OH Precursor Research Articles

SnO-SnO<sub>2</sub> Nanocomposites Based pn Diode: <i>In Situ</i> Synthesis, Characterization and Fabrication of Device for pn Diode Applicability

We report on an approach for the in-situ synthesis (chemical method based) of SnO-SnO2 nanocomposites followed by characterisation (including morphological, chemical, structural and optical) and investigation of the electrical properties of the nanocomposites with reference to the as-synthesized SnO2 nanoparticles. Compared to spherical SnO2 particles, the SnO phase is found existing in the form of sheet like morphology. It has been found that through controlling of the Sn:OH precursor ratio is effective for the achievement of SnO phase. Compared to the pristine SnO2 nanoparticles, the current-voltage (I-V) characteristics of the nanocomposites show the p-n junction characteristics. The observation of rectification ratio 2.05 indicates the excellent rectifying property of the nanocomposites due to the presence of p-type SnO phase. Further, exploration of the I-V characteristics has revealed the dominance of space-charge limited current transport mechanism for the nanocomposites sample. The lattice defects are discovered to be the cause of the transport mechanism in the nanocomposites sample.

Engineering an Almost All-Waterborne System for Transparent yet Superhydrophobic Surfaces with High Liquid Impalement Resistance.

Realistically, green manufacturing of transparent superhydrophobic surfaces (SHSs) and high liquid impalement resistance for outdoor engineering are very necessary but pretty challenging. To address this, an almost all-waterborne system composed of synthesized partially open-cage fluorinated polyhedral oligomeric silsesquioxane bearing a pair of -OH (poc-FPOSS-2OH), silica sol, and resin precursor is engineered. The transparent SHSs facilely formed by this system are featured with the exclusive presence of wrapped silica nanoparticle (SiNP) dendritic networks at solid-gas interfaces. The wrapped SiNP dendritic networks have a small aggregation size and low distribution depth, making SHSs highly transparent. The Si-O polymeric wrappers render mechanical flexibility to SiNP dendritic networks and thus enable transparent SHSs to resist high-speed water jet impinging with a Weber number of ≥19 800 in conjunction with the extremely low-surface-energy poc-FPOSS-2OH, which is the highest liquid impalement resistance so far among waterborne SHSs, and can rival the state-of-the-art solventborne SHSs.

Reconciling the bottom-up and top-down estimates of the methane chemical sink using multiple observations

Abstract. The methane chemical sink estimated by atmospheric chemistry models (bottom-up method) is significantly larger than estimates based on methyl chloroform (MCF) inversions (top-down method). The difference is partly attributable to large uncertainties in hydroxyl radical (OH) concentrations simulated by the atmospheric chemistry models used to derive the bottom-up estimates. In this study, we propose a new approach based on OH precursor observations and a chemical box model. This approach contributes to improving the 3D distributions of tropospheric OH radicals obtained from atmospheric chemistry models and reconciling bottom-up and top-down estimates of the chemical loss of atmospheric methane. By constraining simulated OH precursors with observations, the global mean tropospheric column-averaged air-mass-weighted OH concentration ([OH]trop-M) is ∼10×105 molec. cm−3 (which is 2×105 molec. cm−3 lower than the original model-simulated global [OH]trop-M) and agrees with that obtained by the top-down method based on MCF inversions. With OH constrained by precursor observations, the methane chemical loss is 471–508 Tg yr−1, averaged from 2000 to 2009. The new adjusted estimate is in the range of the latest top-down estimate of the Global Carbon Project (GCP) (459–516 Tg yr−1), contrary to the bottom-up estimates that use the original model-simulated OH fields (577–612 Tg yr−1). The overestimation of global [OH]trop-M and methane chemical loss simulated by the atmospheric chemistry models is caused primarily by the models' underestimation of carbon monoxide and total ozone column, and overestimation of nitrogen dioxide. Our results highlight that constraining the model-simulated OH fields with available OH precursor observations can help improve bottom-up estimates of the global methane sink.

