OH Reaction Research Articles

OH Kinetics with a Range of Nitrogen-Containing Compounds: N-Methylformamide, t-Butylamine, and N-Methyl-propane Diamine.

Emissions of amines and amides to the atmosphere are significant from both anthropogenic and natural sources, and amides can be formed as secondary pollutants. Relatively little kinetic data exist on overall rate coefficients with OH, the most important tropospheric oxidant, and even less on site-specific data which control the product distribution. Structure-activity relationships (SARs) can be used to estimate both quantities. Rate coefficients for the reaction of OH with t-butylamine (k1), N-methyl-1,3-propanediamine (k2), and N-methylformamide (k3) have been measured using laser flash photolysis coupled with laser-induced fluorescence. Proton-transfer-reaction mass spectrometry (PTR-MS) has been used to ensure the reliable introduction of these low-vapor pressure N-containing compounds and to give qualitative information on products. Supporting ab initio calculations are presented for the t-butylamine system. The following rate coefficients have been determined: k1,298K= (1.66 ± 0.20) × 10-11 cm3 molecule-1 s-1, k(T)1 = 1.65 × 10-11 (T/300)-0.69 cm3 molecule-1 s-1, k2,293K = (7.09 ± 0.22) × 10-11 cm3 molecule-1 s-1, and k3,298K = (1.03 ± 0.23) × 10-11 cm3 molecule-1 s-1. For OH + t-butylamine, ab initio calculations predict that the fraction of N-H abstraction is 0.87. The dominance of this channel was qualitatively confirmed using end-product analysis. The reaction of OH with N-methyl-1,3-propanediamine also had a negative temperature dependence, but the reduction in the rate coefficient was complicated by reagent loss. The measured rate coefficient for reaction 3 is in good agreement with a recent relative rate study. The results of this work and the literature data are compared with the recent SAR estimates for the reaction of OH with reduced nitrogen compounds. Although the SARs reproduce the overall rate coefficients for reactions, site-specific agreement with this work and other literature studies is less strong.

Observations of atmospheric oxidation and ozone production in South Korea

South Korea routinely experiences poor air quality with ozone and small particles exceeding air quality standards. To build a better understanding of this problem, in 2016, the KORea-United States cooperative Air Quality (KORUS-AQ) study collected surface and airborne measurements of many chemical species, including the reactive gases hydroxyl (OH) and hydroperpoxyl (HO2). Several different results are reported here. First, OH and HO2 measured on the NASA DC-8 agree to within uncertainties with values calculated by two different box models, both in statistical comparisons and as a function of altitude from the surface to 8 km. These comparisons show substantial scatter, likely due to both variability in instrument performance and the difficulty in interpolating measurements made with frequencies different from those of the model time step. Second, OH and HO2 calculated by a model including HO2 uptake on aerosol particles in the chemical mechanism are inconsistent with observations. Third, in the planetary boundary layer over both ocean and land, measured and model-calculated OH reactivity are sometimes different, and this missing OH reactivity, which is as much as ∼4 s−1, increased from April to June and originated primarily from the Korean peninsula. Fourth, repeated missed approaches at the Seoul Air Base during several days show that the changes in the sum of ozone and nitrogen dioxide are consistent with ozone production rates calculated from HO2 either observed or modeled by the Langley Research Center model.

Simulating nitrate formation mechanisms during PM2.5 events in Taiwan and their implications for the controlling direction

