OH-Initiated Oxidation of Imidazoles in Tropospheric Aqueous-Phase Chemistry

OH Radical Oxidation of Organosulfates in the Atmospheric Aqueous Phase

Organosulfates (OS, ROSO3-), ubiquitous constituents of atmospheric particulate matter (PM), influence both the physicochemical and climatic properties of PM. Although the formation pathways of OS have been extensively researched, only a few studies have investigated the atmospheric fate of this class of compounds. Here, to better understand the reactivity and transformation of OS under cloudwater- and aerosol-relevant conditions, we investigate the hydroxyl radical (OH) oxidation bimolecular rate constants (kOS+OHII) and products of five atmospherically relevant OS as a function of pH and ionic strength: methyl sulfate (MeS), ethyl sulfate (EtS), propyl sulfate (PrS), hydroxyacetone sulfate (HaS) and phenyl sulfate (PhS). Our results show that OS are oxidized by OH with kOS+OHII between 108 - 109 M-1 s-1, which corresponds to atmospheric lifetimes of minutes in aqueous aerosol to days in cloudwater. We find that kOS+OHII increases with carbon chain length (MeS < EtS < PrS) and aromaticity (PrS < PhS), but does not depend on solution pH (2, 9). In addition, we find that whereas the OH reactivity of the aliphatic OS studied here decreases by ∼2× with increasing ionic strength (0-15 M), the reactivity of PhS decreases by ∼10×. The oxidation of EtS and PrS produced organic peroxides (ROOH) as first-generation oxidation products, which subsequently photolyzed; the oxidation of PhS resulted in hydroxylated aromatic products. These results highlight the need for inclusion of OS loss pathways in atmospheric models, and suggest caution in using ambient OS concentration measurements alone to estimate their production rates.

Theoretical analysis of the OH-initiated atmospheric oxidation reactions of imidazole

Imidazoles are present in Earth's atmosphere in both the gas-phase and in aerosol particles, and have been implicated in the formation of brown carbon aerosols. The gas-phase oxidation of imidazole (C3N2H4) by hydroxyl radicals has been shown to be preferentially initiated via OH-addition to position C5, producing the 5-hydroxyimidazolyl radical adduct. However, the fate of this adduct upon reaction with O2 in the atmospheric gas-phase is currently unknown. We employed an automated approach to investigate the reaction mechanism and kinetics of imidazole's OH-initiated gas-phase oxidation, in the presence of O2 and NOx. The explored mechanism included reactions available to first-generation RO2 radicals, as well as alkoxyl radicals produced from RO2 + NO reactions. Product distributions were obtained by assembling and solving a master equation, under conditions relevant to the Earth's atmosphere. Our calculations show a complex, branched reaction mechanism, which nevertheless converges to yield two major closed-shell products: 4H-imidazol-4-ol (4H-4ol) and N,N'-diformylformamidine (FMF). At 298 K and 1 atm, we estimate the yields of 4H-4ol and FMF from imidazole oxidation initiated via OH-addition to position C5 to be 34 : 66, 12 : 85 and 2 : 95 under 10 ppt, 100 ppt and 1 ppb of NO respectively. This work also revealed O2-migration pathways between the α-N-imino peroxyl radical isomers. This reaction channel is fast for the first-generation RO2 radicals, and may be important during the atmospheric oxidation of other unsaturated organic nitrogen compounds as well.

Photochemistry of Imidazole-2-carbaldehyde in Droplets as a Potential Source of H2O2 and Its Oxidation of SO2

Hydrogen peroxide (H2O2) plays a crucial role as an oxidizing agent within the tropospheric environment, making a substantial contribution to sulfate formation in hydrated aerosols and cloud and fog droplets. Field observations show that high levels of H2O2 are often observed in heavy haze events and polluted air. However, the source of H2O2 remains unclear. Here, using the droplets formed in situ by the deliquescence of hygroscopic compounds under a high relative humidity (RH), the formation of H2O2 by the photochemistry of imidazole-2-carbaldehyde (2-IC) under ultraviolet irradiation was explored. The results indicate that 2-IC produces IM-C•-OH and IM-C•═O radicals via H transfer itself to its excited triplet state and generates H2O2 and organic peroxides in the presence of O2, which has an evident oxidizing effect on SO2, suggesting the potential involvement of this pathway in the formation of atmospheric sulfate. H2O2 formation is limited in acidic droplets or droplets containing ammonium ions, and no H2O2 is detected in droplets containing nitrate, whereas droplets containing citric acid have an obvious promotion effect on H2O2 formation. These findings provide valuable insights into the behaviors of atmospheric photosensitizers, the source of H2O2, and the formation of sulfate in atmospheric droplets.

