Nascent energy distribution of the Criegee intermediate CH2OO from direct dynamics calculations of primary ozonide dissociation

Tracking the reaction networks of acetaldehyde oxide and glyoxal oxide Criegee intermediates in the ozone-assisted oxidation reaction of crotonaldehyde

The reaction of unsaturated compounds with ozone (O3) is recognized to lead to the formation of Criegee intermediates (CIs), which play a key role in controlling the atmospheric budget of hydroxyl radicals and secondary organic aerosols. The reaction network of two CIs with different functionality, i.e. acetaldehyde oxide (CH3CHOO) and glyoxal oxide (CHOCHOO) formed in the ozone-assisted oxidation reaction of crotanaldehyde (CA), is investigated over a temperature range between 390 K and 840 K in an atmospheric pressure jet-stirred reactor (JSR) at a residence time of 1.3 s, stoichiometry of 0.5 with a mixture of 1% crotonaldehyde, 10% O2, at an fixed ozone concentration of 1000 ppm and 89% Ar dilution. Molecular-beam mass spectrometry in conjunction with single photon tunable synchrotron vacuum-ultraviolet (VUV) radiation is used to identify elusive intermediates by means of experimental photoionization energy scans and ab initio threshold energy calculations for isomer identification. Addition of ozone (1000 ppm) is observed to trigger the oxidation of CA already at 390 K, which is below the temperature where the oxidation reaction of CA was observed in the absence of ozone. The observed CA + O3 product, C4H6O4, is found to be linked to a ketohydroperoxide (2-hydroperoxy-3-oxobutanal) resulting from the isomerization of the primary ozonide. Products corresponding to the CIs uni- and bi-molecular reactions were observed and identified. A network of CI reactions is identified in the temperature region below 600 K, characterized by CIs bimolecular reactions with species like aldehydes, i.e., formaldehyde, acetaldehyde, and crotonaldehyde and alkenes, i.e., ethene and propene. The region below 600 K is also characterized by the formation of important amounts of typical low-temperature oxidation products, such as hydrogen peroxide (H2O2), methyl hydroperoxide (CH3OOH), and ethyl hydroperoxide (C2H5OOH). Detection of additional oxygenated species such as alcohols, ketene, and aldehydes are indicative of multiple active oxidation routes. This study provides important information about the initial step involved in the CIs assisted oligomerization reactions in complex reactive environments where CIs with different functionalities are reacting simultaneously. It provides new mechanistic insights into ozone-assisted oxidation reactions of unsaturated aldehydes, which is critical for the development of improved atmospheric and combustion kinetics models.

Ultraviolet Photodissociation Dynamics of the 1-Methylallyl Radical

The ultraviolet (UV) photodissociation dynamics of the 1-methylallyl (1-MA) radical were studied using the high-n Rydberg atom time-of-flight (HRTOF) technique in the wavelength region of 226-244 nm. The 1-MA radicals were produced by 193 nm photodissociation of the 3-chloro-1-butene and 1-chloro-2-butene precursor. The 1 + 1 REMPI spectrum of 1-MA agrees with the previous UV absorption spectrum in this wavelength region. Quantum chemistry calculations show that the UV absorption is mainly attributed to the 3pz Rydberg state (perpendicular to the allyl plane). The H atom photofragment yield (PFY) spectrum of 1-MA from 3-chloro-1-butene displays a broad peak around 230 nm, while that from 1-chloro-2-butene peaks at ∼236 nm. The translational energy distributions of the H atom loss product channel, P (ET)'s, show a bimodal distribution indicating two dissociation pathways in 1-MA. The major pathway is isotropic in product angular distribution with β ∼ 0 and has a low fraction of average translational energy in the total excess energy, ⟨fT⟩, in the range of 0.13-0.17; this pathway corresponds to unimolecular dissociation of 1-MA after internal conversion to form 1,3-butadiene + H. The minor pathway is anisotropic with β ∼ -0.23 and has a large ⟨fT⟩ of ∼0.62-0.72. This fast pathway suggests a direct dissociation of the methyl H atom on a repulsive excited state surface or the repulsive part of the ground state surface to form 1,3-butadiene + H. The fast/slow pathway branching ratio is in the range of 0.03-0.08.

