Cycloalkene Ozonolysis: Collisionally Mediated Mechanistic Branching

Ozonolysis can produce long-lived greenhouse gases from commercial refrigerants

Hydrofluoroolefins are being adopted as sustainable alternatives to long-lived fluorine- and chlorine-containing gases and are finding current or potential mass-market applications as refrigerants, among a myriad of other uses. Their olefinic bond affords relatively rapid reaction with hydroxyl radicals present in the atmosphere, leading to short lifetimes and proportionally small global warming potentials. However, this type of functionality also allows reaction with ozone, and whilst these reactions are slow, we show that the products of these reactions can be extremely long-lived. Our chamber measurements show that several industrially important hydrofluoroolefins produce CHF3 (fluoroform, HFC-23), a potent, long-lived greenhouse gas. When this process is accounted for in atmospheric chemical and transport modeling simulations, we find that the total radiative effect of certain compounds can be several times that of the direct radiative effect currently recommended by the World Meteorological Organization. Our supporting quantum chemical calculations indicate that a large range of exothermicity is exhibited in the initial stages of ozonolysis, which has a powerful influence on the CHF3 yield. Furthermore, we identify certain molecular configurations that preclude the formation of long-lived greenhouse gases. This demonstrates the importance of product quantification and ozonolysis kinetics in determining the overall environmental impact of hydrofluoroolefin emissions.

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.

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.

Bridged-ozonolysis of mixed aromatic hydrocarbons and organic amines: Inter-inhibited decay rate, altered product yield and synergistic-effect-enhanced secondary organic aerosol formation

Ozonolysis of aromatic hydrocarbons (AHs) or organic amines (OAs) occurs via different transformation processes, with varying rate constants and contributions to secondary organic aerosol (SOA) formation. However, to date no data is available on the ozonolysis of mixtures of AHs and OAs. This study investigated the kinetics, products and SOA yield from ozonolysis of mixture of trimethylamine with styrene, toluene or m-xylene. In the mixed system, the decay rates of styrene and trimethylamine were (1.32 ± 0.26) × 10^-4 s^-1 and (0.80 ± 0.02) × 10^-4 s^-1, decreasing up to 36.5 % and 54.4 % compared with their respective individual systems. This inter-inhibition of decay rates increased the yield of main products from styrene (i.e. benzaldehyde) by 23.5 % and trimethylamine (i.e. nitromethane) by 346.4 %. Ozonolysis of styrene or trimethylamine produced formaldehyde, which acted as a bridged product connecting the ozonolysis pathways of these two substrates, altering the yields of all products. Ozonolysis of styrene to benzaldehyde determined the increase of SOA particle number concentration (from 9.5 × 10⁵ to 1.9 × 10⁶ particles cm^-3), while trimethylamine ozonolysis to N, N-dimethylformamide contributed to synergistic-effect-enhanced SOA yield (from (64.3 ± 3.5)% to (68.1 ± 4.8)%). The findings provide a novel insight into the kinetics and mechanism of ozonolysis, as well as the resulting SOA formation from mixtures of AHs and OAs, helping to comprehensively understand the transformation and fate of organics in real atmospheric environments.

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

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

The oxidation mechanism of gas-phase ozonolysis of limonene in the atmosphere

Limonene with endo- and exo-double bonds is a significant monoterpene in the atmosphere and has high reactivity towards O3. We investigated the atmospheric oxidation mechanism of limonene ozonolysis using a high level quantum chemistry calculation coupled with RRKM-ME kinetic simulation. The additions of O3 can take place at both the endo- and exo-double bonds with a branching ratio of 0.87 : 0.13, forming four major highly energized CIs* (named Syn-2a*, Syn-2b*, Anti-2b* and Anti-2c*) with the relative higher fractions of 0.21 : 0.35 : 0.27 : 0.11. A yield of 4% for Limona-ketone was obtained as well. For the unimolecular isomerization pathways of limonene + O3 → POZs → CIs* → SOZ, VHP, or dioxirane, five, one, or none of the internal rotations are treated as hindered internal rotors for CIs*. We obtained percentages of 0.59 : 0.18 : 0.14 in total for separate isomerization routes in the formation of VHPs, dioxirane and SOZs from CIs* using the fourth-order Runge-Kutta method. Additionally, a yield of ∼5% was acquired for stabilized CIs compiling the fractions of different addition routes. About ∼10% of stabilized Anti-2b would isomerize to VHP and 90% would isomerize to SOZs. Isomerization to VHPs dominates the fate of stabilized Syn-2a, Syn-2b and Anti-2c. The overall yield of OH radicals was 0.61. Our study suggested a yield of 0.17 for stabilized SOZs and 0.18 for dioxirane, although both their fates are ambiguous.

