OH Formation Rate Research Articles

Challenges in the determination of reactive oxygen species evolving during membrane water electrolysis for in situ ozone production

The treatment of ultrapure water with electrochemically produced O3 is a common means for disinfection yet leads to the formation of a variety of reactive oxygen species (ROS). The present study draws a comprehensive comparison between three commonly used photometric and fluorometric assays for ROS analysis and quantifies the individual signal responses for dissolved O3, ·OH and H2O2, respectively, to account for cross-sensitivities. By calibrating all combinations of assays and analytes, we developed a quantification procedure to reliably determine the actual ROS composition in ultrapure water environments for different operation conditions of a membrane water electrolyzer with PbO2 anodes down to concentrations of 0.97 μg L−1. While the ·OH formation rate can be described linearly over the observed current density range, substantial O3 evolution is only found for current densities of 0.75 A cm−2 and above (up to 3.7 μmol h−1 for J = 1.25 A cm−2). The formation of H2O2 is only observed when an organic carbon source is introduced into the solution. We further quantify the interference of H2O2 with the reading of the oxidation-reduction potential as a common water parameter and elaborate on its validity to monitor the peroxone process when both H2O2 and O3 are present simultaneously.

Electrochemical oxidation degradation of fungicide 5-chloro-2-methyl-4-isothiazoline-3-one (CMIT) in brine of reverse osmosis by a novel Ti/CB@MXene anode

The electrochemical oxidation purification of reverse osmosis concentrated brine was sustainable water purification technology; however, anode oxidation efficiency and capacity limit its development. According to MXene’s high surface area and structure characterization, a novel Ti/ carbon black (CB) @MXene anode was prepared for the first time for electrochemical oxidation degradation of 5-chloro-2-methyl-4-isothiazoline-3-one (CMIT) in reverse osmosis brine, even though MXene was studied more widely in sulfate radical based AOP and photocatalysis. This anode showed slippy surface and high oxygen evolution potential (2.84 V), suggesting it is hard to be fouled and eliminated the side reaction effect. According to the anode fabrication parameters optimization, the best mass ratio of Ti3C2Tx and CB was 1:1, showing the highest oxygen evolution potential. The reaction parameters including current density and plate spacing were optimized as 5.625 mA/cm2 and 1.0 cm, where the best CMIT degradation efficiency was 89.86 %. This performance was better than DSA and similar with BDD anode, however its stability needs to be improved. The indirect oxidation dominated CMIT degradation as 99.69 % and •OH formation rate was 13.81 L·mol−1·s−1, which was approximately 5.04 fold higher than that of traditional Ti anode. Three possible degradation pathways of CMIT were proposed, as below: electron rich sulfur in SN bond was oxidized to form sulfoxide structure, chlorine was replaced by •OH, olefin double bond was attacked by •OH. The formed intermediates showed less ecotoxicity. The Ti/CB@MXene anode showed better potential in electrochemical oxidation treatment of reverse osmosis brine.

Oxidative potential associated with water-soluble components of PM2.5 in Beijing: The important role of anthropogenic organic aerosols

Oxidative stress is the mainstream toxicological mechanism for the adverse health outcomes of ambient aerosols. However, our understanding of the crucial redox-active species affecting the oxidative potential of water-soluble aerosols (OPWS) remains limited. In this study, the OPWS of PM2.5 in Beijing was measured using dithiothreitol (DTT) assay, including DTT consumption rate and ·OH formation rate. OPWS was more closely related to water-soluble organic compounds (WSOC) rather than transition metals. Laboratory simulations were conducted to investigate the effects of individual target species in the context of complex metal-organic interactions. The results showed that reducing WSOC can effectively decrease OPWS, while reducing Cu2+ increased OPWS. Parallel factor analysis demonstrated that OPWS was the most significantly correlated with the highly oxidized humic-like or quinone-like substances. Multiple linear regression showed that aromatic secondary organic carbon (SOC) (34.4%), other primary combustion sources of WSOC (20.0%), primary biomass burning WSOC (19.8%), transition metal ions (12.9%) and biomass burning SOC (12.8%) made significant contributions to DTTV. In addition to the anthropogenic sources of WSOC, the aged biogenic SOC also contributed to OHV, particularly in summer. Reducing anthropogenic WSOC was the key to the effective control of OPWS of PM2.5 in Beijing.

