Secondary organic aerosol from aqueous reactions of green leaf volatiles with organic triplet excited states and singlet molecular oxygen

Aqueous-phase photo-oxidation of selected green leaf volatiles initiated by OH radicals: Products and atmospheric implications

C₅- and C₆- unsaturated oxygenated organic compounds emitted by plants under stress like cutting, freezing or drying, known as Green Leaf Volatiles (GLVs), may clear some of the existing uncertainties in secondary organic aerosol (SOA) budget. The transformations of GLVs are a potential source of SOA components through photo-oxidation processes occurring in the atmospheric aqueous phase. Here, we investigated the aqueous photo-oxidation products from three abundant GLVs (1-penten-3-ol, (Z)-2-hexen-1-ol, and (E)-2-hexen-1-al) induced by OH radicals, carried out in a photo-reactor under simulated solar conditions. The aqueous reaction samples were analyzed using advanced hyphenated mass spectrometry techniques: capillary gas chromatography mass spectrometry (c-GC-MS); and reversed-phase liquid chromatography high resolution mass spectrometry (LC-HRMS). Using carbonyl-targeted c-GC-MS analysis, we confirmed the presence of propionaldehyde, butyraldehyde, 1-penten-3-one, and 2-hexen-1-al in the reaction samples. The LC-HRMS analysis confirmed the presence of a new carbonyl product with the molecular formula C₆H₁₀O₂, which probably bears the hydroxyhexenal or hydroxyhexenone structure. Density functional theory (DFT)-based quantum calculations were used to evaluate the experimental data and obtain insight into the formation mechanism and structures of the identified oxidation products via the addition and hydrogen-abstraction pathways. DFT calculations highlighted the importance of the hydrogen abstraction pathway leading to the new product C₆H₁₀O₂. Atmospheric relevance of the identified products was evaluated using a set of physical property data like Henry's law constant (HLC) and vapor pressure (VP). The unknown product of molecular formula C₆H₁₀O₂ has higher HLC and lower VP than the parent GLV and thus has potential to remain in the aqueous phase leading to possible aqueous SOA formation. Other observed carbonyl products are likely first stage oxidation products and precursors of aged SOA.

Evaluation of Probes to Measure Oxidizing Organic Triplet Excited States in Aerosol Liquid Water

Oxidizing triplet excited states of organic matter (³C*) drive numerous reactions in fog/cloud drops and aerosol liquid water (ALW). Quantifying oxidizing triplet concentrations in ALW is difficult because ³C* probe loss can be inhibited by the high levels of dissolved organic matter (DOM) and copper in particle water, leading to an underestimate of triplet concentrations. In addition, illuminated ALW contains high concentrations of singlet molecular oxygen (¹O₂*), which can interfere with ³C* probes. Our overarching goal is to find a triplet probe that has low inhibition by DOM and Cu(II) and low sensitivity to ¹O₂*. To this end, we tested 12 potential probes from a variety of compound classes. Some probes are strongly inhibited by DOM, while others react rapidly with ¹O₂*. One of the probe candidates, (phenylthiol)acetic acid (PTA), seems well suited for ALW conditions, with mild inhibition and fast rate constants with triplets, but it also has weaknesses, including a pH-dependent reactivity. We evaluated the performance of both PTA and syringol (SYR) as triplet probes in aqueous extracts of particulate matter. While PTA is less sensitive to inhibition than SYR, it results in lower triplet concentrations, possibly because it is less reactive with weakly oxidizing triplets.

Detection of excited triplet species from photolysis of carbonyls: Direct evidence for single oxygen formation in atmospheric environment

Excited triplet species play an important role in the photolytic formation of ¹O₂ from carbonyls, but the related mechanism is still uncertain, due to lack of direct evidence. In this study, steady-state and transient photolysis of eleven carbonyls to produce ¹O₂ was investigated. Dicarbonyl displayed greater ¹O₂ production ability than monocarbonyl, while dicarbonyl containing both ketone and carboxyl groups connected by CC bond (i.e., pyruvic acid (PA)) showed the highest ¹O₂ steady-state concentration ([¹O₂]_SS). For the first time, the production of ³PA* from PA with narrow energy gap was confirmed by laser flash photolysis technique and the second-order decay rate constant of ³PA* was 2.78 × 10⁷ M^-1 s^-1. Quenching results verified the dominant contribution of ³PA* to ¹O₂ production from PA. Addition of inorganic salt or increase in solution pH showed negligible effect on ³PA*, but significantly decreased the [¹O₂]_SS of PA by up to two orders of magnitude, due to reduction of hydrate content. Photolysis of methylglyoxal and dimethylamine mixture led to higher content of excited triplet species at pH ≈ 11 and remarkably enhanced [¹O₂]_SS, which was 2.3 times of that from PA and dimethylamine mixture. These findings provide direct evidence for the contribution of transient species from carbonyls or their product to ¹O₂ formation in atmospheric environment.