Low-pressure and nascent yields of stabilized Criegee intermediates CH2OO and CH3CHOO in ozonolysis of propene

The yields of stabilized Criegee intermediates (sCIs), both CH2OO and CH3CHOO, produced from ozonolysis of propene at low pressures (7-16 Torr) were measured indirectly using cavity ringdown spectroscopy (CRDS) and chemical titration with an excess amount of sulfur dioxide (SO2). The method of monitoring the consumption of SO2 as a scavenger and the production of secondary formaldehyde (HCHO) allows characterization of the total sCI and the stabilized CH2OO yields at low pressure and in a short residence time. Both the total sCI and the stabilized CH2OO yields in the propene ozonolysis were found to decrease with decreasing pressure. By extrapolating the 7-16 Torr measurements to the zero-pressure limit, the nascent yield of the total sCIs was determined to be 25 ± 2%. The ranges of nascent yields of stabilized CH2OO and stabilized CH3CHOO were estimated to be 20-25% and 0-5%, respectively. The branching ratios of the stabilized and high-energy CH2OO* and CH3CHOO* were also determined.

The simplest Criegee intermediate CH2OO reaction with dimethylamine and trimethylamine: kinetics and atmospheric implications

We have used the OH laser-induced fluorescence (LIF) method to measure the kinetics of the simplest Criegee intermediate (CH2OO) reacting with two abundant amines in the atmosphere: dimethylamine ((CH3)2NH) and trimethylamine ((CH3)3N). Our experiments were conducted under pseudo-first-order approximation conditions. The rate coefficients we report are (2.15 ± 0.28) × 10-11 cm3 molecule-1 s-1 for (CH3)2NH at 298 K and 10 Torr, and (1.56 ± 0.23) × 10-12 cm3 molecule-1 s-1 for (CH3)3N at 298 K and 25 Torr with Ar as the bath gas. Both reactions exhibit a negative temperature dependence. The activation energy and pre-exponential factors derived from the Arrhenius equation were (-2.03 ± 0.26) kcal mol-1 and (6.89 ± 0.90) × 10-13 cm3 molecule-1 s-1 for (CH3)2NH, and (-1.60 ± 0.24) kcal mol-1 and (1.06 ± 0.16) × 10-13 cm3 molecule-1 s-1 for (CH3)3N. We propose that the electronegativity of the atom in the co-reactant attached to the C atom of CH2OO, in addition to the dissociation energy of the fragile covalent bonds with H atoms (H-X bond), plays an important role in the 1,2-insertion reactions. Under certain circumstances, the title reactions can contribute to the sink of amines and Criegee intermediates and to the formation of secondary organic aerosol (SOA).

Master Equation Studies of the Unimolecular Decay of Thermalized Methacrolein Oxide: The Impact of Atmospheric Conditions

Master equation simulations of the unimolecular reaction dynamics of the Criegee intermediate methacrolein oxide (MACR oxide) have been performed under a variety of temperature and pressure conditions. These simulations provide insight into how the unimolecular kinetics vary across temperatures spanning the range 288-320 K. This work has incorporated a new potential energy surface and includes the anti-to-syn and cis-to-trans conformational dynamics of MACR oxide, as well as the unimolecular reactions to form dioxirane and dioxole species. The competition between the unimolecular reactivity of MACR oxide and previously documented bimolecular reactivity of MACR oxide with water vapor is explored, focusing on how this competition is affected by changes in atmospheric conditions. The impact on the role of MACR oxide as an atmospheric oxidant of SO₂ is noted.

Kinetics of the Simplest Criegee Intermediate CH2OO Reaction with tert-Butylamine

The kinetics of the simplest Criegee intermediate (CH₂OO) reaction with tert-butylamine ((CH₃)₃CNH₂) was studied under pseudo-first-order conditions with the OH laser-induced fluorescence (LIF) method at the temperature range of 283-318 K and the pressure range of 5-75 Torr. Our pressure-dependent measurement showed that at 5 Torr─the lowest pressure measured in the current experiment─this reaction was under the high-pressure limit condition. At 298 K, the reaction rate coefficient was measured to be (4.95 ± 0.64) × 10^-12 cm³ molecule^-1 s^-1. The title reaction was observed to be negative temperature-dependent; the activation energy of (-2.82 ± 0.37) kcal mol^-1 and the pre-exponential factor of (4.21 ± 0.55) × 10^-14 cm³ molecule^-1 s^-1 were derived from the Arrhenius equation. The rate coefficient of the title reaction is slightly larger than (4.3 ± 0.5) × 10^-12 cm³ molecule^-1 s^-1 of the CH₂OO reaction with methylamine; the electron inductive effect and the steric hindrance effect might play a role in contributing to such difference.

