Methyl-ethyl-substituted Criegee intermediate (MECI) is a four-carbon carbonyl oxide that is formed in the ozonolysis of some asymmetric alkenes. MECI is structurally similar to the isoprene-derived methyl vinyl ketone oxide (MVK-oxide) but lacks resonance stabilization, making it a promising candidate to help us unravel the effects of size, structure, and resonance stabilization that influence the reactivity of atmospherically important, highly functionalized Criegee intermediates. We present experimental and theoretical results from the first bimolecular study of MECI in its reaction with SO2, a reaction that shows significant sensitivity to the Criegee intermediate structure. Using multiplexed photoionization mass spectrometry, we obtain a rate coefficient of (1.3 ± 0.3) × 10-10 cm3 s-1 (95% confidence limits, 298 K, 10 Torr) and demonstrate the formation of SO3 under our experimental conditions. Through high-level theory, we explore the effect of Criegee intermediate structure on the minimum energy pathways for their reactions with SO2 and obtain modified Arrhenius fits to our predictions for the reaction of both syn and anti conformers of MECI with SO2 (ksyn = 4.42 × 1011 T-7.80exp(−1401/T) cm3 s-1 and kanti = 1.26 × 1011 T-7.55exp(−1397/T) cm3 s-1). Our experimental and theoretical rate coefficients (which are in reasonable agreement at 298 K) show that the reaction of MECI with SO2 is significantly faster than MVK-oxide + SO2, demonstrating the substantial effect of resonance stabilization on Criegee intermediate reactivity.
Liu, Tianlin; Elliott, Sarah N.; Zou, Meijun; Vansco, Michael F.; Sojdak, Christopher A.; Markus, Charles R.; Almeida, Raybel; Au, Kendrew; Sheps, Leonid; Osborn, David L.; Percival, Carl J.; Taatjes, Craig A.; Caravan, Rebecca L.; Klippenstein, Stephen J.; Lester, Marsha I.
Alkene ozonolysis generates short-lived Criegee intermediates that are a significant source of hydroxyl (OH) radicals. This study demonstrates that roaming of the separating OH radicals can yield alternate hydroxycarbonyl products, thereby reducing the OH yield. Specifically, hydroxybutanone has been detected as a stable product arising from roaming in the unimolecular decay of the methyl-ethyl-substituted Criegee intermediate (MECI) under thermal flow cell conditions. The dynamical features of this novel multistage dissociation plus a roaming unimolecular decay process have also been examined with ab initio kinetics calculations. Experimentally, hydroxybutanone isomers are distinguished from the isomeric MECI by their higher ionization threshold and distinctive photoionization spectra. Moreover, the exponential rise of the hydroxybutanone kinetic time profile matches that for the unimolecular decay of MECI. A weaker methyl vinyl ketone (MVK) photoionization signal is also attributed to OH roaming. Complementary multireference electronic structure calculations have been utilized to map the unimolecular decay pathways for MECI, starting with 1,4 H atom transfer from a methyl or methylene group to the terminal oxygen, followed by roaming of the separating OH and butanonyl radicals in the long-range region of the potential. Roaming via reorientation and the addition of OH to the vinyl group of butanonyl is shown to yield hydroxybutanone, and subsequent C-O elongation and H-transfer can lead to MVK. A comprehensive theoretical kinetic analysis has been conducted to evaluate rate constants and branching yields (ca. 10-11%) for thermal unimolecular decay of MECI to conventional and roaming products under laboratory and atmospheric conditions, consistent with the estimated experimental yield (ca. 7%).
Well-skipping radical-radical reactions can provide a chain-propagating pathway for formation of polycyclic radicals implicated in soot inception. Here we use controlled pyrolysis in a microreactor to isolate and examine the role of well-skipping channels in the phenyl (C6H5) + propargyl (C3H3) radical-radical reaction at temperatures of 800–1600 K and pressures near 25 Torr. The temperature and concentration dependence of the closed-shell (C9H8) and radical (C9H7) products are observed using electron-ionization mass spectrometry. The flow in the reactor is simulated using a boundary layer model employing a chemical mechanism based on recent rate coefficient calculations. Comparison between simulation and experiment shows reasonable agreement, within a factor of 3, while suggesting possible improvements to the model. In contrast, eliminating the well-skipping reactions from the chemistry mechanism causes a much larger discrepancy between simulation and experiment in the temperature dependence of the radical concentration, revealing that the well-skipping pathways, especially to form indenyl radical, are significant at temperatures of 1200 K and higher. While most C9H7 forms by well-skipping at 25 Torr, an additional simulation indicates that the well-skipping channels only contribute around 3% of the C9Hx yield at atmospheric pressure, thus indicating a negligible role of the well-skipping pathways at atmospheric and higher pressures.