Structure and Electron Configuration of Imidazole-2-carboxaldehyde and Its Excited Triplet: Resonance Raman and Transient Absorption Spectroscopy and DFT Calculation Investigations

Imidazole-2-carboxaldehyde (IC) can be generated in atmospheric waters and absorbs solar radiation in the near UV region to produce its excited triplet state (3IC), which contributes to the formation of a secondary organic aerosol (SOA). The photoreactivity of IC is significantly influenced by its surroundings, such as water and acidic environment, because IC is capable of transforming into gem-diol under above conditions. Meanwhile, the electron configuration of 3IC is critical in elucidating the reaction mechanism of 3IC with other anthropogenic and biogenic volatile organic compounds (VOCs). In this study, steady-state and time-resolved resonance Raman as well as transient absorption spectroscopic experiments were conducted to provide vibrational and kinetic information on IC and 3IC in the presence of water and acid conditions. Using density functional theory (DFT) calculations, the H-bonding at the carbonyl O was confirmed and the hydrated structure of IC and 3IC was determined. 1,4-Cyclohexadiene is a good hydrogen donor, and it has a second-order rate constant of ∼107 M-1 s-1 toward 3IC. The results of CASSCF calculations suggest that the hydrogen abstraction may involve the transition from the ππ* to nπ* triplet state via the surface-crossing point.

Ligand-Controlled Growth of Different Morphological Bimetallic Metal-Organic Frameworks for Enhanced Charge-Storage Performance and Quasi-Solid-State Hybrid Supercapacitors

The present research work facilitates a ligand-mediated effective strategy to achieve different morphological surface structures of bimetallic (Ni and Co) metal-organic frameworks (MOFs) by utilizing different types of organic ligands like terephthalic acid (BDC), 2-methylimidazole (2-Melm), and trimesic acid (BTC). Different morphological structures, rectangular-like nanosheets, petal-like nanosheets, and nanosheet-assembled flower-like spheres (NSFS) of NiCo MOFs, are confirmed from the structural characterization for ligands BDC, 2-Melm, and BTC, respectively. The basic characterization studies like scanning electron microscopy, X-ray diffraction, transmission electron microscopy, and Brunauer-Emmett-Teller revealed that the NiCo MOF prepared by using trimesic acid as the ligand (NiCo MOF_BTC) with a long organic linker exhibits a three-dimensional architecture of NSFS that possesses higher surface area and pore dimensions, which enables better ion kinetics. Also, the NiCo MOF_BTC delivered the highest capacity of 1471.4 C g^-1 (and 408 mA h g^-1) at 1 A g^-1 current density, compared to the other prepared NiCo MOFs and already reported different NiCo MOF structures. High interaction of trimesic acid with the metal ions confirmed from ultraviolet-visible spectroscopy and X-ray photoelectron spectroscopy leads to a NSFS structure of NiCo MOF_BTC. For practical application, an asymmetric supercapacitor device (NiCo MOF_BTC//AC) is fabricated by taking NiCo MOF_BTC and activated carbon as the positive and negative electrode, respectively, where the PVA + KOH gel electrolyte serves as a separator as well as an electrolyte. The device delivered an outstanding energy density of 78.1 Wh kg^-1 at a power density of 750 W kg^-1 in an operating potential window of 1.5 V. In addition, it displays a long cycle life of 5000 cycles with only 12% decay of the initial specific capacitance. Therefore, these findings manifest the morphology control of MOFs by using different ligands and the mechanism behind the different morphologies that will provide an effective way to synthesize differently structured MOF materials for future energy-storage applications.