Electronic Absorption Spectroscopy and Photochemistry of Criegee Intermediates

Interest in Criegee intermediates (CIs), often termed carbonyl oxides, and their role in tropospheric chemistry has grown massively since the demonstration of laboratory-based routes to their formation and characterization in the gas phase. This article reviews current knowledge regarding the electronic spectroscopy of atmospherically relevant CIs like CH₂ OO, CH₃ CHOO, (CH₃ )₂ COO and larger CIs like methyl vinyl ketone oxide and methacrolein oxide that are formed in the ozonolysis of isoprene, and of selected conjugated carbene-derived CIs of interest in the synthetic chemistry community. Of the aforementioned atmospherically relevant CIs, all except CH₂ OO and (CH₃ )₂ COO exist in different conformers which, under tropospheric conditions, can display strikingly different thermal loss rates via unimolecular and bimolecular processes. Calculated photolysis rates based on their absorption properties suggest that solar photolysis will rarely be a significant contributor to the total loss rate for any CI under tropospheric conditions. Nonetheless, there is ever-growing interest in the absorption cross sections and primary photochemistry of CIs following excitation to the strongly absorbing ¹ ππ* state, and how this varies with CI, with conformer and with excitation wavelength. The later part of this review surveys the photochemical data reported to date, including a range of studies that demonstrate prompt photo-induced fission of the terminal O-O bond, and speculates about possible alternate decay processes that could occur following non-adiabatic coupling to, and dissociation from, highly internally excited levels of the electronic ground state of a CI.

Low-Pressure and Nascent Yields of Thermalized Criegee Intermediate in Ozonolysis of Ethene

The yields of thermalized formaldehyde oxide (CH₂OO, the simplest Criegee intermediate) produced from ozonolysis of ethene at low pressures were measured indirectly using cavity ringdown spectroscopy (CRDS) and chemical titration with an excess amount of sulfur dioxide (SO₂). The method of monitoring the consumption of SO₂ as a scavenger allows better characterization of the CH₂OO at low pressure and short residence time. The yield of thermalized CH₂OO from ethene ozonolysis was found to decrease with decreasing pressure. The nascent yield of thermalized CH₂OO was determined to be 20.1 ± 2.5% by extrapolation of the 7-19 Torr measurements to the zero-pressure limit. Kinetic models enable better evaluation and understanding of the different measurement methods of thermalized Criegee intermediates. The information on the low-pressure yields from this work serves as a benchmark for theoretical calculations and facilitates a better understanding of the alkene ozonolysis reaction mechanisms.

Post-Transition-State Direct Dynamics Simulations on the Ozonolysis of Catechol

On-the-fly dynamics simulations are performed for the reaction of catechol + O₃. The post transition state (TS) dynamics is studied at temperatures of 400 and 500 K. The PM7 semiempirical method is employed for calculating the potential energy gradient needed for integrating Hamilton's equations of motion. This semiempirical method provides excellent agreement in terms of energy and geometry of the TSs as well as minimum energy states of the system with respect to B3LYP/6-311+G (2df, 2p) calculated results. In the dynamics, first, a peroxyacid is formed, which further dissociates to different fragments. Four major channels forming CO, CO₂, H₂O, and small carboxylic acid (SCA) fragments are seen in this reaction. Rates of each of the channels and the overall unimolecular reaction are calculated at both temperatures. Branching ratios of all these product channels are calculated and compared with experiment. The minimum energy profile of CO₂, CO, and H₂O channels are calculated. A qualitative estimate of activation energies for all the channels are obtained and compared with the explicit TS energies of three product channels, which ultimately correlate with the reaction probabilities.