Using chemical kinetic modeling and statistical analysis, we investigate the possibility of correlating key chemical "markers"-typically small molecules-formed during very lean (φ ∼0.001) oxidation experiments with near-stoichiometric (φ ∼1) fuel ignition properties. One goal of this work is to evaluate the feasibility of designing a fuel-screening platform, based on small laboratory reactors that operate at low temperatures and use minimal fuel volume. Buras et al. [Combust. Flame 2020, 216, 472-484] have shown that convolutional neural net (CNN) fitting can be used to correlate first-stage ignition delay times (IDTs) with OH/HO2measurements during very lean oxidation in low-T flow reactors with better than factor-of-2 accuracy. In this work, we test the limits of applying this correlation-based approach to predict the low-temperature heat release (LTHR) and total IDT, including the sensitivity of total IDT to the equivalence ratio, φ. We demonstrate that first-stage IDT can be reliably correlated with very lean oxidation measurements using compressed sensing (CS), which is simpler to implement than CNN fitting. LTHR can also be predicted via CS analysis, although the correlation quality is somewhat lower than for first-stage IDT. In contrast, the accuracy of total IDT prediction at φ = 1 is significantly lower (within a factor of 4 or worse). These results can be rationalized by the fact that the first-stage IDT and LTHR are primarily determined by low-temperature chemistry, whereas total IDT depends on low-, intermediate-, and high-temperature chemistry. Oxidation reactions are most important at low temperatures, and therefore, measurements of universal molecular markers of oxidation do not capture the full chemical complexity required to accurately predict the total IDT even at a single equivalence ratio. As a result, we find that φ-sensitivity of ignition delay cannot be predicted at all using solely correlation with lean low-T chemical speciation measurements.
The reactivity of carbonyl oxides has previously been shown to exhibit strong conformer and substituent dependencies. Through a combination of synchrotron-multiplexed photoionization mass spectrometry experiments (298 K and 4 Torr) and high-level theory [CCSD(T)-F12/cc-pVTZ-F12//B2PLYP-D3/cc-pVTZ with an added CCSDT(Q) correction], we explore the conformer dependence of the reaction of acetaldehyde oxide (CH3CHOO) with dimethylamine (DMA). The experimental data support the theoretically predicted 1,2-insertion mechanism and the formation of an amine-functionalized hydroperoxide reaction product. Tunable-vacuum ultraviolet photoionization probing of anti- or anti- + syn-CH3CHOO reveals a strong conformer dependence of the title reaction. The rate coefficient of DMA with anti-CH3CHOO is predicted to exceed that for the reaction with syn-CH3CHOO by a factor of ∼34,000, which is attributed to submerged barrier (syn) versus barrierless (anti) mechanisms for energetically downhill reactions.
Rapid molecular-weight growth of hydrocarbons occurs in flames, in industrial synthesis, and potentially in cold astrochemical environments. A variety of high- and low-temperature chemical mechanisms have been proposed and confirmed, but more facile pathways may be needed to explain observations. We provide laboratory confirmation in a controlled pyrolysis environment of a recently proposed mechanism, radical–radical chain reactions of resonance-stabilized species. The recombination reaction of phenyl (c-C6H5) and benzyl (c-C6H5CH2) radicals produces both diphenylmethane and diphenylmethyl radicals, the concentration of the latter increasing with rising temperature. A second phenyl addition to the product radical forms both triphenylmethane and triphenylmethyl radicals, confirming the propagation of radical–radical chain reactions under the experimental conditions of high temperature (1100–1600 K) and low pressure (ca. 3 kPa). Similar chain reactions may contribute to particle growth in flames, the interstellar medium, and industrial reactors.
