Atmospheric fates of Criegee intermediates in the ozonolysis of isoprene

Insights into the catalytic effect of atmospheric organic trace species on the hydration of Criegee intermediates

The bimolecular reactions between Criegee intermediates (CIs) and atmospheric trace species have been extensively investigated, with a particular focus on the reaction with water, while the catalytic role of atmospheric organic compounds in hydration reactions was often neglected. In this study, we employed quantum chemical calculations and Born-Oppenheimer molecular dynamics (BOMD) simulations to investigate the catalytic effects of atmospheric organic amines, organic acids, and alcohols on the hydration reactions of CIs in the gas phase and at the gas-liquid interface. The catalytic reactions were found to follow a cyclic catalytic structure and a stepwise reaction mechanism. Gas-phase studies revealed that organic acids exhibited stronger catalytic effects compared to amines and alcohols, and the catalytic efficiency of amines and alcohols was similar to those of single water molecule. In addition, the catalytic reaction barriers of organic acids and alcohols were positively correlated with their gas-phase acidity (R2 = 0.94 to 0.97). A negative correlation was observed between the catalytic reaction barrier of amines and their gas-phase basicity (R2 = 0.84 to 0.90) and proton affinity (R2 = 0.84 to 0.92). At the gas-liquid interface, organic acids promoted the formation of hydroxyethyl hydroperoxide (HEHP, CH3CH(OH)(OOH)), organic acid ions, and H3O+, whereas the catalytic hydration of CIs by organic amines resulted in the formation of CH3CH(OH)OO and amine ions. Both HEHP and CH3CH(OH)OO can be further decomposed to form OH and HO2, or participate in new particles formation as precursors. This study complements the research gap on the reaction of CIs with water, providing valuable insights into the atmospheric sources of HEHP and HOx as well as the formation of secondary organic aerosols (SOAs).

The role of solar photolysis in the atmospheric removal of methacrolein oxide and the methacrolein oxide-water van-der Waals complex in pristine environments

Biogenic hydrocarbons are emitted into the Earth's atmosphere by terrestrial vegetation as by-products of photosynthesis. Isoprene is one such hydrocarbon and is the second most abundant volatile organic compound emitted into the atmosphere (after methane). Reaction with ozone represents an important atmospheric sink for isoprene removal, forming carbonyl oxides (Criegee intermediates) with extended conjugation. In this manuscript, we argue that the extended conjugation of these Criegee intermediates enables electronic excitation of these compounds, on timescales that are competitive with their slow unimolecular decay and bimolecular chemistry. We show that the complexation of methacrolein oxide with water enhances the absorption cross section of the otherwise dark S1 state, potentially revealing a new avenue for forming lower volatility compounds via tropospherically relevant photochemistry.

Methanetriol─Formation of an Impossible Molecule

Orthocarboxylic acids─organic molecules carrying three hydroxyl groups at the same carbon atom─have been distinguished as vital reactive intermediates by the atmospheric science and physical (organic) chemistry communities as transients in the atmospheric aerosol cycle. Predicted short lifetimes and their tendency to dehydrate to a carboxylic acid, free orthocarboxylic acids, signify one of the most elusive classes of organic reactive intermediates, with even the simplest representative methanetriol (CH(OH)3)─historically known as orthoformic acid─not previously been detected experimentally. Here, we report the first synthesis of the previously elusive methanetriol molecule in low-temperature mixed methanol (CH3OH) and molecular oxygen (O2) ices subjected to energetic irradiation. Supported by electronic structure calculations, methanetriol was identified in the gas phase upon sublimation via isomer-selective photoionization reflectron time-of-flight mass spectrometry combined with isotopic substitution studies and the detection of photoionization fragments. The first synthesis and detection of methanetriol (CH(OH)3) reveals its gas-phase stability as supported by a significant barrier hindering unimolecular decomposition. These findings progress our fundamental understanding of the chemistry and chemical bonding of methanetriol, hydroxyperoxymethane (CH3OOOH), and hydroxyperoxymethanol (CH2(OH)OOH), which are all prototype molecules in the oxidation chemistry of the atmosphere.

Experimental and theoretical study of Criegee intermediate (CH2OO) reactions with n-butyraldehyde and isobutyraldehyde: kinetics, implications and atmospheric fate

The reactions of the simplest Criegee intermediate (CH2OO) with n-butyraldehyde (nBD) and isobutyraldehyde (iBD) were studied at 253-318 K and (50 ± 2) torr, using Cavity Ring-down spectroscopy (CRDS). The rate coefficients obtained at room temperature were (2.63 ± 0.14) × 10-12 and (2.20 ± 0.21) × 10-12 cm3 molecule-1 s-1 for nBD and iBD, respectively. Both the reactions show negative temperature-dependency, following equations, knBD(T = 253-318 K) = (11.51 ± 4.33) × 10-14 × exp{(918.1 ± 107.2)/T} and kiBD(T = 253-318 K) = (6.23 ± 2.29) × 10-14 × exp{(1051.4 ± 105.2)/T} cm3 molecule-1 s-1. High-pressure limit rate coefficients were determined from theoretical calculations at the CCSD(T)-F12/cc-pVTZ-F12//B3LYP/6-311+G(2df, 2p) level of theory, with <40% deviation from the experimental results at room temperature and above. The kinetic simulations were performed using a master equation solver to predict the temperature-dependency of the rate coefficients at the experimental pressure, as well as to predict the contribution of individual pathways. The major products predicted from the theoretical calculations were formaldehyde and formic acid, along with butyric acid from nBD and isobutyric acid from iBD reactions.

Oligomer formation from cross-reaction of Criegee intermediates in the styrene-isoprene-O3 mixed system

Alkene ozonolysis can produce stabilized Criegee intermediates (SCIs), which play a key role in oligomers' formation. Though styrene and isoprene coexist in the ambient atmosphere as important anthropogenic and biogenic secondary organic aerosol (SOA) precursors, respectively, their cross-reactions have not received attention. This study investigated the interactions of SCIs from styrene and isoprene ozonolysis for the first time. The high-resolution Orbitrap mass spectrometer was used to determine the unique ion mass spectra of the isoprene-styrene-O3 mixture. The results show that the signal intensities of new ions account for >8.4% of total ions in the mass spectra of the styrene-isoprene-O3 mixed system. Styrene and isoprene ozonolysis can produce characteristic C7-SCI and C4-SCI, respectively. C7-SCI and C4-SCI can be involved in the cross-reactions, and the results of tandem mass spectra directly confirmed both C7-SCI and C4-SCI as chain units. The O/C and H/C ratios of cross-products are in the range of 0.38-1.07 and 1.00-1.50, respectively, which are consistent with cross-reaction products. Adding a C7-SCI unit reduces the oligomer's volatility by 1.3-1.4 orders of magnitude lower than adding a C4-SCI unit. Thus, C4-SCI can compete with C7-SCI to react with styrene-derived RO2/RC(O)OH to produce more volatile cross-products, while the less volatile cross-products can be formed when isoprene-derived RO2/RC(O)OH reacted with C7-SCI instead of C4-SCI. The SOA yield of the mixed system is lower than that of the single styrene-O3 system but higher than that of the single isoprene-O3 system. Ambient particles were also collected, and 5 possible SCI-related cross-products were identified. This study illustrates the effects of SCI-related cross-reactions on SOA components and physicochemical properties, providing a basis for future research on SCI-related cross-reactions that frequently occur in the ambient atmosphere.

