Atmospheric Hydroxyl Radical Route Revealed: Interface-Mediated Effects of Mineral-Bearing Microdroplet Aerosol

Hydroxyl radical (·OH) plays a crucial role in atmospheric chemistry, regulating the oxidative potential and aerosol composition. This study reveals an unprecedented source of ·OH in the atmosphere: mineral dust-bearing microdroplet aerosols. We demonstrate that Kaolin clay particles in microdroplet aerosols trigger rapid ·OH production upon solar irradiation, with rates reaching an order of at least 10-3 M s-1. This production rate is several orders of magnitude higher than that of the bulk phase (2.4 × 10-11 M s-1) and previously known pathways. On this basis, the surface-based interfacial ·OH production rate is estimated to be 8.9 × 10-5 mol m-2 s-1 at the air-water-solid interface of 1 μm sized aerosol particles. The enhanced ·OH formation is attributed to the unique features of air-water-solid interfaces, where the lifespan of photoinduced holes was significantly increased due to the presence of strong electric fields at the air-water interface. We further investigated the impacts of various environmental factors and aerosol properties on ·OH production, including light intensity, relative humidity, particle size, and pH. Our findings provide new insights into atmospheric photochemical processes mediated by mineral dust-bearing microdroplet aerosols, which are important contributors to ·OH source in the atmosphere. This work advances our understanding of atmospheric interfacial chemistry and its profound and lasting implications for air quality and climate.

Correlation between photolysis quantum yields and efficiency of hydroxyl radicals generation upon excitation of natural iron(III) carboxylates

Fe(III) complexes with simple carboxylic acids play an important role in environmental photochemistry of atmosphere and natural waters and are widely studied as potential photoactive agents for advanced oxidation processes (AOPs). The paper presents approaches to the correct determination of quantum yields of complexes photolysis (φphot) and •OH generation (φOH) upon excitation of natural Fe3+ carboxylates by UV irradiation. The first approach is based on the application of optical spectroscopy to argon-saturated solutions. This method is less time-consuming compared to standard Fe2+-phenanthroline technique and gives reproducible and correct results. The second approach is based on the use of [FeOH]2+ hydroxocomplex as a reference system with a well-known φOH value and benzene as a selective trap for the •OH radical. Particular attention is paid to the thermal reactions of primary photogenerated species leading to a decrease of φphot and a significant increase of φOH in post-irradiation period in air-equilibrated solutions. For the first time, the quantum yields of the •OH radical have been determined for a wide range of Fe3+ complexes (1:1 composition) with natural carboxylic acids and their correlation with φphot was established. The results allow to overcome the existing discrepancy of known literature data on φphot and φOH appeared due to using of different approaches to determining these values. The developed approaches will be also important for the correct determination of the efficiency of •OH radical generation in the systems used in environmental photochemistry and AOPs.

Constructed electron-dense Mn sites in nitrogen-doped Mn3O4 for efficient catalytic ozonation of pyrazines: Degradation and odor elimination

In this study, N-doped Mn3O4 catalysts (Mn-nN) with electron-dense Mn sites were synthesized and employed in heterogeneous catalytic ozonation (HCO). These catalysts demonstrated excellent performance in pyrazines degradation and odor elimination. The synthesis of Mn-nN was achieved through a facile urea-assisted heat treatment method. Experimental characterization and theoretical analyses revealed that the MnN structures in Mn-nN, played a crucial role in facilitating the formation of electron-dense Mn sites that served as the primary active sites for ozone activation. In particular, Mn-1N exhibited excellent performance in the HCO system, demonstrating the highest 2,5-dimethylpyrazine (2,5-DMP) degradation efficiency. •OH was confirmed as the primary reactive oxygen species involved in the HCO process. The second-order rate constants for 2,5-DMP degradation with O3 and •OH, were determined to be (3.75 ± 0.018) × 10-1 and (6.29 ± 0.844) × 109 M-1 s-1, respectively. Seventeen intermediates were identified through GC-MS analysis during the degradation of 2,5-DMP via HCO process with Mn-1N. The degradation pathways were subsequently proposed by considering these identified intermediates. This study introduces a novel approach to synthesize N-doped Mn3O4 catalysts and demonstrates their efficacy in HCO for the degradation of pyrazines and the elimination of associated odors. The results show that the catalysts are promising for addressing odor-related environmental issues and provide valuable insights about the broader significance of catalytic ozonation processes.

Robust quantification of the burst of OH radicals generated by ambient particles in nascent cloud droplets using a direct-to-reagent approach

Reactive oxygen species (ROS) play a central role in chemistry in cloud water, as well as in other aqueous phases such as lung fluid and in wastewater treatment. Recently, work simulating nascent cloud droplets showed that aerosol particles produce a large burst of OH radicals when they first take up water. This activity stops abruptly, within two minutes. The source of the OH radicals is not well understood, but it likely includes the aqueous phase chemistry of ROS and/or organic hydroperoxides and redox active metals such as iron and copper. ROS and their precursors are in general highly reactive and labile, and thus may not survive during traditional sampling methods, which typically involve multi-hour collection on a filter or direct sampling into water or another collection liquid. Further, these species may further decay during storage. Here, we develop a technique to grow aerosol particles into small droplets and capture the droplets directly into a vial containing the terephthalate probe in water, which immediately scavenges OH radicals produced by aerosol particles. The method uses a Liquid Spot Sampler. Extensive characterization of the approach reveals that the collection liquid picks up substantial OH/OH precursors from the gas phase. This issue is effectively addressed by adding an activated carbon denuder. We then compared OH formation measured with the direct-to-reagent approach vs. filter collection. We find that after a modest correction for OH formed in the collection liquid, the samples collected into the reagent produce about six times those collected on filters, for both PM2.5 and total suspended particulate. This highlights the need for direct-to-reagent measurement approaches to accurately quantify OH production from ambient aerosol particles.