Ongoing progress in synthetic biology, metabolic engineering, and catalysis continues to produce a diverse array of advanced biofuels with complex molecular structure and functional groups. In order to integrate biofuels into existing combustion systems, and to optimize the design of next-generation combustion systems, understanding connections between molecular structure and ignition at low-temperature conditions (< 1000 K) remains a priority that is addressed in part using chemical kinetics modeling. The development of predictive models relies on detailed information, derived from experimental and theoretical studies, on molecular structure and chemical reactivity, both of which influence the balance of chain reactions that occur during combustion – propagation, termination, and branching. In broad context, three main categories of reactions affect ignition behavior: (i) initiation reactions that generate a distribution of organic radicals, R˙; (ii) competing unimolecular decomposition of R˙ and bimolecular reaction of R˙ with O2; (iii) decomposition mechanisms of peroxy radical adducts (ROO˙), including isomerization via ROO˙ ⇌ Q˙OOH. All three categories are influenced by functional groups in different ways, which causes a shift in the balance of chain reactions that unfold over complex temperature- and pressure-dependent mechanisms. The objective of the present review is three-fold: (1) to provide a historical account of research on low-temperature oxidation of biofuels, including initiation reactions, peroxy radical reactions, Q˙OOH-mediated reaction mechanisms, and chain-branching chemistry; (2) to summarize the influence of functional groups on chemical kinetics relevant to chain-branching reactions, which are responsible for the accelerated production of radicals that leads to ignition; (3) to identify areas of research that are needed – experimentally and computationally – to address fundamental questions that remain. Results from experimental, quantum chemical, and chemical kinetics modeling studies are reviewed for several classes of biofuels – alcohols, esters, ketones, acyclic ethers and cyclic ethers – and are compared against analogous results in alkane oxidation. The review is organized into separate sections for each biofuel class, which include studies on thermochemistry and bond dissociation energies, rate coefficients for initiation reactions via H-abstraction and related branching fractions, reaction mechanisms and product formation from reactive intermediates, ignition delay times, and chemical kinetics modeling. Each section is then summarized in order to identify areas for which additional functional group-specific work is required. The review concludes with an outline for research directions for improving the fundamental understanding of biofuel ignition chemistry and related chemical kinetics modeling.
Holland, Rayne; Khan, M.A.H.; Driscoll, Isabel; Chhantyal-Pun, Rabi; Derwent, Richard G.; Taatjes, Craig A.; Orr-Ewing, Andrew J.; Percival, Carl J.; Shallcross, Dudley E.
Trifluoroacetic acid (TFA), a highly soluble and stable organic acid, is photochemically produced by certain anthropogenically emitted halocarbons such as HFC-134a and HFO-1234yf. Both these halocarbons are used as refrigerants in the automobile industry, and the high global warming potential of HFC-134a has promoted regulation of its use. Industries are transitioning to the use of HFO-1234yf as a more environmentally friendly alternative. We investigated the environmental effects of this change and found a 33-fold increase in the global burden of TFA from an annual value of 65 tonnes formed from the 2015 emissions of HFC-134a to a value of 2220 tonnes formed from an equivalent emission of HFO-1234yf. The percentage increase in surface TFA concentrations resulting from the switch from HFC-134a to HFO-1234yf remains substantial with an increase of up to 250-fold across Europe. The increase in emissions greater than the current emission scenario of HFO-1234yf is likely to result in significant TFA burden as the atmosphere is not able to disperse and deposit relevant oxidation products. The Criegee intermediate initiated loss process of TFA reduces the surface level atmospheric lifetime of TFA by up to 5 days (from 7 days to 2 days) in tropical forested regions.
The reactions of carbonyl oxide Criegee intermediates with acids proceed predominantly by an insertion mechanism. We characterise the products from one of the simplest reactions of carbonyl oxides with inorganic acids, CH2OO + hydrogen chloride, which occurs via a 1,2-insertion in the H–Cl bond. Reactions of both HCl and DCl isotopologues yield product signal at the mass of the insertion product chloro(hydroperoxy)methane and a dissociative ionisation peak at the mass of the protonated (or deuteronated) Criegee intermediate. The isotopic composition of the insertion product has been measured for reaction mixtures where both HCl isotopologues are present, and the H/D ratio of the product is consistently higher (by a factor of 1.6 ± 0.3) than that of the reactants. This isotope selectivity in the products has smaller uncertainty than the ratio of measured rate coefficients and suggests a normal (k H > k D) kinetic isotope effect in the reaction. Theoretical kinetics calculations predict a small normal kinetic isotope effect for the overall reaction (k H / k D = 1.35 at 20 Torr N2 and k H / k D = 1.2 at 1 atm N2) but predict a substantial inverse kinetic isotope effect (k D > k H) for the stabilisation fraction, in disagreement with the experimental observation.