Atmospheric degradation of chrysene initiated by OH radical: A quantum chemical investigation

Chrysene, a four-ring polycyclic aromatic hydrocarbon (PAH), is recalcitrant to biodegradation and persistent in the environment due to its low water solubility. Here, we investigated the atmospheric degradation process of chrysene initiated by OH radical in the presence of O₂ and NO_X using quantum chemical calculations. The reaction mechanisms were elucidated by density functional theory (DFT) at M06-2X/6-311++G(3df,2p)//M06-2X/6-311+G(d,p) level, and the kinetics calculations were conducted with Rice-Ramsperger-Kassel-Marcus (RRKM) theory. The results show that the oxidation products of atmospheric chrysene are oxygenated PAHs (OPAHs) and nitro-PAHs (NPAHs), including nitro-chrysene, hydroxychrysene, hydroxychrysenone, 11-benzo[a]fluorenone and dialdehydes. Most of the products have deleterious effects on the environment and human beings due to their acute toxicity, carcinogenicity and mutagenicity. The overall rate constant for the reaction of chrysene with OH radical is 4.48 × 10^-11 cm³ molecule^-1 s^-1 and the atmospheric lifetime of chrysene determined by OH radical is 6.4 h. The present work provided a comprehensive understanding on the degradation mechanisms and kinetics of chrysene, which could help to clarify its atmospheric fate and environmental risks.

Rapid unimolecular reaction of stabilized Criegee intermediates and implications for atmospheric chemistry

Elucidating atmospheric oxidation mechanisms is necessary for estimating the lifetimes of atmospheric species and understanding secondary organic aerosol formation and atmospheric oxidation capacity. We report an unexpectedly fast mechanistic pathway for the unimolecular reactions of large stabilized Criegee intermediates, which involves the formation of bicyclic structures from large Criegee intermediates containing an aldehyde group. The barrier heights of the mechanistic pathways are unexpectedly low – about 2–3 kcal/mol – and are at least 10 kcal/mol lower than those of hydrogen shift processes in large syn Criegee intermediates; and the calculated rate constants show that the mechanistic pathways are 10⁵-10⁹ times faster than those of the corresponding hydrogen shift processes. The present findings indicate that analogous low-energy pathways can now also be expected in other large Criegee intermediates and that oxidative capacity of some Criegee intermediates is smaller than would be predicted by existing models.

Criegee intermediates have received much attention in atmospheric chemistry because of their importance in ozonolysis mechanisms. Here, using quantum mechanical kinetics, the authors reveal an unexpectedly fast mechanistic pathway for unimolecular reactions of large stabilized Criegee intermediates.

Ozonolysis of 3-carene in the atmosphere. Formation mechanism of hydroxyl radical and secondary ozonides

The gas-phase ozonolysis mechanism of 3-carene is investigated using high level quantum chemistry and kinetic calculations. The reaction follows the Criegee mechanism with an initial addition of O₃ to the [double bond splayed left]C[double bond, length as m-dash]C[double bond splayed right] bond, followed by a chain of unimolecular isomerizations, as 3-carene + O₃→ POZs (primary ozonides) → CIs (Criegee intermediates, 4 conformers) → Ps (products). In the course of the reaction, a large excess of energy retained in the POZs* lead to the prompt unimolecular processes in POZs*, CIs*, and Ps*, and only ∼4% of CIs* could be stabilized by collision at 298 K and 760 Torr. From RRKM-ME calculations, the VHPs* could further dissociate to vinoxy-type radical and OH radical, the SOZs* could isomerize to 3-caronic acid, and DIOs* could be stabilized via collision. The fractional yield of OH radical, in the range of 0.56 to 0.59, agrees reasonably well with the previously measured value of 1.06 (with an uncertainty factor of 1.5).