Temperature-Dependent Oxidation of Hydroxylated Aldehydes by •OH, SO4•-, and NO3• Radicals in the Atmospheric Aqueous Phase

T-dependent aqueous-phase rate constants were determined for the oxidation of the hydroxy aldehydes, glyceraldehyde, glycolaldehyde, and lactaldehyde, by the hydroxyl radicals (•OH), the sulfate radicals (SO4•-), and the nitrate radicals (NO3•). The obtained Arrhenius expressions for the oxidation by the •OH radical are: k(T,GLYCERALDEHYDE+OH•) = (3.3 ± 0.1) × 1010 × exp((-960 ± 80 K)/T)/L mol-1 s-1, k(T,GLYCOLALDEHYDE+OH•) = (4.3 ± 0.1) × 1011 × exp((-1740 ± 50 K)/T)/L mol-1 s-1, k(T,LACTALDEHYDE+OH•) = (1.6 ± 0.1) × 1011 × exp((-1410 ± 180 K)/T)/L mol-1 s-1; for the SO4•- radical: k(T,GLYCERALDEHYDE+SO4•-) = (4.3 ± 0.1) × 109 × exp((-1400 ± 50 K)/T)/L mol-1 s-1, k(T,GLYCOLALDEHYDE+SO4•-) = (10.3 ± 0.3) × 109 × exp((-1730 ± 190 K)/T)/L mol-1 s-1, k(T,LACTALDEHYDE+SO4•-) = (2.2 ± 0.1) × 109 × exp((-1030 ± 230 K)/T)/L mol-1 s-1; and for the NO3• radical: k(T,GLYCERALDEHYDE+NO3•) = (3.4 ± 0.2) × 1011 × exp((-3470 ± 460 K)/T)/L mol-1 s-1, k(T,GLYCOLALDEHYDE+NO3•) = (7.8 ± 0.2) × 1011 × exp((-3820 ± 240 K)/T)/L mol-1 s-1, k(T,LACTALDEHYDE+NO3•) = (4.3 ± 0.2) × 1010 × exp((-2750 ± 340 K)/T)/L mol-1 s-1, respectively. Targeted simulations of multiphase chemistry reveal that the oxidation by OH radicals in cloud droplets is important under remote and wildfire influenced continental conditions due to enhanced partitioning. There, the modeled average aqueous •OH concentration is 2.6 × 10-14 and 1.8 × 10-14 mol L-1, whereas it is 7.9 × 10-14 and 3.5 × 10-14 mol L-1 under wet particle conditions. During cloud periods, the aqueous-phase reactions by •OH contribute to the oxidation of glycolaldehyde, lactaldehyde, and glyceraldehyde by about 35 and 29%, 3 and 3%, and 47 and 37%, respectively.

Chemical and Light-Absorption Properties of Water-Soluble Organic Aerosols in Northern California and Photooxidant Production by Brown Carbon Components

Atmospheric brown carbon (BrC) can impact the radiative balance of the earth and form photooxidants. However, the light absorption and photochemical properties of BrC from different sources remain poorly understood. To address this gap, dilute water extracts of particulate matter (PM) samples collected at Davis, CA over one year were analyzed using high resolution aerosol mass spectrometry (HR-AMS) and UV-vis spectroscopy. Positive matrix factorization (PMF) on combined AMS and UV-vis data resolved five water-soluble organic aerosol (WSOA) factors with distinct mass spectra and UV-vis spectra: a fresh and an aged water-soluble biomass burning OA (_WSBBOA_fresh and _WSBBOA_aged) and three oxygenated OA (_WSOOAs). _WSBBOA_fresh is the most light-absorbing, with a mass absorption coefficient (MAC_365 nm) of 1.1 m² g^-1, while the _WSOOAs are the least (MAC_365 nm = 0.01-0.1 m² g^-1). These results, together with the high abundance of _WSBBOAs (∼52% of the WSOA mass), indicate that biomass burning activities such as residential wood burning and wildfires are an important source of BrC in northern California. The concentrations of aqueous-phase photooxidants, i.e., hydroxyl radical (·OH), singlet molecular oxygen (¹O₂*), and oxidizing triplet excited states of organic carbon (³C*), were also measured in the PM extracts during illumination. Oxidant production potentials (PP_OX) of the five WSOA factors were explored. The photoexcitation of BrC chromophores from BB emissions and in OOAs is a significant source of ¹O₂* and ³C*. By applying our PP_OX values to archived AMS data at dozens of sites, we found that oxygenated organic species play an important role in photooxidant formation in atmospheric waters.

Spontaneous dark formation of OH radicals at the interface of aqueous atmospheric droplets

Hydroxyl radical (OH) is a key oxidant that triggers atmospheric oxidation chemistry in both gas and aqueous phases. The current understanding of its aqueous sources is mainly based on known bulk (photo)chemical processes, uptake from gaseous OH, or related to interfacial O3 and NO3 radical-driven chemistry. Here, we present experimental evidence that OH radicals are spontaneously produced at the air-water interface of aqueous droplets in the dark and the absence of known precursors, possibly due to the strong electric field that forms at such interfaces. The measured OH production rates in atmospherically relevant droplets are comparable to or significantly higher than those from known aqueous bulk sources, especially in the dark. As aqueous droplets are ubiquitous in the troposphere, this interfacial source of OH radicals should significantly impact atmospheric multiphase oxidation chemistry, with substantial implications on air quality, climate, and health.

Organic Peroxides in Aerosol: Key Reactive Intermediates for Multiphase Processes in the Atmosphere

Organic peroxides (POs) are organic molecules with one or more peroxide (-O-O-) functional groups. POs are commonly regarded as chemically labile termination products from gas-phase radical chemistry and therefore serve as temporary reservoirs for oxidative radicals (HOx and ROx) in the atmosphere. Owing to their ubiquity, active gas-particle partitioning behavior, and reactivity, POs are key reactive intermediates in atmospheric multiphase processes determining the life cycle (formation, growth, and aging), climate, and health impacts of aerosol. However, there remain substantial gaps in the origin, molecular diversity, and fate of POs due to their complex nature and dynamic behavior. Here, we summarize the current understanding on atmospheric POs, with a focus on their identification and quantification, state-of-the-art analytical developments, molecular-level formation mechanisms, multiphase chemical transformation pathways, as well as environmental and health impacts. We find that interactions with SO2 and transition metal ions are generally the fast PO transformation pathways in atmospheric liquid water, with lifetimes estimated to be minutes to hours, while hydrolysis is particularly important for α-substituted hydroperoxides. Meanwhile, photolysis and thermolysis are likely minor sinks for POs. These multiphase PO transformation pathways are distinctly different from their gas-phase fates, such as photolysis and reaction with OH radicals, which highlights the need to understand the multiphase partitioning of POs. By summarizing the current advances and remaining challenges for the investigation of POs, we propose future research priorities regarding their origin, fate, and impacts in the atmosphere.

