Different pathways of the formation of highly oxidized multifunctional organic compounds (HOMs) from the gas-phase ozonolysis of β-caryophyllene

The gas-phase mechanism of the formation of highly oxidized multifunctional organic compounds (HOMs) from the ozonolysis of β-caryophyllene was investigated in a free-jet flow system at atmospheric pressure and a temperature of 295 ± 2 K. Reaction products, mainly highly oxidized RO2 radicals containing up to 14 oxygen atoms, were detected using chemical ionization – atmospheric pressure interface – time-of-flight mass spectrometry with nitrate and acetate ionization. These highly oxidized RO2 radicals react with NO, NO2, HO2 and other RO2 radicals under atmospheric conditions forming the first-generation HOM closed-shell products. Mechanistic information on the formation of the highly oxidized RO2 radicals is based on results obtained with isotopically labelled ozone (18O3) in the ozonolysis reaction and from hydrogen/deuterium (H/D) exchange experiments of acidic H atoms in the products. The experimental findings indicate that HOM formation in this reaction system is considerably influenced by the presence of a double bond in the RO2 radicals primarily formed from the β-caryophyllene ozonolysis. Three different reaction types for HOM formation can be proposed, allowing for an explanation of the detected main products: (i) the simple autoxidation, corresponding to the repetitive reaction sequence of intramolecular H-abstraction of a RO2 radical, RO2  →  QOOH, and subsequent O2 addition, next forming a peroxy radical, QOOH + O2  →  R′O2; (ii) an extended autoxidation mechanism additionally involving the internal reaction of a RO2 radical with a double bond forming most likely an endoperoxide and (iii) an extended autoxidation mechanism including CO2 elimination. The individual reaction steps of the reaction types (ii) and (iii) are uncertain at the moment. From the product analysis it can be followed that the simple autoxidation mechanism accounts only for about one-third of the formed HOMs. Time-dependent measurements showed that the HOM formation proceeds at a timescale of 3 s or less under the concentration regime applied here. The new reaction pathways represent an extension of the mechanistic understanding of HOM formation via autoxidation in the atmosphere, as recently discovered from laboratory investigations on monoterpene ozonolysis

Highly Oxygenated Molecules from Atmospheric Autoxidation of Hydrocarbons : A Prominent Challenge for Chemical Kinetics Studies

Recent advances in chemical ionization mass spectrometry have allowed the detection of a new group of compounds termed highly oxygenated molecules (HOM). These are atmospheric oxidation products of volatile organic compounds (VOC) retaining most of their carbon backbone, and with O/C ratios around unity. Owing to their surprisingly high yields and low vapor pressures, the importance of HOM for aerosol formation has been easy to verify. However, the opposite can be said concerning the exact formation pathways of HOM from major aerosol precursor VOC. While the role of peroxy radical autoxidation, i.e., consecutive intramolecular H-shifts followed by O-2 addition, has been recognized, the detailed formation mechanisms remain highly uncertain. A primary reason is that the autoxidation process occurs on sub-second timescales and is extremely sensitive to environmental conditions like gas composition, temperature, and pressure. This, in turn, poses a great challenge for chemical kinetics studies to be able to mimic the relevant atmospheric reaction pathways, while simultaneously using conditions suitable for studying the short-lived radical intermediates. In this perspective, we define six specific challenges for this community to directly observe the initial steps of atmospherically relevant autoxidation reactions and thereby facilitate vital improvements in the understanding of VOC degradation and organic aerosol formation. (C) 2017 Wiley Periodicals, Inc.Peer reviewe

Formation of Highly Oxidized Radicals and Multifunctional Products from the Atmospheric Oxidation of Alkylbenzenes

Aromatic hydrocarbons contribute significantly to tropospheric ozone and secondary organic aerosols (SOA). Despite large efforts in elucidating the formation mechanism of aromatic-derived SOA, current models still substantially underestimate the SOA yields when comparing to field measurements. Here we present a new, up to now undiscovered pathway for the formation of highly oxidized products from the OH-initiated oxidation of alkyl benzenes based on theoretical and experimental investigations. We propose that unimolecular H-migration followed by O-2-addition, a so-called autoxidation step, can take place in bicyclic peroxy radicals (BPRs), which are important intermediates of the OH -initiated oxidation of aromatic compounds. These autoxidation steps lead to the formation of highly oxidized multifunctional compounds (HOMs), which are able to form SOA. Our theoretical calculations suggest that the intramolecular H-migration in BPRs of substituted benzenes could be fast enough to compete with bimolecular reactions with HO2 radicals or NO under atmospheric conditions. The theoretical findings are experimentally supported by flow tube studies using chemical ionization mass spectrometry to detect the highly oxidized peroxy radical intermediates and closed-shell products. This new unimolecular BPR route to form HOMs in the gas phase enhances our understanding of the aromatic oxidation mechanism, and contributes significantly to a better understanding of aromatic-derived SOA in urban areas.Peer reviewe