Christianson, Matthew G.; Doner, Anna C.; Davis, Matthew M.; Koritzke, Alanna L.; Turney, Justin M.; Schaefer, Henry F.; Sheps, Leonid; Osborn, David L.; Taatjes, Craig A.; Rotavera, Brandon
Oxiranes are a class of cyclic ethers formed in abundance during low-temperature combustion of hydrocarbons and biofuels, either via chain-propagating steps that occur from unimolecular decomposition of β-hydroperoxyalkyl radicals (β-̇QOOH) or from reactions of HOȮ with alkenes. Ethyloxirane is one of four alkyl-substituted cyclic ether isomers produced as an intermediate from n-butane oxidation. While rate coefficients for β-̇QOOH → ethyloxirane + ȮH are reported extensively, subsequent reaction mechanisms of the cyclic ether are not. As a result, chemical kinetics mechanisms commonly adopt simplified chemistry to describe ethyloxirane consumption by convoluting several elementary reactions into a single step, which may introduce mechanism truncation error—uncertainty derived from missing or incomplete chemistry. The present work provides fundamental insight on reaction mechanisms of ethyloxirane in support of ongoing efforts to minimize mechanism truncation error. Reaction mechanisms are inferred from the detection of products during chlorine atom-initiated oxidation experiments using multiplexed photoionization mass spectrometry conducted at 10 Torr and temperatures of 650 K and 800 K. To complement the experiments, calculations of stationary point energies were conducted using the ccCA-PS3 composite method on ̇R + O2 potential energy surfaces for the four ethyloxiranyl radical isomers, which produced barrier heights for 24 reaction pathways. In addition to products from ̇QOOH → cyclic ether + ȮH and ̇R + O2 → conjugate alkene + HOȮ, both of which were significant pathways and are prototypical to alkane oxidation, other species were identified from ring-opening of both ethyloxiranyl and ̇QOOH radicals. The latter occurs when the unpaired electron is localized on the ether group, causing the initial ̇QOOH structure to ring-open and form a resonance-stabilized ketohydroperoxide-type radical. The present work provides the first analysis of ethyloxirane oxidation chemistry, which reveals that consumption pathways are complex and may require an expansion of submechanisms to increase the fidelity of chemical kinetics mechanisms.
Vansco, Michael F.; Caravan, Rebecca L.; Pandit, Shubhrangshu; Zuraski, Kristen; Winiberg, Frank A.F.; Au, Kendrew; Bhagde, Trisha; Trongsiriwat, Nisalak; Walsh, Patrick J.; Osborn, David L.; Percival, Carl J.; Klippenstein, Stephen J.; Taatjes, Craig A.; Lester, Marsha I.
Isoprene is the most abundant non-methane hydrocarbon emitted into the Earth's atmosphere. Ozonolysis is an important atmospheric sink for isoprene, which generates reactive carbonyl oxide species (R1R2CO+O-) known as Criegee intermediates. This study focuses on characterizing the catalyzed isomerization and adduct formation pathways for the reaction between formic acid and methyl vinyl ketone oxide (MVK-oxide), a four-carbon unsaturated Criegee intermediate generated from isoprene ozonolysis. syn-MVK-oxide undergoes intramolecular 1,4 H-atom transfer to form a substituted vinyl hydroperoxide intermediate, 2-hydroperoxybuta-1,3-diene (HPBD), which subsequently decomposes to hydroxyl and vinoxylic radical products. Here, we report direct observation of HPBD generated by formic acid catalyzed isomerization of MVK-oxide under thermal conditions (298 K, 10 torr) using multiplexed photoionization mass spectrometry. The acid catalyzed isomerization of MVK-oxide proceeds by a double hydrogen-bonded interaction followed by a concerted H-atom transfer via submerged barriers to produce HPBD and regenerate formic acid. The analogous isomerization pathway catalyzed with deuterated formic acid (D2-formic acid) enables migration of a D atom to yield partially deuterated HPBD (DPBD), which is identified by its distinct mass (m/z 87) and photoionization threshold. In addition, bimolecular reaction of MVK-oxide with D2-formic acid forms a functionalized hydroperoxide adduct, which is the dominant product channel, and is compared to a previous bimolecular reaction study with normal formic acid. Complementary high-level theoretical calculations are performed to further investigate the reaction pathways and kinetics.
Doner, Anna C.; Davis, Matthew M.; Koritzke, Alanna L.; Christianson, Matthew G.; Turney, Justin M.; Schaefer, Henry F.; Sheps, Leonid; Osborn, David L.; Taatjes, Craig A.; Rotavera, Brandon
Oxiranes are a class of cyclic ethers formed in abundance during low-temperature combustion of hydrocarbons and biofuels, either via chain-propagating steps that occur from unimolecular decomposition of β-hydroperoxyalkyl radicals (β-˙QOOH) or from reactions of HÒO with alkenes. The cis- and trans-isomers of 2,3-dimethyloxirane are intermediates of n-butane oxidation, and while rate coefficients for β-˙QOOH → 2,3-dimethyloxirane + OH are reported extensively, subsequent reaction mechanisms of the cyclic ethers are not. As a result, chemical kinetics mechanisms commonly adopt simplified chemistry to describe the consumption of 2,3-dimethyloxirane by convoluting several elementary reactions into a single step, which may introduce mechanism truncation error—uncertainty derived from missing or incomplete chemistry. The present research examines the isomerdependence of 2,3-dimethyloxirane reaction mechanisms in support of ongoing efforts to minimize mechanism truncation error. Reaction mechanisms are inferred via the detection of products from Cl-initiated oxidation of both cis-2,3-dimethyloxirane and trans-2,3-dimethyloxirane using multiplexed photoionization mass spectrometry (MPIMS). The experiments were conducted at 10 Torr and temperatures of 650 K and 800 K. To complement the experiments, the enthalpies of stationary points on the ˙R + O2 surfaces were computed at the ccCA-PS3 level of theory. In total, 28 barrier heights were computed on the 2,3-dimethyloxiranylperoxy surfaces. Two notable aspects are low-lying pathways that form resonance-stabilized ketohydroperoxide-type radicals caused by ˙QOOH ring-opening when the unpaired electron is localized adjacent to the ether group, and cis-trans isomerization of ˙R and ˙QOOH radicals, via inversion, which enable reaction pathways otherwise restricted by stereochemistry. Several species were identified in the MPIMS experiments from ring opening of 2,3-dimethyloxiranyl radicals. Neither of the two conjugate alkene isomers prototypical of ˙R + O2 reactions were detected. Products were also identified from decomposition of ketohydroperoxide-type radicals. The present work provides the first analysis of 2,3-dimethyloxirane oxidation chemistry and reveals that consumption pathways are complex and require the expansion of submechanisms in chemical kinetics mechanisms.