Aqueous ·OH Oxidation of Highly Substituted Phenols as a Source of Secondary Organic Aerosol

Biomass burning (BB) releases large quantities of phenols (ArOH), which can partition into cloud/fog drops and aerosol liquid water (ALW), react, and form aqueous secondary organic aerosol (aqSOA). While simple phenols are too volatile to significantly partition into particle water, highly substituted ArOH partition more strongly and might be important sources of aqSOA in ALW. To investigate this, we measured the ^·OH oxidation kinetics and aqSOA yields for six highly substituted ArOH from BB. Second-order rate constants are high, in the range (1.9-14) × 10⁹ M^-1 s^-1 at pH 2 and (14-25) × 10⁹ M^-1 s^-1 at pH 5 and 6. Mass yields of aqSOA are also high, with an average (±1σ) value of 82 (±12)%. ALW solutes have a range of impacts on phenol oxidation by ^·OH: a BB sugar and some inorganic salts suppress oxidation, while a nitrate salt and transition metals enhance oxidation. Finally, we estimated rates of aqueous- and gas-phase formation of SOA from a single highly substituted phenol as a function of liquid water content (LWC), from conditions of cloud/fog (0.1 g-H₂O m^-3) to ALW (10 μg-H₂O m^-3). Formation of aqSOA is significant across the LWC range, although gas-phase ^·OH becomes dominant under ALW conditions. We also see a generally large discrepancy between measured and modeled aqueous ^·OH concentrations across the LWC range.

Assessing the source of the photochemical formation of hydroxylating species from dissolved organic matter using model sensitizers

Dissolved organic matter (DOM) is ubiquitous in natural waters and can facilitate the chemical transformation of many contaminants through the photochemical production of reactive intermediates, such as singlet oxygen (¹O₂), excited triplet state DOM (³DOM*), and hydroxylating species (˙OH and other intermediates of similar reaction chemistry). The formation mechanism of most reactive intermediates is well understood, but this is not the case for the formation of hydroxylating species from DOM. To investigate this chemistry, DOM model sensitizers were irradiated with two different probe compounds (benzene and benzoic acid) at two irradiation wavelengths (254 and 320 nm). The ability of DOM model sensitizers to hydroxylate these arene probes was assessed by measuring rates of formation of the hydroxylated probe compounds (phenol and salicylic acid). Multiple classes of model sensitizers were tested, including quinones, hydroxybenzoic acids, aromatic ketones, and other triplet forming species. Of these classes of model sensitizers, only quinones and hydroxybenzoic acids had a hydroxylating capacity. Methanol quenching experiments were used to assess the reactivity of hydroxylating species. These results have several implications for the systems tested. First, they suggest that the hydroxylating intermediate produced from hydroxybenzoic acid photolysis may not be hydroxyl radical, but a different hydroxylating species. Also, these data prompted investigation of whether quinone photoproducts have a hydroxylating capacity. These results confirm that hydroxybenzoic acids and quinones are important to the photochemical production of hydroxylating species from DOM, but the mechanism by which this occurs for these classes of sensitizers is still elusive.

Photodegradation of an Anilide Fungicide Inpyrfluxam in Water and Nitrate Aqueous Solution

The photodegradation behavior of a new anilide fungicide, inpyrfluxam [3-difluoromethyl-N-[(R)-2,3-dihydro-1,1,3-trimethyl-1H-inden-4-yl]-1-methylpyrazole-4-carboxamide] (1), was investigated in aqueous buffer and nitrate solutions under irradiation with artificial sunlight (λ > 290 nm). In both media, 1 mainly photodegraded via oxidation at the 3'-position of the Indane ring, cleavage of the C-N bond of the amide linkage and N-phenyl ring bond, and finally mineralization to carbon dioxide. No isomerization of 1 occurred at the 3'-position of the Indane ring. In the presence of nitrate ion, which originates from fertilizer in agricultural fields, the degradation of 1 was significantly accelerated as compared with buffer solution, and the reaction rate was strongly correlated with the concentration of hydroxyl radicals derived from the photolysis of nitrate ions. The reaction rate constant of hydroxyl radicals with 1 was determined to be 3.0 × 10¹⁰ /M/s, which was higher than that of hydroxyl radicals with other pesticides possessing aromatic rings.

Photoinduced Uptake and Oxidation of SO2 on Beijing Urban PM2.5

Sulfate, as a major component of aerosol particles, greatly contributes to haze formation and affects global climate change. Although formation pathways of sulfate aerosols from the conversion of SO₂ have been extensively studied, the discrepancy between field observations and model simulations suggests that there are still unknown sulfate sources. Herein, we report for the first time a photoinduced SO₂ uptake and oxidation pathway in Beijing urban PM_2.5 aerosols. In comparison with the NO₂- and O₃-induced SO₂ oxidation pathways, this SO₂ photo-oxidation in Beijing urban PM_2.5 could make an important contribution to the daytime sulfate formation. Reactive species, such as ^•OH radicals and H₂O₂, are the major oxidants leading to sulfate formation in PM_2.5. The water-soluble matter (WSM) and water-insoluble organic matter (WISOM) in PM_2.5 were identified as the main photo-oxidant producers. Our work highlights an important daytime sulfate source in the atmosphere and provides new insight into the photochemical aging of ambient aerosols.

Isomerization and Degradation of Levoglucosan via the Photo-Fenton Process: Insights from Aqueous-Phase Experiments and Atmospheric Particulate Matter

So far, studies on the conversion of stereochemistry under photo-Fenton conditions and their atmospheric implication are still rare. Here, we found that the biomass burning marker, the chiral compound levoglucosan (L), undergoes oxidative degradation under photo-Fenton conditions and can be isomerized into mannosan (M) and galactosan (G) simultaneously. Among the formic acid, acetic acid, and oxalic acid in the degradation products of levoglucosan, it was found that the yield of formation of formic acid in the photo-Fenton pathway can be as high as 86%. It is worth noting that both levoglucosan and its isomers are present in the atmosphere and their concentrations are strongly correlated. At the same time, the range of their concentration ratios, L/(G + M), measured in the photo-Fenton experiments in the laboratory was found to agree well with that measured in atmospheric PM_2.5 samples. However, the sources of L, G, and M in the atmosphere are complex, and the photo-Fenton reaction may be an essential pathway for the distribution of L, G, and M in the atmosphere.