Pressure Stabilization of Criegee Intermediates Formed from Symmetric trans-Alkene Ozonolysis

We explore the pressure dependence of a stabilized Criegee Intermediate (sCI) formation from a sequence of trans-alkene ozonolysis reactions. To study the effect of carbon chain length on the stabilization, we select five symmetric trans-alkenes ranging from trans-2-butene (C₄) through trans-7-tetradecene (C₁₄). We measure the pressure falloff curves for each alkene from 50 to 900 Torr in a flow reactor using conversion of SO₂ to H₂SO₄ with and without an OH scavenger, and subsequent detection of H₂SO₄ with a nitrate chemical ionization mass spectrometer to constrain sCI yields. As the length of the carbon chain increases, we observe a systematic increase in Criegee Intermediate stabilization at a given pressure, along with a systematic decrease in the low-pressure limit. Our results also suggest that for these symmetrical systems the anticonformer of the Criegee Intermediate stabilizes before (at lower pressure than) the syn conformer.

The ozonolysis of cyclic monoterpenes: a computational review

Monoterpenes are prevalent organic compounds emitted to the atmosphere, via biogenic activities in various types of plants. Monoterpenes undergo atmospheric decomposition reactions derived by the potent atmospheric oxidizing agents, OH, O3, and NOx. This review critically surveys literature pertinent to the atmospheric removal of monoterpenes by ozone. In general, the ozonolysis reactions of monoterpenes occur through the so-called Criegee mechanism. These classes of reactions generate a wide array of chemical organic and inorganic low vapor pressure (LVP) species. Carbonyl oxides, commonly known as Criegee intermediates (CIs), are the main intermediates from the gas-phase ozonolysis reaction. Herein, we present mechanistic pathways, reactions rate constants, product profiles, thermodynamic, and kinetic results dictating the ozonolysis reactions of selected monoterpenes (namely carene, camphene, limonene, α-pinene, β-pinene, and sabinene). Furthermore, the unimolecular (vinyl hydroperoxide and ester channels) and bimolecular reactions (cycloaddition, insertion, and radical recombination) of the resulting CIs are fully discussed. The orientations and conformations of the resulting primary ozonides (POZs) and CIs of monoterpenes are classified to reveal their plausible effects on reported thermokinetic parameters.

Hydration and Secondary Ozonide of the Criegee Intermediate of Sabinene

A computational study of the formation of secondary ozonide (SOZ) from the Criegee intermediates (CIs) of sabinene, including hydration reactions with H₂O and 2H₂O, was performed. All of the geometries were optimized at the B3LYP and M06-2X with several basis sets. Further single-point energy calculation at the CCSD(T) was performed. Two major pathways of SOZ formation suggest that it is mainly formed from the sabinene CI and formaldehyde rather than sabina ketone and formaldehyde-oxide. However, in both pathways, the activation energies are within a range of ±5 kJ mol^-1. Furthermore, the hydration reactions of the anti-CI with H₂O and 2H₂O showed that the role of the second water molecule is a mediator (catalyst) in this reaction. The dimer hydration reaction has lower activation energies than the monomer by 60 and 69 kJ mol^-1, at the M06-2X/6-31G(d) and CCSD(T)+CF levels of the theory, respectively. A novel water-mediated vinyl hydroperoxide (VHP) channel from both the monomer and dimer has been investigated. The results indicate that the direct nonmediated VHP formation and dissociation is interestingly more possible than the water-mediated VHP. The density functional theory calculations show that the monomer is faster than the dimer by roughly 22 kJ mol^-1. Further, the infrared spectrum of sabina ketone was calculated at B3LYP/6-311+G(2d,p); the calculated carbonyl stretching of 1727 cm^-1 is in agreement with the experimental range of 1700-1800 cm^-1.

QM/MM studies on ozonolysis of α-humulene and Criegee reactions with acids and water at air–water/acetonitrile interfaces

QM/MM electronic structure calculations reveal important mechanistic insights on the ozonolysis of α-humulene and Criegee reactions with acids and water at air–water/acetonitrile interfaces.

Criegee Chemistry on Aqueous Organic Surfaces

In the troposphere, the fate of gas-phase Criegee intermediates (CIs) is deemed to be determined by their reactions with water molecules. Here it is shown that CIs produced in situ on the surface of water/acetonitrile (W/AN) solutions react competitively with millimolar carboxylic acids. Present experiments probe, via online electrospray mass spectrometry, CIs' chemistry on the surface of α-humulene and β-caryophyllene in W/AN microjets exposed to O₃(g) for <10 μs. Mass-specific identification lets us establish the progeny of products and intermediates generated in the early stages of CIs' reactions with H₂O, D₂O, H₂¹⁸O, and n-alkyl-COOH (n = 1-7). It is found that n-alkyl-COOH competes for CIs with interfacial water, their competitiveness being an increasing function of n. Present findings demonstrate that CIs can react with species other than H₂O on the surface of aqueous organic aerosols due to the low water concentrations prevalent in the outermost interfacial layers.