Peroxyl radicals from diketones enhanced the indirect photochemical transformation of carbamazepine: Kinetics, mechanisms, and products

In surface waters, photogenerated transients (e.g., hydroxyl radicals, carbonate radicals, singlet oxygen and the triplet states of dissolved organic matter) are known to play a role in the transformation of biorecalcitrant carbamazepine (CBZ). Small diketones, such as acetylacetone (AcAc) and butanedione (BD), are naturally abundant and have been proven to be effective precursors of carbon and oxygen centered radicals. However, the photochemical kinetics and mechanisms of coexisting diketones and CBZ are barely known. Herein, the effects of AcAc and BD on the photochemical conversion of CBZ were investigated compared with H2O2 which was the main ·OH precursor in the environment. An enhancing effect was observed for the degradation of CBZ by the addition of diketones. The enhancing effect of diketones was pH-dependent and much more significant than H2O2 under simulated solar irradiation. On the basis of the identification of transient species and the competition kinetic model, organic peroxyl radicals were found to play a dominant role in CBZ photodegradation, and the second-order rate constants of the reaction between CBZ and peroxyl radicals were determined to be approximately 107–108 M–1s–1. Furthermore, mutagenic acridine was found to be the major cumulative intermediate with a yield of > 30% in the presence of diketones, which might be an environmental concern. This work indicates that the coexistence of diketones and persistent organic pollutants might lead to some detrimental effects on aquatic environments if the water is exposed to sunlight.

Overlooked Formation of H2O2 during the Hydroxyl Radical-Scavenging Process When Using Alcohols as Scavengers.

Hydroxyl radical (•OH) is an active species widely reported in studies across many scientific fields, and hence, its reliable analysis is vitally important. Currently, alcohols are commonly used as scavengers for •OH determination. However, the impacts of alcohols on the reliability of •OH detection remain unknown. In this study, we found that adding different types and different amounts of alcohols in water samples treated with ultraviolet irradiation undesirably produced substantial amounts of hydrogen peroxide (H2O2), which is a known •OH precursor. This means that the conventional •OH determination method using alcohols is likely unreliable or even misleading. Through careful investigation, we revealed an overlooked reaction pathway during H2O2 and •OH transformations. Varying oxygen concentrations, pHs, alcohol dosages, and types altered H2O2 formation, which can affect •OH determination accuracy. Among alcohols, n-butanol is the best scavenger because it quenches •OH rapidly but re-forms little H2O2.

Optimization of B2O3 coating process for NCA cathodes to achieve long-term stability for application in lithium ion batteries

Ni-rich cathodes exhibit rapid capacity degradation upon cycling; hence, the development of Ni-rich cathode materials with a stable electrochemical performance for use in next-generation lithium-ion batteries (LIBs) are extremely challenging. In this study, boron trioxide (B2O3)-modified LiNi0.87Co0.10Al0.03O2 (NCA) compounds with high charging–discharging structural stability were prepared using the Ni0.88Co0.095Al0.025OH precursor for application in LIB cathodes. In addition, the sintering temperature of the NCA cathode material was optimized. B2O3 not only filled in the cracks that formed during the first sintering process but also effectively prevented the fragmentation and loss of cathode NCA particles during the charge–discharge process, thus improving their electrochemical properties. The coated particles exhibited sufficient structural integrity and did not exhibit severe cracking during the cycling process. The B2NCA3 cathode with 2.0 wt% B2O3 demonstrated sufficient cyclic stability and rate properties. It exhibited a high specific capacity of 184 mAh g−1 and a capacity retention of 86% after 100 cycles at 0.2 C and 25 °C.

Tailoring Lewis acidic metals and SO42− functionalities on bimetallic Mn-Fe oxo-spinels to exploit supported SO4[rad]− in aqueous pollutant fragmentation

