Multiphase Ozonolysis of Aqueous α-Terpineol

Formation of Chromophores from cis-Pinonaldehyde Aged in Highly Acidic Conditions

Sulfuric acid in the atmosphere can participate in acid-catalyzed and acid-driven reactions, including those within secondary organic aerosols (SOA). Previous studies have observed enhanced absorption at visible wavelengths and significant changes in the chemical composition when SOA was exposed to sulfuric acid. However, the specific chromophores responsible for these changes could not be identified. The goals of this study are to identify the chromophores and determine the mechanism of browning in highly acidified α-pinene SOA by following the behavior of specific common α-pinene oxidation products, namely, cis-pinonic acid and cis-pinonaldehyde, when they are exposed to highly acidic conditions. The products of these reactions were analyzed with ultra-performance liquid chromatography coupled with photodiode array spectrophotometry and high-resolution mass spectrometry, UV-vis spectrophotometry, and nuclear magnetic resonance spectroscopy. cis-Pinonic acid (2) was found to form homoterpenyl methyl ketone (4), which does not absorb visible radiation, while cis-pinonaldehyde (3) formed weakly absorbing 1-(4-(propan-2-ylidene)cyclopent-1-en-1-yl)ethan-1-one (5) and 1-(4-isopropylcyclopenta-1,3-dien-1-yl)ethan-1-one (6) via an acid-catalyzed aldol condensation. This chemistry could be relevant for environments characterized by high sulfuric acid concentrations, for example, during the transport of organic compounds from the lower to the upper atmosphere by fast updrafts.

Water Plays Multifunctional Roles in the Intervening Formation of Secondary Organic Aerosols in Ozonolysis of Limonene: A Valence Photoelectron Spectroscopy and Density Functional Theory Study

Although water may affect aqueous aerosol chemistry, how it intervenes in the formation of secondary organic aerosols (SOAs) at the molecular level remains elusive. Ozonolysis of limonene is one of the most important sources of indoor SOAs. Here, we report the valence electronic properties of limonene aerosols and SOAs derived from limonene ozonolysis (Lim-SOAs) via aerosol vacuum ultraviolet photoelectron spectroscopy, with a focus on the effects of water on Lim-SOAs. The first vertical ionization energy of limonene aerosols is measured to be 8.79 ± 0.07 eV. While water significantly increases the total photoelectron yield of Lim-SOAs, three photoelectron features attributable to Lim-SOAs each exhibit distinct dependence on the fraction of water in aerosols, implying that different formation pathways and molecular origins are involved in the formation of Lim-SOAs. Combined with density functional theory calculation and mass spectrometry measurements, this study reveals that water, particularly the water dimer, enhances the formation of Lim-SOAs by altering the ozonolysis energetics and pathways by intervening in its Criegee chemistry, acting as both a catalyst and a reactant. The atmospheric implication is discussed.

Stability of Terpenoid-Derived Secondary Ozonides in Aqueous Organic Media

1,2,4-Trioxolanes, known as secondary ozonides (SOZs), are key products of ozonolysis of biogenic terpenoids. Functionalized terpenoid-derived SOZs are readily taken up into atmospheric aerosols; however, their condensed-phase fates remain unknown. Here, we report the results of a time-dependent mass spectrometric investigation into the liquid-phase fates of C₁₀ and C₁₃ SOZs synthesized by ozonolysis of a C₁₀ monoterpene alcohol (α-terpineol) in water:acetone (1:1 = vol:vol) mixtures. Isomerization of Criegee intermediates and bimolecular reaction of Criegee intermediates with acetone produced C₁₀ and C₁₃ SOZs, respectively, which were detected as their Na⁺-adducts by positive-ion electrospray mass spectrometry. Use of CD₃COCD₃, D₂O, and H₂¹⁸O solvents enabled identification of three types of C₁₃ SOZs (aldehyde, ketone, and lactol) and other products. These SOZs were surprisingly stable in water:acetone (1:1) mixtures at T = 298 K, with some persisting for at least a week. Theoretical calculations supported the high stability of the lactol-type C₁₃ SOZ formed from the aldehyde-type C₁₃ SOZ via intramolecular rearrangement. The present results suggest that terpenoid-derived SOZs can persist in atmospheric condensed phases, potentially until they are delivered to the epithelial lining fluid of the pulmonary alveoli via inhaled particulate matter, where they may exert hitherto unrecognized adverse health effects.