Wavelength Dependence of the Transformation Mechanism of Sulfonamides Using Different LED Light Sources and TiO2 and ZnO Photocatalysts.

The comparison of the efficiency of the commercially available photocatalysts, TiO2 and ZnO, irradiated with 365 nm and 398 nm light, is presented for the removal of two antibiotics, sulfamethazine (SMT) and sulfamethoxypyridazine (SMP). The •OH formation rate was compared using coumarin, and higher efficiency was proved for TiO2 than ZnO, while for 1,4-benzoquinone in O2-free suspensions, the higher contribution of the photogenerated electrons to the conversion was observed for ZnO than TiO2, especially at 398 nm irradiation. An extremely fast transformation and high quantum yield of SMP in the TiO2/LED398nm process were observed. The transformation was fast in both O2 containing and O2-free suspensions and takes place via desulfonation, while in other cases, mainly hydroxylated products form. The effect of reaction parameters (methanol, dissolved O2 content, HCO3− and Cl−) confirmed that a quite rarely observed energy transfer between the excited state P25 and SMP might be responsible for this unique behavior. In our opinion, these results highlight that “non-conventional” mechanisms could occur even in the case of the well-known TiO2 photocatalyst, and the effect of wavelength is also worth investigating.

Impact of Reaction Parameters and Water Matrices on the Removal of Organic Pollutants by TiO2/LED and ZnO/LED Heterogeneous Photocatalysis Using 365 and 398 nm Radiation.

In this work, the application of high-power LED365nm and commercial, low-price LED398nm for heterogeneous photocatalysis with TiO2 and ZnO photocatalysts are studied and compared, focusing on the effect of light intensity, photon energy, quantum yield, electrical energy consumption, and effect of matrices and inorganic components on radical formation. Coumarin (COU) and its hydroxylated product (7-HC) were used to investigate operating parameters on the •OH formation rate. In addition to COU, two neonicotinoids, imidacloprid and thiacloprid, were also used to study the effect of various LEDs, matrices, and inorganic ions. The transformation of COU was slower for LED398nm than for LED365nm, but r07-HC/r0COU ratio was significantly higher for LED398nm. The COU mineralization rate was the same for both photocatalysts using LED365nm, but a significant difference was observed using LED398nm. The impact of matrices and their main inorganic components Cl− and HCO3− were significantly different for ZnO and TiO2. The negative effect of HCO3− was evident, however, in the case of high-power LED365nm and TiO2, and the formation of CO3•− almost doubled the r07-HC and contributes to the conversion of neonicotinoids by altering the product distribution and mineralization rate.

Study on stretch extinction characteristics of methane/carbon dioxide versus oxygen/carbon dioxide counterflow non-premixed combustion under elevated pressures

As a clean gaseous fuel, methane (the main constituent of natural gas) has been widely used in the lives of residents and industrial processes. The oxy-fuel combustion technology for methane/carbon dioxide is an effective way to reduce the pollutant emissions of NOx, which is also beneficial to the capture of carbon dioxide. Herein, the stretch extinction characteristics of non-premixed flames induced by methane/carbon dioxide mixtures versus oxygen/carbon dioxide mixtures counterflow combustion with various fuel concentrations were studied in high-pressure environments. The computed flame extinction strain rates of oxygen-enriched and air combustions in low fuel concentration range at pressures of 0.1, 0.5 and 0.7 MPa have a good consistency with the experiment results obtained from previous literature. The results show that the increase of flame extinction limit represented by the stretch rate for the oxygen-enriched combustion with increasing pressure is more significant than that for the air combustion. The analyses of reaction pathways, integral of reaction rates and sensitivity coefficients suggest that the OH formation rates of near extinction flames limits can be used to estimate the ratio of the flame extinction strain rate for oxygen-enriched combustion to that for air combustion under high pressures. The reasons for the enhancement of the OH formation rate aroused by increasing pressures were clarified.