Rapid Allylic 1,6 H-Atom Transfer in an Unsaturated Criegee Intermediate

A novel allylic 1,6 hydrogen-atom-transfer mechanism is established through infrared activation of the 2-butenal oxide Criegee intermediate, resulting in very rapid unimolecular decay to hydroxyl (OH) radical products. A new precursor, Z/E-1,3-diiodobut-1-ene, is synthesized and photolyzed in the presence of oxygen to generate a new four-carbon Criegee intermediate with extended conjugation across the vinyl and carbonyl oxide groups that facilitates rapid allylic 1,6 H-atom transfer. A low-energy reaction pathway involving isomerization of 2-butenal oxide from a lower-energy (tZZ) conformer to a higher-energy (cZZ) conformer followed by 1,6 hydrogen transfer via a seven-membered ring transition state is predicted theoretically and shown experimentally to yield OH products. The low-lying (tZZ) conformer of 2-butenal oxide is identified based on computed anharmonic frequencies and intensities of its conformers. Experimental IR action spectra recorded in the fundamental CH stretch region with OH product detection by UV laser-induced fluorescence reveal a distinctive IR transition of the low-lying (tZZ) conformer at 2996 cm^-1 that results in rapid unimolecular decay to OH products. Statistical RRKM calculations involving a combination of conformational isomerization and unimolecular decay via 1,6 H-transfer yield an effective decay rate k_eff(E) on the order of 10⁸ s^-1 at ca. 3000 cm^-1 in good accord with the experiment. Unimolecular decay proceeds with significant enhancement due to quantum mechanical tunneling. A rapid thermal decay rate of ca. 10⁶ s^-1 is predicted by master-equation modeling of 2-butenal oxide at 298 K, 1 bar. This novel unimolecular decay pathway is expected to increase the nonphotolytic production of OH radicals upon alkene ozonolysis in the troposphere.

Improved Computational Modeling of the Kinetics of the Acetylperoxy + HO2 Reaction

The acetylperoxy + HO2 reaction has multiple impacts on the troposphere, with a triplet pathway leading to peracetic acid + O2 (reaction 1a) competing with singlet pathways leading to acetic...

Unimolecular and water reactions of oxygenated and unsaturated Criegee intermediates under atmospheric conditions

Ozonolysis of unsaturated hydrocarbons (VOCs) is one of the main oxidation processes in the atmosphere. The stabilized Criegee intermediates (SCI) formed are highly reactive oxygenated species that potentially influence the...

Spiers Memorial Lecture: theory of unimolecular reactions

One hundred years ago, at an earlier Faraday Discussion meeting, Lindemann presented a mechanism that provides the foundation for contemplating the pressure dependence of unimolecular reactions. Since that time, our...

Identification of the acetaldehyde oxide Criegee intermediate reaction network in the ozone-assisted low-temperature oxidation of trans-2-butene

Uni- and bi-molecular reactions involving Criegee intermediates (CIs) have been the focus of many studies due to the role these molecules play in atmospheric chemistry. The reactivity of CIs is known to strongly depend on their structure. The reaction network of the second simplest CI, acetaldehyde oxide (CH₃CHOO), is investigated in this work in an atmospheric pressure jet-stirred reactor (JSR) during the ozonolysis of trans-2-butene to explore the kinetic pathways relevant to atmospheric chemistry and low-temperature combustion. The mole fraction profiles of reactants, intermediates, and final products are determined by means of molecular-beam mass spectrometry in conjunction with single-photon ionization employing tunable synchrotron-generated vacuum ultraviolet radiation. A network of CI reactions is identified in the temperature region below 600 K, characterized by CI addition to trans-2-butene, water, formaldehyde, formic acid, and methanol. No sequential additions of the CH₃CHOO CI are observed, in contrast with the reactivity of the simplest CI (H₂COO) and the earlier observation of an extensive reaction network with up to four H₂COO sequential additions (Phys. Chem. Chem. Phys., 2019, 21, 7341-7357). Experimental photoionization efficiency scans recorded at 300 K and 425 K and ab initio threshold energy calculations lead to the identification and quantification of previously elusive intermediates, such as ketohydroperoxide and hydroperoxide species. Specifically, the C₄H₈ + O₃ adduct is identified as a ketohydroperoxide (KHP, 3-hydroperoxybutan-2-one, CH₃C(O)CH(CH₃)OOH), while hydroxyacetaldehyde (glycolaldehyde, HCOCH₂OH) formation is attributed to unimolecular isomerization of the CIs. Other hydroperoxide species such as methyl hydroperoxide (CH₃OOH), ethyl hydroperoxide (C₂H₅OOH), butyl hydroperoxide (OOH), hydroperoxyl acetaldehyde (HOOCH₂CHO), hydroxyethyl hydroperoxide (CH₃CH(OH)OOH), but-1-enyl-3-hydroperoxide, and 4-hydroxy-3-methylpentan-2-one (HOCH(CH₃)CH(CH₃)C(O)CH₃) are also identified. Detection of additional oxygenated species such as methanol, ethanol, ketene, and aldehydes suggests multiple active oxidation routes. These results provide additional evidence that CIs are key intermediates of the ozone-unsaturated hydrocarbon reactions providing critical inputs for improved kinetics models.