The long-term downward trend of NOX concentrations does not reflect the reduction of nitrate (NO3−) in Taiwan. Instead, the proportion of NO3− in PM2.5 increased in recent years. To probe the increasing importance of NO3− in PM2.5, this study applied the WRF/CMAQ modeling system to implement a simulation from 16 March 2017 to 30 April 2017, in which 5 p.m.2.5 events with daily average concentrations ≥35 μg m−3 and their corresponding correlation coefficients (R) of simulated and observed PM2.5 above 0.6 were selected for analysis. During the daytime, the reaction of NO2 and OH contributed more than 90% of the total HNO3. After sunset, the high concentrations of NO3 and N2O5 peaked, followed soon by the simultaneous rise of NO3−, aerosol water content, and HNO3 concentrations around midnight, which indicated that the heterogeneous reaction was the main formation mechanism of NO3−, accounting for approximately 30%–90% of total HNO3. At nighttime, the daytime-formed gaseous phase NO3− condensed, and low wind and low boundary layer height favored accumulation; therefore, PM2.5 peaked around the midnight period to the early morning. The sensitivity test showed that doubling and halving the NOX and NH3 emissions could directly lead to the highest production and reduction of NO3−, respectively, followed by doubling and halving NMHC emissions, which caused the highest and lowest O3 concentrations. The correlation analysis of the simulation results showed that the daytime maximum O3 and HNO3 were highly correlated. The relationships between daytime maximum O3, nighttime maximum NO3, N2O5, and HNO3 in pairs were also moderately to highly correlated. This study implies that in addition to direct reduction of NOX or NH3 emissions, controlling O3 is possibly another useful strategy to reduce NO3−. Because NOX emission reduction could conflict with controlling O3, this study suggests to re-examine the determination of NOX-limited and VOCS-limited regions in order to develop strategies for reducing NOX emission and O3 simultaneously.

A novel composite inorganic retarding gel for preventing coal spontaneous combustion

In order to better control the fire problem due to coal oxidations, a novel composite inorganic retarding gel material processed from sodium-based bentonite junction and kaolin, silica, hydrogen peroxide and silicon oxide is developed. For studying the gel's properties the scanning electron microscopy (SEM) and the thermogravimetry (TG-DTG) are used to characterize microscopical structures and mass losses with the temperature change process, respectively. For quantifying the inhibitory effect of the novel gel on the spontaneous combustion process of coal oxidation, laboratory tests such as temperature-programmed oxidation and infrared spectroscopy are carried out. Changes of the gases generated from the treated coal sample by the novel gel under the temperature increasing are analyzed. The inhibitory effects of mud and MgCl2 solutions on coal spontaneous combustion are compared. The results show that the surface of the gel is flat and smooth in microscopic scale. Under the a high-temperature, the gel is stable and is not easy to decompose. In detail, the quality loss is within 10%. The novel gel with a concentration of 20% has a good inhibitory effect on CO release and spontaneous combustion in coal combustion. In addition, the novel gel can significantly reduce the consumption of –OH, –CH and other functional groups in the low-temperature oxidation reaction. Hence, the novel gel is a kind of Inhibitor material with a better comprehension performance. This study provides a novel gel for better preventing and controlling the coal spontaneous combustion hazard.

Modeled Emission of Hydroxyl and Ozone Reactivity from Evaporation of Fragrance Mixtures.

Volatile chemical products (VCPs) account for increasing fractions of organic carbon emitted to the atmosphere, particularly in urban areas. Fragrances are potentially reactive components that are added to many VCPs. To better constrain these emissions, 11 commercially available liquid fragrance mixtures were characterized for their composition and their evaporation modeled. Emissions of mass, hydroxyl reactivity, and ozone reactivity were estimated by modeling under four different scenarios. Fragrance compounds were generally less than one-half the mass of fragrance mixtures, with the balance comprised of solvents and plasticizers and unresolved mass thought to be dominated by plasticizers. The results showed that terpenes and terpenoids account for nearly all of the emitted mass and reactivity while only comprising ∼10% w/w on average of the liquid fragrance mixtures. Most of the reactivity is emitted within hours, with ozone reactivity evolving more rapidly than OH reactivity and comprised almost entirely of terpenes. Limonene, a common fragrance constituent, dominates the reactivity of emitted carbon. Generally, 20-40% of the potential hydroxyl reactivity contained in the fragrance mixture does not evaporate on time scales sufficient to have an impact on local or regional air quality.