Generation of SO4− anchored on metal oxides via radical transfer from •OH to surface SO42− functionality (•OH → SO4−) is singular, unraveled recently, and promising to decompose aqueous refractory contaminants. The core in furthering supported SO4− production is to reduce the energy required to accelerate the rate-determining step of the •OH → SO4− (•OH desorption), while increasing the collision frequency between the •OH precursors (H2O2) and H2O2 activators (Lewis acidic metals) or between SO42−-attacking radicals (•OH) and supported SO4− precursors (SO42−). Herein, Mn-substituted Fe3O4 oxo-spinels (MnXFe3-XO4; MnX) served as reservoirs to accommodate the Lewis acidic Fe/Mn and SO42−, whose properties were tailored by altering the metal compositions (X). The production of supported SO4− via the •OH → SO4− was of high tangibility, as confirmed by their electron paramagnetic resonance spectra coupled with those simulated. A concave trend was observed in the plot of the Lewis acidic strength of Fe/Mn versus X of MnX with the minimum at X ~ 1.5. Hence, Mn1.5 could expedite •OH liberation from the surface most proficiently and therefore exhibited the greatest initial H2O2 scission rate, as corroborated by its lowest energy barrier needed for activating the •OH → SO4−. Meanwhile, a volcano-shaped trend was found in the plot of SO42− concentration versus X of MnX (other than Mn3). This could tentatively increase the collision frequency between •OH and SO42− on the surface of Mn1.5, as partially substantiated by its second largest pre-factor among the catalysts. Therefore, Mn1.5 exhibited the highest phenol consumption rate (-rPHENOL, 0) among the catalysts, which was ~ 20-fold larger than those for SO42−-modified Fe2O3 and NiO, which we reported previously. Mn1.5 also outperformed other catalysts in recycling phenol degradation, fragmenting another pollutant (aniline), and mineralizing phenol/aniline.

Photoinduced Oxidation Reactions at the Air-Water Interface.

Chemistry on water is a fascinating area of research. The surface of water and the interfaces between water and air or hydrophobic media represent asymmetric environments with unique properties that lead to unexpected solvation effects on chemical and photochemical processes. Indeed, the features of interfacial reactions differ, often drastically, from those of bulk-phase reactions. In this Perspective, we focus on photoinduced oxidation reactions, which have attracted enormous interest in recent years because of their implications in many areas of chemistry, including atmospheric and environmental chemistry, biology, electrochemistry, and solar energy conversion. We have chosen a few representative examples of photoinduced oxidation reactions to focus on in this Perspective. Although most of these examples are taken from the field of atmospheric chemistry, they were selected because of their broad relevance to other areas. First, we outline a series of processes whose photochemistry generates hydroxyl radicals. These OH precursors include reactive oxygen species, reactive nitrogen species, and sulfur dioxide. Second, we discuss processes involving the photooxidation of organic species, either directly or via photosensitization. The photochemistry of pyruvic acid and fatty acid, two examples that demonstrate the complexity and versatility of this kind of chemistry, is described. Finally, we discuss the physicochemical factors that can be invoked to explain the kinetics and thermodynamics of photoinduced oxidation reactions at aqueous interfaces and analyze a number of challenges that need to be addressed in future studies.

Infrared Spectroscopic and Kinetic Characterization on the Photolysis of Nitrite in Alcohol-Containing Aqueous Solutions.

Nitrite is regarded as a potential OH and NO precursor in aqueous solution upon ultraviolet photolysis. A step-scan Fourier-transform interferometer was employed to collect the transient infrared difference spectra upon excitation of the sodium nitrite aqueous solution in the presence of methanol and ethanol upon 355 nm pulsed excitation. The photolytic intermediates were proposed to be NO and NO2 via the direct dissociation from NO2- and the rapid reaction of OH and NO2-, respectively. Coupled with the theoretical calculations of the absolute energies and harmonic wavenumbers of relevant species using B3LYP density functional theory with the C-PCM model to account for the medium effect of H2O, a transient band at 1860-2030 cm-1 could be attributed to dissolved N2O3 isomers that could be quickly generated from NO + NO2. On the basis of the predicted thermodynamics, the reactions of alcohols with N2O3 were less thermodynamically favorable than that of water, resulting in a slightly decelerated depletion rate of N2O3 in alcohol-containing aqueous solution. Comparing the transient population of N2O3 in the absence and presence of CH3OH or C2H5OH, the upper-bound bimolecular rate coefficient of NO or NO2 with alcohols is reported as 7.3 × 103 M-1 s-1 for the first time. The spectroscopic and kinetic evidence of the reactivity of alcohols with NO, NO2, and N2O3 are provided to augment the roles of alcohols and NxOy in solution or in aqueous aerosol photochemistry.