Decomposition of multifunctionalized α-alkoxyalkyl-hydroperoxides derived from the reactions of Criegee intermediates with diols in liquid phases

The oxidation of volatile organic compounds in the atmosphere produces organic hydroperoxides (ROOHs) that typically possess not only -OOH but also other functionalities such as -OH and -C(O). Because of their high hydrophilicity and low volatility, such multifunctionalized ROOHs are expected to be taken up in atmospheric condensed phases such as aerosols and fog/cloud droplets. However, the characteristics of ROOHs that control their fates and lifetimes in liquid phases are poorly understood. Here, we report a study of the liquid-phase decomposition kinetics of multifunctionalized α-alkoxyalkyl-hydroperoxides (α-AHs) that possessed an ether, a carbonyl, a hydroperoxide, and two hydroxy groups. These ROOHs were synthesized by ozonolysis of α-terpineol in water in the presence of 1,3-propanediol, 1,4-butanediol, or 1,5-pentanediol. Their decomposition products were detected as chloride anion adducts by electrospray mass spectrometry as a function of reaction time. Experiments using H₂¹⁸O and D₂O revealed that hemiacetal species were α-AH decomposition products that further transformed into other products. The result that the rate coefficients (k) of the decomposition of C₁₅ α-AHs increased exponentially from pH 5.0 to 3.9 was consistent with an H⁺-catalyzed decomposition mechanism. The temperature dependence of k and an Arrhenius plot yielded activation energies (E_a) of 15.7 ± 0.8, 15.0 ± 2.4, and 15.9 ± 0.3 kcal mol^-1 for the decomposition of α-AHs derived from the reaction of α-terpineol CIs with 1,3-propanediol, 1,4-butanediol, and 1,5-pentanediol, respectively. The determined E_a values were compared with those of related ROOHs. We found that alkyl chain length is not a critical factor for the decomposition mechanism, whereas the presence of additional -OH groups would modulate the reaction barriers to decomposition via the formation of hydrogen-bonding with surrounding water molecules. The derived E_a values for the decomposition of the multifunctionalized, terpenoid-derived α-AHs will facilitate atmospheric modeling by serving as representative values for ROOHs in atmospheric condensed phases.

Ozonation of organic compounds in water and wastewater: A critical review

Ozonation has been applied in water treatment for more than a century, first for disinfection, later for oxidation of inorganic and organic pollutants. In recent years, ozone has been increasingly applied for enhanced municipal wastewater treatment for ecosystem protection and for potable water reuse. These applications triggered significant research efforts on the abatement efficiency of organic contaminants and the ensuing formation of transformation products. This endeavor was accompanied by developments in analytical and computational chemistry, which allowed to improve the mechanistic understanding of ozone reactions. This critical review assesses the challenges of ozonation of impaired water qualities such as wastewaters and provides an up-to-date compilation of the recent kinetic and mechanistic findings of ozone reactions with dissolved organic matter, various functional groups (olefins, aromatic compounds, heterocyclic compounds, aliphatic nitrogen-containing compounds, sulfur-containing compounds, hydrocarbons, carbanions, β-diketones) and antibiotic resistance genes.

Decomposition mechanism of α-alkoxyalkyl-hydroperoxides in the liquid phase: temperature dependent kinetics and theoretical calculations

Organic hydroperoxides (ROOHs) play key roles in the atmosphere as a reactive intermediate species. Due to the low volatility and high hydrophilicity, ROOHs are expected to reside in atmospheric condensed phases such as aerosols, fogs, and cloud droplets. The decomposition mechanisms of ROOHs in the liquid phase are, however, still poorly understood. Here we report a temperature-dependent kinetics and theoretical calculation study of the aqueous-phase decompositions of C₁₂ or C₁₃ α-alkoxyalkyl-hydroperoxides (α-AHs) derived from ozonolysis of α-terpineol in the presence of 1-propanol, 2-propanol, and ethanol. We found that the temporal profiles of α-AH signals, detected as chloride-adducts by negative ion electrospray mass spectrometry, showed single-exponential decay, and the derived first-order rate coefficient k for α-AH decomposition increased as temperature increased, e.g., k(288 K) = (5.3 ± 0.2) × 10^-4 s^-1, k(298 K) = (1.2 ± 0.3) × 10^-3 s^-1, k(308 K) = (2.1 ± 1.4) × 10^-3 s^-1 for C₁₃ α-AHs derived from the reaction of α-terpineol Criegee intermediates with 1-propanol in the solution at pH 4.5. Arrhenius plot analysis yielded an activation energy (E _a) of 12.3 ± 0.6 kcal mol^-1. E _a of 18.7 ± 0.3 and 13.8 ± 0.9 kcal mol^-1 were also obtained for the decomposition of α-AHs (at pH 4.5) derived from the reaction of α-terpineol Criegee intermediates with 2-propanol and with ethanol, respectively. Based on the theoretical kinetic and thermodynamic calculations, we propose that a proton-catalyzed mechanism plays a central role in the decomposition of these α-AHs in acidic aqueous organic media, while water molecules may also participate in the decomposition pathways and affect the kinetics. The decomposition of α-AHs could act as a source of H₂O₂ and multifunctionalized species in atmospheric condensed phases.

