OH Reaction Rate Research Articles

Effect of direct water injection on knock suppressing mechanism in a high compression ratio gasoline engine

ABSTRACT A low capacity, boosted gasoline engine is an effective approach to enhancing fuel economy. However, knock combustion poses significant limitations to the practical development of these engines. This study investigated the chemical kinetics and macroscopic properties of engines equipped with direct water injection (DWI) at high compression ratios (CRs) to address knock issues. The results indicated that the generation and consumption pathways of OH and CH2O remain unaffected by water injection. Although water exhibited an opposing effect on the sensitive reactions of OH and CH2O, the production reaction rates of OH and CH2O decrease with water injection. Knock index values at each monitoring point approached zero when the water injection quality exceeds 30%. Additionally, knock suppression effectiveness improved with the advancement of water injection timing. The simulation results indicated that the knock index at high CR decreased from 13.61 to 0.27 when the water/fuel ratio exceeded 30%. The indicated specific fuel consumption decreased by 29.3%, and indicated power increased by 41.8% compared to the original engine. Although NOx and CO emissions increased because of the high CR, the water injection coupled with the compression ratio strategy limited the increase in emissions. These findings indicated that DWI coupled with high CR was advantageous to enhancing fuel economy and reducing emissions in low capacity gasoline engines.

Numerical Study on Chemical Kinetic Characteristics of Counterflow Diffusion Flame Extinction of Methane/Ammonia/Air Flame under High Pressure or Air Preheating Temperature.

Green ammonia has become an increasingly popular fuel in recent years because of its combustion process without carbon oxide release. Adding ammonia to methane fuel for co-combustion has become one of the important research topics in the current combustion field. In the present study, the CH4/NH3/Air counterflow diffusion flame was taken as the research object, and Chemkin-2019 R3 software was used to explore and analyze the flame extinction limit and chemical kinetics characteristics under different ammonia mixing ratios, initial pressures, and air preheating temperatures. It was obtained that the flame extinction stretch rate was decreased by increasing the NH3 mole fraction in the CH4/NH3 mixed fuel. The increase in pressure or air preheating temperature would accelerate the chemical reaction rate of each component in the combustion process, increase the flame extinction limit, and counteract the "stretching effect" of the flame, thus restraining the flame extinguishing phenomenon. The results of a path analysis show that the formation and consumption of OH had an important influence on flame extinction in the chain reaction. The net reaction rate of OH increases with increasing the initial pressure or air preheating temperature, which leads to an increase in flame intensity, combustion stability, and the extinction limit. Furthermore, the function curve between the reaction influences the RIF factor and the stretch rate of the first-to-ten reactions, affected by the heat release of flame combustion, was drawn and quantitatively analyzed. Eventually, a sensitivity analysis of the flame under different working conditions was completed, which found that promoting the forward reaction R39 H + O2<=>O + OH also promotes the positive combustion as a whole when the flame was near extinction. The sensitivity coefficient of R39 in the CH4/NH3/Air flame increases with the growing initial pressure. The increasing air preheating temperature was capable of switching the reaction of R248 NH2 + OH<=>NH + H2O in the CH4/NH3/Air flame from an inhibiting reaction to a promoting reaction, while decreasing the sensitivity coefficient of inhibiting the forward reaction R10 O + CH3<=>H + CH2O, R88 OH + HO2<=>O2 + H2O, and R271 H + NO + M<=>HNO + M. Thus, the inhibition effect of flame extinction was weakened, and the positive progress of combustion was promoted.