Reaction Kinetics of Green Leaf Volatiles with Sulfate, Hydroxyl, and Nitrate Radicals in Tropospheric Aqueous Phase

Green plants exposed to abiotic or biotic stress release C-5 and C-6 unsaturated oxygenated hydrocarbons called Green Leaf Volatiles (GLVs). GLVs partition into tropospheric waters and react to form secondary organic aerosol (SOA). We explored the kinetics of aqueous-phase reactions of 1-penten-3-ol (PENTOL), (Z)-2-hexen-1-ol (HEXOL), and (E)-2-hexen-1-al (HEXAL) with SO4•-, •OH, and NO3•. At 298 K, the rate constants for reactions of PENTOL, HEXOL, and HEXAL with SO4•- were, respectively, (9.4 ± 1.0) × 108 L mol-1 s-1, (2.5 ± 0.3) × 109 L mol-1 s-1, and (4.8 ± 0.2) × 108 L mol-1 s-1; with •OH - (6.3 ± 0.1) × 109 L mol-1 s-1, (6.7 ± 0.3) × 109 L mol-1 s-1, and (4.8 ± 0.3) × 109 L mol-1 s-1; and with NO3• - (1.5 ± 0.15) × 108 L mol-1 s-1, (8.4 ± 2.3) × 108 L mol-1 s-1, and (3.0 ± 0.7) × 107 L mol-1 s-1. The rate constants increased weakly with temperatures ranging from 278 to 318 K. The diffusional limitations of the rate constants appeared significant only for the GLV-•OH reactions. The aqueous-phase reactions appeared negligible in deliquescent aerosol and haze water but not in clouds and rains. The atmospheric lifetimes of GLVs decreased from many days to hours with increasing liquid water content and radicals' concentration.

Evolution of the chromophore aerosols and its driving factors in summertime Xi'an, Northwest China

Atmospheric chromophores have photo-sensitiveness that can participate in photochemical reactions, so they may have the potential to make an important contribution in organic aerosols aging. This study attempts to explain the effects of oxidation reaction and photochemical reaction on atmospheric chromophores. For this study, the summer period (higher sunshine intensity) was selected to observe the mechanisms by the online excitation emission matrix (EEM) fluorescence. The results showed that a lot of secondary organic aerosols were produced in the afternoon, but a large portion of them is non-chromophore. We observed that the secondary chromophores of highly-oxygenated humic-like substances (HULIS) were produced, which suggests a degradation product of less-oxygenated HULIS. The photochemical reaction and oxidation reaction were the important reactions that occur in the afternoon, which drives the oxidation state evolution of the atmospheric chromophores. Atmospheric oxidation processes are the mainly driving reaction for the transformation of atmospheric chromophore. The aged aerosol has a lower fluorescence index and a high degree of humification. It is speculated that the aerosol from night to morning is in the accumulation process dominated by local sources, and then it is mainly in the process of being gradually aged at noon and afternoon. This study will guide to better understand the atmospheric chemical processes of chromophore aerosols and provide guidance for the EEM approach to trace the aerosol aging in the atmosphere.

Diurnal evolutions and sources of water-soluble chromophoric aerosols over Xi'an during haze event, in Northwest China

Atmospheric brown carbon and their chemical behavior potentially impacts the climate and air quality. Due to lack of researches on the atmospheric chromophores by using online experimental instrument, so using the offline EEM approaches to study their types, sources and chemical processes. In this study, PILS-EEM-TOC system (Particle into liquid sampler coupled with excitation-emission matrix and total organic carbon) was developed in order to distinguish the hourly evolutions and sources of water-soluble chromophoric organic matters in atmospheric fine particles. The results suggested that the sources of atmospheric chromophores in winter were primary combustion (~90%) and coal burning, followed by biomass burning and cooking emissions in Xi'an (Northwest China). These atmospheric chromophores decay under the combined action of solar radiation and atmospheric oxidants. Meanwhile, the secondary chromophores were mainly highly-oxygenated humic-like substance (HULIS), produced by atmospheric oxidation reactions with the highest peak in the afternoon. The partly secondary chromophores can also be generated through the Maillard-like reaction in the morning, which depends on the relative humidity of the atmosphere. These findings made a deeper understanding of the sources and transformation of atmospheric brown carbon aerosols.

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.