UV Absorption Spectroscopy of the Conformer-Dependent Reactivity of the Four Carbon Criegee Intermediate of Methyl Vinyl Ketone Oxide: An Ab initio Quantum Dynamics Study

An extended theoretical analysis of the photodissociation dynamics of the four-carbon Criegee intermediate (CH2═CH(CH3)COO) or methyl vinyl ketone oxide, which has four conformers, following excitation to the B state, is presented. Our analysis relies on multireference electronic wave functions combined with a wavepacket propagation treatment for the two coupled B1A' and C1A' electronic states and two nuclear degrees of freedom. For each conformer, the 2D model depends on potential energy surfaces (PESs) along the O-O and C-O-O bending modes for the two lowest excited states, B1A' and C1A', and is sufficiently accurate to reproduce the experimental B1A' ← X1A' absorption spectrum with unprecedented accuracy. It is found that the roles of each conformer are essential in producing a cumulative spectrum, which is close to the recent experimental spectrum. The anti-trans and anti-cis conformers make contributions at the longer and shorter wavelengths of the cumulative spectrum, respectively, while the syn-cis and syn-trans conformers have contributions in the middle wavelength range of the cumulative spectrum of MVK-oxide. The existence of a deep well for each conformer on the PESs of the (diabatic) B state causes a considerable amount of the wavepacket to be reflected by the B state wells, which can explain the oscillatory structures appearing in the long wavelength range of 360-480 nm of the spectrum. The weakly avoided crossings between the B-state and C-state PESs of each conformer appearing within the range of 2.80-3.08 eV excitation energy cause considerable disturbance in the vibronic fine structure of the bands. The results give novel insight into the complex interactions governing this intriguing process.

Carbonyl Oxide Stabilization from Trans Alkene and Terpene Ozonolysis

The pressure dependence of carbonyl oxide (Criegee intermediate) stabilization can be measured via H2SO4 detection using chemical ionization mass spectrometry. By selectively scavenging OH radicals in a flow reactor containing an alkene, O3, and SO2, we measure an H2SO4 ratio related to the Criegee intermediate stabilization, and by performing experiments at multiple pressures, we constrain the pressure dependence of the stabilization. Here, we present results from a set of monoterpenes as well as isoprene, along with previously published results from tetramethylethylene and a sequence of symmetrical trans alkenes. We are able to reproduce the observations with a physically sensible set of parameters related to standard pressure falloff functions, providing both a consistent picture of the reaction dynamics and a method to describe the pressure stabilization following ozonolysis of all alkenes under a wide range of atmospheric conditions.

Detection of a C4 Criegee Intermediate: Fourier-Transform Microwave Spectroscopy of Methacrolein Oxide

Pure rotational transitions of methacrolein oxide (MACRO) were observed by Fourier-transform microwave spectroscopy. Among the four low-lying conformers existing within an energy window of 3 kcal/mol, only the lowest-energy conformer, the anti-trans conformer, was detected in a discharged jet of a 1,3-diiode-2-methylprop-1-ene and O2 mixture diluted in Ar. Nineteen pure rotational transitions, in the frequency range from 10 to 25 GHz, most of them showing A/E splitting due to the methyl-top internal rotation, were observed and analyzed by the XIAM program, yielding the internal rotation barrier of 559 cm-1, which very well agrees with a theoretically calculated value, 558 cm-1, at the CCSD(T)/cc-pVTZ level of theory.

A five-carbon unsaturated Criegee intermediate: synthesis, spectroscopic identification, and theoretical study of 3-penten-2-one oxide

Biogenic alkenes, such as isoprene and α-pinene, are the predominant source of volatile organic compounds (VOCs) emitted into the atmosphere. Atmospheric processing of alkenes via reaction with ozone leads to formation of zwitterionic reactive intermediates with a carbonyl oxide functional group, known as Criegee intermediates (CIs). CIs are known to exhibit a strong absorption (π* ← π) in the near ultraviolet and visible (UV-vis) region due to the carbonyl oxide moiety. This study focuses on the laboratory identification of a five-carbon CI with an unsaturated substituent, 3-penten-2-one oxide, which can be produced upon atmospheric ozonolysis of substituted isoprenes. 3-Penten-2-one oxide is generated in the laboratory by photolysis of a newly synthesized precursor, (Z)-2,4-diiodopent-2-ene, in the presence of oxygen. The electronic spectrum of 3-penten-2-one oxide was recorded by UV-vis induced depletion of the VUV photoionization signal on the parent m/z 100 mass channel using a time-of-flight mass spectrometer. The resultant electronic spectrum is broad and unstructured with peak absorption at ca. 375 nm. To complement the experimental findings, electronic structure calculations are performed at the CASPT2(12,10)/aug-cc-pVDZ level of theory. The experimental spectrum shows good agreement with the calculated electronic spectrum and vertical excitation energy obtained for the lowest energy conformer of 3-penten-2-one oxide. In addition, OH radical products resulting from unimolecular decay of energized 3-penten-2-oxide CIs are detected by UV laser-induced fluorescence. Finally, the experimental electronic spectrum is compared with that of a four-carbon, isoprene-derived CI, methyl vinyl ketone oxide, to understand the effects of an additional methyl group on the associated electronic properties.

Modeling the Ground- and Excited-State Unimolecular Decay of the Simplest Fluorinated Criegee Intermediate, HFCOO, Formed from the Ozonolysis of Hydrofluoroolefin Refrigerants

Hydrofluoroolefins (HFO) are fourth-generation refrigerants designed to function as efficient refrigerants with no ozone depletion potential and zero global warming potential. Despite extensive studies on their chemical and physical properties, the ground- and excited-state chemistry of their atmospheric oxidation products is less well understood. This study focuses on the ground- and excited-state chemistry of the simplest fluorinated Criegee intermediate (CI), fluoroformaldehyde oxide (HFCOO), which is the simplest fluorinated CI formed from the ozonolysis of HFOs. HFCOO contains syn- and anti-conformers, which have Boltzmann populations of, respectively, 87 and 13% at 298 K. For both conformers, the calculated ground-state reaction energy profiles associated with cyclization to form fluorodioxirane is lower than the equivalent unimolecular decay path in the simplest CI, H2COO, with anti-HFCOO returning a barrier height more than half of that of H2COO. The excited-state dynamics reveal that photoexcitation to the bright S2 state of syn-HFCOO and anti-HFCOO is expected to undergo a prompt O-O fission─with the former conformer expected to dissociate with an almost unity quantum yield and to form both O (1D) + HFCO (S0) and O (3P) + HFCO (T1) products. In contrast, photoexcitation of anti-HFCOO is expected to undergo an O-O bond fission with a non-unity quantum yield. The fraction of photoexcited anti-HFCOO that dissociates is predicted to exclusively form O (1D) + HFCO (S0) products, which is in sharp contrast to H2COO. The wider implications of our results are discussed from both physical and atmospheric chemistry perspectives.