Constraining non-methane VOC emissions with TROPOMI HCHO observations: impact on summertime ozone simulation in August 2022 in China

Abstract. Non-methane volatile organic compounds (NMVOC), serving as crucial precursors of O3, have a significant impact on atmospheric oxidative capacity and O3 formation. However, both anthropogenic and biogenic NMVOC emissions remain subject to considerable uncertainty. Here, we extended the Regional multi-Air Pollutant Assimilation System (RAPAS) using the ensemble Kalman filter (EnKF) algorithm to optimize NMVOC emissions in China in August 2022 by assimilating TROPOspheric Monitoring Instrument (TROPOMI) HCHO retrievals. We also simultaneously optimize NOx emissions by assimilating in situ NO2 observations to address the chemical feedback among VOCs–NOx–O3. Furthermore, a process-based analysis was employed to quantify the impact of NMVOC emission changes on various chemical reactions related to O3 formation and depletion. NMVOC emissions exhibited a substantial reduction of 50.2 %, especially in the middle and lower reaches of the Yangtze River, revealing a prior overestimation of biogenic NMVOC emissions due to an extreme heat wave. Compared to the forecast with prior NMVOC emissions, the forecast with posterior emissions significantly improved HCHO simulations, reducing biases by 75.7 %, indicating a notable decrease in posterior emission uncertainties. The forecast with posterior emissions also effectively corrected the overestimation of O3 in forecasts with prior emissions, reducing biases by 49.3 %. This can be primarily attributed to a significant decrease in the RO2+NO reaction rate and an increase in the NO2+OH reaction rate in the afternoon, thus limiting O3 generation. Sensitivity analyses emphasized the necessity of considering both NMVOC and NOx emissions for a comprehensive assessment of O3 chemistry. This study enhances our understanding of the effects of NMVOC emissions on O3 production and can contribute to the development of effective emission reduction policies.

Photochemical behaviour and toxicity evolution of phenylbenzoate liquid crystal monomers in water

Liquid crystal monomers (LCMs) are a group of emerging pollutants that pose potential environmental risks because of their ubiquitous occurrence and toxicity. Understanding their environmental transformation is essential for assessing the ecological risk. In this study, we investigated the photochemical transformation kinetics, mechanism, and photo-induced toxicity of three phenylbenzoate LCMs in water. Their apparent photolytic rate constants were within (0.023 – 0.058) min-1, and the half-lives were < 30.0 min, showing lower persistence in water. Dissolved organic matter significantly inhibited their photolysis because of light-shielding effect and quenching of excited triplet states of LCMs. Their photolysis mainly occurred through excited triplet states, and the reactive oxygen species (i.e., ⋅OH, 1O2 and ⋅O2-) contributed to their degradation. The main photolysis pathways were ester bond cleavage, ⋅OH substitution/addition, and defluorination. Experiments and computational simulation revealed that some ·OH addition/substitution products have similar toxicity with LCMs. Additionally, the ∙OH reaction rate constants (kOH) of LCMs were determined to be > 1 × 109 M-1 s-1, evidence for their high reactivity toward ⋅OH. We have further developed reliable methods to estimate kOH of other phenylbenzoate-like LCMs with quantum chemical calculations. These results are useful for understanding the transformation and fate of LCMs in aquatic environments.

Experimental and kinetic modeling study on laminar flame speeds and emission characteristics of oxy-ammonia premixed flames

The low flame speed and NOx emissions of ammonia are the key to affecting combustion temperature, stability, flame structure, and pollutant control, limiting its application in power generation devices. This study proposes oxy-ammonia combustion (oxygen content in oxidant = 100%, NH3/O2 combustion) as a novel method for significantly enhancing ammonia combustion. A method for measuring the laminar flame speed using the NH* distribution of the Bunsen burner flame is proposed, and the measured data is close to the constant volume combustion bomb. 21 kinetic models were collected and optimized after comparison with the experimental data of constant volume combustion bombs, and results show that Han et al.'s model has higher prediction accuracy for NH3/O2 flames. The preheating temperature was relevant to gas turbine combustors (298.15–500 K), and the equivalence ratio ranged from 0.7 to 1.6. At this extreme condition (oxygen content in oxidant = 100%), the results show that the maximum laminar flame speed reaches 1.25 m/s at ambient pressure and temperature, which is approximately 18 times that of NH3/air combustion, mainly due to the increased reaction rates of OH, H, O and NH2 radicals in the reaction zone (primary ammonia decomposition and H/O radical pool). The NO emission of NH3/O2 combustion at stoichiometric conditions is about 7216 ppmv@15%O2, 1.5 and 13 times that of NH3/air and CH4/air combustion, respectively. NO production and consumption may follow the N2O and NNH mechanism, and HNO is an important precursor for NO formation. Increasing the preheating temperature has less effect on the reaction pathway directions in the reaction network of nitrogen conversion but affects the path flux significantly.