Aqueous-Phase Decomposition of Isoprene Hydroxy Hydroperoxide and Hydroxyl Radical Formation by Fenton-like Reactions with Iron Ions

Isoprene hydroxy hydroperoxides (ISOPOOH) formed by the photooxidation of isoprene under low-NO conditions play an important role in the formation and evolution of secondary organic aerosols, yet multiphase processes of ISOPOOH are poorly understood. By applying electron paramagnetic resonance spectroscopy, we observe that ISOPOOH undergoes aqueous-phase decomposition upon interacting with Fe(II) ions to form OH and organic radicals at room temperature. To reproduce the measured dependence of OH formation on the Fe concentrations by kinetic modeling, we postulate that Fe(II) ions react with ISOPOOH via Fenton-like reactions to form OH radicals with a rate constant of 7.3 × 10^-18 cm³ s^-1. At low concentrations, oxalate forms monocomplexes with Fe(II) ions, which can promote OH formation by ISOPOOH. However, at high concentrations, oxalate scavenges OH radicals, thereby lowering aqueous OH concentrations. These findings provide new insight for the atmospheric fate of ISOPOOH and reactive oxygen species generation in the aqueous phase.

Mineral Dust and Iron Solubility: Effects of Composition, Particle Size, and Surface Area

There is significant iron deposition in the oceans, approximately 14–16 Tg annually from mineral dust aerosols, but only a small percentage (approx. 3%) of it is soluble and, thus, bioavailable. In this work, we examine the effect of mineralogy, particle size, and surface area on iron solubility in pure mineral phases to simulate atmospheric processing of mineral dust aerosols during transport. Pure iron-bearing minerals common to Saharan dust were partitioned into four size fractions (10–2.5, 2.5–1, 1–0.5, and 0.5–0.25 µm) and extracted into moderately acidic (pH 4.3) and acidic (pH 1.7) leaching media to simulate mineral processing during atmospheric transport. Results show that, in general, pure iron-bearing clay materials present an iron solubility (% dissolved Fe/total Fe in the mineral) an order of magnitude higher than pure iron oxide minerals. The relative solubility of iron in clay particles does not depend on particle size for the ranges examined (0.25–10 μm), while iron in hematite and magnetite shows a trend of increasing solubility with decreasing particle size in the acidic leaching medium. Our results indicate that while mineralogy and aerosol pH have an effect on the solubilization of iron from simulated mineral dust particles, surface processes of the aerosol might also have a role in iron solubilization during transport. The surface area of clay minerals does not change significantly as a function of particle size (10–0.25 µm), while the surface area of iron oxides is strongly size dependent. Overall, these results show how mineralogy and particle size can influence iron solubility in atmospheric dust.

Singlet Oxygen Photogeneration in Coastal Seawater: Prospect of Large-Scale Modeling in Seawater Surface and Its Environmental Significance

Chromophoric-dissolved organic matter (CDOM) acts as the precursor to singlet oxygen (¹O₂) in natural waters, while water acts as the main scavenger. In this study, we showed that ¹O₂ in coastal seawater can be successfully predicted from CDOM parameters. The ¹O₂ steady-state concentration [¹O₂]_ss and photoformation rate (R¹O₂) varied by a factor of 6 across 13 sampling stations in the Seto Inland Sea, Japan, ranging from 1.2 to 8.2 × 10^-14 M and 3.32 to 22.7 × 10^-9 M s^-1, respectively. Investigation of CDOM optical properties revealed that CDOM abundance measured as the absorption coefficient at 300 nm (a₃₀₀) had the strongest correlation (r = 0.96, p < 0.001) with [¹O₂]_ss, while parameters indicative of CDOM quality (e.g., spectral slope) did not influence [¹O₂]_ss. A linear relationship between [¹O₂]_ss and a₃₀₀, normalized to a sunlight intensity of 0.91 kW/m², was derived as [¹O₂]_ss (10^-14 M) = 2.12(a₃₀₀) + 0.48. This was then used to predict [¹O₂]_ss using a₃₀₀ values from a subsequent, independent sampling exercise conducted 2 years after the first sampling. There was a good agreement (r = 0.93, p < 0.001) between the predicted values and the experimentally determined values based on a 95% prediction interval plot. Kinetic estimations using [¹O₂]_ss suggest that ¹O₂ mediates the degradation of tetrabromobisphenol A in surface seawater (t_1/2 = 0.63 days) while also contributing to the indirect photolysis of methyl mercury. The findings from this study suggest that large-scale modeling of ¹O₂ generation in surface seawater from CDOM parameters is possible with useful environmental significance for determining the fate of pollutants.

Factors controlling the degradation of hydrogen peroxide in river water, and the role of riverbed sand

Diurnal changes of H₂O₂ in river water during mid-summer were investigated. H₂O₂ in river water increased with the increase in intensity of solar radiation in the morning, and reached a maximum at 14:00, although solar radiation reached a maximum around 12:00. In the afternoon, a gradual decrease in H₂O₂ was observed, and H₂O₂ reached a minimum just before sunrise. Degradation rate constants determined using unfiltered river water samples were 0.081-0.161 h^-1, corresponding to a half-life of 4.3-8.5 h. We simulated diurnal changes in H₂O₂ using a simple formation, accumulation, and degradation model for static water using formation and degradation rate constants. The results of the modeling suggested that in situ degradation rate constants in rivers could be faster than those determined for unfiltered river water samples. Experiments using river sand indicated that riverbed sand could play an important role in H₂O₂ decay in rivers. We discussed the decomposition process of H₂O₂ in rivers.

Photochemistry of the Cloud Aqueous Phase: A Review

This review paper describes briefly the cloud aqueous phase composition and deeply its reactivity in the dark and mainly under solar radiation. The role of the main oxidants (hydrogen peroxide, nitrate radical, and hydroxyl radical) is presented with a focus on the hydroxyl radical, which drives the oxidation capacity during the day. Its sources in the aqueous phase, mainly through photochemical mechanisms with H₂O₂, iron complexes, or nitrate/nitrite ions, are presented in detail. The formation rate of hydroxyl radical and its steady state concentration evaluated by different authors are listed and compared. Finally, a paragraph is also dedicated to the sinks and the reactivity of the HO^• radical with the main compounds found in the cloud aqueous phase. This review presents an assessment of the reactivity in the cloud aqueous phase and shows the significant potential impact that this medium can have on the chemistry of the atmosphere and more generally on the climate.