Synthesis of Small Peptide Nanogels Using Radiation Crosslinking as a Platform for Nano-Imaging Agents for Pancreatic Cancer Diagnosis

Nanoparticle-based drug delivery systems (DDS) have been developed as effective diagnostic and low-dose imaging agents. Nano-imaging agents with particles greater than 100 nm are difficult to accumulate in pancreatic cancer cells, making high-intensity imaging of pancreatic cancer challenging. Peptides composed of histidine and glycine were designed and synthesized. Additionally, aqueous peptide solutions were irradiated with γ-rays to produce peptide nanogels with an average size of 25-53 nm. The mechanisms underlying radiation-mediated peptide crosslinking were investigated by simulating peptide particle formation based on rate constants. The rate constants for reactions between peptides and reactive species produced by water radiolysis were measured using pulse radiolysis. HGGGHGGGH (H9, H-histidine; G-glycine) particles exhibited a smaller size, as well as high formation yield, stability, and biodegradability. These particles were labeled with fluorescent dye to change their negative surface potential and enhance their accumulation in pancreatic cancer cells. Fluorescent-labeled H9 particles accumulated in PANC1 human pancreatic cancer cells, demonstrating that these particles are effective nano-imaging agents for intractable cancers.

T- and pH-dependent OH radical reaction kinetics with glycine, alanine, serine, and threonine in the aqueous phase

Glycine, alanine, serine, and threonine are essential amino acids originating from biological activities. These substances can be emitted into the atmosphere directly. In the present study, the aqueous phase reaction kinetics of hydroxyl radicals (˙OH) with the four amino acids is investigated using the competition kinetics method under controlled temperature and pH conditions. The following T-dependent Arrhenius expressions are derived for the ˙OH reactions with glycine, k(T, H₂A⁺) = (9.1 ± 0.3) × 10⁹ × exp[(-2360 ± 230 K)/T], k(T, HA^±) = (1.3 ± 0.1) × 10¹⁰ × exp[(-2040 ± 240 K)/T]; alanine, k(T, H₂A⁺) = (1.4 ± 0.1) × 10⁹ × exp[(-1120 ± 320 K)/T], k(T, HA^±) = (5.5 ± 0.2) × 10⁹ × exp[(-1300 ± 200 K)/T]; serine, k(T, H₂A⁺) = (1.1 ± 0.1) × 10⁹ × exp[(-470 ± 150 K)/T], k(T, HA^±) = (3.9 ± 0.1) × 10⁹ × exp[(-720 ± 130 K)/T]; and threonine, k(T, H₂A⁺) = (5.0 ± 0.1) × 10¹⁰ × exp[(-1500 ± 100 K)/T], k(T, HA^±) = (3.3 ± 0.1) × 10¹⁰ × exp[(-1320 ± 90 K)/T] (in units of L mol^-1 s^-1). The energy barriers of the ˙OH-induced H atom abstractions were simulated by the density functional theory (DFT) calculation performed with GAUSSIAN using the method of M06-2X and the basis set of 6-311++G(3df,2p). According to the calculation results, the -COOH and -NH₃⁺ groups with strong negative inductive effects increase the energy barriers and thus decrease the ˙OH reaction rate constants. In contrast, the presence of a -OH or -CH₃ group with weak negative or positive inductive effects can reduce energy barriers and hence increase the ˙OH reaction rate constants. To improve the previous structure-activity relationship, the contribution factors of -NH₃⁺ at C_α-atom and C_β-atom are determined as 0.07 and 0.15, respectively. Aqueous phase ˙OH oxidation acts as an important sink of the amino acids in the atmosphere, and can be accurately described by the obtained Arrhenius expressions under atmospheric conditions.