Chhantyal-Pun, Rabi; Khan, M.A.H.; Taatjes, Craig A.; Percival, Carl J.; Orr-Ewing, Andrew J.; Shallcross, Dudley E.
In the context of tropospheric chemistry, Criegee intermediates denote carbonyl oxides with biradical/zwitterionic character (R1R2COO) that form during the ozonolysis of alkenes. First discovered almost 70 years ago, stabilised versions of Criegee intermediates formed via collisional removal of excess energy have interesting kinetic and mechanistic properties. The direct production and detection of these intermediates were not reported in the literature until 2008. However, recent advances in their generation through the ultraviolet irradiation of the corresponding diiodoalkanes in excess O2 and detection by various spectroscopic techniques (photoionisation, ultraviolet, infrared, microwave and mass spectrometry) have shown that these species can react rapidly with closed-shell molecules, in many cases at or exceeding the classical gas-kinetic limit, via multiple reaction pathways. These reactions can be complex, and laboratory measurements of products and the temperature and pressure dependence of the reaction kinetics have also revealed unusual behaviour. The potential role of these intermediates in atmospheric chemistry is significant, altering models of the oxidising capacity of the Earth's atmosphere and the rate of generation of secondary organic aerosol.
Vansco, Michael F.; Caravan, Rebecca L.; Zuraski, Kristen; Winiberg, Frank A.F.; Au, Kendrew; Trongsiriwat, Nisalak; Walsh, Patrick J.; Osborn, David L.; Percival, Carl J.; Khan, M.A.H.; Shallcross, Dudley E.; Taatjes, Craig A.; Lester, Marsha I.
Ozonolysis of isoprene, one of the most abundant volatile organic compounds emitted into the Earth's atmosphere, generates two four-carbon unsaturated Criegee intermediates, methyl vinyl ketone oxide (MVK-oxide) and methacrolein oxide (MACR-oxide). The extended conjugation between the vinyl substituent and carbonyl oxide groups of these Criegee intermediates facilitates rapid electrocyclic ring closures that form five-membered cyclic peroxides, known as dioxoles. This study reports the first experimental evidence of this novel decay pathway, which is predicted to be the dominant atmospheric sink for specific conformational forms of MVK-oxide (anti) and MACR-oxide (syn) with the vinyl substituent adjacent to the terminal O atom. The resulting dioxoles are predicted to undergo rapid unimolecular decay to oxygenated hydrocarbon radical products, including acetyl, vinoxy, formyl, and 2-methylvinoxy radicals. In the presence of O2, these radicals rapidly react to form peroxy radicals (ROO), which quickly decay via carbon-centered radical intermediates (QOOH) to stable carbonyl products that were identified in this work. The carbonyl products were detected under thermal conditions (298 K, 10 Torr He) using multiplexed photoionization mass spectrometry (MPIMS). The main products (and associated relative abundances) originating from unimolecular decay of anti-MVK-oxide and subsequent reaction with O2 are formaldehyde (88 ± 5%), ketene (9 ± 1%), and glyoxal (3 ± 1%). Those identified from the unimolecular decay of syn-MACR-oxide and subsequent reaction with O2 are acetaldehyde (37 ± 7%), vinyl alcohol (9 ± 1%), methylketene (2 ± 1%), and acrolein (52 ± 5%). In addition to the stable carbonyl products, the secondary peroxy chemistry also generates OH or HO2 radical coproducts.