Photochemical production of hydroxyl radical from algal organic matter

Photochemical production of hydroxyl radical (·OH) from algal organic matter (AOM) collected from Lake Torrens in South Australia was examined using a sunlight simulator. The two AOM isolates featured lower molecular weight, lower chromophoric content, and lower SUVA₂₅₄ (0.7 and 0.9, L mgC^-1 m^-1) than the reference Suwannee River hydrophobic acid (SR-HPO), they had considerably higher apparent quantum yields (ϕ_NOM^OH, 3.03 × 10^-5 and 2.18 × 10^-5) than SR-HPO (0.84 × 10^-5). Fluorescence excitation-emission matrix (FEEM) showed that the major components in the AOM were aromatic protein-like and soluble microbial substances. Unique formulas of the two AOM isolates as compared to SR-HPO were revealed using FTICR-MS and classified into four areas, namely protein-like molecules with low O/C (H/C > 1.5, O/C: 0.2-0.4), lignin-derived moieties with low O/C (H/C:1.0-1.5, O/C: 0.1-0.3), protein-like molecules with high O/C (H/C > 1.5, O/C: 0.5-0.7), and carbohydrate derivatives (H/C > 1.5, O/C > 0.7). These unique AOM moieties characterised utilizing FEEM and FTICR-MS were tentatively postulated to contribute to the high ϕ_NOM^OH. To the best of our knowledge, this is the first study performed to both evaluate natural AOM as an efficient photosensitiser of ·OH and propose AOM moieties responsible for the high ϕ_NOM^OH.

A light-driven burst of hydroxyl radicals dominates oxidation chemistry in newly activated cloud droplets

Aerosol particles and their interactions with clouds are one of the most uncertain aspects of the climate system. Aerosol processing by clouds contributes to this uncertainty, altering size distributions, chemical composition, and radiative properties. Many changes are limited by the availability of hydroxyl radicals in the droplets. We suggest an unrecognized potentially substantial source of OH formation in cloud droplets. During the first few minutes following cloud droplet formation, the material in aerosols produces a near-UV light-dependent burst of hydroxyl radicals, resulting in concentrations of 0.1 to 3.5 micromolar aqueous OH ([OH]_aq). The source of this burst is previously unrecognized chemistry between iron(II) and peracids. The contribution of the "OH burst" to total OH in droplets varies widely, but it ranges up to a factor of 5 larger than previously known sources. Thus, this new process will substantially enhance the impact of clouds on aerosol properties.

Kinetics of photocatalytic removal of imidacloprid from water by advanced oxidation processes with respect to nanotechnology

In this study, the kinetics of photocatalytic removal of imidacloprid, a systemic chloronicotinoid insecticide, from water using two advanced oxidation systems (ZnO(normal)/H₂O₂/artificial sunlight and ZnO(nano)/H₂O₂/artificial sunlight) were investigated. Moreover, the effects of pH, insecticide concentration, catalyst concentration, catalyst particle size, and water type on the photocatalytic removal of imidacloprid were evaluated. Furthermore, total mineralization of imidacloprid under these advanced oxidation systems was evaluated by monitoring the decreases in dissolved organic carbon (DOC) concentrations and formation rate of inorganic ions (Cl^- and NO₂ ^-) with irradiation time using total organic carbon (TOC) analysis and ion chromatography to confirm the complete detoxification of imidacloprid in water. The degradation rate of imidacloprid was faster under the ZnO(nano)/H₂O₂/artificial sunlight system than the ZnO(normal)/artificial sunlight system in both pure and river water. The photocatalytic degradation of imidacloprid under both advanced oxidation systems was affected by pH, catalyst concentration, imidacloprid concentration, and water type. Almost complete mineralization of imidacloprid was only achieved in the ZnO(nano)/H₂O₂/artificial sunlight oxidation system. The photogeneration rate of hydroxyl radicals was higher under the ZnO(nano)/H₂O₂/artificial sunlight system than the ZnO(normal)/H₂O₂/artificial sunlight system. Advanced oxidation processes, particularly those using nanosized zinc oxide, can be regarded as an effective photocatalytic method for imidacloprid removal from water.

First evaluation of the effect of microorganisms on steady state hydroxyl radical concentrations in atmospheric waters

Clouds are complex multiphasic media where efficient chemical reactions take place and where microorganisms have been found to be metabolically active. Hydroxyl radical is the main oxidant in cloud water, and more generally in the atmosphere, during the day and drives the cloud oxidative capacity. However, only one measurement of the steady state hydroxyl radical concentrations in cloud water has been reported so far. Cloud chemistry models are used to estimate the hydroxyl radical concentrations with values ranging from 10^-12 to 10^-15 M that are surely overestimated due to a lack of knowledge about the speciation of the organic matter acting as a sink for hydroxyl radicals. The aim of this work is to quantify the concentration of hydroxyl radicals at steady state in rain and cloud waters and to measure the impact of native microflora on this concentration. First, the non-toxicity of terephthalic acid as probe is controlled before the analysis in real atmospheric water samples. Higher concentrations of hydroxyl radicals are found in cloud waters than in rain waters, with a mean value "1.6 ± 1.5" × 10^-16 M and "7.2 ± 5.0" × 10^-16 M for rain and cloud waters respectively and no real impact of microorganisms was observed. This method allows the measurement of steady state hydroxyl radical levels at very low concentrations (down to 10^-17 M) and it is biocompatible, fast and easy to handle. It is a useful tool, complementary to other methods, to give a better overview of atmospheric water oxidant capacity.