Measurement report: Regional characteristics of seasonal and long-term variations in greenhouse gases at Nainital, India, and Comilla, Bangladesh

Abstract. Emissions of greenhouse gases (GHGs) from the Indian subcontinent have increased during the last 20 years along with rapid economic growth; however, there remains a paucity of GHG measurements for policy-relevant research. In northern India and Bangladesh, agricultural activities are considered to play an important role in GHG concentrations in the atmosphere. We performed weekly air sampling at Nainital (NTL) in northern India and Comilla (CLA) in Bangladesh from 2006 and 2012, respectively. Air samples were analyzed for dry-air gas mole fractions of CO2, CH4, CO, H2, N2O, and SF6 and carbon and oxygen isotopic ratios of CO2 (δ13C-CO2 and δ18O-CO2). Regional characteristics of these components over the Indo-Gangetic Plain are discussed compared to data from other Indian sites and Mauna Loa, Hawaii (MLO), which is representative of marine background air. We found that the CO2 mole fraction at CLA had two seasonal minima in February–March and September, corresponding to crop cultivation activities that depend on regional climatic conditions. Although NTL had only one clear minimum in September, the carbon isotopic signature suggested that photosynthetic CO2 absorption by crops cultivated in each season contributes differently to lower CO2 mole fractions at both sites. The CH4 mole fraction of NTL and CLA in August–October showed high values (i.e., sometimes over 4000 ppb at CLA), mainly due to the influence of CH4 emissions from the paddy fields. High CH4 mole fractions sustained over months at CLA were a characteristic feature on the Indo-Gangetic Plain, which were affected by both the local emission and air mass transport. The CO mole fractions at NTL were also high and showed peaks in May and October, while CLA had much higher peaks in October–March due to the influence of human activities such as emissions from biomass burning and brick production. The N2O mole fractions at NTL and CLA increased in June–August and November–February, which coincided with the application of nitrogen fertilizer and the burning of biomass such as the harvest residues and dung for domestic cooking. Based on H2 seasonal variation at both sites, it appeared that the emissions in this region were related to biomass burning in addition to production from the reaction of OH and CH4. The SF6 mole fraction was similar to that at MLO, suggesting that there were few anthropogenic SF6 emission sources in the district. The variability of the CO2 growth rate at NTL was different from the variability in the CO2 growth rate at MLO, which is more closely linked to the El Niño–Southern Oscillation (ENSO). In addition, the growth rates of the CH4 and SF6 mole fractions at NTL showed an anticorrelation with those at MLO, indicating that the frequency of southerly air masses strongly influenced these mole fractions. These findings showed that rather large regional climatic conditions considerably controlled interannual variations in GHGs, δ13C-CO2, and δ18O-CO2 through changes in precipitation and air mass.

Exploring key reaction sites and deep degradation mechanism of perfluorooctane sulfonate via peroxymonosulfate activation under electrocoagulation process

Perfluorooctane sulfonate (PFOS), normally present in groundwater and surface water, is an emerging environmental contaminants, but is extremely difficult to be degraded due to high energy of the C-F bond. Here, an electrocoagulation (EC) technique coupled with peroxymonosulfate (PMS) activation was used to deeply degrade PFOS. Results showed that approximately 100% PFOS was removed from the solution in the monopolar serial (MS) mode within 60 min and achieved a high kinetic rate of 0.074 min−1, which was significantly higher than those of reported studies (Table S3). Energy consumption (2.06 kWh/kg) in the MS mode was significantly lower than that of Al (52.30 kWh/kg) and Zn (213.50 kWh/kg) electrodes, which further confirmed the potential application prospects of EC technique. The quenching experiments, electron spin response (ESR) analysis, and DFT calculations can verify that ·OH was the main radical from the reaction of Fe2+–OH reaction site with PMS. In addition, results from fluorine balance and TOC removal also indicated the complete mineralization and degradation of PFOS in the EC process. Quantum chemical calculations can confirm the PFOS degradation mechanism and key active sites for direct electron transfer and radical attack. After five cycle operations of PFOS degradation, the EC process was still effective in degrading PFOS with a removal efficiency above 98%. Thus, this work provided a novel alternative for the high-effective treatment of PFOS from contaminated environmental water bodies.