Holland, Rayne; Khan, M.A.H.; Chhantyal-Pun, Rabi; Orr-Ewing, Andrew J.; Percival, Carl J.; Taatjes, Craig A.; Shallcross, Dudley E.
Perfluorooctanoic acid, PFOA, is one of the many concerning pollutants in our atmosphere; it is highly resistant to environmental degradation processes, which enables it to accumulate biologically. With direct routes of this chemical to the environment decreasing, as a consequence of the industrial phase out of PFOA, it has become more important to accurately model the effects of indirect production routes, such as environmental degradation of precursors; e.g., fluorotelomer alcohols (FTOHs). The study reported here investigates the chemistry, physical loss and transport of PFOA and its precursors, FTOHs, throughout the troposphere using a 3D global chemical transport model, STOCHEM-CRI. Moreover, this investigation includes an important loss process of PFOA in the atmosphere via the addition of the stabilised Criegee intermediates, hereby referred to as the "Criegee Field. " Whilst reaction with Criegee intermediates is a significant atmospheric loss process of PFOA, it does not result in its permanent removal from the atmosphere. The atmospheric fate of the resultant hydroperoxide product from the reaction of PFOA and Criegee intermediates resulted in a ≈0.04 Gg year-1 increase in the production flux of PFOA. Furthermore, the physical loss of the hydroperoxide product from the atmosphere (i.e., deposition), whilst decreasing the atmospheric concentration, is also likely to result in the reformation of PFOA in environmental aqueous phases, such as clouds, precipitation, oceans and lakes. As such, removal facilitated by the "Criegee Field" is likely to simply result in the acceleration of PFOA transfer to the surface (with an expected decrease in PFOA atmospheric lifetime of ≈10 h, on average from ca. ≈80 h without Criegee loss to 70 h with Criegee loss).
Taatjes, Craig A.; Khan, M.A.H.; Eskola, Arkke J.; Percival, Carl J.; Osborn, David L.; Wallington, Timothy J.; Shallcross, Dudley E.
The reaction of perfluorooctanoic acid with the smallest carbonyl oxide Criegee intermediate, CH 2 OO, has been measured and is very rapid, with a rate coefficient of (4.9 ± 0.8) × 10 -10 cm 3 s -1 , similar to that for reactions of Criegee intermediates with other organic acids. Evidence is shown for the formation of hydroperoxymethyl perfluorooctanoate as a product. With such a large rate coefficient, reaction with Criegee intermediates can be a substantial contributor to atmospheric removal of perfluorocarboxylic acids. However, the atmospheric fates of the ester product largely regenerate the initial acid reactant. Wet deposition regenerates the perfluorocarboxylic acid via condensed-phase hydrolysis. Gas-phase reaction with OH is expected principally to result in formation of the acid anhydride, which also hydrolyzes to regenerate the acid, although a minor channel could lead to destruction of the perfluorinated backbone.
Methanol is a benchmark for understanding tropospheric oxidation, but is underpredicted by up to 100% in atmospheric models. Recent work has suggested this discrepancy can be reconciled by the rapid reaction of hydroxyl and methylperoxy radicals with a methanol branching fraction of 30%. However, for fractions below 15%, methanol underprediction is exacerbated. Theoretical investigations of this reaction are challenging because of intersystem crossing between singlet and triplet surfaces – ∼45% of reaction products are obtained via intersystem crossing of a pre-product complex – which demands experimental determinations of product branching. Here we report direct measurements of methanol from this reaction. A branching fraction below 15% is established, consequently highlighting a large gap in the understanding of global methanol sources. These results support the recent high-level theoretical work and substantially reduce its uncertainties.
Khan, M.A.H.; Lyons, Kyle; Chhantyal-Pun, Rabi; Mcgillen, Max R.; Caravan, Rebecca L.; Taatjes, Craig A.; Orr-Ewing, Andrew J.; Percival, Carl J.; Shallcross, Dudley E.
Acetic acid (CH3COOH) is one of the most abundant carboxylic acids in the troposphere. In the study, the tropospheric chemistry of CH3COOH is investigated using the 3-D global chemistry transport model, STOCHEM-CRI. The highest mixing ratios of surface CH3COOH are found in the tropics by as much as 1.6 ppb in South America. The model predicts the seasonality of CH3COOH reasonably well and correlates with some surface and flight measurement sites, but the model drastically underpredicts levels in urban and midlatitudinal regions. The possible reasons for the underprediction are discussed. The simulations show that the lifetime and global burden of CH3COOH are 1.6–1.8 days and 0.45–0.61 Tg, respectively. The reactions of the peroxyacetyl radical (CH3CO3) with the hydroperoxyl radical (HO2) and other organic peroxy radicals (RO2) are found to be the principal sources of tropospheric CH3COOH in the model, but the model-measurement discrepancies suggest the possible unknown or underestimated sources which can contribute large fractions of the CH3COOH burden. The major sinks of CH3COOH in the troposphere are wet deposition, dry deposition, and OH loss. However, the reaction of CH3COOH with Criegee intermediates is proposed to be a potentially significant chemical loss process of tropospheric CH3COOH that has not been previously accounted for in global modeling studies. Inclusion of this loss process reduces the tropospheric CH3COOH level significantly which can give even larger discrepancies between model and measurement data, suggesting that the emissions inventory and the chemical production sources of CH3COOH are underpredicted even more so in current global models.