Surfactant-Assisted Ozonolysis of Alkenes in Water: Mitigation of Frothing Using Coolade as a Low-Foaming Surfactant

Aqueous-phase ozonolysis in the atmosphere is an important process during cloud and fog formation. Water in the atmosphere acts as both a reaction medium and a reductant during the ozonolysis. Inspired by the atmospheric aqueous-phase ozonolysis, we herein report the ozonolysis of alkenes in water assisted by surfactants. Several types of surfactants, including anionic, cationic, and nonionic surfactants, were investigated. Although most surfactants enhanced the solubility of alkenes in water, they also generated excessive foaming during the ozone bubbling, which led to the loss of products. Mitigation of the frothing was accomplished by using Coolade as a nonionic and low-foaming surfactant. Coolade-assisted ozonolysis of alkenes in water provided the desired carbonyl products in good yields and comparable to those achieved in organic solvents. During the ozonolysis reaction, water molecules trapped within the polyethylene glycol region of Coolade were proposed to intercept the Criegee intermediate to provide a hydroxy hydroperoxide intermediate. Decomposition of the hydroxy hydroperoxide led to formation of the carbonyl product without the need for a reductant typically required for the conventional ozonolysis using organic solvents. This study presents Coolade as an effective surfactant to improve the solubility of alkenes while mitigating frothing during the ozonolysis in water.

Effects of Metal Ions on Aqueous-Phase Decomposition of α-Hydroxyalkyl-Hydroperoxides Derived from Terpene Alcohols

We report the results of a mass spectrometric study of the effects of atmospherically relevant metal ions on the decomposition of α-hydroxyalkyl-hydroperoxides (α-HHs) derived from ozonolysis of α-terpineol in aqueous solutions. By direct mass spectrometric detection of chloride adducts of α-HHs, we assessed the temporal profiles of α-HHs and other products in the presence of metal ions. In addition, reactions between α-HHs and FeCl₂ in the presence of excess DMSO showed that the amount of hydroxyl radicals formed in a mixture of α-terpineol, O₃, and FeCl₂ was 5.7 ± 0.8% of the amount formed in a mixture of H₂O₂ and FeCl₂. The first-order rate constants for the decay of α-HHs produced by ozonolysis of α-terpineol in the presence of 5 mM acetate buffer at a pH of 5.1 ± 0.1 were determined to be (4.5 ± 0.1) × 10^-4 s^-1 (no metal ions), (4.7 ± 0.2) × 10^-4 s^-1 (with 0.05 mM Fe²⁺), (4.7 ± 0.1) × 10^-4 s^-1 (with 0.05 mM Zn²⁺), and (4.8 ± 0.2) × 10^-4 s^-1 (with 0.05 mM Cu²⁺). We propose that in acidic aqueous media, the reaction of α-HHs with Fe²⁺ is outcompeted by H⁺-catalyzed decomposition of α-HHs, which produces the corresponding aldehydes and H₂O₂, which can in turn react with Fe²⁺ to form hydroxyl radicals.

Aqueous-phase fates of α-alkoxyalkyl-hydroperoxides derived from the reactions of Criegee intermediates with alcohols

In the atmosphere, carbonyl oxides known as Criegee intermediates are produced mainly by ozonolysis of volatile organic compounds containing C[double bond, length as m-dash]C double bonds, such as biogenic terpenoids. Criegee intermediates can react with OH-containing species to produce labile organic hydroperoxides (ROOHs) that are taken up into atmospheric condensed phases. Besides water, alcohols are an important reaction partner of Criegee intermediates and can convert them into α-alkoxyalkyl-hydroperoxides (α-AHs), R1R2C(-OOH)(-OR'). Here, we report a study on the aqueous-phase fates of α-AHs derived from ozonolysis of α-terpineol in the presence of methanol, ethanol, 1-propanol, and 2-propanol. The α-terpineol α-AHs and the decomposition products were detected as their chloride adducts by electrospray mass spectrometry as a function of reaction time. Our discovery that the rate of decomposition of α-AHs increased as the pH decreased from 5.9 to 3.8 implied that the decomposition mechanism was catalyzed by H+. The use of isotope solvent experiments revealed that a primary decomposition product of α-AHs in an acidic aqueous solution was a hemiacetal R1R2C(-OH)(-OR') species that was further transformed into other products such as lactols. The proposed H+-catalyzed decomposition of α-AHs, which provides H2O2 and multifunctional species in ambient aerosol particles, may be faster than other degradation processes (e.g., photolysis by solar radiation).