A theoretical study on the photodegradation mechanism of the endocrine disrupting chemical p-nonylphenol induced by [rad]OH in water

Nonylphenol (NP) has attracted significant attention because of its widespread environmental presence, toxicity, and pseudo-estrogen properties. Photolysis is the main degradation pathway of many organic pollutants in water. In this study, the photodegradation mechanism of p-nonylphenol (p-NP) induced by OH in water was investigated using the density functional theory (DFT). Based on the calculations of charges using natural population analysis (NPA), the reaction of OH with NP included the carbon addition of a benzene ring and the hydrogen extraction of a phenolic hydroxyl group. The structures of all reactants, products, and intermediates were optimized at the B3LYP/6-31+G(d,p) level. Using vibrational frequency and intrinsic reaction coordinate analyses, the transition state and its reaction pathway were confirmed. Additionally, the solvent effect results showed the reaction preferably took place in water. Our results provide a theoretical basis for understanding the transformative process and assessing the potential risk of NP in the environment.

Atmospheric photo-oxidation of myrcene: OH reaction rate constant, gas-phase oxidation products and radical budgets

Abstract. The photo-oxidation of myrcene, a monoterpene species emitted by plants, was investigated at atmospheric conditions in the outdoor simulation chamber SAPHIR (Simulation of Atmospheric PHotochemistry In a Large Reaction Chamber). The chemical structure of myrcene consists of one moiety that is a conjugated π system (similar to isoprene) and another moiety that is a triple-substituted olefinic unit (similar to 2-methyl-2-butene). Hydrogen shift reactions of organic peroxy radicals (RO2) formed in the reaction of isoprene with atmospheric OH radicals are known to be of importance for the regeneration of OH. Structure–activity relationships (SARs) suggest that similar hydrogen shift reactions like in isoprene may apply to the isoprenyl part of RO2 radicals formed during the OH oxidation of myrcene. In addition, SAR predicts further isomerization reactions that would be competitive with bimolecular RO2 reactions for chemical conditions that are typical for forested environments with low concentrations of nitric oxide. Assuming that OH peroxy radicals can rapidly interconvert by addition and elimination of O2 like in isoprene, bulk isomerization rate constants of 0.21 and 0.097 s−1 (T=298 K) for the three isomers resulting from the 3′-OH and 1-OH addition, respectively, can be derived from SAR. Measurements of radicals and trace gases in the experiments allowed us to calculate radical production and destruction rates, which are expected to be balanced. The largest discrepancies between production and destruction rates were found for RO2. Additional loss of organic peroxy radicals due to isomerization reactions could explain the observed discrepancies. The uncertainty of the total radical (ROx=OH+HO2+RO2) production rates was high due to the uncertainty in the yield of radicals from myrcene ozonolysis. However, results indicate that radical production can only be balanced if the reaction rate constant of the reaction between hydroperoxy (HO2) and RO2 radicals derived from myrcene is lower (0.9 to 1.6×10-11 cm3 s−1) than predicted by SAR. Another explanation of the discrepancies would be that a significant fraction of products (yield: 0.3 to 0.6) from these reactions include OH and HO2 radicals instead of radical-terminating organic peroxides. Experiments also allowed us to determine the yields of organic oxidation products acetone (yield: 0.45±0.08) and formaldehyde (yield: 0.35±0.08). Acetone and formaldehyde are produced from different oxidation pathways, so that yields of these compounds reflect the branching ratios of the initial OH addition to myrcene. Yields determined in the experiments are consistent with branching ratios expected from SAR. The yield of organic nitrate was determined from the gas-phase budget analysis of reactive oxidized nitrogen in the chamber, giving a value of 0.13±0.03. In addition, the reaction rate constant for myrcene + OH was determined from the measured myrcene concentration, yielding a value of (2.3±0.3)×10-10 cm3 s−1.

