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

Recent advances in mass spectrometry techniques for atmospheric chemistry research on molecular-level

The Earth's atmosphere is composed of an enormous variety of chemical species associated with trace gases and aerosol particles whose composition and chemistry have critical impacts on the Earth's climate, air quality, and human health. Mass spectrometry analysis as a powerful and popular analytical technique has been widely developed and applied in atmospheric chemistry for decades. Mass spectrometry allows for effective detection, identification, and quantification of a broad range of organic and inorganic chemical species with high sensitivity and resolution. In this review, we summarize recently developed mass spectrometry techniques, methods, and applications in atmospheric chemistry research in the past several years on molecular-level. Specifically, new developments of ion-molecule reactors, various soft ionization methods, and unique coupling with separation techniques are highlighted. The new mass spectrometry applications in laboratory studies and field measurements focused on improving the detection limits for traditional and emerging volatile organic compounds, characterizing multiphase highly oxygenated molecules, and monitoring particle bulk and surface compositions.

Extending the Range of Detectable Trace Species with the Fast Polarity Switching of Chemical Ionization Orbitrap Mass Spectrometry

Chemical ionization (CI) atmospheric pressure interface mass spectrometry is a unique analytical technique for its low detection limits, softness to preserve molecular information, and selectivity for particular classes of species. Here, we present a fast polarity switching approach for highly sensitive online analysis of a wide range of trace species in complex samples using selective CI chemistries and high-resolution mass spectrometry. It is achieved by successfully coupling a multischeme chemical ionization inlet (MION) and an Orbitrap Fourier transform mass spectrometer. The capability to flexibly combine ionization chemistries from both polarities effectively extends the detectability compared to using only one ionization chemistry, as commonly used positive and negative reagent ions tend to be sensitive to different classes of species. We tested the performance of the MION-Orbitrap using reactive gaseous organic species generated by α-pinene ozonolysis in an environmental chamber and a standard mixture of 71 pesticides. Diethylammonium and nitrate are used as reagent ions in positive and negative polarities. We show that with a mass resolving power of 280,000, the MION-Orbitrap can switch and measure both polarities within 1 min, which is sufficiently fast and stable to follow the temporal evolution of reactive organic species and the thermal desorption profile of pesticides. We detected 23 of the 71 pesticides in the mixture using only nitrate as the reagent ion. Facilitated by polarity switching, we also detected 47 pesticides using diethylammonium, improving the total number of detected species to 59. For reactive organic species generated by α-pinene ozonolysis, we show that combining diethylammonium and nitrate addresses the need to measure oxygenated molecules in atmospheric environments with a wide range of oxidation states. These results indicate that the polarity switching MION-Orbitrap can promisingly serve as a versatile tool for the nontargeted chemical analysis of trace species in various applications.

Pollution characteristics, source appointment and environmental effect of oxygenated volatile organic compounds in Guangdong-Hong Kong-Macao Greater Bay Area: Implication for air quality management

Same as other bay areas, the Guangdong-Hong Kong-Macao Greater Bay Area (GBA) is also suffering atmospheric composite pollution. Even a series of atmospheric environment management policies have been conducted to win the "blue sky defense battle", the atmospheric secondary pollutants (e.g., O3) originated from oxygenated volatile organic compounds (OVOCs) still threaten the air quality in GBA. However, there lacks a systematic summary on the emission, formation, pollution and environmental effects of OVOCs in this region for further air quality management. This review focused on the researches related to OVOCs in GBA, including their pollution characteristics, detection methods, source distributions, secondary formations, and impacts on the atmosphere. Pollution profile of OVOCs in GBA revealed that the concentration percentage among total VOCs from Guangzhou and Dongguan cities exceeded 50 %, while methanol, formaldehyde, acetone, and acetaldehyde were the top four highest concentrated OVOCs. The detection technique on regional atmospheric OVOCs (e.g., oxygenated organic molecules (OOMs)) underwent an evolution of off-line derivatization method, on-line spectroscopic method and on-line mass spectrometry method. The OVOCs in GBA were mainly from primary emissions (up to 80 %), including vehicle emissions and biomass combustion. The anthropogenic alkenes and aromatics in urban area, and natural isoprene in rural area also made a significant contribution to the secondary emission (e.g., photochemical formation) of OVOCs. About 20 % in average of ROx radicals was produced from photolysis of formaldehyde in comparison with O3, nitrous acid and rest OVOCs, while the reaction between OVOCs and free radical accelerated the NOx-O3 cycle, contributing to 15 %-60 % cumulative formation of O3 in GBA. Besides, the heterogeneous reactions of dicarbonyls generated 21 %-53 % of SOA. This review also provided suggestions for future research on OVOCs in terms of regional observation, analytical method and mechanistic study to support the development of a control and management strategy on OVOCs in GBA and China.