Direct Probing of Criegee Intermediates from Gas-Phase Ozonolysis Using Chemical Ionization Mass Spectrometry

Criegee intermediates (CIs), mainly formed from gas-phase ozonolysis of alkenes, are considered as atmospheric oxidants besides OH and NO3 radicals as well as ozone. Direct CI measurement techniques are inevitably needed for reliable assessment of CIs' role in atmospheric processes. We found that CIs from ozonolysis reactions can be directly probed by means of chemical ionization mass spectrometry with a detection limit of about 10(4)-10(5) molecules cm(-3). Results from quantum chemical calculations support the experimental findings. The simplest CI, CH2OO, is detectable as an adduct with protonated ethers, preferably with protonated tetrahydrofuran. Kinetic measurements yielded k(CH2OO + SO2) = (3.3 +/- 0.9) X 10(-11) and k(CH2OO + acetic acid) = (1.25 +/- 0.30) x 10(-10) cm(3) molecule(-1) s(-1) at 295 +/- 2 K, in very good agreement with recent measurements using diiodomethane photolysis for CH2OO generation. CIs from the ozonolysis of cydohexene, acting as surrogate for cyclic terpenes, are followed as protonated species (CI)H+ using protonated amines as reagent ions. Kinetic investigations indicate a different reactivity of cydohexene-derived CIs compared with that of simple CIs, such as CH2OO. It is supposed that the aldehyde group significantly influences the CI reactivity of the cydohexene-derived CIs. The direct CI detection method presented here should allow study of the formation and reactivity of a wide range of different CIs formed from atmospheric ozonolysis reactions.Peer reviewe

Kinetic modeling studies of SOA formation from α-pinene ozonolysis

This paper describes the implementation of a kinetic gas-particle partitioning approach used for the simulation of secondary organic aerosol (SOA) formation within the SPectral Aerosol Cloud Chemistry Interaction Model (SPACCIM). The kinetic partitioning considers the diffusion of organic compounds into aerosol particles and the subsequent chemical reactions in the particle phase. The basic kinetic partitioning approach is modified by the implementation of chemical backward reaction of the solute within the particle phase as well as a composition-dependent particle-phase bulk diffusion coefficient. The adapted gas-phase chemistry mechanism for α-pinene oxidation has been updated due to the recent findings related to the formation of highly oxidized multifunctional organic compounds (HOMs). Experimental results from a LEAK (Leipziger Aerosolkammer) chamber study for α-pinene ozonolysis were compared with the model results describing this reaction system. The performed model studies reveal that the particle-phase bulk diffusion coefficient and the particle-phase reactivity are key parameters for SOA formation. Using the same particle-phase reactivity for both cases, we find that liquid particles with higher particle-phase bulk diffusion coefficients have 310 times more organic material formed in the particle phase compared to higher viscous semi-solid particles with lower particle-phase bulk diffusion coefficients. The model results demonstrate that, even with a moderate particle-phase reactivity, about 61% of the modeled organic mass consists of reaction products that are formed in the liquid particles. This finding emphasizes the potential role of SOA processing. Moreover, the initial organic aerosol mass concentration and the particle radius are of minor importance for the process of SOA formation in liquid particles. A sensitivity study shows that a 22-fold increase in particle size merely leads to a SOA increase of less than 10%. Due to two additional implementations, allowing backward reactions in the particle phase and considering a composition-dependent particle-phase bulk diffusion coefficient, the potential overprediction of the SOA mass with the basic kinetic approach is reduced by about 40%. HOMs are an important compound group in the early stage of SOA formation because they contribute up to 65% of the total SOA mass at this stage. HOMs also induce further SOA formation by providing an absorptive medium for SVOCs (semi-volatile organic compounds). This process contributes about 27% of the total organic mass. The model results are very similar to the LEAK chamber results. Overall, the sensitivity studies demonstrate that the particle reactivity and the particle-phase bulk diffusion require a better characterization in order to improve the current model implementations and to validate the assumptions made from the chamber simulations. The successful implementation and testing of the current kinetic gas-particle partitioning approach in a box model framework will allow further applications in a 3-D model for regional-scale process investigations

