cis-Pinonic Acid Oxidation by Hydroxyl Radicals in the Aqueous Phase under Acidic and Basic Conditions: Kinetics and Mechanism

Photo-Oxidation of α-Pinene Oxidation Products in Atmospheric Waters - pH- and Temperature-Dependent Kinetic Studies

The atmospheric α-pinene oxidation leads to three carboxylic acids: norpinonic acid (NPA), pinic acid (PA), and 3-methyl-1,2,3-butanetricarboxylic acid (MBTCA). In this study, the OH radical kinetics in the aqueous phase of these carboxylic acids were investigated at different temperatures and pH values of solutions. Activation parameters and the corresponding atmospheric lifetimes of the acids in the troposphere were derived. The overall second-order rate constants for the individual speciation forms of the acids (AH and A- for NPA; AH2, AH- and A2- for PA; and AH3, AH2-, AH2- and A3- for MBTCA) were determined. At 298 K, the rate constants for reactions of protonated forms (AHx) of NPA, PA, and MBTCA with •OH, were (1.5 ± 0.2) × 109 L mol-1 s-1, (2.4 ± 0.1) × 109 L mol-1 s-1, and (4.1 ± 0.6) × 108 L mol-1 s-1, respectively. For the fully deprotonated forms (Ax-) of studied acids, the second-order rate constants were (2.2 ± 0.2) × 109 L mol-1 s-1, (2.8 ± 0.1) × 109 L mol-1 s-1, and (10.2 ± 0.7) × 108 L mol-1 s-1 at 298 K, respectively. It was found that the reactions of NPA and PA with OH radicals are faster than with MBTCA. For MBTCA, the reaction rate depends on pH more strongly at elevated temperatures (>298 K). The atmospheric lifetimes of the acids considered due to their reactivity with •OH were calculated for different model scenarios at a temperature of 283 K and pH = 2 in the aqueous phase. For this purpose, liquid water content (LWC) was used for aerosols and clouds under storm conditions and at various aqueous-phase concentrations of OH radicals. The lifetimes decreased with increasing LWC (from 10-12 m3 m-3 in aerosol to 10-5 m3 m-3 in storms), indicating that the acids undergo significant aqueous processing under realistic atmospheric conditions. Besides, the aerosol systems appeared less effective in removing PA and NPA, with lifetimes ranging from hundreds of days to tens and hundreds of hours, respectively. Clouds were more effective, with lifetimes ranging from tens of hours to a single second or less. MBTCA, which dissolves better in water, was effectively removed in all systems, with the longest lifetime of approximately 90 min.

Photooxidation-Initiated Aqueous-Phase Formation of Organic Peroxides: Delving into Formation Mechanisms

Formation of highly oxygenated molecules (HOMs) such as organic peroxides (ROOR, ROOH, and H2O2) is known to degrade food and organic matter. Gas-phase unimolecular autoxidation and bimolecular RO2 + HO2/RO2 reactions are prominently renowned mechanisms associated with the formation of peroxides. However, the reaction pathways and conditions favoring the generation of peroxides in the aqueous phase need to be evaluated. Here, we identified bulk aqueous-phase ROOHs in varying organic precursors, including a laboratory model compound and monoterpene oxidation products. Our results show that formation of ROOHs is suppressed at enhanced oxidant concentrations but exhibits complex trends at elevated precursor concentrations. Furthermore, we observed an exponential increase in the yield of ROOHs when UV light with longer wavelengths was used in the experiment, comparing UVA, UVB, and UVC. Water-soluble organic compounds represent a significant fraction of ambient cloud-water components (up to 500 μM). Thus, the reaction pathways facilitating the formation of HOMs (i.e., ROOHs) during the aqueous-phase oxidation of water-soluble species add to the climate and health burden of atmospheric particulate matter.