Photosensitization mechanisms at the air-water interface of aqueous aerosols

Photosensitization reactions are believed to provide a key contribution to the overall oxidation chemistry of the Earth's atmosphere. Generally, these processes take place on the surface of aqueous aerosols, where organic surfactants accumulate and react, either directly or indirectly, with the activated photosensitizer. However, the mechanisms involved in these important interfacial phenomena are still poorly known. This work sheds light on the reaction mechanisms of the photosensitizer imidazole-2-carboxaldehyde through ab initio (QM/MM) molecular dynamics simulations and high-level ab initio calculations. The nature of the lowest excited states of the system (singlets and triplets) is described in detail for the first time in the gas phase, in bulk water, and at the air-water interface, and possible intersystem crossing mechanisms leading to the reactive triplet state are analyzed. Moreover, the reactive triplet state is shown to be unstable at the air-water surface in a pure water aerosol. The combination of this finding with the results obtained for simple surfactant-photosensitizer models, together with experimental data from the literature, suggests that photosensitization reactions assisted by imidazole-2-carboxaldehyde at the surface of aqueous droplets can only occur in the presence of surfactant species, such as fatty acids, that stabilize the photoactivated triplet at the interface. These findings should help the interpretation of field measurements and the design of new laboratory experiments to better understand atmospheric photosensitization processes.

Insights into high concentrations of particle-bound imidazoles in the background atmosphere of southern China: Potential sources and influencing factors

Imidazoles are important constituents in atmospheric brown carbon and have gained increasing attention in the past decade. Although imidazoles have been studied widely in laboratories, the sparse field observations severely limit the understanding of imidazole's abundance and sources in the atmosphere. In this study, we measured particle-bound imidazoles and their precursors at a background forest site in the Nanling Mountains of southern China. The average concentration of imidazoles (4.17 ± 3.76 ng/m³) was found to be significantly higher than other background sites worldwide. Further analyses revealed that a majority of imidazoles (59.1%) at the site originated from secondary formation through reactions of dicarbonyls (e.g., glyoxal and methylglyoxal) and reduced nitrogen species, with relatively minor contributions from regional transport (32.8%) and biomass burning (8.1%). In addition, the key factors influencing secondary formation of imidazoles, such as relative humidity, water-soluble inorganic ions, and pH, were analyzed. Our results indicated that the secondary formation of imidazoles can be greatly enhanced under high humidity conditions, particularly during fog events. Overall, this study offers valuable insights into potential sources and influencing factors of ambient imidazoles in background atmospheres.

Photochemical Reactions of Glyoxal during Particulate Ammonium Nitrate Photolysis: Brown Carbon Formation, Enhanced Glyoxal Decay, and Organic Phase Formation

Glyoxal is an important precursor of aqueous secondary organic aerosol (aqSOA). Its photooxidation to form organic acids and oligomers and reactions with reduced nitrogen compounds to form brown carbon (BrC) have been extensively investigated separately, although these two types of reactions can occur simultaneously during the daytime. Here, we examine the reactions of glyoxal during photooxidation and BrC formation in premixed NH₄NO₃ + Glyoxal droplets. We find that nitrate photolysis and photosensitization can enhance the decay rates of glyoxal by a factor of ∼5 and ∼6 compared to those under dark, respectively. A significantly enhanced glyoxal decay rate by a factor of ∼12 was observed in the presence of both nitrate photolysis and photosensitization. Furthermore, a new organic phase was formed in irradiated NH₄NO₃ + Glyoxal droplets, which had no noticeable degradation under prolonged photooxidation. It was attributed to the imidazole oxidation mediated by nitrate photolysis and/or photosensitization. The persistent organic phase suggests the potential to contribute to SOA formation in ambient fine particles. This study highlights that glyoxal photooxidation mediated by nitrate photolysis and photosensitization can significantly enhance the atmospheric sink of glyoxal, which may partially narrow the gap between model predictions and field measurements of ambient glyoxal concentrations.