Accretion Product Formation from Ozonolysis and OH Radical Reaction of alpha-Pinene : Mechanistic Insight and the Influence of Isoprene and Ethylene

alpha-Pinene (C10H16) represents one of the most important biogenic emissions in the atmosphere. Its oxidation products can significantly contribute to the secondary organic aerosol (SOA) formation. Here, we report on the formation mechanism of C-19 and C-20 accretion products from alpha-pinene oxidation, which are believed to be efficient SOA precursors. Measurements have been performed in a free-jet flow system. Detection of RO2 radicals and accretion products was carried out by recent mass spectrometric techniques using different ionization schemes. Observed C-10-RO2 radicals from alpha-pinene ozonolysis were O,O-C10H15(O-2)(x)O-2 with x = 0, 1, 2, 3 and from the OH radical reaction HO-C10H16(O-2)(alpha)O-2 with alpha = 0, 1, 2. All detected C 20 accretion products can be explained via the accretion reaction RO2 + R'O-2 -> ROOR' + O-2 starting from the measured C-10-RO2 radicals. We speculate that C-19 accretion products are formed in an analogous way assuming CH2O elimination. Addition of isoprene (C5H8), producing C-5-RO2 radicals, leads to C-15 accretion products formed via cross-reactions with C-10-RO2 radicals. This process is competing with the formation of C-19/C-20 products from the pure alpha-pinene oxidation. A similar behavior has been observed for ethylene additives that form C-12 accretion products. In the atmosphere, a complex accretion product spectrum from self- and cross-reactions of available RO2 radicals can be expected. Modeling atmospheric conditions revealed that C-19/C-20 product formation is only reduced by a factor of 1.2 or 3.6 in isoprene-dominated environments assuming a 2- or 15-fold isoprene concentration over alpha-pinene, respectively, as present in different forested areas.Peer reviewe

Accretion Product Formation from Self- and Cross-Reactions of RO2 Radicals in the Atmosphere

Hydrocarbons are emitted into the Earth's atmosphere in very large quantities by human and biogenic activities. Their atmospheric oxidation processes almost exclusively yield RO2 radicals as reactive intermediates whose atmospheric fate is not yet fully unraveled. Herein, we show that gas-phase reactions of two RO2 radicals produce accretion products composed of the carbon backbone of both reactants. The rates for accretion product formation are very high for RO2 radicals bearing functional groups, competing with those of the corresponding reactions with NO and HO2. This pathway, which has not yet been considered in the modelling of atmospheric processes, can be important, or even dominant, for the fate of RO2 radicals in all areas of the atmosphere. Moreover, the vapor pressure of the formed accretion products can be remarkably low, characterizing them as an effective source for the secondary organic aerosol.Peer reviewe

First oxidation products from the reaction of hydroxyl radicals with isoprene for pristine environmental conditions

Isoprene, C5H8, inserts about half of the non-methane carbon flux of biogenic origin into the atmosphere. Its degradation is primarily initiated by the reaction with hydroxyl radicals. Here we show experimentally the formation of reactive intermediates and corresponding closedshell products from the reaction of hydroxyl radicals with isoprene for low nitric oxide and low hydroperoxy radical conditions. Detailed product analysis is achieved by mass spectrometric techniques. Quantum chemical calculations support the usefulness of applied ionization schemes. Observed peroxy radicals are the isomeric HO-C5H8O2 radicals and their isomerization products HO-C5H8(O-2)O-2, bearing most likely an additional hydroperoxy group, and in traces HO-C5H8(O-2)(2)O-2 with two hydroperoxy groups. Main closed-shell products from unimolecular peroxy radical reactions are hydroperoxy aldehydes, C5H8O3, and smaller yield products with the composition C5H8O4 and C4H8O5. Detected signals of C10H18O4, C10H18O6, and C5H10O2 stand for products arising from peroxy radical self- and cross-reactions.Peer reviewe

New instrument for measuring atmospheric concentrations of non-OH oxidants of SO2

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