Infrared Spectra of (Z)- and (E)-•CH2C(CH3)CHI Radicals Produced upon Photodissociation of (Z)- and (E)-(CH2I)(CH3)C═CHI in Solid para-Hydrogen

Ozonolysis of isoprene is important in atmospheric chemistry because of the abundant emission of isoprene. This process produces the Criegee intermediates CH₂OO, methyl vinyl ketone oxide (MVKO, C₂H₃C(CH₃)OO), and methacrolein oxide (MACRO, CH₂C(CH₃)CHOO). Gaseous MACRO was recently produced and identified in laboratories after photolysis of a mixture of 1,3-diiodo-2-methyl-prop-1-ene [(CH₂I)(CH₃)C═CHI] and O₂, but the conformation-dependent formation mechanism remains unexplored. We report conformation-distinct IR spectra of (E)- and (Z)-(CH₂I)(CH₃)C═CHI isolated in solid p-H₂. Upon irradiation near 300 nm of (E)- and (Z)-(CH₂I)(CH₃)C═CHI in solid p-H₂ at 3.3 K, 3-iodo-2-methyl-prop-1-en-3-yl [•CH₂C(CH₃)CHI] radicals were characterized, with intense infrared absorption lines at 2991.3, 1458.7, 1434.7, 1317.4, 1190.4, 786.3, 677.9, and 467.2 cm^-1 and additional 11 weaker ones assigned to (E)-•CH₂C(CH₃)CHI and intense lines at 3108.5, 3076.2, 3028.5, 2970.0, 1174.2, 796.0, 683.6, and 609.5 cm^-1 and additional 7 weaker ones to (Z)-•CH₂C(CH₃)CHI. The assignments were derived according to the behaviors of secondary photolysis at 495 and 460 nm and a comparison of the vibrational wavenumbers and IR intensities of the observed lines with those calculated with the B2PLYP-D3/aug-cc-pVTZ-pp method. These observations confirm that only the allylic C-I bond, not the vinylic one, was photodissociated at 290 nm, and in solid p-H₂, the excess energy upon photolysis induced no conformational change. When O₂ was present in the matrix, several intense lines at 1147.5, 1025.7, 914.4, and 728.7 cm^-1, and 4 weaker ones were tentatively assigned to the adduct CH₂C(CH₃)CHIOO; the assignments were supported by ¹⁸O₂ isotopic experiments. Unlike in the gaseous phase, the remaining C-I bond of this adduct could not break to form MACRO because of the efficient quenching in a low-temperature matrix.

Unimolecular Reactions of E-Glycolaldehyde Oxide and Its Reactions with One and Two Water Molecules

The kinetics of Criegee intermediates are important for atmospheric modeling. However, the quantitative kinetics of Criegee intermediates are still very limited, especially for those with hydroxy groups. Here, we calculate rate constants for the unimolecular reaction of E-glycolaldehyde oxide [E-hydroxyethanal oxide, E-(CH₂OH)CHOO], for its reactions with H₂O and (H₂O)₂, and for the reaction of the E-(CH₂OH)CHOO…H₂O complex with H₂O. For the highest level of electronic structure, we use W3X-L//CCSD(T)-F12a/cc-pVDZ-F12 for the unimolecular reaction and the reaction with water and W3X-L//DF-CCSD(T)-F12b/jun-cc-pVDZ for the reaction with 2 water molecules. For the dynamics, we use a dual-level strategy that combines conventional transition state theory with the highest level of electronic structure and multistructural canonical variational transition state theory with small-curvature tunneling with a validated density functional for the electronic structure. This dynamical treatment includes high-frequency anharmonicity, torsional anharmonicity, recrossing effects, and tunneling. We find that the unimolecular reaction of E-(CH₂OH)CHOO depends on both temperature and pressure. The calculated results show that E-(CH₂OH)CHOO…H₂O + H₂O is the dominant entrance channel, while previous investigations only considered Criegee intermediates + (H₂O)₂. In addition, we find that the atmospheric lifetime of E-(CH₂OH)CHOO with respect to 2 water molecules is particularly short with a value of 1.71 × 10^-6 s at 0 km, which is about 2 orders of magnitude shorter than those usually assumed for Criegee intermediate reactions with water dimer. We also find that the OH group in E-(CH₂OH)CHOO enhances its reactivity.

Direct gas-phase formation of formic acid through reaction of Criegee intermediates with formaldehyde

Ozonolysis of isoprene is considered to be an important source of formic acid (HCOOH), but its underlying reaction mechanisms related to HCOOH formation are poorly understood. Here, we report the kinetic and product studies of the reaction between the simplest Criegee intermediate (CH₂OO) and formaldehyde (HCHO), both of which are the primary products formed in ozonolysis of isoprene. By utilizing time-resolved infrared laser spectrometry with the multifunctional dual-comb spectrometers, the rate coefficient k_CH2OO+HCHO is determined to be (4.11 ± 0.25) × 10^-12 cm³ molecule^-1 s^-1 at 296 K and a negative temperature dependence of the rate coefficient is observed and described by an Arrhenius expression with an activation energy of (-1.81 ± 0.04) kcal mol^-1. Moreover, the branching ratios of the reaction products HCOOH + HCHO and CO + H₂O + HCHO are explored. The yield of HCOOH is obtained to be 37-54% over the pressure (15-60 Torr) and temperature (283-313 K) ranges. The atmospheric implications of the reaction CH₂OO + HCHO are also evaluated by incorporating these results into a global chemistry-transport model. In the upper troposphere, the percent loss of CH₂OO by HCHO is found by up to 6% which can subsequently increase HCOOH mixing ratios by up to 2% during December-January-February months.

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.

Concurrent dominant pathways of multifunctional products formed from nocturnal isoprene oxidation

Determination of dominant chemical pathways toward the formation of nocturnal secondary organic aerosols (SOA) remains ambiguous by which nitrogen oxides (NO_x) always affect oxidation of volatile alkenes. Here, comprehensive chamber simulations on dark isoprene ozonolysis were conducted under different nitrogen dioxides (NO₂) mixing ratios to exam multiple functionalized isoprene oxidation products. Aside from that the oxidation processes were concurrently driven by nitrogen radical (NO₃) and small hydroxyl radicals (OH), ozone (O₃) cycloaddition at isoprene was launched initially regardless of NO₂ to rapidly form first-generation oxidation products, i.e., carbonyls and Criegee intermediates (CI) referred to carbonyl oxides. They could further undergo complicated self- and cross-reactions to produce alkylperoxy radicals (RO₂). Corresponding to yields of the C₅H₁₀O₃ tracer, weak OH pathway at night was credited to ozonolysis of isoprene but suppressed by unique NO₃ chemistry. Following the ozonolysis of isoprene, NO₃ played a crucial supplementary role in nighttime SOA formation. The ensuing production of gas-phase nitrooxy carbonyls (the first-generation nitrates) became dominant in the production of a sizeable pool of organic nitrates (RO₂NO₂). By contrast, isoprene dihydroxy dinitrates (C₅H₁₀N₂O₈) were outstanding with the elevated NO₂, related to typical second-generation nitrates. As such, the yielding number concentrations of dark SOA were promoted to approximately 1.8 × 10⁴ cm^-3 but presented a nonlinear relation with excess high-NO₂ condition. This study provides valuable insights into importance of multifunctional organic compounds from alkene oxidation to constitute nighttime SOA.

Water Charge Transfer Accelerates Criegee Intermediate Reaction with H2O- Radical Anion at the Aqueous Interface

Criegee intermediates (CIs) are important carbonyl oxides that may react with atmospheric trace chemicals and impact the global climate. The CI reaction with water has been widely studied and is a main channel for trapping CIs in the troposphere. Previous experimental and computational reports have largely focused on reaction kinetic processes in various CI-water reactions. The molecular-level origin of CI's interfacial reactivity at the water microdroplet surface (e.g., as found in aerosols and clouds) is unclear. In this study, by employing the quantum mechanical/molecular mechanical (QM/MM) Born-Oppenheimer molecular dynamics with the local second-order Møller-Plesset perturbation theory, our computational results reveal a substantial water charge transfer up to ∼20% per water, which creates the surface H₂O⁺/H₂O^- radical pairs to enhance the CH₂OO and anti-CH₃CHOO reactivity with water: the resulting strong CI-H₂O^- electrostatic attraction at the microdroplet surface facilitates the nucleophilic attack to the CI carbonyl by water, which may counteract the apolar hindrance of the substituent to accelerate the CI-water reaction. Our statistical analysis of the molecular dynamics trajectories further resolves a relatively long-lived bound CI(H₂O^-) intermediate state at the air/water interface, which has not been observed in gaseous CI reactions. This work provides insights into what may alter the oxidizing power of the troposphere by the next larger CIs than simple CH₂OO and implicates a new perspective on the role of interfacial water charge transfer in accelerating molecular reactions at aqueous interfaces.

