:

F. Kwabia Tchana, F. Willaert, X. Landsheere, J. M. Flaud, L. Lago, M. Chapuis, P. Roy, L. Manceron. A new, low temperature long-pass cell for mid-IR to THz Spectroscopy and Synchrotron Radiation Use. Rev. Sci. Inst. 84, 093101, (2013). C. Wenger, V. Boudon, M. Rotger, M. Sanzharov, and J.-P. Champion,"XTDS and SPVIEW: Graphical tools for Analysis and Simulation of High Resolution Molecular Spectra", J. Mol. Spectrosc. 251, 102 (2008),

Anomalous sulfur isotope ratios, called mass independent fractionation (MIF), are commonly observed in sedimentary rocks formed more than 2.5 billion years ago. These anomalies likely originated from photochemistry of small, sulfur-containing molecules in the early Earth’s atmosphere. The disappearance of MIF in rocks younger than 2.5 billion years is thought to be evidence of rising O₂ concentrations during the Great Oxygenation Event (GOE), an important milestone in the development of life on Earth. However, the photochemical origin of the pre-GOE anomaly is not well understood. Here, we use a model of the X, B, and B” states of S₂, originally developed by Western, to determine a possible mechanism for isotopologue-selective photodissociation. A model of the rotation-vibration structure of the B-X UV transition shows small perturbations between the bright B state and the dark B” state that vary depending on isotopologue. These perturbations suggest a sequential two-photon mechanism for selective photodissociation. Symmetry (e.g., 32-32 vs. 32-34) may also contribute to MIF. This presentation will primarily focus on the UV spectra of the 32-32 and 32-34 isotopologues. We also examine the possibility that a similar mechanism may be at work in the B-X transition of SO.