Dissociation constants of relevant secondary organic aerosol components in the atmosphere

The presented studies focus on measuring the determination of the acidity constant (pKa) of relevant secondary organic aerosol components. For our research, we selected important oxidation products (mainly carboxylic acids) of the most abundant terpene compounds, such as α-pinene, β-pinene, β-caryophyllene, and δ-3-carene. The research covered the synthesis and determination of the acidity constant of selected compounds. We used three methods to measure the acidity constant, i.e., 1H NMR titration, pH-metric titration, Bates-Schwarzenbach spectrophotometric method. Moreover, the pKa values were calculated with Marvin 21.17.0 software to compare the experimentally derived values with those calculated from the chemical structure. pKa values measured with 1H NMR titration ranged from 3.51 ± 0.01 for terebic acid to 5.18 ± 0.06 for β-norcaryophyllonic acid. Moreover, the data determined by the 1H NMR method revealed a good correlation with the data obtained with the commonly used potentiometric and UV-spectroscopic methods (R2 = 0.92). In contrast, the comparison with in silico results exhibits a relatively low correlation (R2Marvin = 0.66). We found that most of the values calculated with the Marvin Program are lower than experimental values obtained with pH-metric titration with an average difference of 0.44 pKa units. For di- and tricarboxylic acids, we obtained two and three pKa values, respectively. A good correlation with the literature values was observed, for example, Howell and Fisher (1958) used pH-metric titration and measured pKa1 and pKa2 to be 4.48 and 5.48, while our results are 4.24 ± 0.10 and 5.40 ± 0.02, respectively.

Aqueous Radical Initiated Oxidation of an Organic Monolayer at the Air-Water Interface as a Proxy for Thin Films on Atmospheric Aerosol Studied with Neutron Reflectometry

Neutron reflectometry has been used to study the radical initiated oxidation of a monolayer of the lipid 1,2-distearoyl-sn-glycero-3-phosphocholine (DSPC) at the air-solution interface by aqueous-phase hydroxyl, sulfate, and nitrate radicals. The oxidation of organic films at the surface of atmospheric aqueous aerosols can influence the optical properties of the aerosol and consequently can impact Earth's radiative balance and contribute to modern climate change. The amount of material at the air-solution interface was found to decrease on exposure to aqueous-phase radicals which was consistent with a multistep degradation mechanism, i.e., the products of reaction of the DSPC film with aqueous radicals were also surface active. The multistep degradation mechanism suggests that lipid molecules in the thin film degrade to form progressively shorter chain surface active products and several reactive steps are required to remove the film from the air-solution interface. Bimolecular rate constants for oxidation via the aqueous phase OH radical cluster around 1010 dm3 mol-1 s-1. Calculations to determine the film lifetime indicate that it will take ∼4-5 days for the film to degrade to 50% of its initial amount in the atmosphere, and therefore attack by aqueous radicals on organic films could be atmospherically important relative to typical atmospheric aerosol lifetimes.

Aging of α-Pinene Secondary Organic Aerosol by Hydroxyl Radicals in the Aqueous Phase: Kinetics and Products

The reaction of hydroxyl radicals (OH) with a water-soluble fraction of the α-pinene secondary organic aerosol (SOA) was investigated using liquid chromatography coupled with negative electrospray ionization mass spectrometry. The SOA was generated by the dark ozonolysis of α-pinene, extracted into the water, and subjected to chemical aging by the OH. Bimolecular reaction rate coefficients (k_OH) for the oxidation of terpenoic acids by the OH were measured using the relative rate method. The unaged SOA was dominated by the cyclobutyl-ring-retaining compounds, primarily cis-pinonic, cis-pinic, and hydroxy-pinonic acids. Aqueous oxidation by the OH resulted in the removal of early-stage products and dimers, including well-known oligomers with MW = 358 and 368 Da. Furthermore, a 2- to 5-fold increase in the concentration of cyclobutyl-ring-opening products was observed, including terpenylic and diaterpenylic acids and diaterpenylic acid acetate as well as some of the newly identified OH aging markers. At the same time, results obtained from the kinetic box model showed a high degree of SOA fragmentation following the reaction with the OH, which indicates that non-radical reactions occurring during the evaporation of water likely contribute to the high yields of terpenoic _aqSOAs reported previously. The estimated atmospheric lifetimes showed that in clouds, terpenoic acids react with the OH exclusively in the aqueous phase. Aqueous OH aging of the α-pinene SOA results in a 10% increase of the average O/C ratio and a 3-fold decrease in the average k_OH value, which is likely to affect the cloud condensation nuclei activity of the _aqSOA formed after the evaporation of water.