Enthalpies of formation for Criegee intermediates: A correlation energy convergence study

Criegee intermediates, formed from the ozonolysis of alkenes, are known to have a role in atmospheric chemistry, including the modulation of the oxidizing capacity of the troposphere. Although studies have been conducted since their discovery, the synthesis of these species in the laboratory has ushered in a new wave of investigations of these structures, both theoretically and experimentally. In some of these theoretical studies, high-order corrections for correlation energy are included to account for the mid multi-reference character found in these systems. Many of these studies include a focus on kinetics; therefore, the calculated energies should be accurate (<1 kcal/mol in error). In this research, we compute the enthalpies of formation for a small set of Criegee intermediates, including higher-order coupled cluster corrections for correlation energy up to coupled cluster with perturbative quintuple excitations. The enthalpies of formation for formaldehyde oxide, anti-acetaldehyde oxide, syn-acetaldehyde oxide, and acetone oxide are presented at 0 K as 26.5, 15.6, 12.2, and 0.1 kcal mol^-1, respectively. Additionally, we do not recommend the coupled cluster with perturbative quadruple excitations [CCSDT(Q)] energy correction, as it is approximately twice as large as that of the coupled cluster with full quadruple excitations (CCSDTQ). Half of the CCSDT(Q) energy correction may be included as a reliable, cost-effective estimation of CCSDTQ energies for Criegee intermediates.

Electronic Spectroscopy and Dissociation Dynamics of Vinyl-Substituted Criegee Intermediates: 2-Butenal Oxide and Comparison with Methyl Vinyl Ketone Oxide and Methacrolein Oxide Isomers

The 2-butenal oxide Criegee intermediate [(CH₃CH═CH)CHOO], an isomer of the four-carbon unsaturated Criegee intermediates derived from isoprene ozonolysis, is characterized on its first π* ← π electronic transition and by the resultant dissociation dynamics to O (¹D) + 2-butenal [(CH₃CH═CH)CHO] products. The electronic spectrum of 2-butenal oxide under jet-cooled conditions is observed to be broad and unstructured with peak absorption at 373 nm, spanning to half maxima at 320 and 420 nm, and in good accord with the computed vertical excitation energies and absorption spectra obtained for its lowest energy conformers. The distribution of total kinetic energy released to products is ascertained through velocity map imaging of the O (¹D) products. About half of the available energy, deduced from the theoretically computed asymptotic energy, is accommodated as internal excitation of the 2-butenal fragment. A reduced impulsive model is introduced to interpret the photodissociation dynamics, which accounts for the geometric changes between 2-butenal oxide and the 2-butenal fragment, and vibrational activation of associated modes in the 2-butenal product. Application of the reduced impulsive model to the photodissociation of isomeric methyl vinyl ketone oxide reveals greater internal activation of the methyl vinyl ketone product arising from methyl internal rotation and rock, which is distinctly different from the dissociation dynamics of 2-butenal oxide or methacrolein oxide.

An Experimental and Master Equation Investigation of Kinetics of the CH2OO + RCN Reactions (R = H, CH3, C2H5) and Their Atmospheric Relevance

We have performed direct kinetic measurements of the CH₂OO + RCN reactions (R = H, CH₃, C₂H₅) in the temperature range 233-360 K and pressure range 10-250 Torr using time-resolved UV-absorption spectroscopy. We have utilized a new photolytic precursor, chloroiodomethane (CH₂ICl), whose photolysis at 193 nm in the presence of O₂ produces CH₂OO. Observed bimolecular rate coefficients for CH₂OO + HCN, CH₂OO + CH₃CN, and CH₂OO + C₂H₅CN reactions at 296 K are (2.22 ± 0.65) × 10^-14 cm³ molecule^-1 s^-1, (1.02 ± 0.10) × 10^-14 cm³ molecule^-1 s^-1, and (2.55 ± 0.13) × 10^-14 cm³ molecule^-1 s^-1, respectively, suggesting that reaction with CH₂OO is a potential atmospheric degradation pathway for nitriles. All the reactions have negligible temperature and pressure dependence in the studied regions. Quantum chemical calculations (ωB97X-D/aug-cc-pVTZ optimization with CCSD(T)-F12a/VDZ-F12 electronic energy correction) of the CH₂OO + RCN reactions indicate that the barrierless lowest-energy reaction path leads to a ring closure, resulting in the formation of a 1,2,4-dioxazole compound. Master equation modeling results suggest that following the ring closure, chemical activation in the case of CH₂OO + HCN and CH₂OO + CH₃CN reactions leads to a rapid decomposition of 1,2,4-dioxazole into a CH₂O + RNCO pair, or by a rearrangement, into a formyl amide (RC(O)NHC(O)H), followed by decomposition into CO and an imidic acid (RC(NH)OH). The 1,2,4-dioxazole, the CH₂O + RNCO pair, and the CO + RC(NH)OH pair are atmospherically significant end products to varying degrees.

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.

Organic acid formation in the gas-phase ozonolysis of α,β-unsaturated ketones

Organic acids are key species in determining the radiative properties of the atmosphere due to their contribution to particle formation. Reported discrepancies between field measurements and modelling suggest significant missing sources. Herein, we present a mechanistic investigation on the gas-phase ozonolysis of ethyl vinyl ketone (EVK, 1-penten-3-one), which we chose as a model compound for α,β-unsaturated ketones. Experiments were performed in a 1080 L quartz-glass reaction chamber (QUAREC) at 990 ± 15 mbar and 298 ± 2 K (r. h. ≪ 0.1%) and analysed via long-path FTIR spectrometry and PTR-ToF-MS. The experiments were performed in the presence of an excess of CO to suppress the chemistry of OH radicals. For a comprehensive picture, in selected experiments, SO₂ was also added to the reaction system to scavenge the stabilized Criegee intermediates (sCIs) and to investigate their formation yield. Combining the results of both set-ups allowed us to quantify 2-oxobutanal, for which we report vapour-phase FTIR spectra. In addition, we introduce the first-ever infrared spectra of perpropionic acid, which was also positively identified in the EVK + O₃ system. A detailed analysis of the experimental findings allowed us to link the identified reaction products (acetaldehyde, ethyl hydroperoxide, and perpropionic acid) to known bimolecular reactions of RO₂ radicals. Thereby, it is shown that the EVK + O₃ reaction yields formic acid, HC(O)OH, and propionic acid, C₂H₅C(O)OH, and their formation is not covered by mechanisms reported in the literature. Three different pathways accounting for their formation from chemically activated CIs are proposed and possible implications for the ozonolysis of α,β-unsaturated ketones in the atmosphere are discussed.

Effect of NO2 on nocturnal chemistry of isoprene: Gaseous oxygenated products and secondary organic aerosol formation

As one of the most abundant non-methane hydrocarbon in the atmosphere, isoprene has attracted lots of attention on its oxidation processes and environmental effects. However, less is known about the nocturnal chemistry of isoprene with multiple oxidants coexisting in the atmosphere. Besides, though highly oxygenated molecules (HOMs) have recently been recognized to contribute to secondary organic aerosol (SOA) formation, the specific contribution of measured HOMs on SOA formation in isoprene oxidation has not been well established. In this study, the oxidation of isoprene was simulated under dark and various NO₂/O₃ conditions. Plenty of oxidation products were identified by combining two state-of-the-art time-of-flight mass spectrometers, and more species with high C and N numbers and low volatilities were detected under high NO₂ conditions. The nocturnal oxidation of isoprene was found to be governed by synergic effects of multiple oxidants, including O₃, NO₃•, and •OH at the same time, and the oxidation proportions changed with NO₂. NO₂ promoted the formation of most N-containing products especially N2 products, because of the decisive role of NO₃• on their formation. Nevertheless, some products such as C₅H₁₀O_3-5, C₅H₁₁NO₆, and C₁₀H₁₆N₂O_10,11 showed a better correlation with HO₂NO₂ rather than NO₂/O₃, indicating the importance of HO₂• chemistry on the oxidation products formation. Though the concentration of measured oxygenated products was dominated by volatile and semi-volatile organic compounds, the low- and extremely low-volatile organic compounds contributed over 97 % to the SOA formation potential. However, challenges still exist in accurately simulating SOA formation from the measured oxygenated molecules to match the measurement, and further comprehensive characterization of oxidation products in both gas and aerosol phases at the molecular level is needed.