Lewis Acid Strength of Interfacial Metal Sites Drives CH3 OH Selectivity and Formation Rates on Cu-Based CO2 Hydrogenation Catalysts.

CH3 OH formation rates in CO2 hydrogenation on Cu-based catalysts sensitively depend on the nature of the support and the presence of promoters. In this context, Cu nanoparticles supported on tailored supports (highly dispersed M on SiO2 ; M=Ti, Zr, Hf, Nb, Ta) were prepared via surface organometallic chemistry, and their catalytic performance was systematically investigated for CO2 hydrogenation to CH3 OH. The presence of Lewis acid sites enhances CH3 OH formation rate, likely originating from stabilization of formate and methoxy surface intermediates at the periphery of Cu nanoparticles, as evidenced by metrics of Lewis acid strength and detection of surface intermediates. The stabilization of surface intermediates depends on the strength of Lewis acid M sites, described by pyridine adsorption enthalpies and 13 C chemical shifts of -OCH3 coordinated to M; these chemical shifts are demonstrated here to be a molecular descriptor for Lewis acid strength and reactivity in CO2 hydrogenation.

Catalytic pathways and mechanistic consequences of water during vapor phase hydrogenation of butanal on Ru/SiO2

Kinetic assessments, Fourier-transform infrared spectroscopy, and H/D kinetic and isotopic exchange studies establish the sequence of H-addition steps and reveal how water induces a shift in their kinetic relevance during butanal hydrogenation on Ru/SiO2. Without water, hydrogenation turnovers are slower than the H/D exchange rates at the carbonyl carbon and increase linearly with butanal and H2 pressures. Thus, hydrogenation occurs via quasi-equilibrated H-addition to the carbonyl carbon, which forms an alkoxy intermediate, before the second, kinetically relevant H-addition to the oxygen. Water vapor does not alter CH formation rates but promotes the OH formation rates by stabilizing the transition state through hydrogen bonds, making the first H-addition that forms the CH bond kinetically relevant, thus decreasing the apparent order to H2. This study provides the mechanistic context behind the water promotional effects during butanal hydrogenation, as the chemical potential of water increases and hydrogen bonding prevails and mediates catalysis.

Effect of Straw Return on Hydroxyl Radical Formation in Paddy Soil.

Straw return, as an important agricultural management measure, is receiving growing attention. Hydroxyl radical (•OH) can be produced when subsurface soil interacts with oxygen, but the effects of straw incorporation on •OH formation have rarely been evaluated. In this study, we found that straw return had a significant effect on soil properties. Soil pH and redox potential (Eh) decreased while electronic conductivity (EC) showed an increment. Dissolved organic carbon content of soil initially increased and then decreased to the same level as the control by the end of the experiment of 120days. Moreover, Fe(II) formation was promoted by straw return under anaerobic conditions. •OH was produced in the flooded paddy soil when exposed to oxygen, which correlated well with Fe(II) content. The effect of rape (Brassica campestris L.) straw on •OH formation rate was more evident as compared to wheat (Triticum aestivum L.) straw, suggesting a potentially more profound influence of rape straw return on pollutant transformation in paddy soils.

Mechanistic and Kinetic Understanding of the UV254 Photolysis of Chlorine and Bromine Species in Water and Formation of Oxyhalides.