Interfacial Extraction to Trap and Characterize the Criegee Intermediates from Phospholipid Ozonolysis

Criegee intermediates (CIs) play a significant role in cell membrane peroxidation, but their identification remains elusive at the molecular level. Herein, we combined interfacial extraction and sonic spray ionization mass spectrometry to study the oxidation reaction of 1-palmitoyl-2-oleoyl-sn-glycero-3-phospho-(1'-rac-glycerol) (POPG) mediated by ozone (O₃) at/near the surface of a hung water droplet. On-line interfacial extraction and ionization provided a snapshot of the short-lived CIs. Experiments in which the content of water was varied provided evidence for the formation of CIs, which has not been previously observed. Capture experiments using 5,5-dimethyl-pyrroline N-oxide (DMPO) indicated that CIs could be selectively characterized, and the extracted ion current (EICs) of CIs vs DMPO-CI adducts further confirmed the successful observation of CIs. Theoretical calculation suggested that surface ozonolysis of POPG was mainly mediated by anti-CI. These results open a new route for aqueous surface reactive species identification, and benefit toward the understanding of disease development associated with cell oxidative stress mediated by CIs.

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.

Probing autoxidation of oleic acid at air-water interface: A neglected and significant pathway for secondary organic aerosols formation

Fatty acids have been proposed to be a potential source of precursors for SOAs, but the autoxidation process was neglected in the oxidation studies. Here, the autoxidation of oleic acid was explored using microdroplet mass spectrometry. Bulk solution, concentration and solvent composition experiments provided direct evidences for that the autoxidation occurred at or near the air-water interface. The kinetic data showed an acceleration at this interface and was comparable to ozonation, indicating that autoxidation is an important pathway for SOAs formation. In addition, intermediates/products were captured and identified using tandem mass spectrometry, spin-trapping and quenched agents. The autoxidation mechanism was divided into addition intermediates (AIs) and Criegee intermediates (CIs) pathways mediated by hydroxyl radicals (OH). The CI chemistry which is ubiquitous in gas phase was observed at the air-water interface, and this leaded to the mass/volume loss of aerosols. Inversely, the AI chemistry caused the increase of mass, density and hygroscopicity of aerosols. AI chemistry was dominated compared to CI chemistry, but varied by concerning aerosol sizes, ultraviolet light (UV) and charge. Moreover, the MS approach of selectively probing the interfacial substances at the scale of sub-seconds opens new opportunities to study heterogeneous chemistry in atmosphere.

Reaction Kinetics of Criegee Intermediates with Nitric Acid

This work investigated the reaction kinetics of HNO₃ with four Criegee intermediates (CIs): CH₂OO, (CH₃)₂COO, methyl vinyl ketone oxide (MVKO), and methacrolein oxide (MACRO). Our results show that these reactions are extremely fast with rate coefficients of (1.51 ± 0.45) × 10^-10, (3.54 ± 1.06) × 10^-10, (3.93 ± 1.18) × 10^-10, and (3.0 ± 1.0) × 10^-10 cm³ s^-1 for reactions of HNO₃ with CH₂OO, (CH₃)₂COO, syn-MVKO, and anti-MACRO, respectively. This is consistent with previous results for the reactions between CIs and carboxylic acids, but the rate coefficient of CH₂OO + HNO₃ in the literature [Foreman Angew. Chem. 2016, 128, 10575] was found to be overestimated by a factor of 3.6. In addition, we did not observe any significant pressure dependence in the HNO₃ reactions with CH₂OO and (CH₃)₂COO under 100-400 Torr. Our results indicate that in a dry area with severe NO_x pollution, the reactions of CIs with HNO₃ and their products may be worthy of attention, but these reactions may be insignificant under high-humidity conditions. However, CI reactions with HNO₃ may not play an important role in the atmospheric removal processes of HNO₃ because of the low concentrations of CIs.