Formation of extremely low-volatility organic compounds from styrene ozonolysis: Implication for nucleation

Styrene is a highly reactive compound with the dual nature of aromatics and olefins. This work presents evidence for formation of extremely low-volatility organic compounds (ELVOCs) from styrene ozonolysis. The molecules of ELVOCs were analyzed using a high-resolution orbitrap mass spectrometer. The results show that ELVOCs were oligomers characterized by stabilized Criegee radicals (SCIs) as chain units. The addition of C₆H₅CHOO (SCI1) or CH₂OO (SCI2) can dramatically decrease the oligomers' volatility. At low relative humidity (RH), ELVOCs are mainly formed from the reaction of RO₂ radical, C₆H₅OO·, with SCI1 and SCI2; however, ELVOCs are primarily produced by the reaction between benzoic acid and SCI1 at high RH. Ambient particles were also collected to propose the probable oligomers from styrene-SCI. Our results suggest that styrene-SCI derived ELVOCs may act as nucleating agents, potentially providing an experimental basis for nucleation events that frequently occur in urban areas.

The photoisomerization mechanism of methacrolein oxide (MACR-OO): the cyclic dioxole formation pathway revealed

Methacrolein oxide (MACR-OO), the isopropenyl substituted Criegee intermediate (CI), is one product of isoprene ozonolysis. In this work, we report MACR-OO's photo-isomerization paths with electronic structure calculation at the CASSCF and MS-CASPT2 levels and trajectory surface-hopping (TSH) nonadiabatic dynamics simulation at the CASSCF level. Our calculated results show that the ring-closure is the dominant photo-induced unimolecular isomerization of MACR-OO in the S₁ state. In addition, a new photo-induced ring-closure to heterocyclopentane dioxole in syn_syn-MACR-OO is found. The findings of MACR-OO are expected to deepen the understanding of the substituted CIs and their photochemistry.

Comparing the Excited State Dynamics of CH2OO, the Simplest Criegee Intermediate, Following Vertical versus Adiabatic Excitation

Ab initio molecular dynamics studies of CH₂OO molecules following excitation to the minimum-energy geometry of the strongly absorbing S₂ (¹ππ*) state reveal a much richer range of behaviors than just the prompt O-O bond fission, with unity quantum yield and retention of overall planarity, identified in previous vertical excitation studies from the ground (S₀) state. Trajectories propagated for 100 fs from the minimum-energy region of the S₂ state show a high surface hopping (nonadiabatic coupling) probability between the near-degenerate S₂ and S₁ (¹nπ*) states at geometries close to the S₂ minimum, which enables population transfer to the optically dark S₁ state. Greater than 80% of the excited population undergoes O-O bond fission on the S₂ or S₁ potential energy surfaces (PESs) within the analysis period, mostly from nonplanar geometries wherein the CH₂ moiety is twisted relative to the COO plane. Trajectory analysis also reveals recurrences in the O-O stretch coordinate, consistent with the resonance structure observed at the red end of the parent S₂-S₀ absorption spectrum, and a small propensity for out-of-plane motion after nonadiabatic coupling to the S₁ PES that enables access to a conical intersection between the S₁ and S₀ states and cyclization to dioxirane products.

Secondary organic aerosol formation from photooxidation of C3H6 under the presence of NH3: Effects of seed particles

Frequently-occurred secondary organic aerosols (SOAs) under low-NO_x conditions contribute to the winter haze episodes and remain unclear in the abundant presence of NH₃. Here, the effects of CaCl₂ seed particles on the photooxidation of low-molecular-weight C₃H₆ with co-existing NO₂ and NH₃ were highlighted and investigated through a chamber-simulation study equipped with high-resolution proton-transfer-reaction time-of-flight mass spectrometer (PTR-ToF-MS). The influences of NH₃ are often overestimated to exclusively enhance SOA yields under a low-[NO₂]₀ condition. Instead, the seeds played a central role in the heterogeneous formation of SOAs in this reaction with two orders of magnitudes higher than that in the absence of seeds at relative humidity (RH) of 82%. Interestedly, the O₃ production was unchanged whether the seeds existed or not, small changes in the production of O₃ were observed whether the seeds existed or not, indicating that the gas-phase conversions of C₃H₆ and NO_x into C1-C3 oxygenated volatile organic compounds (OVOCs) and nitrogen-containing compounds (NOCs) were not affected by seed particles. Given that the ensuing formation of these low-volatile compounds was condensed into nucleation on the seeds, the explosive growth of C₃H₆ SOAs was then stimulated in the addition of NH₃. Besides NO₂ photolysis, the producing O₃ was related to the formation of secondary carbonyls such as formaldehyde and then was consumed in the ·OH generation of approximately 3.40 × 10^-12 molecules cm^-3. This study provides a new insight to better understand the new gas-to-particle formation mechanisms when the haze pollution outbreaks in the complex air mixture.

An ab initio quantum dynamics simulation of UV Absorption Spectrum of Methyl Vinyl Ketone Oxide

The spectroscopy of the four-carbon Criegee intermediate, methyl vinyl ketone oxide (MVK-oxide), following UV excitation to the B state (corresponding to the first $\pi^* \leftarrow \pi$ electronic transition) is studied theoretically, relied on single reference electronic wave function and a quantum dynamical approach for the nuclear motion. Two interacting electronic states $B{}^1\rm{A'}$ and $C{}^1\rm{A'}$ together with two nuclear degrees of freedom (\ce{O-O} stretching and \ce{C-O-O} bending modes) are considered in the dynamical approach. The computed absorption spectrum is found to agree well with the available experimental recordings. The existence of the weak oscillatory structures in the absorption spectrum are argued likely due to the fact that the excitation energy range of the first $\pi^* \leftarrow \pi$ electronic transition is below the energy barrier of the diabatic B state ($\sim 215$~nm) and thus below the sufficiently deep well occurring at the intermediate \ce{O-O} distances. Thissuggests the recurrences of the wavepacket, which might be considered as origin of the weak oscillatory structures in the absorption spectrum. The computed electronic excitation profile of MVK-oxide is predicted to peak at $373$ nm.

Absolute photodissociation cross sections of thermalized methyl vinyl ketone oxide and methacrolein oxide

Methyl vinyl ketone oxide (MVKO) and methacrolein oxide (MACRO) are resonance-stabilized Criegee intermediates which are formed in the ozonolysis reaction of isoprene, the most abundant unsaturated hydrocarbon in the atmosphere. The absolute photodissociation cross sections of MVKO and MACRO were determined by measuring their laser depletion fraction at 352 nm, which was deduced from their time-resolved UV-visible absorption spectra. After calibrating the 352 nm laser fluence with the photodissociation of NO₂, for which the absorption cross section and photodissociation quantum yield are well known, the photodissociation cross sections of thermalized (299 K) MVKO and MACRO at 352 nm were determined to be (3.02 ± 0.60) × 10^-17 cm² and (1.53 ± 0.29) × 10^-17 cm², respectively. Using their reported spectra and photodissociation quantum yields, their peak absorption cross sections were deduced to be (3.70 ± 0.74) × 10^-17 cm² (at 371 nm, MVKO) and (3.04 ± 0.58) × 10^-17 cm² (at 397 nm, MACRO). These values agree fairly with our theoretical predictions and are substantially larger than those of smaller, alkyl-substituted Criegee intermediates (CH₂OO, syn-CH₃CHOO, (CH₃)₂COO), revealing the effect of extended conjugation. With their cross sections, we also quantified the synthesis yields of MVKO and MACRO in the present experiment to be 0.22 ± 0.10 (at 299 K and 30-700 torr) and 0.043 ± 0.019 (at 299 K and 500 torr), respectively, relative to their photolyzed precursors. The lower yield of MACRO can be related to the high endothermicity of its formation channel.

