Re-examining ammonia addition to the Criegee intermediate: converging to chemical accuracy

Theory shows ammonia is unlikely to be significant in Criegee chemistry and demonstrates the importance of perturbative quadruple excitations in Criegee chemistry.

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.

Kinetics of the simplest Criegee intermediate reaction with ozone studied using a mid-infrared quantum cascade laser spectrometer

The first experimental study of Criegee intermediate reactions with ozone, clarifying its role in atmosphere and laboratory studies of ozonolysis.

Kinetic studies of C1 and C2 Criegee intermediates with SO2 using laser flash photolysis coupled with photoionization mass spectrometry and time resolved UV absorption spectroscopy

Kinetics of CH₂OO + SO₂ confirmed over a wide range of [SO₂]. Acetaldehyde observed as a major product of the reaction of CH₃CHOO + SO₂.

Selective deuteration illuminates the importance of tunneling in the unimolecular decay of Criegee intermediates to hydroxyl radical products

Ozonolysis of alkenes, an important nonphotolytic source of hydroxyl (OH) radicals in the atmosphere, proceeds through unimolecular decay of Criegee intermediates. Here, we report a large kinetic isotope effect associated with the rate-limiting hydrogen-transfer step that releases OH radicals for a prototypical Criegee intermediate, CH3CHOO. IR excitation of selectively deuterated syn-CD3CHOO is shown to result in deuterium atom transfer and release OD radical products. Vibrational activation of syn-CD3CHOO is coupled with direct time-resolved detection of OD products to measure a 10-fold slower rate of unimolecular decay upon deuteration in the vicinity of the transition state barrier, which is confirmed by microcanonical statistical theory that incorporates quantum mechanical tunneling. The corresponding kinetic isotope effect of ∼10 is attributed primarily to the decreased probability of D-atom vs. H-atom transfer arising from tunneling. Master equation modeling is utilized to compute the thermal unimolecular decay rates for selectively and fully deuterated syn methyl-substituted Criegee intermediates under atmospheric conditions. At 298 K (1 atm), tunneling is predicted to enhance the thermal decay rate of syn-CH3CHOO compared with the deuterated species, giving rise to a significant kinetic isotope effect of ∼50.

An Estimation of the Levels of Stabilized Criegee Intermediates in the UK Urban and Rural Atmosphere Using the Steady-State Approximation and the Potential Effects of These Intermediates on Tropospheric Oxidation Cycles

Levels of the stabilized Criegee Intermediate (sCI), produced via the ozonolysis of unsaturated volatile organic compounds (VOCs), were estimated at two London urban sites (Marylebone Road and Eltham) and one rural site (Harwell) in the UK over the period of 1998-2012. The steady-state approximation was applied to data obtained from the NETCEN (National Environmental Technology Centre) database, and the levels of annual average sCI were estimated to be in the range of 30-3000 molecules cm-3 for UK sites. A consistent diurnal cycle of sCI concentration is estimated for the UK sites with increasing levels during daylight hours, peaking just after midday. The seasonal pattern of sCI shows higher levels in spring with peaks around May due to the higher levels of O3. The ozone weekend effect resulted in higher sCI in UK urban areas during weekend. The sCI data were modeled using the information provided by the Air Quality Improvement Research Program (AQIRP) and found that the modeled production was five- to six-fold higher than our estimated data, and therefore the estimated sCI concentrations in this study are thought to be lower estimates only. Compared with nighttime, 1.3- to 1.8-fold higher sCI exists under daytime conditions. Using the levels of sCI estimated at Marylebone Road, globally the oxidation rates of NO2 + sCI (22.4 Gg/yr) and SO2 + sCI (37.6 Gg/yr) in urban areas can increase their effect in the troposphere and potentially further alter the oxidizing capacity of the troposphere. Further investigations of modeled sCI show that CH3CHOO (64%) and CH2OO (13%) are dominant among all contributing sCI at the UK sites.

Online Quantification of Criegee Intermediates of α-Pinene Ozonolysis by Stabilization with Spin Traps and Proton-Transfer Reaction Mass Spectrometry Detection