Superoxide Formation from Aqueous Reactions of Biogenic Secondary Organic Aerosols

Reactive oxygen species (ROS) play a central role in aqueous-phase processing and health effects of atmospheric aerosols. Although hydroxyl radical (^•OH) and hydrogen peroxide (H₂O₂) are regarded as major oxidants associated with secondary organic aerosols (SOA), the kinetics and reaction mechanisms of superoxide (O₂^•-) formation are rarely quantified and poorly understood. Here, we demonstrate a dominant formation of O₂^•- with molar yields of 0.01-0.03% from aqueous reactions of biogenic SOA generated by ^•OH photooxidation of isoprene, β-pinene, α-terpineol, and d-limonene. The temporal evolution of ^•OH and O₂^•- formation is elucidated by kinetic modeling with a cascade of aqueous reactions including the decomposition of organic hydroperoxides, ^•OH oxidation of primary or secondary alcohols, and unimolecular decomposition of α-hydroxyperoxyl radicals. Relative yields of various types of ROS reflect a relative abundance of organic hydroperoxides and alcohols contained in SOA. These findings and mechanistic understanding have important implications on the atmospheric fate of SOA and particle-phase reactions of highly oxygenated organic molecules as well as oxidative stress upon respiratory deposition.

Temperature Dependence of Aqueous-Phase Decomposition of α-Hydroxyalkyl-Hydroperoxides

Ozonolysis of unsaturated organic species with water produces α-hydroxyalkyl-hydroperoxides (α-HHs), which are reactive intermediates that lead to the formation of H₂O₂ and multifunctionalized species in atmospheric condensed phases. Here, we report temperature-dependent rate coefficients (k) for the aqueous-phase decomposition of α-terpineol α-HHs at 283-318 K and terpinen-4-ol α-HHs at 313-328 K. The temporal profiles of α-HH signals, detected as chloride adducts by negative-ion electrospray mass spectrometry, showed single-exponential decay, and the derived first-order k for α-HH decomposition increased as temperature increased, e.g., k(288 K) = (4.7 ± 0.2) × 10^-5, k(298 K) = (1.5 ± 0.4) × 10^-4, k(308 K) = (3.4 ± 0.9) × 10^-4, k(318 K) = (1.0 ± 0.2) × 10^-3 s^-1 for α-terpineol α-HHs at pH 6.1. Arrhenius plot analysis yielded activation energies of 17.9 ± 0.7 (pH 6.1) and 17.1 ± 0.2 kcal mol^-1 (pH 6.2) for the decomposition of α-terpineol and terpinen-4-ol α-HHs, respectively. Activation energies of 18.6 ± 0.2 and 19.2 ± 0.5 kcal mol^-1 were also obtained for the decomposition of α-terpineol α-HHs in acidified water at pH 5.3 and 4.5, respectively. Theoretical kinetic and thermodynamic calculations confirmed that both water-catalyzed and proton-catalyzed mechanisms play important roles in the decomposition of these α-HHs. The relatively strong temperature dependence of k suggests that the lifetime of these α-HHs in aqueous phases (e.g., aqueous aerosols, fog, cloud droplets, wet films) is controlled not only by the water content and pH but also by the temperature of these media.

Indoor aerosol water content and phase state in U.S. residences: impacts of relative humidity, aerosol mass and composition, and mechanical system operation

Hygroscopic particulate matter (PM) constituents promote uptake of aerosol water (AW), depending on relative humidity (RH), which can constrain qualities such as organic aerosol (OA) phase state and inorganic aerosol (IA) deliquescence and efflorescence. This work provides a first incorporation of AW predictions into residential indoor PM simulations. The indoor model, IMAGES, which simulates factored OA concentrations and thermodynamics using the 2D-volatility basis set, was expanded to predict speciated IA concentrations, AW with κ-Köhler theory of hygroscopic growth, and OA phase state with glass transition temperatures. Since RH is the largest driver of AW and varies with meteorology, simulations were conducted using a database of historical ambient weather and pollution records spanning the sixteen U.S. climate zones, facilitating assessment of seasonal and regional trends. Over this diverse simulation set, the residential indoor AW mass was ∼10 to 100 times smaller than dry PM mass. This relative AW amount indoors was about ∼10 times smaller than outdoors, since indoor-emitted aerosol is likely less hygroscopic. The indoor OA phase state was typically semisolid, suggesting kinetic limitations might inhibit thermodynamic OA partitioning equilibrium from being established indoors. Residences in hot and humid climates during the summertime may have liquid indoor OA, while amorphous solid indoor OA can exist in cold climates. Deliquescence and efflorescence of recirculated IA within HVAC systems during cooling or heating, respectively, was also modeled. Oftentimes, two IA populations with different histories existing as wet or dry aerosol were generated by HVAC operation depending on indoor and outdoor environmental conditions and the HVAC operating mode.