Synchronized Two-Color Time-Resolved Dual-Comb Spectroscopy for Quantitative Detection of HOx Radicals Formed from Criegee Intermediates

Criegee intermediates, derived from ozonolysis of alkenes and recognized as key species in the production of nonphotolytic free radicals, play a crucial role in atmospheric chemistry. Here, we present a spectrometer based on synchronized two-color time-resolved dual-comb spectroscopy, enabling simultaneous spectral acquisitions in two molecular fingerprint regions near 2.9 and 7.8 μm. Upon flash photolysis of CH₂I₂/O₂/N₂ gas mixtures, multiple reaction species, involving the simplest Criegee intermediates (CH₂OO), formaldehyde (CH₂O), hydroxyl (OH) and hydroperoxy (HO₂) radicals are simultaneously detected with microsecond time resolution. The concentration of each molecule can be determined based on high-resolution rovibrational absorption spectroscopy. With quantitative detection and simulation of temporal concentration profiles of the targeted molecules at various conditions, the underlying reaction mechanisms and pathways related to the formation of the HO_x radicals, which can be generated from decomposition of initially energized and vibrationally excited Criegee intermediates, are explored. This approach capable of achieving multispectral measurements with simultaneously high spectral and temporal resolutions opens up the opportunities for quantification of transient intermediates and products, thus, enabling elucidation of complex reaction mechanisms.

Wall losses of oxygenated volatile organic compounds from oxidation of toluene: Effects of chamber volume and relative humidity

Vapor wall losses can affect the yields of secondary organic aerosol. The effects of surface-to-volume (S/V) ratio and relative humidity (RH) on the vapor-wall interactions were investigated in this study. The oxygenated volatile organic compounds (OVOCs) were generated from toluene-H₂O₂ irradiations. The average gas to wall loss rate constant (k_gw) of OVOCs in a 400 L reactor (S/V = 7.5 m^-1) is 2.47 (2.41 under humid conditions) times higher than that in a 5000 L reactor (S/V = 3.6 m^-1) under dry conditions. In contrast, the average desorption rate constant (k_wg) of OVOCs in 400 L reactor is only 1.37 (1.20 under humid conditions) times higher than that in 5000 L reactor under dry conditions. It shows that increasing the S/V ratio can promote the wall losses of OVOCs. By contrast, the RH effect on k_gw is not prominent. The average k_gw value under humid conditions is almost the same as under dry conditions in the 400 L (5000 L) reactor. However, increasing RH can decrease the desorption rates. The average k_wg value under dry conditions is 1.45 (1.27) times higher than that under humid conditions in the 400 L (5000 L) reactor. The high RH can increase the partitioning equilibrium timescales and enhance the wall losses of OVOCs.

Formation reaction mechanism and infrared spectra of anti-trans-methacrolein oxide and its associated precursor and adduct radicals

Methacrolein oxide (MACRO) is an important carbonyl oxide produced in ozonolysis of isoprene, the most abundantly-emitted non-methane hydrocarbon in the atmosphere. We employed a step-scan Fourier-transform infrared spectrometer to investigate the source reaction of MACRO in laboratories. Upon UV irradiation of precursor CH₂IC(CH₃)CHI (1), the CH₂C(CH₃)CHI radical (2) was detected, confirming the fission of the allylic C‒I bond rather than the vinylic C‒I bond. Upon UV irradiation of (1) and O₂ near 21 Torr, anti-trans-MACRO (3a) was observed to have an intense OO-stretching band near 917 cm^-1, much greater than those of syn-CH₃CHOO and (CH₃)₂COO, supporting a stronger O‒O bond in MACRO because of resonance stabilization. At increased pressure (86‒346 Torr), both reaction adducts CH₂C(CH₃)CHIOO (4) and (CHI)C(CH₃)CH₂OO (5) radicals were observed, indicating that O₂ can add to either carbon of the delocalized propenyl radical moiety of (2). The yield of MACRO is significantly smaller than other carbonyl oxides.

Catalytic role of formaldehyde in particulate matter formation

Formaldehyde (HCHO), the simplest and most abundant carbonyl in the atmosphere, contributes to particulate matter (PM) formation via two in-cloud processing pathways. First, in a catalytic pathway, HCHO reacts with hydrogen peroxide (H2O2) to form hydroxymethyl hydroperoxide (HMHP), which rapidly oxidizes dissolved sulfur dioxide (SO2,aq) to sulfate, regenerating HCHO. Second, HCHO reacts with dissolved SO2,aq to form hydroxymethanesulfonate (HMS), which upon oxidation with the hydroxyl radical (OH) forms sulfate and also reforms HCHO. Chemical transport model simulations using rate coefficients from laboratory studies of the reaction rate of HMHP with SO2,aq show that the HMHP pathways reduce the SO2 lifetime by up to a factor of 2 and contribute up to ∼18% of global sulfate. This contribution rises to >50% in isoprene-dominated regions such as the Amazon. Combined with recent results on HMS, this work demonstrates that the one-carbon molecules HMHP and HCHO contribute significantly to global PM, with HCHO playing a crucial catalytic role.

Enhanced uptake of methacrolein at the acidic nanoparticle interface: Adsorption, heterogeneous reaction and impact for the secondary organic aerosol formation

Considering the moderate acidity of aerosols, the formation of secondary organic aerosols (SOA) through acid-catalyzed heterogeneous reactions has become a recent concern. However, the detailed information on the multiphase chemistry of organic compounds adsorbed onto acidic aerosols remains uncertain. In this work, we investigated the multiphase chemical processes between methacrolein (MACR) and sulfuric acid (SA) and their relationship with SOA formation. Results show that the aqueous nanoparticle interface, especially when it is an acidic nanoparticle interface, is a perfect area to adsorb and accommodate MACR. The occurrence percentage of MACR on the interface is more than 70%. With the increase of SA concentration, the first solvation shell changed from only water to the mixture of SA and water, which facilitates the heterogeneous hydration reaction of MACR. Compared with the neutral nanoparticle interface, the acidic nanoparticle interface exhibits a better ability to uptake and accommodate gaseous carbonyl species. Moreover, SA can catalyze the hydration reaction of MACR inside the aqueous media, and the resulting oligomers contribute to the formation and growth of SOA. The hydration reaction indirectly promotes the continuous adsorption of MACR at the acidic nanoparticle interface. The rate constant shows a positive altitude dependence, and acid-catalyzed reactions have an important impact on environmental chemistry, such as cloud SOA formation, within the range of about 2-6 km. This study reports a complete description of the heterogeneous interactions between unsaturated carbonyl species and acidic nanoparticles by using molecular dynamics and quantum chemistry methods, aiming to provide some insights for the further study on heterogeneous chemistry and its role in the formation of tropospheric SOA.

