Mitchio Okumura

The gas-phase oxidation of ethylene oxide (EtO, C₂H₄O) by the hydroxyl radical (OH) is the primary atmospheric pathway for removing this known carcinogen from the troposphere. The rate coefficient of the OH + EtO reaction was measured at room temperature (296 K) in a temperature- and pressure-controlled reactor using pulsed laser photolysis-laser-induced fluorescence (PLP-LIF). Time-resolved pseudo-first order OH decays, generated by photolyzing H₂O₂, HNO₃, or (CH₃)₃COOH at 248 nm, were measured using LIF in EtO/N₂ mixtures at total pressures of 67 and 267 hPa. The observed rate coefficient, k₁(296 K) = (1.09 ± 0.10) × 10^-13 cm³ molecule^-1 s^-1, is approximately 25% faster than the current recommended value. These results are compared with previous room-temperature studies, and atmospheric lifetimes of EtO with respect to OH are discussed.

The peroxychloroformyl (ClC(O)OO) radical is proposed to be a key intermediate involved in the CO₂ cycle, the polysulfur cycle, and the sulfur oxidation cycle in Venus's upper atmosphere. A better understanding of the reactivity of ClC(O)OO is essential to reduce uncertainties in simulations of Venus's photochemical models. However, relevant kinetic studies remain scarce as a result of the lack of suitable spectroscopic methods for gas-phase studies. In this work, the electronic structure of ClC(O)OO was explored through a synergistic combination of experimental and theoretical approaches. Under C_s symmetry, the lowest-excited 1²A' state is bound with an adiabatic excitation energy in the near-infrared (NIR) region. Based on the parameters calculated in this work, the simulated NIR absorption bands suggest the possibility of cis- and trans-isomer selective measurements. In addition, using a multipass transient absorption UV-Vis spectrometer, we recorded the UV spectrum of gas-phase ClC(O)OO in the 210-320 nm region. The absorption band exhibits a peak near 218 nm with an absolute cross-section approximately of σ(218 nm) ≈ 5 × 10^-18 cm², along with a shoulder near 255 nm. The spectrum can be fitted to two Gaussian profiles, which were assigned to the 1²A″ ← X²A″ and 2²A″ ← X²A″ transitions. Corresponding orbital transitions were rationalized by a simplified molecular orbital diagram of the -C(O)OO functional group. Our results suggest that the ClC(O)OO radical possesses reactivity similar to those of other acyl peroxy radicals.

Thionyl chloride (Cl2SO) is widely used as a chlorinating reagent in both industrial and scientific applications due to its ability to provide two chlorine atoms. However, relevant studies on the role of the intermediate sulfinyl chloride (ClSO) radical in Cl2SO applications are sparse, as the ClSO radical was assumed to be thermally unstable, with an estimated Cl–S bond energy of ∼5 kcal mol−1. In this study, we determined the Cl–S bond energy of the ClSO radical by measuring its ionization energy using photoionization mass spectrometry interfaced with synchrotron radiation, further supported by performing high-accuracy extrapolated ab initio thermochemistry calculations. The Cl–SO bond energy was found to be 54.2 ± 0.9 kcal mol−1, which was a factor of 10 larger than the previous estimations, indicating that the thermal decomposition rate of ClSO was negligible. As a result, kinetic simulations revealed that the ClSO concentrations can be orders of magnitude higher than previously assumed in a variety of applications. This highlights the unconsidered importance of ClSO chemistry. Better control of ClSO radicals could improve efficiency and reduce Cl2SO waste for semiconductor fabrication, battery development, and pharmaceutical research, as well as planetary/exoplanetary atmospheres where chlorine and sulfur oxide are present.