This study investigated the UV254 photolysis of free available chlorine and bromine species in water. The intrinsic quantum yields for •OH and X• (X = Cl or Br) generation were determined by model fitting of formaldehyde formation using a tert-butanol assay to be 0.61/0.45 for HOCl/OCl- and 0.32/0.43 for HOBr/OBr-. The steady-state •OH concentration in UV/HOX was higher than that in UV/OX- by a factor of 23.3 and 7.8 for Cl and Br, respectively. This was attributed to the different •OH consumption rate by HOCl versus OCl-, while for HOBr/OBr-, both the •OH formation and consumption rates were implied. This was supported by a k of 1.4 × 108 M-1 s-1 for the •OH reaction with HOCl, which was >14 times less than the k for •OH reactions with OCl-, HOBr, and OBr-. Formation of ClO3- and BrO3- was found to be significant with apparent quantum yields of 0.12-0.23. A detailed mechanistic study on the formation of XO3- including a new pathway involving XO• is presented, which has important implications as the level of XO3- can exceed the regulation (BrO3-) or guideline (ClO3-) values during UV/halogen oxidant water treatment. Our new kinetic models well simulate the experimental results for the halogen oxidant decomposition, probe compound degradation, and formation of ClO3- and BrO3-.

The pollution levels, variation characteristics, sources and implications of atmospheric carbonyls in a typical rural area of North China Plain during winter

Atmospheric carbonyls were measured at a typical rural area of the North China Plain (NCP) from November 13 to December 24, 2017 to investigate the pollution characteristics, sources and environmental implications. Fifteen carbonyls were detected, and formaldehyde, acetaldehyde and acetone accounted for about 81% at most. The concentration of the total carbonyls in heavily polluted days was twice more than that in clean days. In contrast to other carbonyls, m-tolualdehyde exhibited relatively high concentrations in the clean days in comparison with the polluted days. The ratios of three principal carbonyls to CO showed similar daily variations at different pollution levels with significant daytime peaks. Multiple linear regression analysis revealed that the contributions of background, primary and secondary sources to three principal carbonyls showed similar variation trends from the clean level to the heavily polluted level. The OH formation rate of formaldehyde showed a similar variation trend to its photodegradation rate, reaching the peak value at noon, which is important to maintain relatively high OH levels to initiate the oxidation of various gas-phase pollutants for secondary pollutant formation at the rural site. OH radical consumption rate and ozone formation potential (OFP) calculations showed that formaldehyde and acetaldehyde were the dominant oxidative species among measured carbonyls. As for OH radical consumption, n-butyraldehyde and m-tolualdehyde were important contributors, while for ozone formation potential, n-butyraldehyde and propionaldehyde made significant contributions. In addition, the contribution of carbonyl compounds to secondary organic aerosol (SOA) formation was also important and needs further investigation.

Budget of nitrous acid and its impacts on atmospheric oxidative capacity at an urban site in the central Yangtze River Delta region of China

Abstract In this study, we used a wet chemistry based long-path absorption spectroscopy method to measure HONO in Changzhou, in the central Yangtze River Delta region (YRD) of China, from April 3–24, 2017. During the observation period, the average HONO mixing ratio was 1.55 ± 1.21 ppbv. In addition, the average OH formation rates of the photolysis of HONO, O3, HCHO and H2O2 along with ozonolysis of alkenes were 7.84 × 106, 2.02 × 107, 7.41 × 105, 3.79 × 105 and 1.51 × 106 molecules cm−3 s−1, respectively. At nighttime, the average conversion rate from NO2 to HONO was determined to be ~0.018 h−1. In this work, the primary emission rate of HONO was determined by the ratios of HONO to nitrogen oxides (NOx = NO + NO2) within freshly emitted plumes (NO/NOx > 0.85) and a value of ~0.69% was obtained. The sources of HONO were further investigated through a box model utilizing the Master Chemical Mechanism. The simulation results show that primary emissions contributed only ~12.3% of the total HONO budget during daytime but a substantial portion (31.4%) at night. Comparing to heterogeneous HONO sources, the gas-phase NO + OH reaction was the less important HONO source, with a contribution of 14.2% at night and 28.7% during the daytime. Heterogeneous reactions of NO2 on various surfaces (mostly the ground surface) were responsible for most of the observed HONO, i.e., 34.7% during the day and 54.4% at night. Overall, nighttime HONO can be reasonably explained by aforementioned mechanisms. However, daytime HONO cannot be fully accounted for without the consideration of nitrate photolysis, which contributed ~24.1% of the daytime HONO according to the model simulation. Our findings highlight that photolysis of particulate nitrate could be an important source of daytime HONO, providing a channel to cycle NO and OH radical back into the photochemical system and further enhancing the atmospheric oxidative capacity within the air shed of a typical industrial zone of China.