Acidity and the multiphase chemistry of atmospheric aqueous particles and clouds

The acidity of aqueous atmospheric solutions is a key parameter driving both the partitioning of semi-volatile acidic and basic trace gases and their aqueous-phase chemistry. In addition, the acidity of atmospheric aqueous phases, e.g., deliquesced aerosol particles, cloud, and fog droplets, is also dictated by aqueous-phase chemistry. These feedbacks between acidity and chemistry have crucial implications for the tropospheric lifetime of air pollutants, atmospheric composition, deposition to terrestrial and oceanic ecosystems, visibility, climate, and human health. Atmospheric research has made substantial progress in understanding feedbacks between acidity and multiphase chemistry during recent decades. This paper reviews the current state of knowledge on these feedbacks with a focus on aerosol and cloud systems, which involve both inorganic and organic aqueous-phase chemistry. Here, we describe the impacts of acidity on the phase partitioning of acidic and basic gases and buffering phenomena. Next, we review feedbacks of different acidity regimes on key chemical reaction mechanisms and kinetics, as well as uncertainties and chemical subsystems with incomplete information. Finally, we discuss atmospheric implications and highlight the need for future investigations, particularly with respect to reducing emissions of key acid precursors in a changing world, and the need for advancements in field and laboratory measurements and model tools.

Aromatic Carbonyl and Nitro Compounds as Photosensitizers and Their Photophysical Properties in the Tropospheric Aqueous Phase

Secondary organic aerosol formation in the atmospheric aqueous/particulate phase by photosensitized reactions is currently subject to uncertainties. To understand the impact of photosensitized reactions, photophysical and -chemical properties of photosensitizers, kinetic data, and reaction mechanisms of these processes are required. The photophysical properties of acetophenones, benzaldehydes, benzophenones, and naphthalenes were investigated in aqueous solution using laser flash excitation. Quantum yields of excited photosensitizers were determined giving values between 0.06-0.80 at 298 K and pH = 5. Molar absorption coefficients (ε_max(³PS*) = (0.8-13) × 10⁴ L mol^-1 cm^-1), decay rate constants in water (k_1st = (9.4 ± 0.5) × 10² to (2.2 ± 0.1) × 10⁵ s^-1), and quenching rate constants with oxygen (k_q(O₂) = (1.7 ± 0.1-4.4 ± 0.4) × 10⁹ L mol^-1 s^-1) of the excited triplet states were determined at 298 K and pH = 5. Photosensitized reactions of carboxylic acids and alkenes show second-order rate constants in the range of (37 ± 7.0-0.55 ± 0.1) × 10⁴ and (27 ± 5.0-0.04 ± 0.01) × 10⁸ L mol^-1 s^-1. The results show that different compound classes act differently as a photosensitizer and can be a sink for certain organic compounds in the atmospheric aqueous phase.

Five-Membered Heterocycles as Potential Photosensitizers in the Tropospheric Aqueous Phase: Photophysical Properties of Imidazole-2-carboxaldehyde, 2-Furaldehyde, and 2-Acetylfuran

Photosensitized reactions of organic compounds in the atmospheric aqueous and particle phase might be potential sources for secondary organic aerosol (SOA) formation, addressed as aqueous SOA. However, data regarding the photophysical properties of photosensitizers, their kinetics, as well as reaction mechanisms of such processes in the aqueous/particle phase are scarce. The present study investigates the determination of the photophysical properties of imidazole-2-carboxaldehyde, 2-furaldehyde, and 2-acetylfuran as potential photosensitizers using laser flash excitation in aqueous solution. Quantum yields of the formation of the excited photosensitizers were obtained by a scavenging method with thiocyanate, resulting in values between 0.86 and 0.96 at 298 K and pH = 5. The time-resolved absorbance spectra of the excited photosensitizers were measured, and their molar attenuation coefficients were determined ranging between (0.30 and 1.4) × 10⁴ L mol^-1 cm^-1 at their absorbance maxima (λ_max = 335-440 nm). Additionally, the excited photosensitizers are quenched by water and molecular oxygen, resulting in quenching rate constants of k_1st = (1.0 ± 0.2-1.8 ± 0.2) × 10⁵ s^-1 and k_q(O₂) = (2.1 ± 0.2-2.7 ± 0.2) × 10⁹ L mol^-1 s^-1, respectively.