Aqueous-phase oxidation of syringic acid emitted from biomass burning: Formation of light-absorbing compounds

Syringic acid is a methoxyphenol model compound derived from biomass burning, and its photooxidation processes have important effects on atmospheric chemistry. However, its aqueous-phase photochemistry remains unclear. In this study, we systematically report the photooxidation of syringic acid induced by OH radicals in the aqueous phase. Employing the relative rate technique, the bimolecular rate constant for syringic acid reaction with OH radicals was acquired to be (1.1 ± 0.3) × 10¹⁰ M^-1 s^-1. Notably, colored products were formed as the reaction progressed. Furthermore, the UV-vis and fluorescence spectra confirmed the formation of light-absorbing organic species, and the results agreed well with previous results on atmospheric and natural humic-like substances (HULIS). The photooxidation products were detected by high performance liquid chromatography mass spectrometry (HPLC/MS), and a possible reaction mechanism was proposed. The aqueous-phase reaction of syringic acid would undergo functionalization process forming a hydroxylation product that enhances the degree of oxidation of aqueous secondary organic aerosol (aqSOA), and goes through dimerization process by C-C or C-O coupling of phenoxy radicals which may conduce to the formation of HULIS. These findings suggest that the photooxidation of syringic acid is an important pathway for highly oxygenated phenolic aqSOA formation, providing a secondary source for HULIS in a liquid phase or in deliquescent particles surrounded by a layer of water.

One-year measurements of secondary organic aerosol (SOA) markers in the Paris region (France): Concentrations, gas/particle partitioning and SOA source apportionment

Twenty-five biogenic and anthropogenic secondary organic aerosol (SOA) markers have been measured over a one-year period in both gaseous and PM₁₀ phases in the Paris region (France). Seasonal and chemical patterns were similar to those previously observed in Europe, but significantly different from the ones observed in America and Asia due to dissimilarities in source precursor emissions. Nitroaromatic compounds showed higher concentrations in winter due to larger emissions of their precursors originating from biomass combustion used for residential heating purposes. Among the biogenic markers, only isoprene SOA marker concentrations increased in summer while pinene SOA markers did not display any clear seasonal trend. The measured SOA markers, usually considered as semi-volatiles, were mainly associated to the particulate phase, except for the nitrophenols and nitroguaiacols, and their gas/particle partitioning (GPP) showed a low temperature and OM concentrations dependency. An evaluation of their GPP with thermodynamic model predictions suggested that apart from equilibrium partitioning between organic phase and air, the GPP of the markers is affected by processes suppressing volatility from a mixed organic and inorganic phase, such as enhanced dissolution in aerosol aqueous phase and non-equilibrium conditions. SOA marker concentrations were used to apportion secondary organic carbon (SOC) sources applying both, an improved version of the SOA-tracer method and positive matrix factorization (PMF) Total SOC estimations agreed very well between both models, except in summer and during a highly processed Springtime PM pollution event in which systematic underestimation by the SOA tracer method was evidenced. As a first approach, the SOA-tracer method could provide a reliable estimation of the average SOC concentrations, but it is limited due to the lack of markers for aged SOA together with missing SOA/SOC conversion fractions for several sources.