Biogenic alkenes, which are among the most abundant volatile organic compounds in the atmosphere, are readily oxidized by ozone. Characterizing the reactivity and kinetics of the first-generation products of these reactions, carbonyl oxides (often named Criegee intermediates), is essential in defining the oxidation pathways of organic compounds in the atmosphere but is highly challenging due to the short lifetime of these zwitterions. Here, we report the development of a novel online method to quantify atmospherically relevant Criegee intermediates (CIs) in the gas phase by stabilization with spin traps and analysis with proton-transfer reaction mass spectrometry. Ozonolysis of α-pinene has been chosen as a proof-of-principle model system. To determine unambiguously the structure of the spin trap adducts with α-pinene CIs, the reaction was tested in solution, and reaction products were characterized with high-resolution mass spectrometry, electron paramagnetic resonance, and nuclear magnetic resonance spectroscopy. DFT calculations show that addition of the Criegee intermediate to the DMPO spin trap, leading to the formation of a six-membered ring adduct, occurs through a very favorable pathway and that the product is significantly more stable than the reactants, supporting the experimental characterization. A flow tube set up has been used to generate spin trap adducts with α-pinene CIs in the gas phase. We demonstrate that spin trap adducts with α-pinene CIs also form in the gas phase and that they are stable enough to be detected with online mass spectrometry. This new technique offers for the first time a method to characterize highly reactive and atmospherically relevant radical intermediates in situ.

OH production from the photolysis of isoprene-derived peroxy radicals: cross-sections, quantum yields and atmospheric implications

The OH radical production from the near-ultraviolet photolysis of peroxy radicals derived from isoprene has been investigated.

Atmospheric Chemistry of Criegee Intermediates: Unimolecular Reactions and Reactions with Water

Criegee intermediates are produced in the ozonolysis of unsaturated hydrocarbons in the troposphere, and understanding their fate is a prerequisite to modeling climate-controlling atmospheric aerosol formation. Although some experimental and theoretical rate data are available, they are incomplete and partially inconsistent, and they do not cover the tropospheric temperature range. Here, we report quantum chemical rate constants for the reactions of stabilized formaldehyde oxide (CH₂OO) and acetaldehyde oxide (syn-CH₃CHOO and anti-CH₃CHOO) with H₂O and for their unimolecular reactions. Our results are obtained by combining post-CCSD(T) electronic structure benchmarks, validated density functional theory potential energy surfaces, and multipath variational transition state theory with multidimensional tunneling, coupled-torsions anharmonicity, and high-frequency anharmonicity. We consider two different types of reaction mechanisms for the bimolecular reactions, namely, (i) addition-coupled hydrogen transfer and (ii) double hydrogen atom transfer (DHAT). First, we show that the MN15-L exchange-correlation functional has kJ/mol accuracy for the CH₂OO + H₂O and syn-CH₃CHOO + H₂O reactions. Then we show that, due to tunneling, the DHAT mechanism is especially important in the syn-CH₃CHOO + H₂O reaction. We show that the dominant pathways for reactions of Criegee intermediates depend on altitude. The results we obtain eliminate the discrepancy between experiment and theory under those conditions where experimental results are available, and we make predictions for the full range of temperatures and pressures encountered in the troposphere and stratosphere. The present results are an important cog in clarifying the atmospheric fate and oxidation processes of Criegee intermediates, and they also show how theoretical methods can provide reliable rate data for complex atmospheric processes.

Communication: Real time observation of unimolecular decay of Criegee intermediates to OH radical products

In the atmosphere, a dominant loss process for carbonyl oxide intermediates produced from alkene ozonolysis is also an important source of hydroxyl radicals. The rate of appearance of OH radicals is revealed through direct time-domain measurements following vibrational activation of prototypical methyl-substituted Criegee intermediates under collision-free conditions. Complementary theoretical calculations predict the unimolecular decay rate for the Criegee intermediates in the vicinity of the barrier for 1,4 hydrogen transfer that leads to OH products. Both experiment and theory yield unimolecular decay rates of ca. 10(8) and 10(7) s(-1) for syn-CH3CHOO and (CH3)2COO, respectively, at energies near the barrier. Tunneling through the barrier, computed from high level electronic structure theory and experimentally validated, makes a significant contribution to the decay rate. Extension to thermally averaged unimolecular decay of stabilized Criegee intermediates under atmospheric conditions yields rates that are six orders of magnitude slower than those evaluated directly in the barrier region.