Singlet Oxygen Quantum Yields in Environmental Waters

Singlet oxygen (¹O₂) is a reactive oxygen species produced in sunlit waters via energy transfer from the triplet states of natural sensitizers. There has been an increasing interest in measuring apparent ¹O₂ quantum yields (Φ_Δ) of aquatic and atmospheric organic matter samples, driven in part by the fact that this parameter can be used for environmental fate modeling of organic contaminants and to advance our understanding of dissolved organic matter photophysics. However, the lack of reproducibility across research groups and publications remains a challenge that significantly limits the usability of literature data. In the first part of this review, we critically evaluate the experimental techniques that have been used to determine Φ_Δ values of natural organic matter, we identify and quantify sources of errors that potentially explain the large variability in the literature, and we provide general experimental recommendations for future studies. In the second part, we provide a qualitative overview of known Φ_Δ trends as a function of organic matter type, isolation and extraction procedures, bulk water chemistry parameters, molecular and spectroscopic organic matter features, chemical treatments, wavelength, season, and location. This review is supplemented with a comprehensive database of Φ_Δ values of environmental samples.

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.

Identification of species and sources of atmospheric chromophores by fluorescence excitation-emission matrix with parallel factor analysis

It is essential to fully understand the physicochemical properties and sources of atmospheric chromophores to evaluate their impacts on environmental quality and global climate. Three-dimensional excitation-emission matrix (EEM) fluorescence spectroscopy is an important method for directly characterizing the occurrences, origins, and chemical behaviors of atmospheric chromophores. However, there is still a lack of adequate information on the sources and chemical structures of EEM-defined chromophores. This situation limits the extensive application of the EEM method in the study of atmospheric chromophores. Under these adverse conditions, this work uses the analysis of EEM data by the parallel factor (PARAFAC) analysis model and a comprehensive comparison of the types and abundances of different chromophores in different aerosol samples (combustion source samples, secondary organic aerosols, and ambient aerosols) to demonstrate that the EEM method can distinguish among different chromophore types and aerosol sources. Indeed, approximately half of all fluorescent substances can be attributed to specific chemicals and sources. These findings provide an important basis for the study of the sources and chemical processes of atmospheric chromophores by the EEM approach.

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.

Reactive Oxygen Species Production from Secondary Organic Aerosols: The Importance of Singlet Oxygen

Organic aerosols are subjected to atmospheric processes driven by sunlight, including the production of reactive oxygen species (ROS) capable of transforming their physicochemical properties. In this study, secondary organic aerosols (SOA) generated from aromatic precursors were found to sensitize singlet oxygen (¹O₂), an arguably underappreciated atmospheric ROS. Specifically, we quantified ¹O₂, OH radical, and H₂O₂ quantum yields within photoirradiated solutions of laboratory-generated SOA from toluene, biphenyl, naphthalene, and 1,8-dimethylnaphthalene. At 5 mg_C L^-1 of SOA extracts, the average steady-state concentrations of ¹O₂ and of OH radicals in irradiated solutions were 3 ± 1 × 10^-14 M and 3.6 ± 0.9 × 10^-17 M, respectively. Furthermore, ROS quantum yields of irradiated ambient PM₁₀ extracts were comparable to those from laboratory-generated SOA, suggesting a similarity in ROS production from both types of samples. Finally, by using our measured ROS concentrations, we predict that certain organic compounds found in aerosols, such as amino acids, organo-nitrogen compounds, and phenolic compounds have shortened lifetimes by more than a factor of 2 when ¹O₂ is considered as an additional sink. Overall, our findings highlight the importance of SOA as a source of ¹O₂ and its potential as a competitive ROS species in photooxidation processes.

Halogen Radical Oxidants in Natural and Engineered Aquatic Systems

Photochemical reactions contribute to the transformation of contaminants and biogeochemically important substrates in environmental aquatic systems. Recent research has demonstrated that halogen radicals (e.g., Cl^•, Br^•, Cl₂^•-, BrCl^•-, Br₂^•-) impact photochemical processes in sunlit estuarine and coastal waters rich in halides (e.g., chloride, Cl^-, and bromide, Br^-). In addition, halogen radicals participate in contaminant degradation in some engineered processes, including chlorine photolysis for drinking water treatment and several radical-based processes for brine and wastewater treatment. Halogen radicals react selectively with substrates (with bimolecular rate constants spanning several orders of magnitude) and via several potential chemical mechanisms. Consequently, their role in photochemical processes remains challenging to assess. This review presents an integrative analysis of the chemistry of halogen radicals and their contribution to aquatic photochemistry in sunlit surface waters and engineered treatment systems. We evaluate existing data on the generation, speciation, and reactivity of halogen radicals, as well as experimental and computational approaches used to obtain this data. By evaluating existing data and identifying major uncertainties, this review provides a basis to assess the impact of halogen radicals on photochemical processes in both saline surface waters and engineered treatment systems.