Criegee intermediates (CIs), carbonyl oxides formed in ozonolysis of alkenes, play key roles in the troposphere. The decomposition of CIs can be a significant source of OH to the tropospheric oxidation cycle especially during nighttime and winter months. A variety of model-measurement studies have estimated surface-level stabilized Criegee intermediate (sCI) concentrations on the order of 1 × 104 cm-3 to 1 × 105 cm-3, which makes a non-negligible contribution to the oxidising capacity in the terrestrial boundary layer. The reactions of sCI with the water monomer and the water dimer have been found to be the most important bimolecular reactions to the tropospheric sCI loss rate, at least for the smallest carbonyl oxides; the products from these reactions (e.g. hydroxymethyl hydroperoxide, HMHP) are also of importance to the atmospheric oxidation cycle. The sCI can oxidise SO2 to form SO3, which can go on to form a significant amount of H2SO4 which is a key atmospheric nucleation species and therefore vital to the formation of clouds. The sCI can also react with carboxylic acids, carbonyl compounds, alcohols, peroxy radicals and hydroperoxides, and the products of these reactions are likely to be highly oxygenated species, with low vapour pressures, that can lead to nucleation and SOA formation over terrestrial regions.
The initial oxidation products of methyl butyrate (MB) and ethyl butyrate (EB) are studied using a time- and energy-resolved photoionization mass spectrometer. Reactions are initiated with Cl radicals in an excess of oxygen at a temperature of 550 K and a pressure of 6 Torr. Ethyl crotonate is the sole isomeric product that is observed from concerted HO2-elimination from initial alkylperoxy radicals formed in the oxidation of EB. Analysis of the potential energy surface of each possible alkylperoxy radical shows that the CH3CH(OO)CH2C(O)OCH2CH3 (RγO2) and CH3CH2CH(OO)C(O)OCH2CH3 (RβO2) radicals are the isomers that could undergo this concerted HO2-elimination. Two lower-mass products (formaldehyde and acetaldehyde) are observed in both methyl and ethyl butyrate reactions. Secondary reactions of alkylperoxy radicals with HO2 radicals can decompose into the aforementioned products and smaller radicals. These pathways are the likely explanation for the formation of formaldehyde and acetaldehyde.
Methyl vinyl ketone (MVK) and methacrolein (MACR) are important intermediate products in atmospheric degradation of volatile organic compounds, especially of isoprene. This work investigates the reactions of the smallest Criegee intermediate, CH2OO, with its co-products from isoprene ozonolysis, MVK and MACR, using multiplexed photoionization mass spectrometry (MPIMS), with either tunable synchrotron radiation from the Advanced Light Source or Lyman-α (10.2 eV) radiation for photoionization. CH2OO was produced via pulsed laser photolysis of CH2I2 in the presence of excess O2. Time-resolved measurements of reactant disappearance and of product formation were performed to monitor reaction progress; first order rate coefficients were obtained from exponential fits to the CH2OO decays. The bimolecular reaction rate coefficients at 300 K and 4 Torr are k(CH2OO + MVK) = (5.0 ± 0.4) × 10-13 cm3 s-1 and k(CH2OO + MACR) = (4.4 ± 1.0) × 10-13 cm3 s-1, where the stated ±2σ uncertainties are statistical uncertainties. Adduct formation is observed for both reactions and is attributed to the formation of a secondary ozonides (1,2,4-trioxolanes), supported by master equation calculations of the kinetics and the agreement between measured and calculated adiabatic ionization energies. Kinetics measurements were also performed for a possible bimolecular CH2OO + CO reaction and for the reaction of CH2OO with CF3CHCH2 at 300 K and 4 Torr. For CH2OO + CO, no reaction is observed and an upper limit is determined: k(CH2OO + CO) < 2 × 10-16 cm3 s-1. For CH2OO + CF3CHCH2, an upper limit of k(CH2OO + CF3CHCH2) < 2 × 10-14 cm3 s-1 is obtained.
Direct kinetic and product studies of Criegee Intermediates reveal insertion and addition mechanisms for multiple co-reactant species. Observation of these highly oxygenated low volatility products indicate the potential role of Criegee Intermediate chemistry in molecular weight growth, and subsequently, secondary organic aerosol formation.