Theoretical kinetics predictions for NH2 + HO2

Recent modeling studies of NH3 oxidation, which are motivated by the prospective role of ammonia as a zero-carbon fuel, have indicated significant discrepancies between existing literature mechanisms. In this study high level theoretical kinetics predictions have been obtained for the reaction of NH2 with HO2, which has previously been highlighted as an important reaction with high sensitivity and high uncertainty. The potential energy surface is explored with coupled cluster calculations including large basis sets and high-level corrections to yield high accuracy (∼0.2 kcal/mol) estimates of the stationary point energies. Variational transition state theory is used to predict the microcanonical rate constants, which are then incorporated in master equation treatments of the temperature and pressure dependent kinetics. For the radical-radical channels, the microcanonical rates are obtained from variable reaction coordinate transition state theory implementing directly evaluated multireference electronic energies. The analysis yields predictions for the total rate constant as well as the branching to the NH3 + O2, H2NO + OH, and HNO + H2O channels. Rate constants are also reported for the H2NO + OH reaction as they arise naturally from the analysis. The rate constant and branching fraction determined in this work for the NH2 + HO2 reaction deviate significantly from values used in most previous modeling studies. The fact that the main product channel is chain terminating, rather than propagating, has strong implications for modeling NH3 ignition and oxidation, in particular at intermediate temperatures and elevated pressure.

Evolution of OH reactivity in NO-free volatile organic compound photooxidation investigated by the fully explicit GECKO-A model

Abstract. OH reactivity (OHR) is an important control on the oxidative capacity in the atmosphere but remains poorly constrained in many environments, such as remote, rural, and urban atmospheres, as well as laboratory experiment setups under low-NO conditions. For an improved understanding of OHR, its evolution during oxidation of volatile organic compounds (VOCs) is a major aspect requiring better quantification. We use the fully explicit Generator of Explicit Chemistry and Kinetics of Organics in the Atmosphere (GECKO-A) model to study the OHR evolution in the NO-free photooxidation of several VOCs, including decane (an alkane), m-xylene (an aromatic), and isoprene (an alkene). Oxidation progressively produces more saturated and functionalized species. Total organic OHR (including precursor and products, OHRVOC) first increases for decane (as functionalization increases OH rate coefficients) and m-xylene (as much more reactive oxygenated alkenes are formed). For isoprene, C=C bond consumption leads to a rapid drop in OHRVOC before significant production of the first main saturated multifunctional product, i.e., isoprene epoxydiol. The saturated multifunctional species in the oxidation of different precursors have similar average OHRVOC per C atom. The latter oxidation follows a similar course for different precursors, involving fragmentation of multifunctional species to eventual oxidation of C1 and C2 fragments to CO2, leading to a similar evolution of OHRVOC per C atom. An upper limit of the total OH consumption during complete oxidation to CO2 is roughly three per C atom. We also explore the trends in radical recycling ratios. We show that differences in the evolution of OHRVOC between the atmosphere and an environmental chamber, and between the atmosphere and an oxidation flow reactor (OFR), can be substantial, with the former being even larger, but these differences are often smaller than between precursors. The Teflon wall losses of oxygenated VOCs in chambers result in large deviations of OHRVOC from atmospheric conditions, especially for the oxidation of larger precursors, where multifunctional species may suffer substantial wall losses, resulting in significant underestimation of OHRVOC. For OFR, the deviations of OHRVOC evolution from the atmospheric case are mainly due to significant OHR contribution from RO2 and lack of efficient organic photolysis. The former can be avoided by lowering the UV lamp setting in OFR, while the latter is shown to be very difficult to avoid. However, the former may significantly offset the slowdown in fragmentation of multifunctional species due to lack of efficient organic photolysis.