β-FeOOH self-supporting electrode for efficient electrochemical anodic oxidation process

In this work, β-FeOOH was synthesized and grown on carbon paper with the assistance of dopamine (PDA) via a facile hydrothermal method, producing β-FeOOH self-supporting electrode eventually. Electrochemical anodic oxidation performance to methyl orange (MO) solution using β-FeOOH anode was investigated and the major influencing factors such as current density, initial pH value and initial MO concentration on MO degradation efficiency were further explored. Experimental results suggested that 99.4% degradation rate of MO could be achieved only after 25 min electrolysis, its pseudo first-order reaction kinetic constant was 11.3 ⅹ 10^-2 min^-1 and the COD removal ratio was 37.3% after 120 min electrolysis under optimized conditions: current density was 10 mA cm^-2, initial pH value was 3 and initial MO concentration was 10 mg L^-1. At the same time, β-FeOOH electrode also exhibited a high cycling stability and the MO removal ratio was still keeping at 84.9% after eight cycles. Moreover, this electrode showed efficient decomposition performance to multiple simulated pollutants, indicating the well potential practical application values of β-FeOOH electrode. At last, the proposed degradation mechanism of MO was evaluated according to the analyzing results of UV-vis and HPLC-MS to MO solution under different degradation durations.

Mechanism of atmospheric organic amines reacted with ozone and implications for the formation of secondary organic aerosols

Organic amines are one of the most important nitrogen-containing compounds in the atmosphere, and their reactions with tropospheric ozone contribute significantly to the formation of secondary organic aerosols (SOA). However, the chemical pathways of their reaction with atmospheric ozone are poorly understood. This study investigates the atmospheric ozonolysis mechanism of two typical organic amines-diethylamine and triethylamine using experimental and theoretical methods. Intermediate results from GC-MS and PTR-TOF-MS analysis confirm the formation of eight and eleven nitrogen- and oxygen-containing products during the ozonolysis of diethylamine and triethylamine, respectively. N-ethylethanimine (56.5% in average) or acetaldehyde (64.9% in average) is formed as the dominant product from the ozonolysis of each organic amine. Ozonolysis pathway results indicate that the conversion to N-ethylethanimine is the dominant pathway for diethylamine ozonolysis. At the same time, triethylamine prefers the initial transformation to diethylamine with the discharge of acetaldehyde and then converts to N-ethylethanimine. Higher SOA mass concentration is obtained from the ozonolysis of triethylamine than diethylamine, probably because the former releases a larger amount of intermediate products, especially acetaldehyde. Our results provide a deep insight into the atmospheric processing of organic amines via ozonolysis and the implications of this mechanism for SOA formation.

Secondary organic aerosol formation from 3C⁎-initiated oxidation of 4-ethylguaiacol in atmospheric aqueous-phase

In this study, we investigated aqueous-phase triplet excited states (³C^⁎)-induced photo-degradation of 4-ethylguaiacol (EG) under both simulated sunlight and ultraviolet (UV) light irradiations. Through quencher experiments, the relative contributions of reactive oxygen species (ROS, such as ¹O₂/O₂^-/·OH) and ³C^⁎ were calculated and results showed three reactive species, e.g., ³C^⁎, ¹O₂ and O₂^-, all seemed to play important roles in the photo-degradation of EG, but contribution from ·OH was relatively minor. High steady-state ¹O₂ concentration after 1 h irradiation further revealed the major contribution of ¹O₂ to photo-degradation under Xe light irradiation. The degradation experiment under three saturated gases (air, O₂ and N₂) showed that the degradation rate in air-saturated condition was the largest owing to synergistic effect of ¹O₂ and ³C^⁎. Oxidative capacity of aqueous secondary organic aerosol (aqSOA) increased with reaction time by monitoring oxygen-to‑carbon (O/C) ratio and carbon oxidation state (OS_c) via an aerodyne soot particle aerosol mass spectrometer (SP-AMS). Moreover, aqSOA mass yields were calculated via SP-AMS data. The UV-vis spectral change suggested formation of light-absorbing organics at first stage under simulated sunlight irradiation. Based on the identified products and the reactive intermediates, we postulated that ³C^⁎-induced oxidation might be attributed to direct reactions by ³C^⁎ and ¹O₂, chemical reaction by ROS, as well as oligomerization via H-abstraction. To the best of our knowledge, this is the first time to explore systematically reaction pathways of 4-ethylguaiacol under ³C^∗ radical on the basis of thorough analysis of products and reactive species. Our findings highlight the impacts of aqSOA from biomass burning emissions on air quality and climate change.