The effect of ultrasonic treatment on the phase composition of hydroxylapatite synthesized using a hydrothermal technique

The influence of the duration of ultrasonic treatment after hydrothermal synthesis of hydroxylapatite (HA) samples synthesized in the Ca(NO 3 ) 2 ‒(NH 4 ) 2 HPO 4 ‒NH 4 OH precursor system on the phase composition and crystallinity of the synthesized samples is considered. HA samples were investigated using X-ray diffraction and infrared Fourier spectroscopy. The study showed that ultrasonic treatment, regardless of the duration, does not affect the phase composition and crystallinity of the obtained HA samples. In all the studied samples, the presence of the HA phase Ca 10 (PO 4 ) 6 (OH) 2 with a crystallinity of 0,97 was established.

Water does not catalyze the reaction of OH radicals with ethanol.

Recent experiments suggested that water catalyzes the reaction of OH radicals with alcohols, while another work showed the opposite result. Here, we resolve this disagreement and show that heterogeneous oxidation systematically biased the work showing the catalytic effect and corroborate that water does not catalyze the reaction of OH with alcohols.

Kinetic study on the effect of di-tert-butyl peroxide and 2-ethylhexyl nitrate on the reactivity of toluene primary reference fuels

ABSTRACT Fuel additives have shown to provide an important contribution to control the auto-ignition process of the homogeneous charge compression ignition (HCCI) and reactivity-controlled compression ignition (RCCI). In this work a kinetic modeling study has been conducted to study the effects of di-tert-butyl peroxide (DTBP) and 2-ethylhexyl nitrate (EHN) on the reactivity of toluene primary reference fuels (TPRF). The kinetic analysis at engine-like conditions showed that for TPRF, the first-stage heat release was similar for the two additives. However, in the case of EHN the OH precursor nitrous acid (common isomer HONO) was formed in significant amounts during the induction period which eventually lead to a faster high-temperature ignition. This was in contrast to primary reference fuels where the ignition enhancing effect of DTBP was more significant compared to EHN as a result of a more pronounced first-stage heat release when adding DTBP and a lesser amount of nitrous acid formed when adding EHN compared to a TPRF. Using a TPRF as gasoline surrogate fuel could reproduce the trends in RON as function of added EHN and DTBP for a commercial gasoline, indicating the importance to include fuel sensitivity (RON-MON) in a surrogate fuel.

Achieving Rich Mixed-Valence Polysulfide/Carbon Nanotube Films toward Ultrahigh Volume Energy Density and Largely Deformable Pseudocapacitors

In this work, new insights into dependence of electrochemical performance enhancement on transition metals' rich mixed valence and their atomic ratio as well as redox active polysulfides are proposed. Especially, the influence of atomic ratio is further demonstrated by both experiments and density functional theoretical calculation where increasing Co/S leads to the enlargement of both interatom distance and hole diameter in a MnxCoySz cell. We rationally designed and prepared novel flexible electrodes of a rich mixed-valence polysulfide MnxCoySz/carbon nanotube film (CNTF) through acid activation of a dense CNTF into a hydrogel-like conductive matrix, growth of the MnxCoy(CO3)0.5OH precursor on each CNT, and controlled sulfidation. Nanostructure control allows us to obtain fast electron/ion transfer and increased availability of active sites/interfaces. The optimal MnCo9S10/CNTF shows a specific capacitance reaching 450 F cm-3 at 10 mA cm-2, much higher than reported values for CNT-based electrodes. Also, it exhibits remarkable cycling stability with only 1.6% capacity loss after 10 000 cycles at a high current density of 80 mA cm-2. An all-solid-state asymmetric supercapacitor (ASC) applying MnCo9S10/CNTF delivers an exceptionally high volumetric energy density of 67 mW h cm-3 (at 10 W cm-3). Particularly, integrated electric sources with adjustable output voltages can be obtained by connecting several ASCs in series, and there are no structural failure and capacity loss during repeated large-angle twisting and vigorous hammering. This work provides a general route to energy storage devices with ultrahigh volumetric energy density and outstanding reliability for wearable electronics.