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.

Computational investigation into the gas-phase ozonolysis of the conjugated monoterpene α-phellandrene

Reaction with ozone is a major atmospheric sink for α-phellandrene, a monoterpene found in both indoor and outdoor environments, however experimental literature concerning the reaction is scarce. In this study, high-level G4(MP2) quantum chemical calculations are used to theoretically characterise the reaction of ozone with both double bonds in α-phellandrene for the first time. Results show that addition of ozone to the least substituted double bond in the conjugated system is preferred. Following addition, thermal and chemically activated unimolecular reactions, including the so-called hydroperoxide and ester or 'hot' acid channels, and internal cyclisation reactions, are characterised to major first generation products. Conjugation present in α-phellandrene allows two favourable Criegee intermediate reaction pathways to proceed that have not previously been considered in the literature; namely a 1,6-allyl resonance stabilised hydrogen shift and intramolecular dioxirane isomerisation to an epoxide. These channels are expected to play an important role alongside conventional routes in the ozonolysis of a-phellandrene. Computational characterisation of the potential energy surface thus provides insight into this previously unstudied system, and will aid future mechanism development and experimental interpretation involving α-phellandrene and structurally similar species, to which the results are expected to extend.

Atmospheric oxidation of phenanthrene initiated by OH radicals in the presence of O2 and NOx - A theoretical study

Phenanthrene (Phe) is one of the most abundant polycyclic aromatic hydrocarbons (PAHs) observed in polluted urban atmosphere. The most important atmospheric loss process of Phe is the reaction with OH radicals. The present work investigated OH radical-initiated atmospheric degradation of Phe in the presence of O2 and NOx. The possible reaction mechanism was elucidated by density functional theory (DFT) calculations. Calculations show that the main products are a series of ring-retaining and ring-opening oxygenated PAHs containing phenanthrol, phenanthones, phenanthrenequinone, and dialdehydes. Rice-Ramsperger-Kassel-Marcus (RRKM) theory was employed to evaluate the rate constants for the initial steps of Phe with OH. The atmospheric lifetime of Phe relative to gas-phase reactions with OH is estimated to be 4.6h, based on the calculated overall rate constant of 3.02×10(-11)cm(3) molecule(-1)s(-1) at 298K and 1atm. Combined with available experimental data, this work also provides a comprehensive investigation of the formation mechanism of oxygenated PAHs in the atmospheric oxidation process of phenanthrene and should help to clarify its potential health risk.

The formation of highly oxidized multifunctional products in the ozonolysis of cyclohexene

The prompt formation of highly oxidized organic compounds in the ozonolysis of cyclohexene (C6H10) was investigated by means of laboratory experiments together with quantum chemical calculations. The experiments were performed in borosilicate glass flow tube reactors coupled to a chemical ionization atmospheric pressure interface time-of-flight mass spectrometer with a nitrate ion (NO3(-))-based ionization scheme. Quantum chemical calculations were performed at the CCSD(T)-F12a/VDZ-F12//ωB97XD/aug-cc-pVTZ level, with kinetic modeling using multiconformer transition state theory, including Eckart tunneling corrections. The complementary investigation methods gave a consistent picture of a formation mechanism advancing by peroxy radical (RO2) isomerization through intramolecular hydrogen shift reactions, followed by sequential O2 addition steps, that is, RO2 autoxidation, on a time scale of seconds. Dimerization of the peroxy radicals by recombination and cross-combination reactions is in competition with the formation of highly oxidized monomer species and is observed to lead to peroxides, potentially diacyl peroxides. The molar yield of these highly oxidized products (having O/C > 1 in monomers and O/C > 0.55 in dimers) from cyclohexene ozonolysis was determined as (4.5 ± 3.8)%. Fully deuterated cyclohexene and cis-6-nonenal ozonolysis, as well as the influence of water addition to the system (either H2O or D2O), were also investigated in order to strengthen the arguments on the proposed mechanism. Deuterated cyclohexene ozonolysis resulted in a less oxidized product distribution with a lower yield of highly oxygenated products and cis-6-nonenal ozonolysis generated the same monomer product distribution, consistent with the proposed mechanism and in agreement with quantum chemical modeling.