Molecular-Weight Growth in Ozone-Initiated Low-Temperature Oxidation of Methyl Crotonate

We report experiments of ozone-initiated low-temperature oxidation of methyl crotonate (MC, CH₃-CH═CH-C(O)OCH₃) from 420 to 660 K in a near-atmospheric-pressure jet-stirred reactor using photoionization molecular-beam mass spectrometry as a sampling technique. In this temperature regime, no typical low-temperature combustion (LTC) reactions have been observed for MC when oxygen (O₂) is used as the oxidizer. Upon ozone addition, significant oxidation of methyl crotonate is found. On the basis of experimentally observed energy-dependent mass peaks in combination with temperature-dependent mole fraction profiles and photoionization efficiency curves, we provide new insights into the methyl crotonate ozonolysis reaction network. The observed MC + O₃ products, C₅H₈O₅, are found to be related to the keto-hydroperoxides resulting from the isomerization of the primary ozonide. Evidence is also provided that molecular growth mainly results from cycloaddition reactions of the Criegee intermediate into aldehydes and alkenes as well as addition reactions of the Criegee intermediates to the double bond of methyl crotonate and sequential decomposition into ketones. Furthermore, species that contribute in large amounts to the low-temperature oxidation of methyl crotonate, like H₂O₂, CH₃OOH, CH₃OH, and HC(O)OH, are identified, and their mole fractions are reported. Additionally, preliminary modeling is performed which qualitatively captures the observed NTC behavior and reveals future research opportunities.

Decomposition of the simplest ketohydroperoxide in the ozonolysis of ethylene

Hydroperoxides from the ozonolysis of alkenes, in addition to Criegee intermediates, have been proposed as an atmospheric source of OH radicals in the absence of sunlight, but have remained largely elusive due to their reactivity. A weak peroxide bond enables facile OH elimination, and subsequent β-scission can lead to a variety of decomposition products depending on the nature of the peroxide. In this paper we explore this process theoretically for the simplest ketohydroperoxide, hydroperoxyacetaldehyde (HPA), which is believed to be formed in the ozonolysis of ethylene. Despite it being the most stable C₂H₄O₃ species in this reaction scheme, lower in energy than the starting materials by around 100 kcal mol^-1, HPA has only been directly observed once in the ozonolysis of ethylene by photoionization mass spectrometry appearance energy. Here we report predictions of the rotational spectrum of HPA conducted in support of microwave spectroscopy experiments. We suggest a new dissociation path from HPA to glyoxal [HOOCH₂CHO → HCOCH₂O + OH → CHOCHO + H], supported by thermochemical calculations. We encourage the search for glyoxal using complementary experimental methods, and suggest possible future experimental directions. Evidence of glyoxal formation from ethylene ozonolysis might provide evidence of this underappreciated path in an important and long studied reaction system.

Trends in stabilisation of Criegee intermediates from alkene ozonolysis

Criegee Intermediates (CI), formed in the ozonolysis of alkenes, play a central role in tropospheric chemistry as an important source of radicals, with stabilised CI (SCI) able to participate in bimolecular reactions, affecting climate through the formation of inorganic and organic aerosol. However, total SCI yields have only been determined for a few alkene systems, while speciated SCI yields from asymmetrical alkenes are almost entirely unknown. Here we report for the first time a systematic experimental exploration of the stabilisation of CH2OO and (CH3)2COO CI, formed from ten alkene-ozone systems with a range of different sizes and structures, under atmospherically relevant conditions in the EUPHORE chamber. Experiments in the presence of excess SO2 (an SCI scavenger) determined total SCI yields from each alkene-ozone system. Comparison of primary carbonyl yields in the presence/absence of SO2 determined the stabilisation fraction of a given CI. The results show that the stabilisation of a given CI increases as the size of the carbonyl co-product increases. This is interpreted in terms of the nascent population of CI formed following decomposition of the primary ozonide (POZ) having a lower mean energy distribution when formed with a larger carbonyl co-product, as more of the energy from the POZ is taken by the carbonyl. These findings have significant implications for atmospheric modelling of alkene ozonolysis. Higher stabilisation of small CI formed from large alkenes is expected to lead to lower radical yields from CI decomposition, and higher SCI concentrations, increasing the importance of SCI bimolecular reactions.