Full-scale comparison of UV/H2O2 and UV/Cl2 advanced oxidation: The degradation of micropollutant surrogates and the formation of disinfection byproducts

The photolysis of chlorine by UV light leads to the formation of the hydroxyl radicals (OH) as well as reactive chlorine species (RCS) that can be effective as advanced oxidation processes (AOPs) for water treatment. Much of the research to date has been done at laboratory- or bench-scale. This study reports results from a model that demonstrates that the relative effectiveness of the UV/Cl2 AOP compared to the more traditional UV/H2O2 AOP is a function of optical path length. As such, the relative effectiveness of the two treatment options evaluated at small scale may not reflect the relative performance at full-scale, making results previously obtained at small-scale potentially less scalable. This study therefore compares the performance of UV/Cl2 to UV/H2O2 at a full-scale water treatment plant, using sucralose and caffeine as spiked surrogates for contaminants that are reactive solely to OH radicals, and to both OH and RCS, respectively. pH was varied between 6.5 and 8.0. The results demonstrated that when using a medium pressure UV lamp, UV/Cl2 might lead to approximately twice the production of OH radicals as UV/H2O2 at pH 6.5 when using the same molar oxidant concentration, but adding chlorine to the UV reactor at pH 8.0 had a negligible impact on OH radical concentration in comparison to UV alone. The study also confirmed previous small-scale results that RCS can be a major contributor to UV/Cl2 treatment for compounds such as caffeine that are susceptible to RCS, with UV/Cl2 effective at both pH 6.5 and 8.0 for such compounds. Disinfection byproducts were monitored, with adsorbable organohalide (AOX) formation increasing by approximately 10 μg-Cl/L due to chlorine photolysis, but only at pH 6.5 and not at pH 8.0. This implies that UV/Cl2 might increase AOX mostly due to reaction between OH and organic precursors to make them more reactive with chlorine, and not due to RCS. The formation of specific DBPs of current or emerging regulatory interest was minimal under all conditions, except for chlorate. Chlorate yields were in the order of 6–18% of the photolysed chlorine.

PH-dependent production of molecular chlorine, bromine, and iodine from frozen saline surfaces

Abstract. The mechanisms of molecular halogen production from frozen saline surfaces remain incompletely understood, limiting our ability to predict atmospheric oxidation and composition in polar regions. In this laboratory study, condensed-phase hydroxyl radicals (OH) were photochemically generated in frozen saltwater solutions that mimicked the ionic composition of ocean water. These hydroxyl radicals were found to oxidize Cl−, Br−, and I−, leading to the release of Cl2, Br2, I2, and IBr. At moderately acidic pH (buffered between 4.5 and 4.8), irradiation of ice containing OH precursors (either of hydrogen peroxide or nitrite ion) produced elevated amounts of I2. Subsequent addition of O3 produced additional I2, as well as small amounts of Br2. At lower pH (1.7–2.2) and in the presence of an OH precursor, rapid dark conversion of I− to I2 occurred from reactions with hydrogen peroxide or nitrite, followed by substantial photochemical production of Br2 upon irradiation. Exposure to O3 under these low pH conditions also increased production of Br2 and I2; this likely results from direct O3 reactions with halides, as well as the production of gas-phase HOBr and HOI that subsequently diffuse to frozen solution to react with Br− and I−. Photochemical production of Cl2 was only observed when the irradiated sample was composed of high-purity NaCl and hydrogen peroxide (acting as the OH precursor) at pH = 1.8. Though condensed-phase OH was shown to produce Cl2 in this study, kinetics calculations suggest that heterogeneous recycling chemistry may be equally or more important for Cl2 production in the Arctic atmosphere. The condensed-phase OH-mediated halogen production mechanisms demonstrated here are consistent with those proposed from recent Arctic field observations of molecular halogen production from snowpacks. These reactions, even if slow, may be important for providing seed halogens to the Arctic atmosphere. Our results suggest the observed molecular halogen products are dependent on the relative concentrations of halides at the ice surface, as we only observe what diffuses to the air–surface interface.