Infrared Characterization of the Products and Rate Coefficient of the Reaction between Criegee Intermediate CH2OO and HNO3

The reactions of Criegee intermediates with HNO₃ are important in the polluted urban atmosphere because of their large rate coefficients and the significant concentration of HNO₃. Employing a step-scan Fourier-transform spectrometer, we recorded infrared spectra of transient species and end products in the reaction CH₂OO + HNO₃ upon irradiation of a flowing mixture of CH₂I₂/HNO₃/N₂/O₂ at 308 nm. Eight bands at 1686, 1426, 1348, 1294, 1052, 965, 891, and 825 cm^-1 were assigned to the absorption of the adduct nitrooxymethyl hydroperoxide (NMHP, NO₃CH₂OOH). Additional products from two dissociation channels were observed. Four bands at 1709, 1325, 1276, and 886 cm^-1 were assigned to H₂C(O)ONO₂ (with coproduct OH), produced from the fission of the O-O bond of internally hot NMHP (NMHP*). Simultaneous detection of H₂CO (1746 cm^-1), NO₂ (1617 cm^-1), and HO₂ (1392 and 1098 cm^-1) indicated a direct cleavage of the N-OC and C-OO bonds of NMHP*. The relative yields of these three channels in pressure range 10-150 Torr were estimated. At 10 Torr, the absorption of internally excited HNO₃ near 885 and 1320 cm^-1 was also detected at an early stage of the reaction. We investigated also the rate coefficient of the reaction CH₂OO + HNO₃ by probing the temporal profiles of the formation of NMHP and NO₂ under total pressures of 40 and 70 Torr at 298 K. The rate coefficient k_HNO3 = (2.4 ± 0.4) × 10^-10 cm³ molecule^-1 s^-1 is less than half the only literature value, (5.4 ± 1.0) × 10^-10 cm³ molecule^-1 s^-1, reported by Foreman et al. (Angew. Chem. Int. Ed. 2016, 55, 10419-10422).

Kinetics of the Reactions of CH2OO with Acetone, α-Diketones, and β-Diketones

Rate constants for the reactions between the simplest Criegee intermediate, CH₂OO, with acetone, the α-diketones biacetyl and acetylpropionyl, and the β-diketones acetylacetone and 3,3-dimethyl-2,4-pentanedione have been measured at 295 K. CH₂OO was produced photochemically in a flow reactor by 355 nm laser flash photolysis of diiodomethane in the presence of excess oxygen. Time-dependent concentrations were measured using broadband transient absorption spectroscopy, and the reaction kinetics was characterized under pseudo-first-order conditions. The bimolecular rate constant for the CH₂OO + acetone reaction is measured to be (4.1 ± 0.4) × 10^-13 cm³ s^-1, consistent with previous measurements. The reactions of CH₂OO with the β-diketones acetylacetone and 3,3-dimethyl-2,5-pentanedione are found to have broadly similar rate constants of (6.6 ± 0.7) × 10^-13 and (3.5 ± 0.8) × 10^-13 cm³ s^-1, respectively; these values may be cautiously considered as upper limits. In contrast, α-diketones react significantly faster, with rate constants of (1.45 ± 0.18) × 10^-11 and (1.29 ± 0.15) × 10^-11 cm³ s^-1 measured for biacetyl and acetylpropionyl. The potential energy surfaces for these 1,3-dipolar cycloaddition reactions are characterized at the M06-2X/aug-cc-pVTZ and CBS-QB3 levels of theory and provide additional support to the observed experimental trends. The reactivity of carbonyl compounds with CH₂OO is also interpreted by application of frontier molecular orbital theory and predicted using Hammett substituent constants. Finally, the results are compared with other kinetic studies of Criegee intermediate reactions with carbonyl compounds and discussed within the context of their atmospheric relevance.