First Measurements of Organic Triplet Excited States in Atmospheric Waters

Photooxidants chemically transform organic compounds in atmospheric drops and particles. Photooxidants such as hydroxyl radical (^•OH) and singlet molecular oxygen (¹O₂*) have been characterized in cloud and fog drops, but there are no measurements of the triplet excited states of organic matter (³C*). These "triplets", which are formed from excitation of chromophoric dissolved organic matter (CDOM), i.e., brown carbon, are difficult to measure because they are a mixture of species instead of a single entity. Here, we use a two-probe technique to measure the steady-state concentrations, rates of photoformation, and quantum yields of oxidizing triplet states during simulated-sunlight illumination of bulk fog waters. Concentrations of ³C* are (0.70-15) × 10^-14 M with an average (±σ) value of 5.0 (±5.1) × 10^-14 M. The average ³C* photoformation rate is 130 (±130) μM h^-1, while the average quantum yield is 3.7 (±4.5)%. Based on our previous measurements of ^•OH and ¹O₂* in the same fog samples, the ratio of the steady-state concentrations for ¹O₂*:³C*:^•OH is approximately 3:1:0.04, respectively. At our measured concentrations, triplet excited states can be the dominant aqueous oxidants for organic compounds such as phenols from biomass combustion.

A survey of photogeochemistry

The participation of sunlight in the natural chemistry of the earth is presented as a unique field of study, from historical observations to prospects for future inquiry. A compilation of known reactions shows the extent of light-driven interactions between naturally occurring components of land, air, and water, and provides the backdrop for an outline of the mechanisms of these phenomena. Catalyzed reactions, uncatalyzed reactions, direct processes, and indirect processes all operate in natural photochemical transformations, many of which are analogous to well-known biological reactions. By overlaying photochemistry and surface geochemistry, complementary approaches can be adopted to identify natural photochemical reactions and discern their significance in the environment.

Concentrations of a triplet excited state are enhanced in illuminated ice

Photochemical reactions influence the fates and lifetimes of organic compounds in snow and ice, both through direct photoreactions and via photoproduced transient species such as hydroxyl radical (˙OH) and, perhaps, triplet excited states of organic compounds (i.e., triplets). While triplets can be important oxidants in atmospheric drops and surface waters, little is known of this class of oxidants in frozen samples. To investigate this, we examined the photoreaction of phenol with the triplet state of 3,4-dimethoxybenzaldehyde (³DMB*), a product from biomass combustion, in illuminated laboratory ices. Our results show that the rate of phenol loss due to ³DMB* is, on average, increased by a factor of 95 ± 50 in ice compared to the equivalent liquid sample. We find that this experimentally measured freeze concentration factor, F_EXP, is independent of total solute concentration and temperature, in contrast to what is expected from a liquid-like region whose composition follows freezing point depression. We also find that F_EXP for triplets is independent of pH, although the rates of phenol loss increase with decreasing pH in both solution and ice. The enhancement in the rate of phenol loss in/on ice indicates that concentrations of triplet excited states are enhanced in ice relative to solution and suggests that this class of oxidants might be a significant sink for organics in snow and ice.

Characterization of Chromophoric Water-Soluble Organic Matter in Urban, Forest, and Marine Aerosols by HR-ToF-AMS Analysis and Excitation-Emission Matrix Spectroscopy

Chromophoric water-soluble organic matter in atmospheric aerosols potentially plays an important role in aqueous reactions and light absorption by organics. The fluorescence and chemical-structural characteristics of the chromophoric water-soluble organic matter in submicron aerosols collected in urban, forest, and marine environments (Nagoya, Kii Peninsula, and the tropical Eastern Pacific) were investigated using excitation-emission matrices (EEMs) and a high-resolution aerosol mass spectrometer. A total of three types of water-soluble chromophores, two with fluorescence characteristics similar to those of humiclike substances (HULIS-1 and HULIS-2) and one with fluorescence characteristics similar to those of protein compounds (PLOM), were identified in atmospheric aerosols by parallel factor analysis (PARAFAC) for EEMs. We found that the chromophore components of HULIS-1 and -2 were associated with highly and less-oxygenated structures, respectively, which may provide a clue to understanding the chemical formation or loss of organic chromophores in atmospheric aerosols. Whereas HULIS-1 was ubiquitous in water-soluble chromophores over different environments, HULIS-2 was abundant only in terrestrial aerosols, and PLOM was abundant in marine aerosols. These findings are useful for further studies regarding the classification and source identification of chromophores in atmospheric aerosols.

Aqueous benzene-diols react with an organic triplet excited state and hydroxyl radical to form secondary organic aerosol

Benzene-diols are oxidized rapidly by hydroxyl radical and the triplet excited state of an aromatic carbonyl to efficiently form SOA in the aqueous phase.