Venus has a thick atmosphere with many of its chemical and photochemical processes yet to be fully understood. While it mainly consists of relatively inert CO₂ and N₂, the trace gases, which include chlorine and sulfur compounds, are responsible for Venus's rich and diverse chemistry and photochemistry. Given that chlorine has two stable isotopes and sulfur has four, there are plenty of opportunities for naturally occurring isotopic effects in Venusian atmospheric chemistry. Oxygen, carbon, and hydrogen also typically participate in Venusian chlorine and sulfur chemistry, providing further diversity in isotopes to study.Using high-level computational chemistry methods in synergy with experimental kinetics, we model temperature-dependent rate coefficients. Furthermore, the computational chemistry approaches are used to model mass-dependent rate coefficients to study how isotopic fingerprints in the reactants and products from the chemistry may arise. I will present our recent work involving CO oxidation in the Venusian atmosphere and how the measurement of isotopic signals can inform us which chemical mechanism dominates CO oxidation on Venus. The discrepancy between models and observations of O₂ abundance in the Venusian mesosphere remains an issue. The Cl radical oxidation of CO may explain the missing O₂ in the Venusian mesosphere, as the cycle consumes O₂ efficiently: Cl + CO ↔ ClCO + O₂ → ClC(O)OO → ClO + CO₂. The initial equilibrium between Cl and CO is identified as the bottleneck for propagating the radical oxidation cycle.We show how the isotopic fingerprint in CO from the Cl radical oxidation cycle is ripe for detection on Venus through microwave spectroscopy and how important getting the isotopic ratio correct is for retrievals of other properties. Alternative CO oxidation mechanisms are also explored, and other isotopic fingerprints may be investigated, for example, for the ClO radical, which has been observed in the Venusian atmosphere. Our work provides quantitative numbers for isotopic fractionation, which are needed to interpret which chemical processes dominate on Venus. Fractionation of naturally occurring isotopes on Venus provides detectable signatures for future missions through either spectroscopy (IR or microwave) or mass spectrometry. Notably, NASA’s DAVINCI mission will include a mass spectrometer in its payload.Our work is a collaboration between theory and experiments, providing a detailed mechanistic and quantitative understanding of CO chemistry in the Venusian mesosphere. It is additionally relevant to understanding the isotopic fractionation of the atmosphere of Archean Earth (pre-oxygen era) and Noachian Mars, where large mass-independent signatures of CO related compounds can be found, e.g., in Martian soil carbon.

OH+NO is an important termolecular association reaction in the troposphere and stratosphere that influences the atmospheric ozone budget. In this study, rate coefficients for the reaction of OH + NO + M → HONO + M were measured under conditions relevant to the troposphere/lower stratosphere over a temperature range of 228-298 K and pressure range of 50-750 Torr using N₂ as a bath gas. Time-resolved kinetics were studied by pulsed laser photolysis-laser-induced fluorescence (PLP-LIF) detecting OH by laser-induced fluorescence. Data for the temperature range 258-298 K were fit to two falloff expressions, with the JPL expressions (k_1,0^N₂ = 7.37 × 10^-31(T/300 K)^-2.90 cm⁶ molecule^-2 s^-1 and k_1,∞ = 3.44 × 10^-11(T/300 K)^-0.1cm³ molecule^-1 s^-1) and IUPAC expression (k_1,0^N₂ = 6.80 × 10^-31(T/300 K)^-2.81 cm⁶ molecule^-2 s^-1, F_C = 0.81, k_1,∞ = 1.96 × 10^-11(T/300 K)^-0.3 cm³ molecule^-1 s^-1). At temperatures T < 258 K, the measured rate coefficients were significantly higher than the IUPAC and JPL fits. To accommodate the rate coefficient deviation from the two expressions, data across the entire temperature range (228-298 K) was fit with two approaches. First, rate coefficients were fit with an empirical modification by adding a second falloff term to the JPL expression with a second low-pressure rate coefficient of k_1,0^N₂ = 5.20 × 10^-35(T/300 K)^-30.4 cm⁶ molecule^-2 s^-1. Second, k_1,0^N₂, k_1,∞, and n were fit globally to the entire temperature data set, but F_C was varied for each individual temperature, which increased with decreasing temperature. In the second portion of the study, the influence of H₂O on the reaction rate was investigated using a N₂-H₂O mixture as the bath gas at 50 Torr and 273 and 298 K. The JPL and IUPAC falloff expressions were modified to include H₂O as a third-body collisional partner consistent with a nonlinear mixture model. Fits to the data yielded the low pressure termolecular rate coefficients in H₂O, k_1,0^H₂O = 3.81 × 10^-30(T/300 K)^-6.04 and k_1,0^H₂O = 3.31 × 10^-30(T/300 K)^-5.81 cm⁶ molecule^-2 s^-1, respectively. Experimental data were fit using MESMER give energy relaxation parameters of <ΔE_{down,295 K, N2}> = 170 ± 10 cm^-1 and <ΔE_{down,295 K,H2O}> = 634 ± 20 cm^-1, indicating that H₂O is a 4× more efficient collisional quencher than N₂ alone. The modified JPL expressions with the newly derived low pressure rate coefficients were implemented into a STOCHEM-CRI atmospheric model. Predictions of HONO concentrations with the new rates were up to 15% higher in remote tropical regions.

