After almost 16 years in low-Earth orbit, the ACE satellite (aka SCISAT) is making near-real time measurements of numerous trace gases, thin clouds, aerosols and temperature by solar occultation. A high inclination orbit gives coverage of tropical, mid-latitude 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 data for the vertical distribution of trace gases, and for temperature and pressure. Aerosols and clouds are monitored by their infrared spectra and through the extinction of solar radiation using two filtered imagers. Our new version of FTS processing, v.4.0, retrieves 44 molecules including 19 halogen-containing gases, in addition to 24 isotopologues, and features new routine data products SO₂, ClO, HFC-134a, HFC-23, acetone, acetonitrile, PAN (peroxyacetyl nitrate) and low altitude CO₂. At low altitudes (5.5 to 17.5 km) collision-induced absorption spectra of nitrogen provide the pointing (tangent altitude of the field-of-view). When combined with the Canadian weather service model, the pointing yields our new low altitude CO₂ data product. ACE monitors the Montreal Protocol substances that deplete the ozone layer, and all of the main greenhouse gases, including CO₂, responsible for climate change. See http://www.ace.uwaterloo.ca for more information.

Near global ozone isotopologue distributions have been determined from infrared solar occultation measurements of the Atmospheric Chemistry Experiment (ACE) satellite mission. ACE measurements are made with a high resolution Fourier transform spectrometer. Annual and seasonal latitudinal fractionation (δ value) distributions of the ozone isotopologues ¹⁶O¹⁶O¹⁸O, ¹⁶O¹⁸O¹⁶O and ¹⁶O¹⁷O¹⁶O were obtained. Asymmetric ozone (¹⁶O¹⁶O¹⁸O) shows higher fractionation compared to symmetric ozone (¹⁶O¹⁸O¹⁶O). The maximum ozone fractionation occurs in the tropical stratosphere as expected. An enhancement of the heavy ozone isotopologues is also seen in the Antarctic polar vortex.

An analysis of theoretical and experimental H₂¹⁶O line shape parameters and line intensities in the 22 230 - 22 721 cm⁻¹ window is presented. This visible region is often used in the retrieval of water vapor in the terrestrial atmosphere as there is minimum interference from other molecules and it is therefore vital to have highly accurate water parameters available. The HITRAN2016 database (Gordon et al. (2017)) is an important resource for performing such retrievals and for H₂¹⁶O, the visible section of the database contains data from numerous sources, both theoretical and experimental. We compute two sets of cross sections, one using only theoretical sources, the other using data only from HITRAN2016. Both sets are compared to the cross-sections generated based on the atmospheric observation of this visible region by Harder et al. (1997). Neither set modeled the entire observed spectrum better than the other, however, each set did provide lower residuals than the other for particular regions. Despite using only approximate broadening parameters in the first set of theoretical cross sections, an inter-comparison with HITRAN2016 allowed us to identify in-accurate broadening parameters from a common source. This region of HITRAN2016 combines both the theoretical intensities of Barber and Tennyson (2006), also known as BT2, with experimental measurements of Tolchenov et al. (2005). Inconsistencies between the theoretical line intensities were clearly apparent. Replacing both the incorrect line shape parameters and line intensities with values from alternative sources or estimated using different approaches has improved the overall residual on the HITRAN cross sections. This will be important for the future retrievals of atmospheric water vapor.

The near-infrared bands of carbon dioxide (CO₂) play an important role in point source as well as remote sensing measurements. Here we have measured high accuracy line intensities for the (30012)←(00001), (30013)←(00001), and (30014)←(00001) bands near 1.6 μm. Three separate cavity ring-down spectrometers were employed: a frequency-agile, rapid scanning spectrometer and two frequency-stabilized spectrometers. Through this combination of instruments, we have reached relative combined standard uncertainties as low as 0.1% for the band intensities. I will discuss these measurements as well as comparisons to existing spectroscopic models and databases. Finally, I will present the results of atmospheric retrievals using these line intensities.

Gas-phase ozonolysis is a major degradation mechanism of alkenes in the Earth's atmosphere and forms Criegee intermediates (CIs), carbonyl oxides, as reactive intermediates. The chemistry of CIs plays a central role in controlling the budgets of many tropospheric species including OH, organic acids, and secondary organic aerosols (SOA). The reaction of CIs with organic acids can provide a pathway in which alkenes are converted to low-volatility compounds and thus contribute to the formation of SOA. Here we report spectroscopic investigation, through pure rotational spectroscopy, of the reaction between the simplest Criegee intermediate, CH₂OO, and the simplest organic acid, the formic acid, HCOOH. In this experiment, CH₂OO molecules have been generated in the discharged plasma of a CH₂I₂/O₂ mixture, which containing a small amount of HCOOH enough to react with CH₂OO. The resulting products (including CH₂OO) were characterized by Fourier-transform microwave (FTMW) spectroscopy. Rotational transitions in the 6-40 GHz frequency range were observed by FTMW spectroscopy together with MW-mmW and MW-MW double-resonance techniques. Preliminary results for the reaction of both conformers of the methyl-substituted Criegee intermediate, CH₃CHOO, with formic acid are also presented.