Photochemical Mechanism and Control Strategy Optimization for Summertime Ozone Pollution in Yining City

To explore the formation mechanism of the ozone （O3） and emission reduction strategy in a northwestern city， an extensive field campaign was conducted in summertime in 2021 in Yining City， in which the 0-D box model incorporating the latest explicit chemical mechanism （MCMv3.3.1） was applied for the first time to quantify the O3-NOx-VOCs sensitivity and formulate a precise O3 control strategy in this city. The results showed that： ① the three indicators ［i.e.， O3 formation potential （OFP）， ·OH reaction rate （k·OH）， and relative incremental reactivity （RIR）］ jointly indicated that alkenes， oxygenated volatile organic compounds （OVOCs）， and aromatics were the highest contributors among anthropogenic volatile organic compounds （AVOC） to O3 formation， and the contribution of biogenic volatile organic compounds （BVOC） also could not be ignored. Additionally， the results based on RIR calculation implied that that the acetaldehyde， ethylene， and propylene were the most sensitive individual VOCs species in Yining City. ② The in-situ photochemical O3 variations were primarily influenced by the local photochemical production and export process horizontally to downwind areas or vertically to the upper layer， and the reaction pathways of HO2·+ NO and ·OH + NO2 contributed the most to the gross Ox photochemical production （60%） and photochemical destruction production （53%）， respectively. Hence， the reduction in local emissions for O3 precursors was more essential to alleviate O3 pollution in this city. ③ The outcome based on RIR（NOx） / RIR（AVOC） and EKMA jointly suggested that the photochemical regime in this city can be considered a transitional regime that was also nearly a VOCs-limited regime. Detailed mechanism modeling based on multiple scenarios further suggested that the NOx and VOCs synergic emission reduction strategies was helpful to alleviate O3 pollution. These results are useful to provide policy-related guidance for other cities facing similar O3 pollution in northwest China.

Experimental study on flame spreading over locally wetted thin combustibles

This study proposed a better fire-extinguishing strategy by investigating the effectiveness of wetting operations. It also serves as a first step toward developing physical-based discrete fuel flame spread models using a newly developed small-scale test device. This study examined downward flame spread through a dry narrow path sandwiched by wetted zones formed with thin paper, with different levels of wetting and dried layer width. The dynamic flame behavior and the effect of the presence of adjacent wetting zones were studied both experimentally and numerically. The experimental observations revealed three burning patterns, namely unburned, partially burned, and burned-out regions, depending on various locally wetting fuel arrangements. Additionally, the accelerating flame spread of the wetting case was experimentally reported for the first time. One-dimensional computational simulations suggest that this accelerating phenomenon could be attributed to the chemical reaction triggered by the introduction of water. The reaction rates of OH + CO ↔ H + CO2 and H + O2 + H2O ↔ HO2 + H2O increased due to the water vapor, thereby increasing the heat production rate to further promote the preheating of the downward unburned region and flame spread rate. These findings and conclusions shed more light on the physical and chemical effects of water on flame spreading, highlighting another possibility of water accelerating flame propagation.