Temperature- and pressure-dependent rate coefficient measurement for the reaction of CH2OO with CH3CH2CHO

The rate coefficients of the CH₂OO + CH₃CH₂CHO reaction were studied at temperatures and pressures in the range of 283–318 K and 5–200 Torr.

Pressure-Dependent Kinetics of the Reaction between CH3OO and OH Focusing on the Product Yield of Methyltrioxide (CH3OOOH)

The reaction kinetics of methyl peroxy radical (CH₃OO) and hydroxyl radical (OH) has attracted an increasing level of interest in the past decade, while the branching yields of various product channels are still under debate. In this work, a comprehensive theoretical effort was made to investigate the branching yield of the stabilized methyltrioxide (CH₃OOOH, TRIOX) adduct, which has recently been a research focus. Our computed branching ratio of TRIOX at 298 K and 760 Torr is ∼0.04, in agreement with the result of multiplexed photoionization mass spectrometry. We show that the large branching yield obtained in an early theoretical study mainly originated from the collision-induced strong stabilization presented in their simulation. Our findings clarify the controversial product yield results for this important species in recent studies. The computed rate constants over wide temperature and pressure ranges allow better integration of this reaction into global atmospheric models and low-temperature combustion kinetic models.

Calculation of the absolute photoionization cross-sections for C1-C4 Criegee intermediates and vinyl hydroperoxides

Criegee Intermediates (CIs) and their isomer Vinyl Hydroperoxides (VHPs) are crucial intermediates in the ozonolysis of alkenes. To better understand the underlying chemistry of CIs and VHPs, progress has been made to detect and identify them by photoionization mass spectrometric experiments. Further reliable quantitative information about these elusive intermediates requires their photoionization cross sections. The present work systematically investigated the near-threshold absolute photoionization cross-sections for ten C1-C4 CIs and VHPs, i.e., formaldehyde oxide (CH₂OO), acetaldehyde oxide (syn-/anti-CH₃CHOO), acetone oxide ((CH₃)₂COO), syn-CH₃-anti-(cis-CH=CH₂)COO, syn-CH₃-anti-(trans-CH=CH₂)COO and vinyl hydroperoxide (CH₂CHOOH), 2-hydroperoxypropene (CH₂=C(CH₃)OOH), syn-CH₂ = anti-(cis-CH=CH₂)-COOH, syn-CH₂ = anti-(trans-CH=CH₂)COOH. The adiabatic ionization energies (AIEs) were calculated at the DLPNO-CCSD(T)/CBS level with uncertainties of less than 0.05 eV. The calculated AIEs for C1-C4 CIs and VHPs vary from 8.75 to 10.0 eV with the AIEs decreasing as the substitutions increase. Franck-Condon factors were calculated with the double Duschinsky approximation and the ionization spectra were obtained based on the calculated ionization energies. Pure electronic photoionization cross sections are calculated by the frozen-core Hartree-Fock (FCHF) approximation. The final determined absolute cross sections are around 4.5-6 Mb for the first and second ionization of CIs and 15-25 Mb for VHPs. It is found that the addition of a methyl group or an unsaturated vinyl substitution for the CIs does not substantially change the absolute value of their cross sections.

Identification of the Criegee intermediate reaction network in ethylene ozonolysis: impact on energy conversion strategies and atmospheric chemistry

The reaction network of the simplest Criegee intermediate (CI) CH₂OO has been studied experimentally during the ozonolysis of ethylene.