Study on ozonolysis of asymmetric alkenes with matrix isolation and FT-IR spectroscopy

O₃ and alkenes are important reactants in the formation of SOA in the atmosphere. The intermediates and reaction mechanism of ozonation of alkene is an important topic in atmospheric chemistry. In this study, the low-temperature matrix isolation was used to capture the intermediates such as Primary ozonides (POZs), Criegee Intermediates (CIs), and Secondary ozonides (SOZs) generated from ozonation of 2-methyl-1-butene (2M1B) and 2-methyl-2-butene (2M2B). The results have been identified by the vacuum infrared spectroscopy and theoretical calculation. Our results show that during the ozonation of asymmetric alkenes, two kinds of CIs and more than two kinds of SOZs were generated due to the different decomposition modes of POZs. The infrared absorption peaks of (CH₃)₂COO and CH₃CH₂C(CH₃)OO for O-O telescopic vibration was determined to be 889 cm^-1 and 913 cm^-1, respectively. Using the merged jet method, it was found that a large amount of HCHO was produced during the ozonation of 2M1B, and glyoxal and methylglyoxal were produced in the ozonation of 2M2B. Our findings highlight the importance of asymmetric alkene ozonolysis reactions in producing CIs, further improving the understanding of the generation of CIs from ozonation of alkenes.

Kinetics and Thermodynamics of Reactions Involving Criegee Intermediates: An Assessment of Density Functional Theory and Ab Initio Methods Through Comparison with CCSDT(Q)/CBS Data

Reactions involving Criegee intermediates (CIs, R₁ R₂ COO) are important in atmospheric ozonolysis models. In recent years, density functional theory (DFT) and CCSD(T)-based ab initio methods are increasingly being used for modeling reaction profiles involving CIs. We obtain highly accurate CCSDT(Q)/CBS reaction energies and barrier heights for ring-closing reactions involving atmospherically important CIs (R₁ /R₂ = H, Me, OH, OMe, F, CN, cyclopropene, ethylene, acetaldehyde, and acrolein). We use this benchmark data to evaluate the performance of DFT, double-hybrid DFT (DHDFT), and ab initio methods for the kinetics and thermodynamics of these reactions. We find that reaction energies are more challenging for approximate theoretical procedures than barrier heights. Overall, taking both reaction energies and barrier heights into account, only one of the 58 considered DFT methods (the meta-GGA MN12-L) attains near chemical accuracy, with root-mean-square deviations (RMSDs) of 3.5 (barrier heights) and 4.7 (reaction energies) kJ mol^-1 . Therefore, MN12-L is recommended for investigations where CCSD(T)-based methods are not computationally feasible. For reaction barrier heights performance does not strictly follow Jacob's Ladder, for example, DHDFT methods do not perform better than conventional DFT methods. Of the ab initio methods, the cost-effective CCSD(T)/CBS(MP2) approach gives the best performance for both reaction energies and barrier heights, with RMSDs of 1.7 and 1.4 kJ mol^-1 , respectively. All the considered Gaussian-n methods show good performance with RMSDs below the threshold of chemical accuracy for both reaction energies and barrier heights, where G4(MP2) shows the best overall performance with RMSDs of 2.9 and 1.5 kJ mol^-1 , respectively. © 2019 Wiley Periodicals, Inc.

Synthesis, Electronic Spectroscopy, and Photochemistry of Methacrolein Oxide: A Four-Carbon Unsaturated Criegee Intermediate from Isoprene Ozonolysis

Ozonolysis of isoprene, one of the most abundant volatile organic compounds in the earth's atmosphere, generates the four-carbon unsaturated methacrolein oxide (MACR-oxide) Criegee intermediate. The first laboratory synthesis and direct detection of MACR-oxide is achieved through reaction of photolytically generated, resonance-stabilized iodoalkene radicals with oxygen. MACR-oxide is characterized on its first π* ← π electronic transition using a ground-state depletion method. MACR-oxide exhibits a broad UV-visible spectrum peaked at 380 nm with weak oscillatory structure at long wavelengths ascribed to vibrational resonances. Complementary theory predicts two strong π* ← π transitions arising from extended conjugation across MACR-oxide with overlapping contributions from its four conformers. Electronic promotion to the 1¹ππ* state agrees well with experiment, and results in nonadiabatic coupling and prompt release of O ¹D products observed as anisotropic velocity-map images. This UV-visible detection scheme will enable study of its unimolecular and bimolecular reactions under thermal conditions of relevance to the atmosphere.