Construction of high-efficient visible photoelectrocatalytic system for carbamazepine degradation: Kinetics, degradation pathway and mechanism

This study aimed to construct a photoelectrocatalytic (PEC) reaction system based on the self-made reduced TiO2 NTAs (r-TNAs) photoanode and activated carbon/Polytetrafluoroethylene (AC/PTFE) cathode. It would be observed clearly that the degradation rate constant of carbamazepine (CBZ) over r-TNAs(photoanode)-AC/PTFE(cathode) PEC system (0.04961 min−1) was even higher than that of r-TNAs(photoanode)-Pt(cathode) PEC system (0.04602 min−1) with the assistance of visible light irradiation and +0.4 V external potential. Besides, in order to obtain optimized conditions, the influence of key parameters such as pH value, electric current density and electrolyte concentration were studied. Most importantly, photoelectrochemical (PECH) properties, reactive oxide species contribution, OH formation rate and CBZ degradation pathway were determined. The results illustrated that the excellent PEC degradation performance depended on the excellent photocatalytic property of r-TNAs photoanode and electron transfer property of photoelectrodes in r-TNAs(photoanode)-AC/PTFE(cathode) PEC system. Therefore, the study demonstrated that the r-TNAs(photoanode)-AC/PTFE(cathode) PEC system could be expected to replace metal-catalyzed cathodes depending on its excellent PEC performance activity and low cost as well as the reaction system possessed objective and practical application prospect.

CO2 Hydrogenation on Cu/Al2 O3 : Role of the Metal/Support Interface in Driving Activity and Selectivity of a Bifunctional Catalyst.

Selective hydrogenation of CO2 into methanol is a key sustainable technology, where Cu/Al2 O3 prepared by surface organometallic chemistry displays high activity towards CO2 hydrogenation compared to Cu/SiO2 , yielding CH3 OH, dimethyl ether (DME), and CO. CH3 OH formation rate increases due to the metal-oxide interface and involves formate intermediates according to advanced spectroscopy and DFT calculations. Al2 O3 promotes the subsequent conversion of CH3 OH to DME, showing bifunctional catalysis, but also increases the rate of CO formation. The latter takes place 1) directly by activation of CO2 at the metal-oxide interface, and 2) indirectly by the conversion of formate surface species and CH3 OH to methyl formate, which is further decomposed into CH3 OH and CO. This study shows how Al2 O3 , a Lewis acidic and non-reducible support, can promote CO2 hydrogenation by enabling multiple competitive reaction pathways on the oxide and metal-oxide interface.

PdAu-catalyzed oxidation through in situ generated H2O2 in simulated produced water

Abstract Most wastewater recovered from hydraulically fractured oil and gas wells (i.e. produced water) is transported to government-permitted, salt-water disposal units (SWDs) and discarded via downhole injection. However, there is a limited availability of disposal wells in some states and growing interest over future options for beneficial reuse. One alternative to using SWD facilities is to recycle the water for further use in oilfield operations. Residual oil and grease are one contaminant class in produced water where cost-effective treatment technologies are lacking. In this work, we studied the ability of alumina-supported bimetallic PdAu to degrade organic compounds at room temperature and atmospheric pressure via the catalytic formation of H2O2. Similar to monometallic Pd and Au catalysts, the PdAu catalyst produced H2O2 and hydroxyl radicals in the presence of oxygen and formic acid. The bimetallic catalyst was the most active in terms of initial OH formation rate, and when phenol was present, PdAu showed the highest rate of phenol degradation. We assessed the promotional and inhibitory effects of other species present in produced water including ferrous ion concentration, pH and salt concentration on catalytic phenol oxidation. PdAu was catalytically active for phenol degradation in simulated produced water at salinities as high as ˜0.3 M (˜16,000 ppm). The combination of air-formic acid-bimetallic catalyst is an intriguing approach for the degradation of organics in contaminated water at low pH and moderate salinity.