Easily Recoverable, Micrometer-Sized TiO2 Hierarchical Spheres Decorated with Cyclodextrin for Enhanced Photocatalytic Degradation of Organic Micropollutants

Micrometer-sized titanium dioxide hierarchical spheres (TiO₂-HS) were assembled from nanosheets to address two common limitations of photocatalytic water treatment: (1) inefficiency associated with scavenging of oxidation capacity by nontarget water constituents and (2) energy-intensive separation and recovery of the photocatalyst slurry. These micrometer-sized spheres are amenable to low-energy separation, and over 99% were recaptured from both batch and continuous flow reactors using microfiltration. Using nanosheets as building blocks resulted in a large specific surface area-3 times larger than that of commercially available TiO₂ powder (Evonik P25). Anchoring food-grade cyclodextrin onto TiO₂-HS (i.e., CD-TiO₂-HS) provided hydrophobic cavities to entrap organic contaminants for more effective utilization of photocatalytically generated reactive oxygen species. CD-TiO₂-HS removed over 99% of various contaminants with dissimilar hydrophobicity (i.e., bisphenol A, bisphenol S, 2-naphthol, and 2,4-dichlorophenol) within 2 h under a low-intensity UVA input (3.64 × 10^-6 einstein/L/s). As with other catalyst (including TiO₂ slurry), periodic replacement or replenishment would be needed to maintain high treatment efficiency (e.g., we demonstrate full reactivation through simple reanchoring of CD). Nevertheless, this task would be offset by significant savings in photocatalyst separation. Thus, CD-TiO₂-HS is an attractive candidate for photocatalytic water and wastewater treatment of recalcitrant organic pollutants.

Photochemical oxidation of di-n-butyl phthalate in atmospheric hydrometeors by hydroxyl radicals from nitrous acid

The photochemical oxidation of di-n-butyl phthalate (DBP) by ^•OH radicals from nitrous acid (HONO) in atmospheric hydrometeors was explored by two techniques, steady-state irradiation, and laser flash photolysis (LFP). The effects of atmospheric liquid parameters on DBP transformation were systematically evaluated, showing that DBP does not react with HONO directly and ^•OH-initiated reactions are crucial steps for consumption and transformation of DBP. Two reaction channels are operative: ^•OH addition and hydrogen atom abstraction. The overall rate constant for the reaction of DBP with ^•OH is 5.7 × 10⁹ M^-1 s^-1, and its specific rate constant for addition is 3.7 × 10⁹ M^-1 s^-1 determined by using laser flash photolysis technique. Comparing the individual reaction rate constant for aromatic ring addition with the total rate constant, the majority of the ^•OH radicals (about 65%) attack the aromatic ring. The major transformation products were identified by GC-MS, and the trends of their yields derived from both ring addition and H-abstraction with time are discussed. These results provide important insights into the photochemical transformation of DBP in atmospheric hydrometeors and contribute to atmospheric aerosol chemistry.

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.

Improving the characterization of dissolved organic carbon in cloud water: Amino acids and their impact on the oxidant capacity

Improving our understanding of cloud chemistry depends on achieving better chemical characterization (90% of the organic carbon [OC] fraction remains uncharacterized) and, consequently, assessing the reactivity of this complex system. In this manuscript, we report for the first time the concentrations of 16 amino acids (AAs) in 25 cloud water samples. The concentrations of individual AAs ranged from a few nM up to ~2.0 μM, and the average contribution of AAs corresponded to 9.1% (4.4 to 21.6%) of the dissolved OC (DOC) concentration. Considering their occurrence and concentrations, AAs were expected to represent an important hydroxyl radical (HO^•) sink in aqueous cloud samples. In this work, we estimated that approximately 17% (from 7 to 36%) of the hydroxyl radical-scavenging ability of the DOC could be attributed to the presence of AAs, whereas comparing the AAs suggested that an average of 51% (from 22 to 80%) of their reactivity with HO^• could account for the presence of tryptophan. These results clearly demonstrate that the occurrence and reactivity of AAs must be considered to better estimate the chemical composition and oxidant capacity of the cloud aqueous phase.

Enrichment of 13C in diacids and related compounds during photochemical processing of aqueous aerosols: New proxy for organic aerosols aging

To investigate the applicability of compound specific stable carbon isotope ratios (δ¹³C) of organics in assessment of their photochemical aging in the atmosphere, batch UV irradiation experiments were conducted on two ambient (anthropogenic and biogenic) aerosol samples in aqueous phase for 0.5–120 h. The irradiated samples were analyzed for δ¹³C of diacids, glyoxylic acid (ωC₂) and glyoxal. δ¹³C of diacids and related compounds became larger with irradiation time (i.e., aging), except for few cases. In general, δ¹³C of C₂-C₄ diacids showed an increasing trend with decreasing chain length. Based on δ¹³C of diacids and related compounds and their relations to their concentrations, we found that C₂ and C₃ are enriched with ¹³C during the photochemical decomposition and production from their higher homologues and oxoacids. Photochemical breakdown of higher (≥C₃) to lower diacids is also important in the enrichment of ¹³C in C₃-C₉ diacids whereas their production from primary precursors causes depletion of ¹³C. In case of ωC₂ and glyoxal, their photochemical production and further oxidation to highly oxygenated compounds both cause the enrichment of ¹³C. This study reveals that δ¹³C of diacids and related compounds can be used as a proxy to trace the aging of organic aerosols during long-range atmospheric transport.

Siderophores in Cloud Waters and Potential Impact on Atmospheric Chemistry: Photoreactivity of Iron Complexes under Sun-Simulated Conditions

In the present work, the photoreactivity of a mixture of iron(III)–pyoverdin (Fe(III)–Pyo) complexes was investigated under simulated cloud conditions. Pyoverdins are expected to complex ferric ions naturally present in cloudwater, thus modifying their availability and photoreactivity. The spectroscopic properties and photoreactivity of Fe(III)-Pyo were investigated, with particular attention to their fate under solar irradiation, also studied through simulations. The photolysis of the Fe(III)–Pyo complex leads to the generation of Fe(II), with rates of formation (RFe(II)f) of 6.98 and 3.96 × 10–9 M s–1 at pH 4.0 and 6.0, respectively. Interestingly, acetate formation was observed during the iron-complex photolysis, suggesting that fragmentation can occur after the ligand-to-metal charge transfer (LMCT) via a complex reaction mechanism. Moreover, photogenerated Fe(II) represent an important source of hydroxyl radical via the Fenton reaction in cloudwater. This reactivity might be relevant for the estimation of the rates of formation and steady-state concentrations of the hydroxyl radical by cloud chemistry models and for organic matter speciation in the cloud aqueous phase. In fact, the conventional models, which describe the iron photoreactivity in terms of iron–aqua and oxalate complexes, are not in accordance with our results.