Evaluation of OH and HO<sub>2</sub> concentrations and their budgets during photooxidation of 2-methyl-3-butene-2-ol (MBO) in the atmospheric simulation chamber SAPHIR

Abstract. Several previous field studies have reported unexpectedly large concentrations of hydroxyl and hydroperoxyl radicals (OH and HO2, respectively) in forested environments that could not be explained by the traditional oxidation mechanisms that largely underestimated the observations. These environments were characterized by large concentrations of biogenic volatile organic compounds (BVOC) and low nitrogen oxide concentration. In isoprene-dominated environments, models developed to simulate atmospheric photochemistry generally underestimated the observed OH radical concentrations. In contrast, HO2 radical concentration showed large discrepancies with model simulations mainly in non-isoprene-dominated forested environments. An abundant BVOC emitted by lodgepole and ponderosa pines is 2-methyl-3-butene-2-ol (MBO), observed in large concentrations for studies where the HO2 concentration was poorly described by model simulations. In this work, the photooxidation of MBO by OH was investigated for NO concentrations lower than 200 pptv in the atmospheric simulation chamber SAPHIR at Forschungszentrum Jülich. Measurements of OH and HO2 radicals, OH reactivity (kOH), MBO, OH precursors, and organic products (acetone and formaldehyde) were used to test our current understanding of the OH-oxidation mechanisms for MBO by comparing measurements with model calculations. All the measured trace gases agreed well with the model results (within 15 %) indicating a well understood mechanism for the MBO oxidation by OH. Therefore, the oxidation of MBO cannot contribute to reconciling the unexplained high OH and HO2 radical concentrations found in previous field studies.

Secondary Organic Aerosol Formation from Ambient Air at an Urban Site in Beijing: Effects of OH Exposure and Precursor Concentrations.

Secondary organic aerosol (SOA) is an important component of atmospheric fine particles (PM2.5), while the key factors controlling SOA formation in ambient air remain poorly understood. In this work, the SOA formation in Beijing urban ambient air was investigated using an oxidation flow reactor (OFR) with high concentrations of OH radicals. The SOA formation potential increased significantly with the increase of ambient PM2.5 concentration during the observation. The optimum ambient exposure time, which is the aging time equivalent to atmospheric oxidation (with similar OH exposure) associated with the peak SOA formation, varied between 2 and 4 days in this study. The OA enhancement in this study was much higher than that of developed countries under different environmental conditions. The higher OA enhancement is probably due to the higher concentrations of volatile organic compounds (VOCs) in the urban air of Beijing. This might also have occurred because fragmentation did not dominate in the oxidation of OA, and did not result in negative OA enhancement on highly polluted days compared to relatively clean days with similar exposure time. These results suggested that under typical ambient conditions, high concentrations of VOC precursors might contribute to sustained organic aerosol growth and long duration haze events in Beijing.

Correlations Between the Sonochemical Production Rate of Hydrogen and the Maximum Temperature and Pressure Reached in Acoustic Bubbles

Water sonolysis generates hydrogen through acoustic cavitation. In this work, based on a model for a reactive acoustic bubble, correlations between the sonochemical production of hydrogen and the maximum temperature and pressure reached in the bubble at the violent collapse have been made. The computational analysis has been performed for more than 800 points obtained by combining various cavitation parameters, i.e., frequency, acoustic intensity, liquid temperature, and ambient bubble radius. The simulation results showed that hydrogen production rate progressed linearly with the bubble temperature and pressure rise up to plateaus, which begin at 3500 ± 200 K and 100 ± 10 atm. Analyzing the progress of \(\text {H}^{{\cdot }}\) and \(^{{\cdot }} \)OH (\(\text {H}_{2}\) precursors) as function of bubble temperature and pressure showed very similar evolutions as those obtained for \(\text {H}_{2}\) with the same optimums at 3500 ± 200 K and 100 ± 10 atm. Consequently, in addition to the quench of hydrogen formation at very high bubble temperatures through the reaction \(\text {H}_{2}+\,^{{\cdot }}{\hbox {OH}}\rightarrow \text {H}_{2}\hbox {O}+\text {H}^{{\cdot }}\), the existing optimum temperature and pressure for \(\text {H}_{2}\) production may also be due the hard consumption of their precursors (\(^{{\cdot }}\)OH and \(\text {H}^{{\cdot }})\) above 3500 K and 100 atm.