Synergetic effect of TiO2 and Fe3+ as co-catalysts for enhanced phenol degradation in pulsed discharge system

In this work, the synergetic effect of TiO2 and Fe3+ in pulsed discharge plasma has systematically investigated using phenol as the probe molecule. The dominant effects of TiO2 and Fe3+ dosage were firstly studied, and then phenol degradation was investigated in four parallel experiments including plasma alone, plasma/Fe3+, plasma/TiO2 and plasma/Fe3+/TiO2. The experimental results showed that the phenol removal efficiency in plasma/Fe3+/TiO2 system increased by 25% in comparison with plasma alone, which were only 9% and 10% in plasma/TiO2 and plasma/Fe3+ system, indicating a significantly synergistic effect between Fe3+ and TiO2. To illustrate the synergetic effect of TiO2 and Fe3+ ions, the TiO2 structural characterization was analyzed by XPS, UV–vis spectra and XRD, and Fe2+ andOH concentration was also determined during discharge process. The Fe3+ ions and excited nitrogen from plasma discharge were doped on TiO2 particles, which narrowed the band gap of TiO2 from 3.0eV to 2.0eV and enlarged the absorption edge at around 600nm, and therefore enhanced the photocatalytic activity in the visible light. The co-doping of Fe3+ and nitrogen significantly increased the separation rate of photo-generated electrons and holes and prolonged their lifetime. The photoelectron-transfer pathway was blocked by Fe3+, the Fe3+ ions could be changed to Fe2+ ions using photoelectron on TiO2 surface, inducing Fenton-like reaction for the enhancement of the OH formation rate in the plasma/TiO2/Fe3+ system. The concentration of OH increased from 14.8×10−5molL−1 in the plasma/TiO2 system to 20.6×10−5molL−1 in the plasma/TiO2/Fe3+ system.

Solar-induced generation of singlet oxygen and hydroxyl radical in sewage wastewaters

Singlet oxygen (1O2) and hydroxyl radical (·OH) play an important role in the degradation of pollutants in surface waters. However, the mechanism underlying the photochemical generation of 1O2 and ·OH in wastewaters is poorly known. Here we studied the photo-induced generation of 1O2 and ·OH in different sewage treatment plant units. The correlation between the generation of 1O2 and ·OH and the water constituents was discussed. Our results show that in sewage units the 1O2 formation rate ranges from 2.19 × 10−8 to 6.74 × 10−8 mol L−1 s−1, and the ·OH formation rate ranges from 1.7 × 10−11 to 3.06 × 10−10 mol L−1 s−1. The average 1O2 formation rates in the various sewage units are similar to those in wetland and estuarine waters containing rich dissolved organic matter and 2–4 times higher than those in lake and seawater samples. The average ·OH formation rates of the sewage units are 5–50 times higher than for other water samples reported. The ·OH generation rate increased with the iron content with a correlation coefficient of 0.85, which indicates that the photo-Fenton reaction plays a dominant role in ·OH generation in sewage wastewater.

Hematite facet confined ferrous ions as high efficient Fenton catalysts to degrade organic contaminants by lowering H2O2 decomposition energetic span