Identification of Isoprene Oxidation Reaction Products via Anion Photoelectron Spectroscopy

We present a study on the oxidation of isoprene under several different conditions that may model both atmospheric and combustion chemistry. Anions, formed by passing isoprene/oxidant gas mixtures through a pulsed discharge generating a range of species, are separated via mass spectrometry and characterized by anion photoelectron (PE) spectroscopy supported by computations. Specifically, a UV-irradiated isoprene/O₂ mixture, which additionally produces O₃, and an isoprene/O₂/H₂ mixture, which generates ^•OH when passed through the discharge, were sampled. The mass spectra of ions generated under both conditions show the production of intact molecular ions, ion-molecule complexes (e.g., O₂^-, O₄^-, and O₂^-·isoprene), and singly deprotonated species (e.g., deprotonated isoprene, C₅H₇^-). In addition, both smaller and oxidized fragments are observed using both gas mixtures, though relative abundances differ. From the UV-irradiated isoprene/O₂ gas mixture, additional intact molecular products of reactions initiated by ozonolysis of isoprene, methylglyoxal, and dimethylglyoxal were observed. Fragmentation and oxidation of isoprene observed in both gas mixtures included species with m/z 39, 53, 67, 69, and 83 that we attribute to a series of alkyl- and alkenoxide-based anions. The coexistence of intact molecules and complexes with fragments and reaction products demonstrates the versatility of this ion source as a simple and efficient anion formation method for studying species that may be relevant in atmospheric and combustion chemistry.

Photodissociation dynamics of methyl vinyl ketone oxide: A four-carbon unsaturated Criegee intermediate from isoprene ozonolysis

The electronic spectrum of methyl vinyl ketone oxide (MVK-oxide), a four-carbon Criegee intermediate derived from isoprene ozonolysis, is examined on its second π* ← π transition, involving primarily the vinyl group, at UV wavelengths (λ) below 300 nm. A broad and unstructured spectrum is obtained by a UV-induced ground state depletion method with photoionization detection on the parent mass (m/z 86). Electronic excitation of MVK-oxide results in dissociation to O (¹D) products that are characterized using velocity map imaging. Electronic excitation of MVK-oxide on the first π* ← π transition associated primarily with the carbonyl oxide group at λ > 300 nm results in a prompt dissociation and yields broad total kinetic energy release (TKER) and anisotropic angular distributions for the O (¹D) + methyl vinyl ketone products. By contrast, electronic excitation at λ ≤ 300 nm results in bimodal TKER and angular distributions, indicating two distinct dissociation pathways to O (¹D) products. One pathway is analogous to that at λ > 300 nm, while the second pathway results in very low TKER and isotropic angular distributions indicative of internal conversion to the ground electronic state and statistical unimolecular dissociation.

Understanding the properties of methyl vinyl ketone and methacrolein at the air-water interface: Adsorption, heterogeneous reaction and environmental impact analysis

Air-water interfaces are ubiquitous in nature, as manifested in the form of the surfaces of oceans, lakes, and atmospheric aqueous aerosols. The aerosol droplets interface, in particular, plays a critical role in numerous atmospheric chemistry processes. Methyl vinyl ketone (MVK) and methacrolein (MACR), two abundant volatile organic compounds, are the significant precursors of Criegee intermediates and secondary organic aerosol. In this work, the physicochemical properties of MVK and MACR at the air-water interface are studied from a theoretical perspective. The free energy wells of MVK and MACR occur at the air-water interface, and the absorption probabilities of them are 71% and 67%, respectively. Repulsion dominates the interactions between MVK/MACR and water molecules in the bulk region, while attraction is dominant at the interface. The two molecules tend to tilt at the interface, with the CC bond exposed at the outer interface. The most likely reaction scenario of O₃-initiated MVK/MACR reaction in the troposphere is also determined for the first time. Based on the molecular dynamics simulation results, the activity sequence of MVK + O₃ is given at four different environments by the density functional theory method: air-water interface, mineral clusters interface, bulk solution, and homogeneous gas. The interfacial water molecule can catalyze the reaction of MVK with O₃, and the rate constant at the air-water interface is ~6 times larger than that on the mineral surface model. Compared with mineral particles, aqueous particles play a more significant role in modifying the reaction properties of atmospheric organic species.

Gas-Phase Oxidation Rates and Products of 1,2-Dihydroxy Isoprene

1,2-Dihydroxy isoprene (1,2-DHI), a product of isoprene oxidation from multiple chemical pathways, is produced in the atmosphere in large quantities; however, its chemical fate has not been comprehensively studied. Here, we perform chamber experiments to investigate its gas-phase reactions. We find that the reactions of 1,2-DHI with OH radicals and ozone are rapid (k_OH = 8.0 (±1.3) × 10^-11 cm³ molecule^-1 s^-1; k_O3 = 7.2 (±1.1) × 10^-18 cm³ molecule^-1 s^-1). Reaction with OH, which dominates 1,2-DHI loss, leads primarily to fragmentation and radical recycling; major products under both high- and low-NO conditions include hydroxyacetone, glycolaldehyde, and 2,3-dihydroxy-2-methyl-propanal (DHMP). Radical-terminating hydroperoxide formation from the peroxy radical (RO₂) reaction with HO₂ and organonitrate formation from RO₂ + NO are not observed in the gas phase, possibly due to low volatility; constraints for their branching ratios are instead derived by mass balance. We also measure secondary organic aerosol mass yields from 1,2-DHI (0-23%) and show that oxidation in the presence of aqueous particles leads to formic and acetic acid production. Finally, we incorporate results into GEOS-Chem, a global chemical transport model, to compute the global production (25.3 Tg a^-1) and gas-phase loss (20.2 Tg a^-1) of 1,2-DHI and show that its oxidation provides non-negligible contributions to the atmospheric budgets of hydroxyacetone, glycolaldehyde, hydroxymethyl hydroperoxide, formic acid, and DHMP.

Substituent Effect in the Reactions between Criegee Intermediates and 3-Aminopropanol

Via intramolecular H atom transfer, 3-aminopropanol is more reactive toward Criegee intermediates, in comparison with amines or alcohols. Here we accessed the substituent effect of Criegee intermediates in their reactions with 3-aminopropanol. Through real-time monitoring the concentrations of two Criegee intermediates with their strong UV absorption at 340 nm, the experimental rate coefficients at 298 K (100-300 Torr) were determined to be (1.52 ± 0.08) × 10^-11 and (1.44 ± 0.22) × 10^-13 cm³ s^-1 for the reactions of 3-aminopropanol with (CH₃)₂COO (acetone oxide) and CH₂CHC(CH₃)OO (methyl vinyl ketone oxide), respectively. Compared to our previous experimental value for the reaction with syn-CH₃CHOO, (1.24 ± 0.13) × 10^-11 cm³ s^-1, we can see that the methyl substitution at the anti position has little effect on the reactivity while the vinyl substitution causes a drastic decrease in the reactivity. Our theoretical calculations based on CCSD(T)-F12 energies reproduce this 2-order-of-magnitude decrease in the rate coefficient caused by the vinyl substitution. Using the activation strain model, we found that the interaction of Criegee intermediates with 3-aminopropanol is weaker for the case of vinyl substitution. This effect can be further rationalized by the delocalization of the lowest unoccupied molecular orbital for the vinyl-substituted Criegee intermediates. These results would help us better estimate the impact of similar reactions like the reactions of Criegee intermediates with water vapor, some of which could be difficult to measure experimentally but can be important in the atmosphere. We also found that the B2PLYP-D3BJ/aug-cc-pVTZ calculation can reproduce the CCSD(T)-F12 reaction barrier energies within ca. 1 kcal mol^-1, indicating that the use of the B2PLYP-D3BJ method is promising for future predictions of the reactions of larger Criegee intermediates.