Low-Temperature Oxidation of Ethylene by Ozone in a Jet-Stirred Reactor

Ethylene oxidation initiated by ozone addition (ozonolysis) is carried out in a jet-stirred reactor from 300 to 1000 K to explore the kinetic pathways relevant to low-temperature oxidation. The temperature dependencies of species' mole fractions are quantified using molecular-beam mass spectrometry with electron ionization and single-photon ionization employing tunable synchrotron-generated vacuum-ultraviolet radiation. Upon ozone addition, significant ethylene oxidation is found in the low-temperature regime from 300 to 600 K. Here, we provide new insights into the ethylene ozonolysis reaction network via identification and quantification of previously elusive intermediates by combining experimental photoionization energy scans and ab initio threshold energy calculations for isomer identification. Specifically, the C₂H₄ + O₃ adduct C₂H₄O₃ is identified as a keto-hydroperoxide (hydroperoxy-acetaldehyde, HOOCH₂CHO) based on the calculated and experimentally observed ionization energy of 9.80 (±0.05) eV. Quantification using a photoionization cross-section of 5 Mb at 10.5 eV results in 5 ppm at atmospheric conditions, which decreases monotonically with temperature until 550 K. Other hydroperoxide species that contribute in larger amounts to the low-temperature oxidation of C₂H₄, like H₂O₂, CH₃OOH, and C₂H₅OOH, are identified and their temperature-dependent mole fractions are reported. The experimental evidence for additional oxygenated species such as methanol, ketene, acetaldehyde, and hydroxy-acetaldehyde suggest multiple active oxidation routes. This experimental investigation closes the gap between ozonolysis at atmospheric and elevated temperature conditions and provides a database for future modeling.

Unimolecular decomposition kinetics of the stabilised Criegee intermediates CH2OO and CD2OO

Decomposition kinetics of stabilised CH2OO and CD2OO Criegee intermediates have been investigated as a function of temperature (450-650 K) and pressure (2-350 Torr) using flash photolysis coupled with time-resolved cavity-enhanced broadband UV absorption spectroscopy. Decomposition of CD2OO was observed to be faster than CH2OO under equivalent conditions. Production of OH radicals following CH2OO decomposition was also monitored using flash photolysis with laser-induced fluorescence (LIF), with results indicating direct production of OH in the v = 0 and v = 1 states in low yields. Master equation calculations performed using the Master Equation Solver for Multi-Energy well Reactions (MESMER) enabled fitting of the barriers for the decomposition of CH2OO and CD2OO to the experimental data. Parameterisations of the decomposition rate coefficients, calculated by MESMER, are provided for use in atmospheric models and implications of the results are discussed. For CH2OO, the MESMER fits require an increase in the calculated barrier height from 78.2 kJ mol-1 to 81.8 kJ mol-1 using a temperature-dependent exponential down model for collisional energy transfer with ΔEdown = 32.6(T/298 K)1.7 cm-1 in He. The low- and high-pressure limit rate coefficients are k1,0 = 3.2 × 10-4(T/298)-5.81exp(-12 770/T) cm3 s-1 and k1,∞ = 1.4 × 1013(T/298)0.06exp(-10 010/T) s-1, with median uncertainty of ∼12% over the range of experimental conditions used here. Extrapolation to atmospheric conditions yields k1(298 K, 760 Torr) = 1.1+1.5-1.1 × 10-3 s-1. For CD2OO, MESMER calculations result in ΔEdown = 39.6(T/298 K)1.3 cm-1 in He and a small decrease in the calculated barrier to decomposition from 81.0 kJ mol-1 to 80.1 kJ mol-1. The fitted rate coefficients for CD2OO are k2,0 = 5.2 × 10-5(T/298)-5.28exp(-11 610/T) cm3 s-1 and k2,∞ = 1.2 × 1013(T/298)0.06exp(-9800/T) s-1, with overall error of ∼6% over the present range of temperature and pressure. The extrapolated k2(298 K, 760 Torr) = 5.5+9.2-5.5 × 10-3 s-1. The master equation calculations for CH2OO indicate decomposition yields of 63.7% for H2 + CO2, 36.0% for H2O + CO and 0.3% for OH + HCO with no significant dependence on temperature between 400 and 1200 K or pressure between 1 and 3000 Torr.

Kinetics of the reaction of the simplest Criegee intermediate with ammonia: a combination of experiment and theory

The negative temperature dependence of the rate coefficient for CH₂OO + NH₃ reaction was observed using an OH laser-induced fluorescence method.