Concentration and degradation of alternative biocides and an insecticide in surface waters and their major sinks in a semi-enclosed sea, Japan

A mass distribution model was used to predict the fate of Diuron, Irgarol 1051 and Fenitrothion in Seto Inland Sea which is located in western Japan. This was done by using concentration, degradation, and literature data. Diuron and Irgarol 1051 in Seto Inland Sea are mainly derived from antifouling paints used for ships and boats. On the other hand Fenitrothion exclusively comes from land via rivers and atmospheric deposition. The total inputs/yr to Seto Inland Sea were found to be 104 tons, 7.65 tons and 5.14 tons for Diuron, Irgarol 1051 and Fenitrothion, respectively. The pesticide residence times were 0.26 yr, 0.36 yr and 0.17 yr for Diuron, Irgarol 1051 and Fenitrothion, respectively. Photodegradation was faster than biodegradation. In seawater, the half-life ranges were 37.9-57.3 d for photodegradation. In the same seawater the half-life ranges were 1650-2394 d for biodegradation. Photodegradation is effective in surface water (0-5 m depth) while biodegradation occurs throughout the entire water column. Plankton and fishes accumulate these pesticides significantly. The pesticides are deposited (sorbed and buried with) sediments (between 74 and 87% of total input amounts). The open ocean is an important sink accounting for between 8 and 17% of the total pesticide input amounts while photo- and biodegradation accounts for a small percentage.

Temperature dependence of the photochemical formation of hydroxyl radical from dissolved organic matter

The temperature dependence of the photochemical production of the hydroxyl radical (•OH) from dissolved organic matter (DOM) was investigated by measuring the apparent temperature dependence of the quantum yield (Φa) for this process. Temperature dependent Φa values were analyzed using the Arrhenius equation. Apparent activation energies obtained for DOM isolates purchased from the International Humic Substances Society ranged from 16 to 34 kJ mol(-1). Addition of 40 units mL(-1) catalase, used to hinder the hydrogen peroxide (H2O2)-dependent pathway to •OH, did not impact the observed activation energy. However, an increase in activation energy was observed in lower molecular weight DOM obtained by size fractionation. We also measured the temperature dependence of p-benzoquionone photolysis as a model compound for DOM and observed no temperature dependence (slope p = 0.41) for the formation of phenol from oxidation of benzene (the •OH probe used), but a value of about 10 kJ mol(-1) for p-benzoquinone loss, which is consistent with formation of a quinone-water exciplex. These data provide insight into DOM photochemistry as well as provide parameters useful for modeling steady state •OH concentrations in natural systems.

Photoformation rate, steady-state concentration and lifetime of nitric oxide radical (NO·) in a eutrophic river in Higashi-Hiroshima, Japan

Monthly measurements (January-December 2013) of the photoformation rate, steady-state concentration and lifetime of nitric oxide radical (NO·) in the Kurose River in Higashi-Hiroshima City, Japan, were obtained. Each month, river water samples were collected at six different stations (upstream to downstream). NO· was quantified using 4, 5-diaminofluorescein-2 (DAF-2) as a probe and triazolofluorescein (DAF-2T) as a standard. Results show that NO· photoformation rate ranged from 0.01 to 35.4 (×10(-10) M s(-1)). The radical steady-state concentration in the river ranged from 0.02 to 68.5 (×10(-11) M). There was a strong correlation (r(2)=0.95) between NO· photoformation rate and the nitrite concentration in the river suggesting that this anion is a major NO· precursor. On average, 98% of the photoformed NO· came from river nitrite, and this was calculated using the photoformation rate constant {5.7×10(-5) M(NO·)s(-1) M(NO2(-))(-1)} of NO· from the anion concentration found in the study. The NO· lifetime ranged from 0.05 to 1.3 s in the river and remained fairly stable in the upstream and downstream samples. The ·OH radical, which was quantified during the study, had a photoformation rate of 0.01-13.4 (×10(-10) M s(-1)) and a steady-state concentration of 0.04-119 (×10(-16) M) with a lifetime that ranged from 0.3 to 22.5 (×10(-6) s). ·OH only accounted for ⩽0.0011% of the total NO· scavenged, showing that it was not a major sink for river NO·.

Sources of hydroxyl radical in headwater streams from nitrogen-saturated forest

Hydroxyl radical (HO) photoformation rate (RHO) was determined in headwater stream samples from nitrogen (N)-saturated forests, (1) to quantify the sources of HO in headwater streams and (2) to evaluate the nitrate NO3(-)-induced enhancement of HO formation in stream water caused by N saturation in forested watersheds. Stream water fulvic acid extracted from the forested watersheds was used to quantify the contribution of dissolved organic matter (DOM) to RHO. The results showed that almost all (97%; 81-109%) RHO sources in our headwater stream samples were quantitatively elucidated; the photolysis of NO3(-) (55%; 34-75%), nitrite [N(III)] (2%; 0.5-5.2%), and DOM-derived HO formation, from which photo-Fenton reactions (18%; 12-26%) and the direct photolysis of fluorescent dissolved organic matter (FDOM) (22%; 10-40%), was successfully separated. FDOM, which accounted for 53% (24-96%) of DOM in total organic carbon bases, was responsible for HO formation in our headwater streams. High NO3(-) leaching caused by N saturation in forested watersheds increased RHO in the headwaters, indicating that N-saturated forest could significantly change photoinduced and biogeochemical processes via enhanced HO formation in downstream water.

Estimating hydroxyl radical photochemical formation rates in natural waters during long-term laboratory irradiation experiments

In this study it was observed that, during long-term irradiations (>1 day) of natural waters, the methods for measuring hydroxyl radical (˙OH) formation rates based upon sequentially determined cumulative concentrations of photoproducts from probes significantly underestimate actual ˙OH formation rates. Performing a correction using the photodegradation rates of the probe products improves the ˙OH estimation for short term irradiations (<1 day), but not long term irradiations. Only the 'instantaneous' formation rates, which were obtained by adding probes to aliquots at each time point and irradiating these sub-samples for a short time (≤2 h), were found appropriate for accurately estimating ˙OH photochemical formation rates during long-term laboratory irradiation experiments. Our results also showed that in iron- and dissolved organic matter (DOM)-rich water samples, ˙OH appears to be mainly produced from the Fenton reaction initially, but subsequently from other sources possibly from DOM photoreactions. Pathways of ˙OH formation in long-term irradiations in relation to H2O2 and iron concentrations are discussed.