Atmospheric fates of Criegee intermediates in the ozonolysis of isoprene

We use a large laboratory, modeling, and field dataset to investigate the isoprene + O3 reaction, with the goal of better understanding the fates of the C1 and C4 Criegee intermediates in the atmosphere. Although ozonolysis can produce several distinct Criegee intermediates, the C1 stabilized Criegee (CH2OO, 61 ± 9%) is the only one observed to react bimolecularly. We suggest that the C4 Criegees have a low stabilization fraction and propose pathways for their decomposition. Both prompt and non-prompt reactions are important in the production of OH (28% ± 5%) and formaldehyde (81% ± 16%). The yields of unimolecular products (OH, formaldehyde, methacrolein (42 ± 6%) and methyl vinyl ketone (18 ± 6%)) are fairly insensitive to water, i.e., changes in yields in response to water vapor (≤4% absolute) are within the error of the analysis. We propose a comprehensive reaction mechanism that can be incorporated into atmospheric models, which reproduces laboratory data over a wide range of relative humidities. The mechanism proposes that CH2OO + H2O (k(H2O)∼ 1 × 10(-15) cm(3) molec(-1) s(-1)) yields 73% hydroxymethyl hydroperoxide (HMHP), 6% formaldehyde + H2O2, and 21% formic acid + H2O; and CH2OO + (H2O)2 (k(H2O)2∼ 1 × 10(-12) cm(3) molec(-1) s(-1)) yields 40% HMHP, 6% formaldehyde + H2O2, and 54% formic acid + H2O. Competitive rate determinations (kSO2/k(H2O)n=1,2∼ 2.2 (±0.3) × 10(4)) and field observations suggest that water vapor is a sink for greater than 98% of CH2OO in a Southeastern US forest, even during pollution episodes ([SO2] ∼ 10 ppb). The importance of the CH2OO + (H2O)n reaction is demonstrated by high HMHP mixing ratios observed over the forest canopy. We find that CH2OO does not substantially affect the lifetime of SO2 or HCOOH in the Southeast US, e.g., CH2OO + SO2 reaction is a minor contribution (<6%) to sulfate formation. Extrapolating, these results imply that sulfate production by stabilized Criegees is likely unimportant in regions dominated by the reactivity of ozone with isoprene. In contrast, hydroperoxide, organic acid, and formaldehyde formation from isoprene ozonolysis in those areas may be significant.

Kinetics of stabilised Criegee intermediates derived from alkene ozonolysis: reactions with SO2, H2O and decomposition under boundary layer conditions

The removal of SO2 in the presence of alkene-ozone systems has been studied for ethene, cis-but-2-ene, trans-but-2-ene and 2,3-dimethyl-but-2-ene, as a function of humidity, under atmospheric boundary layer conditions. The SO2 removal displays a clear dependence on relative humidity for all four alkene-ozone systems confirming a significant reaction for stabilised Criegee intermediates (SCI) with H2O. The observed SO2 removal kinetics are consistent with relative rate constants, k(SCI + H2O)/k(SCI + SO2), of 3.3 (±1.1) × 10(-5) for CH2OO, 26 (±10) × 10(-5) for CH3CHOO derived from cis-but-2-ene, 33 (±10) × 10(-5) for CH3CHOO derived from trans-but-2-ene, and 8.7 (±2.5) × 10(-5) for (CH3)2COO derived from 2,3-dimethyl-but-2-ene. The relative rate constants for k(SCI decomposition)/k(SCI + SO2) are -2.3 (±3.5) × 10(11) cm(-3) for CH2OO, 13 (±43) × 10(11) cm(-3) for CH3CHOO derived from cis-but-2-ene, -14 (±31) × 10(11) cm(-3) for CH3CHOO derived from trans-but-2-ene and 63 (±14) × 10(11) cm(-3) for (CH3)2COO. Uncertainties are ±2σ and represent combined systematic and precision components. These values are derived following the approximation that a single SCI is present for each system; a more comprehensive interpretation, explicitly considering the differing reactivity for syn- and anti-SCI conformers, is also presented. This yields values of 3.5 (±3.1) × 10(-4) for k(SCI + H2O)/k(SCI + SO2) of anti-CH3CHOO and 1.2 (±1.1) × 10(13) for k(SCI decomposition)/k(SCI + SO2) of syn-CH3CHOO. The reaction of the water dimer with CH2OO is also considered, with a derived value for k(CH2OO + (H2O)2)/k(CH2OO + SO2) of 1.4 (±1.8) × 10(-2). The observed SO2 removal rate constants, which technically represent upper limits, are consistent with decomposition being a significant, structure dependent, sink in the atmosphere for syn-SCI.

Theoretical study of the reactions of Criegee intermediates with ozone, alkylhydroperoxides, and carbon monoxide

The reaction of Criegee intermediates with hydroperoxides yields exotic ether oxides, as well as oligomers.

Kinetics of the unimolecular reaction of CH2OO and the bimolecular reactions with the water monomer, acetaldehyde and acetone under atmospheric conditions

The rate coefficients of the unimolecular reaction of CH₂OO and the bimolecular reactions with the water monomer and carbonyls were measured.

Direct evidence for a substantive reaction between the Criegee intermediate, CH2OO, and the water vapour dimer

The C1 Criegee intermediate, CH₂OO, reaction with water vapour has been studied.