pH Dependence of the OH Reactivity of Organic Acids in the Aqueous Phase

Photochemical processing taking place in atmospheric aqueous phases serves as both a source and a sink of organic compounds. In aqueous environments, acid-base chemistry and, by extension, aqueous-phase pH, are an important yet often neglected factors to consider when investigating the kinetics of organic compounds. We have investigated the aqueous-phase OH-oxidation of pinic acid, cis-pinonic acid, limononic acid, and formic acid (FA) as a function of pH. We have also extended our studies to other organic acids (OAs) present in the water-soluble fraction of secondary organic aerosol (SOA) arising from the ozonolysis of α-pinene. Although all the OAs exhibited larger OH reactivities at pH 10, the pH dependence was dramatically different between FA, the smallest OA, and those that contained more than eight carbons. A kinetic box model was also employed to characterize our photoreactor and to provide confidence to our results. Our finding shows that the atmospheric lifetimes of small OAs (e.g., FA) are highly sensitive to cloud water pH. However, those of larger OAs and many other OAs in α-pinene SOA are affected to a much less extent. These results are of great importance for the simplification of cloud water chemistry models.

Physicochemical Properties of Terebic Acid, MBTCA, Diaterpenylic Acid Acetate, and Pinanediol as Relevant α-Pinene Oxidation Products

The physicochemical properties and the synthesis of four α-pinene oxidation products, terebic acid, 3-methyl-1,2,3-butanetricarboxylic acid (MBTCA), diaterpenylic acid acetate (DTAA), and pinanediol, are presented in this study. The physicochemical properties encompass thermal properties, solubility in water, and dissociation constant (pK _a) for the investigated compounds. It was found that terebic acid exhibits a relatively high melting temperature of 449.29 K, whereas pinanediol revealed a low melting temperature of 329.26 K. The solubility in water was determined with the dynamic method and the experimental results were correlated using three different mathematical models: Wilson, NRTL, and UNIQUAC equations. The results of the correlation indicate that the Wilson equation appears to work the best for terebic acid and pinanediol. The calculated standard deviation was for 3.79 for terebic acid and 1.25 for pinanediol. In contrast, UNIQUAC was the best mathematical model for DTAA and MBTCA. The calculated standard deviation was 0.57 for DTAA and 2.21 for MBTCA. The measured water solubility increased in the following order: pinanediol > DTAA ≥ MBTCA > terebic acid, which affects their multiphase aging chemistry in the atmosphere. Moreover, acidity constants (pK _a) at 298, 303, and 308 K were determined for DTAA with the Bates-Schwarzenbach spectrophotometric method. The pK _a values obtained at 298, 303, and 308 K were found to be 3.76, 3.85, and 3.88, respectively.

A comprehensive investigation of aqueous-phase photochemical oxidation of 4-ethylphenol

Secondary organic aerosol (SOA) species formed in atmospheric aqueous phases is recently recognized as an important contributor to fine aerosols, which is known to be a prominent human health risk factor internationally. This work, for the first time, systematically investigated aqueous-phase photochemical oxidation of 4-ethylphenol (4-EP) - a model compound from biomass burning and a surrogate of intermediate volatility organic compounds, under both ultraviolet (UV) (Hg lamp) and simulated sunlight (Xe lamp). We found that 4-EP could degrade upon hydroxal radical (OH) oxidation under UV light nearly 15 times faster than that under simulated sunlight, but large aqueous SOA (aqSOA) yields (108%-122%) were observed under both situations. AqSOA masses and oxidation states continuously increased under simulated sunlight, yet they increased first then decreased quickly under UV light. We proposed a reaction scheme based on identified products, showing that oligomerization, functionalization and fragmentation all can occur during 4-EP oxidation. Our results demonstrate that OH radical may suppress oligomerization and functionalization, but is favorable for fragmentation. Under UV light with H₂O₂ (high OH), fragmentation was dominant, producing more volatile and smaller molecules, and less aqSOA in later oxidation; Under simulated sunlight with H₂O₂ (moderate OH), functionalization that can form hydroxylated monomer was more important. Moreover, 4-EP oxidation by the organic triplet excited state (³C*) could form species with stronger visible light absorptivity than those from OH-mediated oxidation, and the absorptivity showed positive link with contents of humic-like substances.