Bannan, Thomas J.; Murray Booth, A.; Le Breton, Michael; Bacak, Asan; Muller, Jennifer B.A.; Leather, Kimberley E.; Khan, M.A.H.; Lee, James D.; Dunmore, Rachel E.; Hopkins, James R.; Fleming, Zoe L.; Sheps, Leonid; Taatjes, Craig A.; Shallcross, Dudley E.; Percival, Carl J.
Following measurements in the winter of 2012, formic acid (HCOOH) and nitric acid (HNO3) were measured using a chemical ionization mass spectrometer (CIMS) during the Summer Clean Air for London (ClearfLo) campaign in London, 2012. Consequently, the seasonal dependence of formic acid sources could be better understood. A mean formic acid concentration of 1.3 ppb and a maximum of 12.7 ppb was measured which is significantly greater than that measured during the winter campaign (0.63 ppb and 6.7 ppb, respectively). Daily calibrations of formic acid during the summer campaign gave sensitivities of 1.2 ion counts s-1 parts per trillion (ppt) by volume-1 and a limit of detection of 34 ppt. During the summer campaign, there was no correlation between formic acid and anthropogenic emissions such as NOx and CO or peaks associated with the rush hour as was identified in the winter. Rather, peaks in formic acid were observed that correlated with solar irradiance. Analysis using a photochemical trajectory model has been conducted to determine the source of this formic acid. The contribution of formic acid formation through ozonolysis of alkenes is important but the secondary production from biogenic VOCs could be the most dominant source of formic acid at this measurement site during the summer.
Comprehensive chemical kinetics models used in the simulation of hydrocarbon and biofuel oxidation rely on accurate prescription of the underlying reaction mechanisms and rate parameters of associated elementary reactions. For practical transportation fuels, such models contain thousands of elementary reactions, which collectively define chain-initiation, -propagation, -branching, and -inhibition pathways. In the low-temperature regime, below approximately 1000 K where R + O2 reactions dominate, primary oxidation intermediates including cyclic ethers, carbonyls, and conjugate alkenes are formed in abundance via unimolecular decomposition of either chemically activated or thermalized radicals, specifically organic peroxy (ROO) or hydroperoxyalkyl species (QOOH). Experimental results from multiplexed photoionization mass spectrometry (MPIMS) experiments are detailed herein for several intermediates, derived initially from R + O2 reactions of hydrocarbons and biofuels, and show that intermediate species formed in the initial steps of oxidation undergo similar reactions to those of the parent molecule, including through QOOH-mediated pathways. Products from QOOH decomposition via chain-inhibition and chain-propagation pathways, namely conjugate alkenes, carbonyls, and cyclic ethers, are detected directly. Despite such rich chemistry involving QOOH radicals, most comprehensive chemical kinetics models neglect the complete description of primary oxidation intermediates, and rather consider a restricted number of reaction pathways. It is suggested that exclusion of the details of the oxidation of these intermediate products may affect the interpretation of combustion simulations using such models.
The rapid reaction of the smallest Criegee intermediate, CH2OO, with water dimers is the dominant removal mechanism for CH2OO in the Earth's atmosphere, but its products are not well understood. This reaction was recently suggested as a significant source of the most abundant tropospheric organic acid, formic acid (HCOOH), which is consistently underpredicted by atmospheric models. However, using time-resolved measurements of reaction kinetics by UV absorption and product analysis by photoionization mass spectrometry, we show that the primary products of this reaction are formaldehyde and hydroxymethyl hydroperoxide (HMHP), with direct HCOOH yields of less than 10%. Incorporating our results into a global chemistry-transport model further reduces HCOOH levels by 10-90%, relative to previous modeling assumptions, which indicates that the reaction CH2OO + water dimer by itself cannot resolve the discrepancy between the measured and predicted HCOOH levels.
Product formation from the low-temperature oxidation of two isotopologues of the proposed biofuel butanone was studied via multiplexed photoionization mass spectrometry (MPIMS) at 500 and 700 K to elucidate product branching ratios for R and QOOH pathways. Products were identified and branching ratios quantified for a number of species, with the aid of ab initio calculations. Chain-inhibiting C-C β-scission of R and select chain-propagating channels are discussed. Whilst methyl vinyl ketone and HOO, (from chain-inhibiting pathways) were found to be major products, chain propagation pathways leading to carbonyl and cyclic ether species following OH-elimination from QOOH were found to be pertinent at both temperatures. At 700 K, R C-C β-scission was significantly enhanced, as evident in the branching ratios, however the formation of QOOH-derived chain-propagation products remained relevant.