Formation of Polycyclic Aromatic Hydrocarbon Oxidation Products in α-Pinene Secondary Organic Aerosol Particles Formed through Ozonolysis

Accurate long-range atmospheric transport (LRAT) modeling of polycyclic aromatic hydrocarbons (PAHs) and PAH oxidation products (PAH-OPs) in secondary organic aerosol (SOA) particles relies on the known chemical composition of the particles. Four PAHs, phenanthrene (PHE), dibenzothiophene (DBT), pyrene (PYR), and benz(a)anthracene (BaA), were studied individually to identify and quantify PAH-OPs produced and incorporated into SOA particles formed by ozonolysis of α-pinene in the presence of PAH vapor. SOA particles were characterized using real-time in situ instrumentation, and collected on quartz fiber filters for offline analysis of PAHs and PAH-OPs. PAH-OPs were measured in all PAH experiments at equal or greater concentrations than the individual PAHs they were produced from. The total mass of PAH and PAH-OPs, relative to the total SOA mass, varied for different experiments on individual parent PAHs: PHE and 6 quantified PHE-OPs (3.0%), DBT and dibenzothiophene sulfone (4.9%), PYR and 3 quantified PYR-OPs (3.1%), and BaA and benz(a)anthracene-7,12-dione (0.26%). Further exposure of PAH-SOA to ozone generally increased the concentration ratio of PAH-OPs to PAH, suggesting longer atmospheric lifetimes for PAH-OPs, relative to PAHs. These data indicate that PAH-OPs are formed during SOA particle formation and growth.

Absolute Infrared Absorption Cross Section of the Simplest Criegee Intermediate Near 1285.7 cm-1

The ν₄ fundamental of the simplest Criegee intermediate, CH₂OO, has been monitored with high-resolution infrared (IR) transient absorption spectroscopy under total pressures of 4-94 Torr. This IR spectrum provides an unambiguous identification of CH₂OO and is potentially useful to determine the number density of CH₂OO in various laboratory studies. Here we utilized an ultraviolet (UV) and IR coupled spectrometer to measure the UV and IR absorption spectra of CH₂OO simultaneously; the absolute IR cross section can then be determined by using a known UV cross section. Due to significant pressure broadening in the studied pressure range, we integrated the IR absorption spectra between 1285.2 and 1286.4 cm^-1 (covering the Q branch), and then we converted this integrated absorbance to the absolute integral IR cross section of CH₂OO (for the Q branch); its absolute value is (3.7 ± 0.6) × 10^-19 cm·molecule^-1 or 2.2 ± 0.4 km·mol^-1. The whole rotational band (P, Q, and R branches) can be adequately simulated by using the precise spectroscopic parameters from the literature, yielding the absolute integral IR cross section (full ν₄ band) to be 19.2 ± 3.5 km·mol^-1. For a practical detection of CH₂OO, this work also reports the peak cross section as a function of total pressure (4-94 Torr O₂). At low pressure (≤4 Torr), where the pressure broadening is insignificant, the absorption cross section of the highest peak is (6.2 ± 0.9) × 10^-18 cm²·molecule^-1 (at the system line width of 0.004 cm^-1 fwhm).

Criegee intermediates and their impacts on the troposphere

Criegee intermediates (CIs), carbonyl oxides formed in ozonolysis of alkenes, play key roles in the troposphere. The decomposition of CIs can be a significant source of OH to the tropospheric oxidation cycle especially during nighttime and winter months. A variety of model-measurement studies have estimated surface-level stabilized Criegee intermediate (sCI) concentrations on the order of 1 × 10⁴ cm^-3 to 1 × 10⁵ cm^-3, which makes a non-negligible contribution to the oxidising capacity in the terrestrial boundary layer. The reactions of sCI with the water monomer and the water dimer have been found to be the most important bimolecular reactions to the tropospheric sCI loss rate, at least for the smallest carbonyl oxides; the products from these reactions (e.g. hydroxymethyl hydroperoxide, HMHP) are also of importance to the atmospheric oxidation cycle. The sCI can oxidise SO₂ to form SO₃, which can go on to form a significant amount of H₂SO₄ which is a key atmospheric nucleation species and therefore vital to the formation of clouds. The sCI can also react with carboxylic acids, carbonyl compounds, alcohols, peroxy radicals and hydroperoxides, and the products of these reactions are likely to be highly oxygenated species, with low vapour pressures, that can lead to nucleation and SOA formation over terrestrial regions.