Rate coefficients for the gas‐phase OH + furan (C4H4O) reaction between 273 and 353 K

AbstractRate coefficients, k1, for the gas‐phase OH radical reaction with the heterocyclic ether C4H4O (1,4‐epoxybuta‐1,3‐diene, furan) were measured over the temperature range 273–353 K at 760 Torr (syn. air). Experiments were performed using: (i) the photochemical smog chamber THALAMOS (thermally regulated atmospheric simulation chamber, IMT NE, Douai‐France) equipped with Fourier Transform Infrared (FTIR) and Selected Ion Flow Tube Mass Spectrometry (SIFT‐MS) detection methods and (ii) a photochemical reactor coupled with FTIR spectroscopy (PCR, University of Crete, Greece). k1(273–353 K) was measured using a relative rate (RR) method, in which the loss of furan was measured relative to the loss of reference compounds with well‐established OH reaction rate coefficients. k1(273–353 K) was found to be well represented by the Arrhenius expression (1.30 ± 0.12) × 10−11 exp[(336 ± 20)/T] cm3 molecule−1 s−1, with k1(296 K) measured to be (4.07 ± 0.32) × 10−11 cm3 molecule−1 s−1. The k1(296 K) and pre‐exponential quoted error limits are 2σ and include estimated systematic errors in the reference rate coefficients. The observed negative temperature dependence is consistent with a reaction mechanism involving the OH radical association to a furan double bond. Quantum mechanical molecular calculations show that OH addition to the α‐carbon (ΔHr(296 K) = −121.5 kJ mol−1) is thermochemically favored over the β‐carbon (ΔHr(296 K) = −52.9 kJ mol−1) addition. The OH‐furan adduct was found to be stable over the temperature range of the present measurements. Maleic anhydride (C4H2O3) was identified as a minor reaction product, 3% lower‐limit yield, demonstrating a non‐ring‐opening active reaction channel. The present results are critically compared with results from previous studies of the OH + furan reaction rate coefficient. The infrared spectrum of furan was measured as part of this study and its estimated climate metrics are reported.

Reynolds number-based global modification of EDC constants and simulation of a syngas and a piloted CH4/air flames

The Eddy Dissipation Concept (EDC), as a promising flame prediction model, is reviewed comprehensively. Instead of strong modifications of the standard EDC constants to fit experiments, a modification method of the EDC constants based on the fuel jet Reynolds number is presented in this work and used to predict a simple jet nonpremixed syngas Sandia Flame CHN–B and a piloted CH4/air partially premixed Sandia Flame D. Compared with the experiments, the predictions show the two flames are properly reproduced with the modified EDC constants. Some over-predictions might be attributed to the under-estimation of turbulence. The microscale and the macroscale Damköhler numbers are presented showing decreasing trends with the increased Reynolds number. Three clear combustion regions in the two flames are firstly identified from the results of the net reaction rate of OH. The reaction heat in the fuel-rich region is found larger than that in the fuel-lean region for Flame D. The contours of the net reaction rate of OH show a stronger reaction region in the fuel-lean side for Flame CHN–B and in the fuel-rich side for Flame D. The predictions with the modified EDC constants agree with the experiments better than those with the standard EDC constants.

OH reaction rate coefficients, infrared spectra, and climate metrics for (E)‐ and (Z)‐ 2‐perfluoroheptene (2‐C7F14) and 3‐perfluoroheptene (3‐C7F14)

AbstractIn this work, perfluoroheptene 2‐ and 3‐C7F14 stereoisomer specific gas‐phase OH reaction rate coefficients, k, were measured at 296 K in ∼600 Torr (He bath gas) using a relative rate (RR) method. Gas‐chromatography (GC) with electron capture detection (ECD) was used for the separation and detection of the stereoisomers. Rate coefficients for (E)‐2‐C7F14, (Z)‐2‐C7F14, (E)‐3‐C7F14, and (Z)‐3‐C7F14 were measured to be (in units of 10−13 cm3 molecule−1 s−1) (3.60 ± 0.51), (2.22 ± 0.21), (3.43 ± 0.47), and (1.48 ± 0.19), respectively, where the uncertainties include estimated systematic errors. Rate coefficients for the (E)‐ stereoisomers were found to be systematically greater than the (Z)‐ stereoisomers by a factor of 1.6 and 2.3 for 2‐C7F14 and 3‐C7F14, respectively. Atmospheric lifetimes with respect to OH radical reaction for (E)‐2‐C7F14, (Z)‐2‐C7F14, (E)‐3‐C7F14, (Z)‐3‐C7F14 were estimated to be ∼33, ∼56, ∼36, and ∼86 days, respectively, for an average OH radical concentration of 1 × 106 molecule cm−3. Quantitative infrared absorption spectra were measured as part of this work. Complimentary theoretically calculated infrared absorption spectra using density functional theory (DFT) are included in this work. The theoretical spectra were used to evaluate stereoisomer climate metrics. Radiative efficiencies (adjusted) and global warming potentials (GWPs, 100‐year time‐horizon), were estimated to be 0.12, 0.19, 0.12, and 0.23 W m−2 ppb−1 and 1.9, 5.1, 2.1, 9.3 for (E)‐2‐C7F14, (Z)‐2‐C7F14, (E)‐3‐C7F14, (Z)‐3‐C7F14, respectively. Atmospheric degradation mechanisms are discussed.