Pure rotational transitions of the methyl-vinyl Ciregee intermediate have been observed by FTMW spectroscopy. The species was produced by discharging a mixture, 1,3-diiodo-but-2-en and O₂ diluted in Ar. Among four possible isomers for this species with energy less than 3 kcal/mol, only the lowest energy isomer, the syn-trans isomer was detected. Thirty rotational transitions with internal rotation splitting for the methyl top were observed. The observed frequencies were analyzed by the XIAM program,¹ yielding the rotational constants, which agree very well for the lowest energy isomer, giving definite assignment of the isomer observed. Furthermore, the internal rotation barrier was determined to be 702.8(28) cm⁻¹, which also reasonably agrees with that of an ab initio calculation 680 cm⁻¹ at CCSD(T)/cc-pVTZ.
1. H. Hartwig and H. Dreizler, Z. Narturforsch. A 51, 923 (1996).

Carbonyl oxides, which are known as Criegee intermediates, are important intermediates produced in ozonolysis of unsaturated hydrocarbons.R. Criegee, Angew. Chem. Int. Edit. 14, 745 (1975).riegee intermediates react readily with other atmospheric species such as HNO₃, SO₂, (H₂O)₂ and HCOOH, leading to production of OH, aerosols and organic acids in the atmosphere. The reaction coefficient between CH₂OO and HNO₃ was reported to be 5.4×10⁻¹⁰ cm³ molecule⁻¹ s⁻¹ at 298 K. E. S. Foreman, K. M. Kapnas and C. Murray, Angew. Chem. Int. Edit. 55, 10419 (2016).heoretical calculations also predict a similar rate coefficient for CH₂OO + HNO₃, the reaction goes through a barrierless path to form nitrooxymethyl hydroperoxide (NMHP, NO₃CH₂OOH). Besides, due to large exothermicity(-184.9 kJ mol⁻¹), internally excited NMHP might decompose further to CH₂ONO₃ and OH.P. Raghunath, Y. P. Lee and M. C. Lin, J. Phys. Chem. A 121, 3871 (2017).n this work, we utilized a step-scan FTIR coupled with a multipass White cell to record time-resolved IR absorption spectra of the reactants and products during the reaction of CH₂OO with HNO₃ in a flow system with total pressure about 10 Torr. CH₂OO was produced from the reaction of CH₂I + O₂; CH₂I was produced from photolysis of CH₂I₂ at 308 nm.O. Welz, J. D. Savee, D. L. Osborn, S. S. Vasu,C. J. Percival, D. E. Shallcross and C. A. Taatjes, Science 335, 204 (2012).he IR absorption spectra were recorded at instrumental resolution 0.3 cm⁻¹. Newly observed bands at 825, 967, 1053, 1294, 1348, 1424, 1686 and 3587 cm⁻¹can be assigned to NMHP. The observed wavenumbers and relative intensities agree with the anharmonic vibrational wavenumbers and IR intensities predicted with the B3LYP/aug-cc-pVTZ method. In addition, we also observed several bands with clear rotational structure, which can be assigned to the absorption of NO₂, H₂CO and HO₂. Observation of these species indicates that another decomposition route for excited NMHP might exist. Furthermore, absorption bands of unternally excited HNO₃ was also observed at low pressure, indicating that decomposition of pre-reaction complex can excite HNO₃. By probing the formation of NMHP and NO₂, the rate coefficient of this reaction was determined to be (5.3±0.8)×10⁻¹⁰ cm³ molecule⁻¹ s⁻¹.

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Footnotes:

R. Criegee, Angew. Chem. Int. Edit. 14, 745 (1975).C E. S. Foreman, K. M. Kapnas and C. Murray, Angew. Chem. Int. Edit. 55, 10419 (2016).T P. Raghunath, Y. P. Lee and M. C. Lin, J. Phys. Chem. A 121, 3871 (2017).I O. Welz, J. D. Savee, D. L. Osborn, S. S. Vasu,C. J. Percival, D. E. Shallcross and C. A. Taatjes, Science 335, 204 (2012).T

Collisional energy transfer between a metastable ozone molecule and an inert collider such as an argon atom is a key step in the formation process of ozone. Understanding these collisional cooling dynamics may provide insight into the “ozone isotopic anomaly”, which is the observation of larger than expected concentrations of certain heavy ozone isotopologues in the atmosphere. Although many explanations to understand this phenomenon have been put forward previously, quantitative prediction/understanding is still lacking. One of the major reasons is the lack of an accurate potential energy surface (PES) for the system. We have recently constructed a new and accurate 3D PES of O₃ + Ar and computed bound states of the complex within the rigid rotor approximation. S. Sur, E. Quintas-Sánchez, S. A. Ndengué, R. Dawes "Development of a potential energy surface for the O₃-Ar system: Rovibrational states of the complex", Submitted, PCCP (2019)n this work, we now present the dynamics of collisions between this triatomic asymmetric top molecule and a heavy atom: O₃-Ar. The MultiConfiguration Time Dependent Hartree (MCTDH) method was used to study the scattering between the O₃ molecule and Ar atom. The state-to-state probabilities from the 0₀₀ rotational state to low lying excited rotational states as well as the state-to-state cross sections are determined for the system. The rate coefficients obtained for ¹⁶O¹⁶O¹⁶O-Ar, are compared with the rate coefficients obtained for the ¹⁶O¹⁶O¹⁸O-Ar isotopologue. The lowered symmetry in ¹⁶O¹⁶O¹⁸O-Ar results in roughly double the density of allowed states due to nuclear spin statistics for bosons, which impacts the scattering dynamics.

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Footnotes:

 S. Sur, E. Quintas-Sánchez, S. A. Ndengué, R. Dawes "Development of a potential energy surface for the O₃-Ar system: Rovibrational states of the complex", Submitted, PCCP (2019)I