Chemical Morphology Controls Reactivity of OH Radicals at the Air-Ice Interface.

At the air-ice interface, some aromatic compounds such as benzene and anthracene are surprisingly unreactive toward OH. This may be a consequence of the poor solvation of these compounds at the interface, resulting in clustering there. We test this hypothesis by comparing the reaction of OH with pyrene, a 4-ring polyaromatic hydrocarbon (PAH), to reactions of OH with the more water-soluble compounds coumarin and 7-hydroxycoumarin (7OHC). We observe that OH reacts readily with coumarin and 7OHC at both liquid and frozen air-water interfaces. Pyrene, a much less soluble compound, reacts with OH at the liquid surface but not at the air-ice interface. We report evidence of pyrene aggregation at the ice surface based on its broadened and red-shifted emission spectrum alongside fluorescence mapping of anthracene, a closely related 3-ring PAH, which shows bunching at the ice surface. By contrast, fluorescence mapping shows that coumarin is fairly homogeneously distributed at the air-ice interface. Together, these results suggest that the limited reactivity of some compounds toward OH at the ice surface may be a consequence of their propensity to self-aggregate, demonstrating that chemical morphology can play an important role in reactions at the ice surface.

A quantitative understanding of total OH reactivity and ozone production in a coastal industrial area during the Yokohama air quality study (AQUAS) campaign of summer 2019

Field measurements performed in the Keihin coastal industrial area during summer 2019 were used to characterize the air quality and ozone production potential. The total hydroxyl radical (OH) reactivity was found to be moderate, peaking with a 2-h average of 29.0 s−1, corresponding to an OH lifetime of ∼34 ms. The average hourly total OH reactivity was 11.4 ± 5.2 s−1 (average ± standard deviation), and the measured OH sinks (from over 90 trace species) accounted for 92.3% of the total OH reactivity. Compared with a previous series of campaigns in urban/suburban Japan, this field work investigated an area often overlooked for atmospheric observations and found the smallest missing OH reactivity (on average, 0.88 s−1). The near-zero missing OH reactivity indicated that photochemical procedures were less important to the Yokohama air quality study (AQUAS–Yokohama) than the ubiquitous substantial secondary products in other areas. This study also demonstrated that oceanic air masses may quickly flush coastal industrial areas and that a limited activity of photochemistry due to elevated NOx effectively terminates OH radical recycling. A comparison with other campaigns in Japan exhibited a positive correlation with oxidants (ozone and formaldehyde), indicating secondary products have a critical role in explaining missing OH reactivity. The ozone production potential was corresponding to a volatile organic compound (VOC)-limited regime, while the missing OH reactivity led to an ∼8% increase in ozone formation. Finally, a positive correlation was found between the ozone production rate and the reactivity ratio between VOCs and NOx, in both the current study and various other field campaigns in urban areas.

Effect of Ozone, Clothing, Temperature, and Humidity on the Total OH Reactivity Emitted from Humans.

People influence indoor air chemistry through their chemical emissions via breath and skin. Previous studies showed that direct measurement of total OH reactivity of human emissions matched that calculated from parallel measurements of volatile organic compounds (VOCs) from breath, skin, and the whole body. In this study, we determined, with direct measurements from two independent groups of four adult volunteers, the effect of indoor temperature and humidity, clothing coverage (amount of exposed skin), and indoor ozone concentration on the total OH reactivity of gaseous human emissions. The results show that the measured concentrations of VOCs and ammonia adequately account for the measured total OH reactivity. The total OH reactivity of human emissions was primarily affected by ozone reactions with organic skin-oil constituents and increased with exposed skin surface, higher temperature, and higher humidity. Humans emitted a comparable total mixing ratio of VOCs and ammonia at elevated temperature-low humidity and elevated temperature-high humidity, with relatively low diversity in chemical classes. In contrast, the total OH reactivity increased with higher temperature and higher humidity, with a larger diversity in chemical classes compared to the total mixing ratio. Ozone present, carbonyl compounds were the dominant reactive compounds in all of the reported conditions.