In this study, we demonstrate that ferrous ions confined on the hematite facets can significantly promote the H2O2 decomposition to produce •OH for more efficient organic contaminants degradation than the unconfined counterparts, while hematite nanorods with exposed {001} and {110} facets exhibit better confining effect than nanoplates with exposed {001} facets. Experimental results revealed that the H2O2 decomposition efficiency of hematite facet confined ferrous ions was affected by the amount of surface confined ferrous ions, and also governed by the binding mode of ferrous ions on the hematite facets. We interestingly found that the polar {110} facets could confine ferrous ions of higher density with a five-coordination binding mode and thus lower the H2O2 decomposition energetic span more efficiently than the nonpolar {001} facets, which confined ferrous ions with a six-coordination binding mode. The specific surface area normalized •OH formation rate constants were, respectively, 5.50×10−3 and 1.04×10−2s−1 for hematite nanoplates and nanorods confined ferrous ions, which were 1.2 and 2.2 times that (4.75×10−3s−1) of the unconfined counterpart. Moreover, rhodamine B could be efficiently degraded in the presence of hematite (0.4gL−1), Fe2+ (5.0×10−5molL−1) and H2O2 (5.0×10−5molL−1) at pH 4.7, along with 23.6% and 72.5% of H2O2 consumption efficiencies for hematite nanoplates and nanorods, respectively. Meanwhile, 6.6% and 11.8% of nitrogen in rhodamine B were converted to NO3− in the hematite nanoplates and nanorods confined ferrous ions Fenton systems, respectively. It was found that N-deethylation, destruction of chromophores, opening-ring and mineralization occurred during the confined ferrous Fenton rhodamine B oxidation process. This study can deepen our understanding on the enhanced reactivity of ferrous ions bound on the surface of iron oxides, and also shed light on the design of high efficient heterogeneous Fenton catalysts.

Formation of hydroxyl radicals from photolysis of secondary organic aerosol material

Abstract. This paper demonstrates that OH radicals are formed by photolysis of secondary organic aerosol (SOA) material formed by terpene ozonolysis. The SOA is collected on filters, dissolved in water containing a radical trap (benzoic acid), and then exposed to ultraviolet light in a photochemical reactor. The OH formation rates, which are similar for both α-pinene and limonene SOA, are measured from the formation rate of p-hydroxybenzoic acid as measured using offline HPLC analysis. To evaluate whether the OH is formed by photolysis of H2O2 or organic hydroperoxides (ROOH), the peroxide content of the SOA was measured using the horseradish peroxidase-dichlorofluorescein (HRP-DCF) assay, which was calibrated using H2O2. The OH formation rates from SOA are 5 times faster than from the photolysis of H2O2 solutions whose concentrations correspond to the peroxide content of the SOA solutions, assuming that the HRP-DCF signal arises from H2O2 alone. The higher rates of OH formation from SOA are likely due to ROOH photolysis, but we cannot rule out a contribution from secondary processes as well. This result is substantiated by photolysis experiments conducted with t-butyl hydroperoxide and cumene hydroperoxide which produce over 3 times more OH than photolysis of equivalent concentrations of H2O2. Relative to the peroxide level in the SOA and assuming that the peroxides drive most of the ultraviolet absorption, the quantum yield for OH generation from α-pinene SOA is 0.8 ± 0.4. This is the first demonstration of an efficient photolytic source of OH in SOA, one that may affect both cloud water and aerosol chemistry.

Dark Formation of Hydroxyl Radical in Arctic Soil and Surface Waters

Hydroxyl radical (•OH) is a highly reactive and unselective oxidant in atmospheric and aquatic systems. Current understanding limits the role of DOM-produced •OH as an oxidant in carbon cycling mainly to sunlit environments where •OH is produced photochemically, but a recent laboratory study proposed a sunlight-independent pathway in which •OH forms during oxidation of reduced aquatic dissolved organic matter (DOM) and iron. Here we demonstrate this non-photochemical pathway for •OH formation in natural aquatic environments. Across a gradient from dry upland to wet lowland habitats, •OH formation rates increase with increasing concentrations of DOM and reduced iron, with highest •OH formation predicted at oxic-anoxic boundaries in soil and surface waters. Comparison of measured vs expected electron release from reduced moieties suggests that both DOM and iron contribute to •OH formation. At landscape scales, abiotic DOM oxidation by this dark •OH pathway may be as important to carbon cycling as bacterial oxidation of DOM in arctic surface waters.