Ozonolysis of β-Caryophyllonic and Limononic Acids in the Aqueous Phase: Kinetics, Product Yield, and Mechanism

Ozonolysis of β-caryophyllonic (BCA) and limononic (LA) acids in the aqueous-phase was investigated. The rate coefficients (k_ozone) measured for the BCA + ozone (O₃) reaction at 295 ± 2 K were 4.8 ± 0.6 × 10⁵ M^-1 s^-1 at pH = 2 and 6.0 ± 0.3 × 10⁵ M^-1 s^-1 at pH = 8. The UV-vis absorption cross sections (σ, cm² molecule^-1) for BCA and LA in water were also measured. Atmospheric lifetimes of BCA and LA due to reactions with O₃, hydroxyl radicals (OH), and due to photolysis were calculated. Lifetime estimates indicate that the aqueous-phase processing of both terpenoic acids studied in this work would be relevant in the atmosphere. In cloudwater, BCA is more likely to react with O₃ with some possible contribution from the oxidation by OH, whereas the opposite is true for LA. Products of BCA and LA ozonolysis were quantified with LC-MS as well as with the UV-vis assays for quantification of formaldehyde and hydroperoxides. Oxygenated derivatives of BCA and LA that were produced following aqueous ozonolysis were identified as keto-BCA and keto-LA, respectively. Additionally, large quantities of intramolecular secondary ozonides and α-acyloxyhydroperoxy aldehydes were tentatively identified as products of aqueous ozonolysis of the two unsaturated terpenoic acids investigated.

Theoretical study of the cis-pinonic acid and its atmospheric hydrolysate participation in the atmospheric nucleation

cis-Pinonic acid (CPA), one of the major photooxidation products of α-pinene, is believed to contribute to the formation of aerosols formed over forested areas. In the current study, we implement quantum chemical calculation to investigate the interaction between sulfuric acid (SA) and CPA as well as the hydrolysate of CPA (HCPA) in the presence of water or ammonia in the atmosphere. The lowest free energy configurations, reactants, transition states, intermediates, and products were optimized at 298/278K and 1atm at the M06-2X/6-311+G(3df,3pd) level. Our results show that one CPA molecule might initially nucleate with SA molecules and subsequently participate in the formation and growth of the new particle in the form of HCPA. More than one HCPA molecule may be involved in the critical nuclei. Furthermore, the hydrolysis reaction of CPA can be effectively catalyzed by SA and nitric acid (NA) in presence of water, which significantly increases the HCPA content in the atmosphere and subsequently promotes the particle nucleation. Overall, the current study elucidates a new mechanism of atmospheric nucleation driven by CPA and its hydrolysate.

Aqueous-Phase Oxidation of Terpene-Derived Acids by Atmospherically Relevant Radicals

Terpene-derived acids formed through the atmospheric gas-phase oxidation of terpenes are able to efficiently undergo a phase transfer into the aqueous phase. The subsequent aqueous-phase oxidation of such compounds has not been intensely studied. Accordingly, the aqueous-phase second-order rate constants of the oxidation reactions of cis-pinonic acid (CPA) and (+)-camphoric acid (+CA) with hydroxyl radicals (^•OH), nitrate radicals (NO₃^•), and sulfate radicals (SO₄^•-) were investigated as a function of temperature and pH in the present study. For CPA and +CA the following ^•OH reaction rate constants at T = 298 K are determined: k_second(CPA, pH<2) = (2.8 ± 0.1) × 10⁹ L mol^-1 s^-1, k_second(CPA, pH>8) = (2.7 ± 0.3) × 10⁹ L mol^-1 s^-1, k_second(+CA, pH<2) = (2.1 ± 0.1) × 10⁹ L mol^-1 s^-1, k_second(+CA, pH=5.3) = (2.7 ± 0.3) × 10⁹ L mol^-1 s^-1, k_second(+CA, pH>8) = (2.7 ± 0.1) × 10⁹ L mol^-1 s^-1. In order to assess the atmospheric impact of the aqueous-phase oxidation of such compounds, atmospheric aqueous-phase lifetimes were calculated for two model scenarios based on CAPRAM 3.0i. The aqueous-phase oxidation under remote conditions emerges to be the most favored pathway with lifetimes of 5 ± 1 h.