Aqueous OH kinetics of saturated C6–C10 dicarboxylic acids under acidic and basic conditions between 283 and 318 K; new structure-activity relationship parameters

Bimolecular reaction rate coefficients (kOH) for reaction of hydroxyl radicals (OH) with adipic, pimelic, suberic, azelaic, sebacic and cis-pinonic acids were measured in aqueous phase for the neutral (pH = 2) and completely deprotonated (pH = 9) forms of the acids listed. Reactions were carried out in bulk solution between 283 and 318 K. Camphoric acid was used as a reference compound for measuring kOH values using a relative rate method, and with hydrogen peroxide (H2O2) as a photochemical source of OH. Concentrations of the reactants were monitored with liquid chromatography coupled with electrospray tandem mass spectrometry (LC-ESI/MS2).The kOH values measured were between 1 and 8 × 10−9 M−1s−1; only adipic and pimelic acids showed a significant pH-dependence to their OH reactivity. The kinetic data acquired and the data reported in the literature for camphoric, glutaric and succinic acids were used to optimize new structure-activity relationship (SAR) parameters. The SAR developed here allowed accurate prediction of kOH values for aliphatic carboxylic acids in the temperature range between 283 and 318 K. The accuracy of the developed SAR was evaluated against the experimental kOH values measured in this work and obtained from the literature data. Moreover, this work is the first to accurately predict the temperature-dependent kOH values for carboxylic acids.

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

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

Atmospheric oxidation of <i>α</i>,<i>β</i>-unsaturated ketones: kinetics and mechanism of the OH radical reaction

Abstract. The OH-radical-initiated oxidation of 3-methyl-3-penten-2-one and 4-methyl-3-penten-2-one was investigated in two atmospheric simulation chambers at 298±3 K and 990±15 mbar using long-path FTIR spectroscopy. The rate coefficients of the reactions of 3-methyl-3-penten-2-one and 4-methyl-3-penten-2-one with OH radicals were determined to be (6.5±1.2)×10-11 and (8.1±1.3)×10-11 cm3molecule-1s-1, respectively. To enlarge the kinetics data pool the rate coefficients of the target species with Cl atoms were determined to be (2.8±0.4)×10-10 and (3.1±0.4)×10-10 cm3molecule-1s-1, respectively. The mechanistic investigation of the OH-initiated oxidation focuses on the RO2+NO reaction. The quantified products were acetoin, acetaldehyde, biacetyl, CO2 and peroxyacetyl nitrate (PAN) for the reaction of 3-methyl-3-penten-2-one with OH radicals and acetone, methyl glyoxal, 2-hydroxy-2-methylpropanal, CO2 and peroxyacetyl nitrate (PAN) for the reaction of 4-methyl-3-penten-2-one with OH, respectively. Based on the calculated product yields an upper limit of 0.15 was determined for the yield of RONO2 derived from the OH reaction of 4-methyl-3-penten-2-one. By contrast, no RONO2 formation was observed for the OH reaction of 3-methyl-3-penten-2-one. Additionally, a simple model is presented to correct product yields for secondary processes.