Homogeneous and heterogeneous reactions of anthracene with selected atmospheric oxidants

The reactions of gas-phase anthracene and suspended anthracene particles with O3 and O3-NO were conducted in a 200-L reaction chamber, respectively. The secondary organic aerosol (SOA) formations from gas-phase reactions of anthracene with O3 and O3-NO were observed. Meanwhile, the size distributions and mass concentrations of SOA were monitored with a scanning mobility particle sizer (SMPS) during the formation processes. The rapid exponential growths of SOA reveal that the atmospheric lifetimes of gas-phase anthracene towards O3 and O3-NO are less than 20.5 and 4.34 hr, respectively. The particulate oxidation products from homogeneous and heterogeneous reactions were analyzed with a vacuum ultraviolet photoionization aerosol time-of-flight mass spectrometer (VUV-ATOFMS). Gas chromatograph/mass spectrometer (GC/MS) analyses of oxidation products of anthracene were carried out for assigning the time-of-flight (TOF) mass spectra of products from homogeneous and heterogeneous reactions. Anthrone, anthraquinone, 9,10-dihydroxyanthracene, and 1,9,10-trihydroxyanthracene were the ozonation products of anthracene, while anthrone, anthraquinone, 9-nitroanthracene, and 1,8-dihydroxyanthraquinone were the main products of anthracene with O3-NO.

Pressure dependent mechanistic branching in the formation pathways of secondary organic aerosol from cyclic-alkene gas-phase ozonolysis

Pressure shows a strong influence on aerosol yields from the gas phase ozonolysis of terpenes and other cyclic alkenes.

Formation of organic acids from the gas-phase ozonolysis of terpinolene

Gas-phase ozonolysis of terpinolene was studied in static chamber experiments using gas chromatography coupled to mass spectrometric and flame ionisation detection to separate and detect products. Two isomers of C(7)-diacids and three isomers of C(7)-aldehydic acids were identified in the condensed phase after derivatisation. Possible mechanisms of formation of these acids were investigated using different OH radical scavengers and relative humidities, and were compared to those reported earlier for the ozonolysis of beta-pinene. In addition, branching ratios for some of the individual reaction steps, e.g. the branching ratio between the two hydroperoxide channels of the C(7)-CI, were deduced from the quantitative product yield data. Branching ratios for POZ decomposition and the stabilisation/decomposition of the C(7-)CI were also obtained from measurements of the C(7) primary carbonyl product.

The gas-phase ozonolysis of unsaturated volatile organic compounds in the troposphere

The gas-phase reactions of ozone with unsaturated hydrocarbons are significant sources of free radical species (including *OH) and particulate material in the Earth's atmosphere. In this tutorial review, the kinetics, products and mechanisms of these reactions are examined, starting with a discussion of the original mechanism proposed by Criegee and following with a summary presentation of the complex, free radical-mediated reactions of carbonyl oxide (Criegee) intermediates. The contribution of ozone-terpene reactions to the atmospheric burden of secondary organic aerosol material is also discussed from the viewpoint of the formation of non-volatile organic acid products from the complex chemistry of ozone with alpha-pinene. Throughout the article, currently accepted understanding is supported through the presentation of key experimental results, and areas of persistent or new uncertainty are highlighted.

Quantum chemical and master equation studies of the methyl vinyl carbonyl oxides formed in isoprene ozonolysis

Methyl vinyl carbonyl oxide is an important intermediate in the reaction of isoprene and ozone and may be responsible for most of the (*)OH formed in isoprene ozonolysis. We use CBS-QB3 calculations and RRKM/master equation simulations to characterize all the pathways leading to the formation of this species, all the interconversions among its four possible conformers, and all of its irreversible isomerizations. Our calculations, like previous studies, predict (*)OH yields consistent with experiment if thermalized syn-methyl carbonyl oxides form (*)OH quantitatively. Natural bond order analysis reveals that the vinyl group weakens the C=O bond of the carbonyl oxide, making rotation about this bond accessible to this chemically activated intermediate. The vinyl group also allows one conformer of the carbonyl oxide to undergo electrocyclization to form a dioxole, a species not previously considered in the literature. Dioxole formation, which has a CBS-QB3 reaction barrier of 13.9 kcal/mol, is predicted to be favored over vinyl hydroperoxide formation, dioxirane formation, and collisional stabilization. Our calculations also predict that two dioxole derivatives, 1,2-epoxy-3-butanone and 3-oxobutanal, should be major products of isoprene ozonolysis.