Fluorescent Molecular Probes for Detection of One-Electron Oxidants Photochemically Generated by Dissolved Organic Matter.

We report a dual probe system based on 4'-substituted biphenyl-2-carboxylic acids (BPAs) for analysis of photooxidants generated by dissolved organic matter. The BPA probes are converted to the corresponding benzocoumarins (BZCs) at different rates depending on the mechanism of oxidation; thus, two probes used simultaneously can differentiate strong triplet excited state sensitizers from hydroxylating species such as hydroxyl radical (•OH) present in dissolved organic matter (DOM). Comparison of the ratios of BZC-CH3 and BZC-CF3 product formation using model photooxidants such as NaNO2, a •OH precursor, and model triplet sensitizer lumichrome gave a range of 2 to 250. Application of these probes to DOM isolates and whole natural waters afforded intermediate ratios. Although the oxidation potential of BPAs (>ca. 1.80 V SHE) is significantly higher than the estimated average reduction potential of typical 3CDOM* samples, these results have demonstrated the presence of a small pool of oxidants in the selected DOM isolates and whole water samples that is capable of oxidizing aromatic carboxylates. As an analytical tool, this probe pair can be used between pH 4-6 without affecting the product formation ratio and may find applications in various systems involving complex mixtures of photochemically produced oxidants of differing natures.

Investigation of the <i>β</i>-pinene photooxidation by OH in the atmosphere simulation chamber SAPHIR

Abstract. Besides isoprene, monoterpenes are the non-methane volatile organic compounds (VOCs) with the highest global emission rates. Due to their high reactivity towards OH, monoterpenes can dominate the radical chemistry of the atmosphere in forested areas. In the present study the photochemical degradation mechanism of β-pinene was investigated in the Jülich atmosphere simulation chamber SAPHIR (Simulation of Atmospheric PHotochemistry In a large Reaction Chamber). One focus of this study is on the OH budget in the degradation process. Therefore, the SAPHIR chamber was equipped with instrumentation to measure radicals (OH, HO2, RO2), the total OH reactivity, important OH precursors (O3, HONO, HCHO), the parent VOC β-pinene, its main oxidation products, acetone and nopinone and photolysis frequencies. All experiments were carried out under low-NO conditions ( ≤ 300 ppt) and at atmospheric β-pinene concentrations ( ≤ 5 ppb) with and without addition of ozone. For the investigation of the OH budget, the OH production and destruction rates were calculated from measured quantities. Within the limits of accuracy of the instruments, the OH budget was balanced in all β-pinene oxidation experiments. However, even though the OH budget was closed, simulation results from the Master Chemical Mechanism (MCM) 3.2 showed that the OH production and destruction rates were underestimated by the model. The measured OH and HO2 concentrations were underestimated by up to a factor of 2, whereas the total OH reactivity was slightly overestimated because the model predicted a nopinone mixing ratio which was 3 times higher than measured. A new, theory-derived, first-generation product distribution by Vereecken and Peeters (2012) was able to reproduce the measured nopinone time series and the total OH reactivity. Nevertheless, the measured OH and HO2 concentrations remained underestimated by the numerical simulations. These observations together with the fact that the measured OH budget was closed suggest the existence of unaccounted sources of HO2. Although the mechanism of additional HO2 formation could not be resolved, our model studies suggest that an activated alkoxy radical intermediate proposed in the model of Vereecken and Peeters (2012) generates HO2 in a new pathway, whose importance has been underestimated so far. The proposed reaction path involves unimolecular rearrangement and decomposition reactions and photolysis of dicarbonyl products, yielding additional HO2 and CO. Further experiments and quantum chemical calculations have to be made to completely unravel the pathway of HO2 formation.