Effect of senescence on biogenic volatile organic compound fluxes in wheat plants

Exchanges of biogenic volatile organic compounds (BVOC) between plants and the atmosphere are likely to vary, in amount and composition, between different plant species but also for a single plant during its development. However, the effect of plant development stages, including senescence, on BVOC exchanges remains poorly investigated, especially in the case of crop plants. We investigated the BVOC exchange patterns for wheat plants, the most grown crop species worldwide, during seed maturation, senescence and after harvest. Fluxes were measured online, in situ, at the plant scale by combining automated chambers and a Proton Transfer - Reaction - Quadrupole ion guide - Time of Flight - Mass Spectrometer (PTR-Qi-Tof-MS). The high resolution and sensitivity of this method enabled the measurement of a large mass spectrum of compounds emitted at very small amounts, allowing a precise characterization of BVOC exchanges. We found that the overall BVOC emissions increased twofold during the senescence stage compared to the maturation stage. Methanol was found to be the most emitted compound (49–60% of the overall flux on a molar basis) followed by acetone (7.5–8.2% of the overall flux on a molar basis) during each developmental stage investigated. Acetaldehyde was another major emitted compound contributing mainly during late senescence to the overall flux (9.7%). When normalized for temperature and light conditions, most BVOC emissions increased during senescence, showing a clear effect of senescence on BVOC exchanges. Chamber emissions were comparable to whole ecosystem fluxes measured at the same site by eddy covariance the previous year. The OH reactivity of the emitted compounds was evaluated based on known reaction rate constants and was the largest during the first senescence stage, peaking at 12 s−1 in the chambers. The results of this study show the need for considering plant phenology when computing BVOC emissions from crops.

Formation of Substituted Alkyls as Precursors of Peroxy Radicals with a Rapid H-Shift in the Atmosphere.

Long straight-chain alkyl peroxy (ROO) radicals substituted with C═C and oxo functional groups are expected to undergo a rapid hydrogen shift (H-shift), which is a critical step in the atmospheric autoxidation mechanism. The existence of a weak tertiary C-H bond plays a key role in the rapid H-shift. Here, the reaction kinetics between OH and two typical long straight-chain functionalized volatile organic compounds, 3-methyl-1-hexene (3-MH) and 2-methylpentanal (2-MP), was theoretically investigated to reveal the fate of the weak C-H bond. The results indicate that the most favored reaction pathways are direct consumption (H-abstraction of 2-MP) and indirect destruction (addition of OH to 3-MH) of the "weak" tertiary C-H bond. The yields of abstraction pathways producing precursors of ROO radicals that undergo rapid H-shifts are computed to be less than 10% for both 3-MH + OH and 2-MP + OH reactions.

Isotope fractionation of diethyl phthalate during oxidation degradation by persulfate activated with zero-valent iron

This study reports the 13C and 2H isotope fractionation associated with the oxidation of diethyl phthalate (DEP) by persulfate (PS) activated with zero-valent iron (ZVI) using three concentration levels (0.2, 0.5 and 1.0 g L−1) at different pH values, 3, 7 and 11, respectively. The results showed that the degradation of DEP followed a pseudo first-order kinetics. The fastest degradation was found at neutral conditions (pH 7). Similar carbon and hydrogen isotope fractionation (εC and εH) was observed during the oxidation of DEP by ZVI activated PS at pH 3, 7 and 11. At ZVI concentration of 0.5 g L−1, the correlation of 13C and 2H fractionation (Λ) were obtained to be 12.7 ± 3.5, 11.1 ± 4.2 and 12.0 ± 2.9 at pH 3, 7 and 11, respectively. The concentration of ZVI has no effect on the correlation of 13C and 2H fractionation (Λ). In addition, radical quenching approach and electron paramagnetic resonance (EPR) were combined to explore the dominant radical species in the ZVI activated PS reaction, and hydroxyl radical (•OH) was found to be the predominant radical at all pH studied. The results of CSIA show the addition of •OH to the aromatic ring of DEP is the main reaction mechanism, which is consistent with the results of radical quenching experiment and EPR study. Carbon and hydrogen apparent kinetic isotope effects (AKIEs) obtained from •OH reactions with DEP supported the hypothesis of CH bond cleavage. Thus, carbon and hydrogen isotope enrichment factors clearly distinguish the different reaction mechanisms and hence, are a promising approach to improve understanding of radical species reaction pathways for chemical oxidation of DEP.