Computational Study of the Reaction between Biogenic Stabilized Criegee Intermediates and Sulfuric Acid

We have postulated a mechanism for the reaction of sulfuric acid with stabilized Criegee intermediates (sCIs). We have computed Gibbs free energies for the reaction of sulfuric acid with two biogenic sCIs and three smaller model species. We have also calculated Gibbs free energies for two competing sink reactions. Due to the large size of the biogenic sCIs, the computations have been performed at the relatively modest B3LYP/6-31G(d,p) and B3LYP/6-311+G(2d,p) levels. However, single-point RI-CC2/def2-QZVPP calculations for the (CH3)(2)COO model species are in good agreement with the B3LYP results. The reaction is found to be strongly exothermic for all studied species. Activation barrier calculations on the (CH3)(2)COO model species further indicate that the reaction with sulfuric acid may proceed significantly faster than the sink reaction with water. If the same applies to the biogenic sCIs, the proposed reactions could account for some part of the organically assisted new particle formation events observed in the atmosphere.

Secondary organic aerosol from limona ketone: insights into terpene ozonolysis via synthesis of key intermediates

Limona ketone was synthesized to explore the secondary organic aerosol (SOA) formation mechanism from limonene ozonolysis and also to test group-additivity concepts describing the volatility distribution of ozonolysis products from similar precursors. Limona ketone SOA production is indistinguishable from alpha-pinene, confirming the expected similarity. However, limona ketone SOA production is significantly less intense than limonene SOA production. The very low vapor pressure of limonene ozonolysis products is consistent with full oxidation of both double bonds in limonene and furthermore with production of products other than ketones after oxidation of the exo double bond in limonene. Mass-balance constraints confirm that ketone products from exo double-bond ozonolysis have a minimal contribution to the ultimate product yield. These results serve as the foundation for an emerging framework to describe the effect on volatility of successive generations of organic compounds in the atmosphere.

Secondary organic aerosol production from terpene ozonolysis. 1. Effect of UV radiation

We report secondary organic aerosol (SOA) yields from the ozonolysis of alpha-pinene under both dark and UV-illuminated conditions. Exposure to UV light reduces SOA yield by 20-40%, with a maximum reduction in yield coinciding with a minimum in the amount of terpene consumed (15-30 ppb). The data are consistent with a constant absolute reduction in the yield of approximately 0.03. Gas chromatography mass spectrometry analysis of filter samples indicates that the major products found in alpha-pinene SOA include organic acids (e.g., pinic acid), keto acids (e.g., pinonic acid), and hydroxy keto acids (e.g., 10-hydroxypinonic acid). Analysis of filter-based results suggests that yield reduction is a result of the formation of a more volatile product distribution when experiments are conducted in the presence of UV light. These results implythat previous "dark bag" experiments may overestimate SOA generation from monoterpenes and also that SOA generation in the atmosphere may depend significantly on actinic flux.

Critical factors determining the variation in SOA yields from terpene ozonolysis: A combined experimental and computational study

A substantial fraction of the total ultrafine particulate mass is comprised of organic compounds. Of this fraction, a significant subfraction is secondary organic aerosol (SOA), meaning that the compounds are a by-product of chemistry in the atmosphere. However, our understanding of the kinetics and mechanisms leading to and following SOA formation is in its infancy. We lack a clear description of critical phenomena; we often don't know the key, rate limiting steps in SOA formation mechanisms. We know almost nothing about aerosol yields past the first generation of oxidation products. Most importantly, we know very little about the derivatives in these mechanisms; we do not understand how changing conditions, be they precursor levels, oxidant concentrations, co-reagent concentrations (i.e., the VOC/NOx ratio) or temperature will influence the yields of SOA. In this paper we explore the connections between fundamental details of physical chemistry and the multitude of steps associated with SOA formation, including the initial gas-phase reaction mechanisms leading to condensible products, the phase partitioning itself, and the continued oxidation of the condensed-phase organic products. We show that SOA yields in the alpha-pinene + ozone are highly sensitive to NOx, and that SOA yields from beta-caryophylene + ozone appear to increase with continued ozone exposure, even as aerosol hygroscopicity increases as well. We suggest that SOA yields are likely to increase substantially through several generations of oxidative processing of the semi-volatile products.