Proton-Catalyzed Decomposition of α-Hydroxyalkyl-Hydroperoxides in Water

In the atmosphere, most biogenic terpenes undergo ozonolysis in the presence of water to form reactive α-hydroxyalkyl-hydroperoxides (α-HHs), and the lifetimes of these α-HHs are a key parameter for understanding the processes that occur during the aging of atmospheric particles. We previously reported that α-HHs generated by ozonolysis of terpenes decompose in water to give H₂O₂ and the corresponding aldehydes, which undergo hydration to form gem-diols. Herein, we report that this decomposition process was dramatically accelerated by acidification of the water with oxalic, acetic, hexanoic, cis-pinonic, or hydrochloric acid. In acidic solution, the temporal profiles of the α-HHs, detected as their chloride adducts by electrospray mass spectrometry, showed single-exponential decays in the pH range from 4.1 to 6.1, and the first-order rate coefficients (k) for the decays increased with decreasing pH. The lifetime of the α-HH derived from α-terpineol was 128 min (k = (1.3 ± 0.4) × 10^-4 s^-1) at pH 6.1 but only 8 min (k = (2.1 ± 0.1) × 10^-3 s^-1) at pH 4.1. Because the rate coefficients increased as the pH decreased and the increase depended on pH rather than on the properties of the acid, we propose that the decomposition of the α-HHs in water was specifically catalyzed by H⁺. Fast H⁺-catalyzed decomposition of α-HHs could be an important source of H₂O₂ and multifunctionalized compounds found in ambient atmospheric particles.

Effects of pH on Interfacial Ozonolysis of α-Terpineol

Acidity changes the physical properties of atmospheric aerosol particles and the mechanisms of reactions that occur therein and on the surface. Here, we used surface-sensitive pneumatic ionization mass spectrometry to investigate the effects of pH on the heterogeneous reactions of aqueous α-terpineol (C₁₀H₁₇OH), a representative monoterpene alcohol, with gaseous ozone. Rapid (≤10 μs) ozonolysis of α-terpineol produced Criegee intermediates (CIs, zwitterionic/diradical carbonyl oxides) on the surface of water microjets. We studied the effects of microjet bulk pH (1-11) on the formation of functionalized carboxylate and α-hydroxy-hydroperoxide chloride adduct (HH-Cl^-) products generated by isomerization and hydration of α-terpineol CIs, respectively. Compared with the signal at pH ≈ 6, the mass spectral signal of HH-Cl^- was less intense under both basic and more acidic conditions, whereas the intensity of the functionalized carboxylate signal increased with increasing pH up to 4 and then remained constant. The decrease of HH-Cl^- signals at bulk pH values of >6 is attributable to the accumulation of OH^- at the air-water interface that suppresses the relative abundance of hydrophilic HH and Cl^-. The present study suggests that α-terpineol in ambient aqueous organic aerosols will be converted into much lower volatile and potentially toxic organic hydroperoxides during the heterogeneous ozonolysis.

Degradation of α-terpineol in aqueous solution by UV/H2O2: kinetics, transformation products and pathways

The monoterpene alcohol α-terpineol is extensively used as the foaming agent in mineral processing and can be released to environment along with the wastewater. This study evaluated the feasibility of eliminating α-terpineol in water by ultraviolet irradiation (UV) in combination with hydrogen peroxide (H₂O₂). Within an H₂O₂ dose of 10 mg/L and an UV fluence of 64.8 J/cm², more than 95% of the α-terpineol can be removed. The reactions fitted well to pseudo-first-order kinetics, and the apparent rate constant was 0.0678 min^-1. The effects of matrix species including various anions and humic acid (HA), were evaluated. The degradation rate decreased significantly with the addition of bicarbonate and HA. Further verification was carried out with three types of real water samples. In the ground water and the surface water, the degradation rate decreased likely due to the presence of natural organic matter. Finally, possible degradation pathways were proposed based on the identification of transformation products, and the occurrence of two main transformation products were monitored. This study demonstrated that the UV/H₂O₂ is an effective technology for the degradation of α-terpineol in water.

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.