Kinetics of Limonene Secondary Organic Aerosol Oxidation in the Aqueous Phase

Twenty semivolatile organic compounds that contribute to limonene secondary organic aerosol (SOA) were synthesized in the flow-tube reactor. Kinetics of the aqueous-phase oxidation of the synthesized compounds by hydroxyl radicals (OH) and ozone (O₃) were investigated at 298 ± 2 K using the relative rate method. Oxidized organic compounds identified as the major components of limonene SOA were quantified with liquid chromatography coupled to the electrospray ionization and quadrupole tandem mass spectrometry (LC-ESI/MS/MS). The bimolecular rate coefficients measured for the oxidation products of limonene are k_OH = 2-5 × 10⁹ M^-1 s^-1 for saturated and k_OH = 1-2 × 10¹⁰ M^-1 s^-1 for unsaturated compounds. Ozonolysis reaction bimolecular rate coefficients obtained for the unsaturated compounds in the aqueous phase are between 2 and 6 × 10⁴ M^-1 s^-1. The results obtained in this work also indicate that oxidation of limonene carboxylic acids by OH was about a factor of 2 slower for the carboxylate ions than for the protonated acids while the opposite was true for the ozonolysis. The data acquired provided new insights into kinetics of the limonene SOA processing in the aqueous phase. Ozonolysis of limonene SOA also increased the concentration of dimers, most likely due to reactions of the stabilized Criegee intermediates with the other, stable products. These results indicate that aqueous-phase oxidation of limonene SOA by OH and O₃ will be relevant in clouds, fogs, and wet aerosols.

Limononic Acid Oxidation by Hydroxyl Radicals and Ozone in the Aqueous Phase

Kinetics and mechanism of limononic acid (3-isopropenyl-6-oxoheptanoic acid, LA) oxidation by hydroxyl radicals (OH) and ozone (O₃) were studied in the aqueous phase at 298 ± 2 K. These reactions were investigated using liquid chromatography coupled to the electrospray ionization and quadrupole tandem mass spectrometry (LC-ESI/MS/MS). The rate coefficients determined for LA + OH reaction were: 1.3 ± 0.3 × 10¹⁰ M^-1 s^-1 at pH = 2 and 5.7 ± 0.6 × 10⁹ M^-1 s^-1 at pH = 10. The rate coefficient determined for LA ozonolysis was 4.2 ± 0.2 × 10⁴ M^-1 s^-1 at pH = 2. The calculated Henry's law constant (H) for LA was ca. 6.3 × 10⁶ M × atm^-1, thereby indicating that in fogs and clouds with LWC = 0.3-0.5 g × m^-3 LA will reside entirely in the aqueous phase. Calculated atmospheric lifetimes due to reaction with OH and O₃ strongly indicate that aqueous-phase oxidation can be important for LA under realistic atmospheric conditions. Under acidic conditions, the aqueous-phase oxidation of LA by OH will dominate over reaction with O₃, whereas the opposite is more likely when pH ≥ 4.5. The aqueous-phase oxidation of LA produced keto-limononic acid and a number of low-volatility products, such as hydroperoxy-LA and α-hydroxyhydroperoxides.