In Venus’s mesosphere, the observation/model discrepancy of molecular oxygen, O₂, abundance has been a long-standing puzzle. Chlorine atoms have been proposed as a catalyst to oxidize carbon monoxide through the formation of chloroformyl radicals (ClCO), removing O₂ and ultimately generating CO₂. However, relevant kinetic studies of this catalytic cycle are scarce and highly uncertain. In this work, we report the spectrum of the ClCO radical between 210–520 nm using a multipass UV–Vis spectrometer coupled to a pulsed-laser photolysis flow reactor at 236–294 K temperature and 50–491 Torr pressure ranges. High-level ab initio calculations were performed to simulate the observed spectrum and to investigate the electronic structure. In addition, we observed the formation of molecular chlorine, Cl₂, and phosgene, Cl₂CO, suggesting that both the terminal chlorine and the central carbon in the ClCO radical are reactive towards chlorine atoms. Most importantly, the reported spectrum will enable future measurements of essential kinetic parameters related to ClCO radicals, which are important in regulating the O₂ abundance in Venus’s mesosphere.

The cross reaction between the free radicals OH and HO₂ plays a critical role in a wide range of environments, including combustion and the atmospheres of Earth and Mars. In the middle atmosphere on Earth, it is the ultimate terminating step of odd-hydrogen chemistry, which has significant implications for the abundance of ozone. Recent laboratory measurements have brought established rate coefficients into question, introducing additional uncertainties into chemical models of the upper stratosphere and mesosphere. In this work, we present measurements of the rate coefficient from 265-350 K using high-resolution infrared and ultraviolet kinetic spectroscopy. A scheme was developed which almost entirely avoids interfering reactions, providing a direct measurement of the decay of OH due to HO₂ and reduces uncertainties associated with absolute calibration of OH and HO₂ concentrations. We find a room temperature rate coefficient of k(295) = (9.0 ± 1.9) × 10^-11 cm³ molecule^-1 s^-1 (1σ error) and a slight temperature dependence over 265-330 K with an E/R of -155 ± 50 K.

We present direct frequency comb cavity ring-down spectroscopy with Vernier filtering as a straightforward approach to sensitive and multiplexed trace gas detection. The high finesse cavity acts both to extend the interaction length with the sample and as a spectral filter, alleviating the need for dispersive elements or an interferometer. In this demonstration, a free running interband cascade laser was used to generate a comb centered at 3.3 μm covering ∼10 cm^-1 (300 GHz), which was coupled into a high-finesse cavity, providing 0.9 km of effective path length. The Vernier configuration transmitted the comb modes in sequence, and ring-down decays could be initiated either with an acousto-optic modulator or by scanning the cavity length beyond the adiabatic limit, with trade-offs between duty cycle and sensitivity. This implementation achieved a figure of merit of 4.3 × 10^-8 cm^-1 Hz^-1/2 per spectral element, and measurements of toluene demonstrated its ability to observe trace concentrations of a continuum absorber. This method does not require anything beyond a conventional single-mode cavity ring-down instrument other than an optical frequency comb, making it readily accessible to a wide range of applications.

Cavity-enhanced dual-comb spectroscopy promises broadband, high-resolution, and highly sensitive spectroscopic measurements on sub-millisecond time scales, making it highly attractive for trace gas monitoring. In this work, we demonstrate cavity-enhanced dual-comb spectroscopy in the molecular fingerprint region using two quantum cascade lasers (QCLs) operating as optical frequency combs centered at 1063cm⁻¹ spanning 56cm⁻¹. The high-finesse bow-tie cavity provided a 285 m effective path length, and the high power-per-mode of the QCL combs granted a strong multi-heterodyne signal of the swept-cavity transmission. This ultimately resulted in a noise equivalent absorption per spectral element of 1.8×10⁻⁹cm⁻¹Hz^−1/2, when considering the active measurement time. Measurements of the ν₈ fundamental band of methanol determined concentrations as low as 1.3 ppm in a single shot, which were captured in a 15 ms sweep of the cavity PZT. The detection limit after averaging 30 measurements was 20 ppb, which took 450 ms in measurement time and 70 s in wall time. This demonstrates the potential of cavity-enhanced dual-comb spectroscopy in challenging applications such as free radical kinetics and environmental monitoring.