The favorable routes for the hydrolysis of CH2OO with (H2O)n (n = 1-4) investigated by global minimum searching combined with quantum chemical methods

The hydrolysis reaction of CH₂OO with water and water clusters is believed to be a dominant sink for the CH₂OO intermediate in the atmosphere. However, the favorable route for the hydrolysis of CH₂OO with water clusters is still unclear. Here global minimum searching using the Tsinghua Global Minimum program has been introduced to find the most stable geometry of the CH₂OO(H₂O)_n (n = 1-4) complex firstly. Then, based on these stable complexes, favorable hydrolysis of CH₂OO with (H₂O)_n (n = 1-4) has been investigated using the quantum chemical method of CCSD(T)-F12a/cc-pVDZ-F12//B3LYP/6-311+G(2d,2p) and canonical variational transition state theory with small curvature tunneling. The calculated results have revealed that, although the contribution of CH₂OO + (H₂O)₂ is the most obvious in the hydrolysis of CH₂OO with (H₂O)_n (n = 1-4), the hydrolysis of CH₂OO with (H₂O)₃ is not negligible in atmospheric gas-phase chemistry as its rate is close to the rate of the CH₂OO + H₂O reaction. The calculated results also show that, in a clean atmosphere, the CH₂OO + (H₂O)_n (n = 1-2) reaction competes well with the CH₂OO + SO₂ reaction at 298 K when the concentrations of (H₂O)_n (n = 1-2) range from 20% relative humidity (RH) to 100% RH, and SO₂ is 2.46 × 10¹¹ molecules per cm³. Meanwhile, when the RH is higher than 40%, it is a new prediction that the CH₂OO + (H₂O)₃ reaction can also compete well with the CH₂OO + SO₂ reaction at 298 K. Besides, Born-Oppenheimer molecular dynamics simulation results show that all the favorable channels of the CH₂OO + (H₂O)_n (n = 1-3) reaction cannot react on a time scale of 100 ps in the NVT simulation. However, the NVE simulation results show that the CH₂OO + (H₂O)₃ reaction can be finished well at 8.5 ps, indicating that the gas phase reaction of CH₂OO + (H₂O)₃ is not negligible in the atmosphere. Overall, the present results have provided a definitive example of how the favorable hydrolysis of important atmospheric species with (H₂O)_n (n = 1-4) takes place, which will stimulate one to consider the favorable hydrolysis of water and water clusters with other Criegee intermediates and other important atmospheric species.

Functionalized Hydroperoxide Formation from the Reaction of Methacrolein-Oxide, an Isoprene-Derived Criegee Intermediate, with Formic Acid: Experiment and Theory

Methacrolein oxide (MACR-oxide) is a four-carbon, resonance-stabilized Criegee intermediate produced from isoprene ozonolysis, yet its reactivity is not well understood. This study identifies the functionalized hydroperoxide species, 1-hydroperoxy-2-methylallyl formate (HPMAF), generated from the reaction of MACR-oxide with formic acid using multiplexed photoionization mass spectrometry (MPIMS, 298 K = 25 °C, 10 torr = 13.3 hPa). Electronic structure calculations indicate the reaction proceeds via an energetically favorable 1,4-addition mechanism. The formation of HPMAF is observed by the rapid appearance of a fragment ion at m/z 99, consistent with the proposed mechanism and characteristic loss of HO2 upon photoionization of functional hydroperoxides. The identification of HPMAF is confirmed by comparison of the appearance energy of the fragment ion with theoretical predictions of its photoionization threshold. The results are compared to analogous studies on the reaction of formic acid with methyl vinyl ketone oxide (MVK-oxide), the other four-carbon Criegee intermediate in isoprene ozonolysis.

Low molecular weight dicarboxylic acids, oxocarboxylic acids and α-dicarbonyls as ozonolysis products of isoprene: Implication for the gaseous-phase formation of secondary organic aerosols

Oxidation of isoprene, a major biogenic volatile organic compound emitted from forest canopies, is a potential source of oxalic acid; the dominant species in organic aerosols. We evaluated here ozonolysis of isoprene in dry darkness as a source of oxalic (C₂), malonic (C₃) and succinic (C₄) acids. We found that oxalic acid and methylglyoxal are dominant products within 10 min of reaction followed by glyoxylic, malonic or succinic acids. Interestingly, molecular distributions of oxidation products from early reactions (9-29 min) were characterized by the predominance of methylglyoxal followed by C₂, which became dominant after 30 min. The isoprene-derived secondary organic aerosols (SOAs) showed chemical evolution with reaction time towards the molecular characteristics of dicarboxylic acids similar to those of ambient aerosols (C₂>C₃≥C₄). The carbon-based relative abundances of methylglyoxal decreased steadily (40%→30%), while those of C₂ increased with reaction time (15%→25%), but no such variations persisted for glyoxal (6-10%). This finding means that methylglyoxal is more important intermediate of oxalic acid than glyoxal. In contrast, smaller variability and lower concentrations of pyruvic and glyoxylic acids than other intermediates indicate that oxalic acid formation under dry conditions follows a different pathway than in aqueous-phase heterogeneous chemistry usually invoked for cloud/fog/atmospheric waters. Here, we propose new reaction schemes for high levels of methylglyoxal and oxalic acid via gas-phase chemical reactions with ozone and OH radicals to better interpret the ambient SOA composition. Furthermore, the relative abundances of C₂ exhibit small variability from 1 to 8 h, suggesting its stable character towards the oxidation by hydroxyl radicals.

Explosive Secondary Aerosol Formation during Severe Haze in the North China Plain

Severe haze events with exceedingly high-levels of fine aerosols occur frequently over the past decades in the North China Plain (NCP), exerting profound impacts on human health, weather, and climate. The development of effective mitigation policies requires a comprehensive understanding of the haze formation mechanisms, including identification and quantification of the sources, formation, and transformation of the aerosol species. Haze evolution in this region exhibits distinct physical and chemical characteristics from clean to polluted periods, as evident from increasing stagnation and relative humidity, but decreasing solar radiation as well as explosive secondary aerosol formation. The latter is attributed to highly elevated concentrations of aerosol precursor gases and is reflected by rapid increases in the particle number and mass concentrations, both corresponding to nonequilibrium chemical processes. Considerable new knowledge has been acquired to understand the processes regulating haze formation, particularly in light of the progress in elucidating the aerosol formation mechanisms. This review synthesizes recent advances in understanding secondary aerosol formation, by highlighting several critical chemical/physical processes, that is, new particle formation and aerosol growth driven by photochemistry and aqueous chemistry as well as the interaction between aerosols and atmospheric stability. Current challenges and future research priorities are also discussed.

Surprisingly long lifetime of methacrolein oxide, an isoprene derived Criegee intermediate, under humid conditions

Ozonolysis of isoprene, the most abundant alkene, produces three distinct Criegee intermediates (CIs): CH₂OO, methyl vinyl ketone oxide (MVKO) and methacrolein oxide (MACRO). The oxidation of SO₂ by CIs is a potential source of H₂SO₄, an important precursor of aerosols. Here we investigated the UV-visible spectroscopy and reaction kinetics of thermalized MACRO. An extremely fast reaction of anti-MACRO with SO₂ has been found, k_SO2 = (1.5 ± 0.4) × 10^-10 cm³ s^-1 (±1σ, σ is the standard deviation of the data) at 298 K (150 - 500 Torr), which is ca. 4 times the value for syn-MVKO. However, the reaction of anti-MACRO with water vapor has been observed to be quite slow with an effective rate coefficient of (9 ± 5) × 10^-17 cm³ s^-1 (±1σ) at 298 K (300 to 500 Torr), which is smaller than current literature values by 1 or 2 orders of magnitude. Our results indicate that anti-MACRO has an atmospheric lifetime (best estimate ca. 18 ms at 298 K and RH = 70%) much longer than previously thought (ca. 0.3 or 3 ms), resulting in a much higher steady-state concentration. Owing to larger reaction rate coefficient, the impact of anti-MACRO on the oxidation of atmospheric SO₂ would be substantial, even more than that of syn-MVKO.