A general scavenging rate constant for reaction of hydroxyl radical with organic carbon in atmospheric waters

Hydroxyl radical (OH) is an important oxidant in atmospheric aqueous phases such as cloud and fog drops and water-containing aerosol particles. We find that numerical models nearly always overestimate aqueous hydroxyl radical concentrations because they overpredict its rate of formation and, more significantly, underpredict its sinks. To address this latter point, we examined OH sinks in atmospheric drops and aqueous particles using both new samples and an analysis of published data. Although the molecular composition of organic carbon, the dominant sink of OH, is extremely complex and poorly constrained, this sink behaves very similarly in different atmospheric waters and even in surface waters. Thus, the sink for aqueous OH can be estimated as the concentration of dissolved organic carbon multiplied by a general scavenging rate constant [kC,OH = (3.8 ± 1.9) × 10(8) L (mol C)(-1) s(-1)], a simple process that should significantly improve estimates of OH concentrations in atmospheric drops and aqueous particles.

Reactive uptake and photo-Fenton oxidation of glycolaldehyde in aerosol liquid water

The reactive uptake and aqueous oxidation of glycolaldehyde were examined in a photochemical flow reactor using hydrated ammonium sulfate (AS) seed aerosols at RH = 80%. The glycolaldehyde that partitioned into the aerosol liquid water was oxidized via two mechanisms that may produce aqueous OH: hydrogen peroxide photolysis (H2O2 + hν) and the photo-Fenton reaction (Fe (II) + H2O2 + hν). The uptake of 80 (±10) ppb glycolaldehyde produced 2-4 wt % organic aerosol mass in the dark (kH* = (2.09-4.17) × 10(6) M atm(-1)), and the presence of an OH source increased the aqueous uptake by a factor of 4. Although the uptake was similar in both OH-aging mechanisms, photo-Fenton significantly increased the degree of oxidation (O/C = 0.9) of the aerosols compared to H2O2 photolysis (O/C = 0.5). Aerosol organics oxidized by photo-Fenton and H2O2 photolysis resemble ambient "aged" and "fresh" OA, respectively, after the equivalent of 2 h atmospheric aging. No uptake or changes in particle composition occurred on dry seed aerosol. This work illustrates that photo-Fenton chemistry efficiently forms highly oxidized organic mass in aerosol liquid water, providing a possible mechanism to bridge the gap between bulk-phase experiments and ambient particles.

Online TOC analysis based on reagent-free oxidation of dissolved organic matter using a mercury lamp-pass-through photoreactor

The reagent-free mineralization of dissolved organic matter (DOM) in river water was achieved within 1 min using a lamp-pass-through photoreactor containing a narrow reaction tube (2 mm i.d.) passing through a 40 W mercury lamp. The structure efficiently irradiated the sample solution in the tube with vacuum ultraviolet (VUV; 185 nm) light from the lamp, which rapidly decomposed the DOM with hydroxyl radicals generated efficiently from the water and oxygen that are naturally present in the solution. The photoreactor was also applicable to oxidizing reagent-free online toatal organic carbon (TOC) analysis of DOM in river-water samples using a non-dispersive infrared radiation detector after acidification of the sample using 20 mmol L(-1) phosphoric acid. The detection limit for phthalate at the injection of 390 μL was 6.2 μg of carbon L(-1). The repeatability, as expressed by the relative standard deviation, was 2.5% for thrice-repeated analyses of a river sample with 1.85 mg of carbon L(-1).

Photochemical formation of hydroxyl radical from effluent organic matter

The photochemical formation of hydroxyl radical (HO•) from effluent organic matter (EfOM) was evaluated using three bulk wastewater samples collected at different treatment facilities under simulated sunlight. For the samples studied, the formation rates of HO•(R(HO•)) were obtained from the formation rate of phenol following the hydroxylation of benzene. The values of R(HO•) ranged from 2.3 to 3.8 × 10(-10) M s(-1) for the samples studied. The formation rate of HO• from nitrate photolysis (R(NO3)(HO•)) was determined to be 3.0 × 10(-7) M(HO)• M(NO3)(-1) s(-1). The HO• production rate from EfOM (R(EfOM)(HO•)) ranged from 0.76 to 1.3 × 10(-10) M s(-1). For the wastewater samples studied, R(EfOM)(HO•) varied from 1.5 to 2.4 × 10(-7) M(HO)• M(C)(-1) (s-1) on molarcarbon basis, which was close to HO• production from nitrate photolysis. The apparent quantum yield for the formation of HO• from nitrate (Φ(NO3-HO•)(a)) was determined as 0.010 ± 0.001 for the wavelength range 290-400 nm in ultrapure water. The apparent quantum yield for HO• formation in EfOM (Φ(EfOM-HO•)(a)) ranged from 6.1 to 9.8 × 10(-5), compared to 2.99 to 4.56 × 10(-5) for organic matter (OM) isolates. The results indicate that wastewater effluents could produce significant concentrations of HO•, as shown by potential higher nitrate levels and relatively higher quantum yields of HO• formation from EfOM.

Protective and curative effects of foliar-spray Fenton solutions against cucumber (Cucumis sativus, L.) powdery mildew

Various Fenton solutions have been developed for advanced oxidation processes in wastewater treatment. In this study, conventional Fenton solutions such as hydrogen peroxide (HOOH) + Fe(2+) (Mix 1) or HOOH + Fe(3+) (Mix 2), and a new type of solution, HOOH + Fe(3+) + oxalic acid (Mix 3), were used as foliar sprays against powdery mildew on cucumber caused by Sphaerotheca fuliginea. Three Fenton solutions, plus a fungicide, fenarimol, were used to cure and/or protect the plant from powdery mildew under greenhouse conditions. Determination of the ·OH photoformation rate of these Fenton solutions revealed that Mix 3 had a photoformation rate 3.6 - 4.3 times higher than those of Mix 1 and 2. Application of fenarimol and Mix 3 to each plant once a week for three weeks resulted in high curative effects for already-diseased plants. Double spraying with fenarimol and the Mix 3 solutions 1-7 days before S. fuliginea inoculation resulted in the protective effects continuing for up to 20 days after spraying. When the disease reemerged at 20 days post inoculation, one reapplication of the spray suppressed the disease for another 7 days. Overall, fenarimol and Mix 3 were most effective for both the protection and suppression of the disease. These results implied that Mix 3 had fungicidal effects similar to fenarimol; therefore, the use of a Fenton solution such as Mix 3 may offer new possibilities for disease control.