Effects of EGR on combustion and emission characteristics of PODE/methanol RCCI mode at high load

In order to extend the high-load operation range of polyoxymethylene dimethyl ethers (PODE)/methanol dual-fuel reactivity controlled compression ignition (RCCI) combustion mode, a numerical model of in-cylinder combustion was established by coupling PODE/methanol chemical reaction kinetics with CONVERGE software. And then the influence mechanism of exhaust gas recirculation (EGR) addition on in-cylinder combustion process, combustion stability and pollutant formation of the RCCI engine under high load and pilot injection strategy were investigated. The results show that with the increase of EGR ratio, the peak values of the first heat release formed by PODE pilot injection combustion and in-cylinder pressure decrease, the combustion start timing and CA50 phase are gradually delayed. While the peak value of the second heat release formed by PODE main injection combustion at low EGR ratio and the combustion duration remain unchanged. Meanwhile, as the EGR ratio increases from 0 to 30 %, the brake thermal efficiency (BTE) decreases from 46.54 % to 44.40 %. And the in-cylinder equivalent ratio and temperature field distributions become uniform, and both high-temperature region (above 1800 K) and NOx generation area are reduced, which decreases NOx emissions by 42.6 % without increasing soot emissions. The methanol mixture at squish zone is subjected to the high-temperature and high-pressure of pilot injection combustion, resulting in obvious pressure oscillation. While EGR addition decreases the generation of HCO and OH active radicals, ignition points and reaction rate, which effectively inhibits methanol spontaneous combustion and knock phenomenon, and decreases the ringing intensity from 2.82 MW/m2 to 1.50 MW/m2. The pilot injection strategy coupled with 18 % EGR can extend the indicated mean effective pressure to 1.22 MPa. However, the load further extension is mainly limited by the sharp decline of BTE.

The effect of reaction mechanism on OH* chemiluminescence in methane inverse diffusion flame

The OH* chemiluminescence is closely related to flame properties in the study of combustion diagnosis. In this work, numerical investigation of the OH* reaction mechanism in methane inverse diffusion flames for evaluating the OH* chemiluminescence is presented. The two-dimensional distribution of OH* emission intensity in laminar inverse diffusion flames was obtained by CCD imaging system and agreed well with the numerical simulation results. By changing oxygen/fuel equivalence ratios, the effect of elementary reactions on OH* chemiluminescence was analyzed. The results show that the OH* distribution depends upon the formation reactions, quenching reactions and diffusion characteristics. The OH* is generated by R1 (H + O + M <=> OH* + M) and R2 (CH + O2 <=> OH* + CO). The trend of R1 distribution is consistent with the OH* molar concentration distribution at different oxygen/fuel equivalence ratios. Although the R1 reaction rate is strongest at the root of flame, the OH* molar concentration is relatively low due to the strong quenching reactions, where the OH* is mainly quenched with H2O and O2. In the pure diffusion zone, the generation and destruction reaction rate of OH* is unbalanced. At the root of the flame, OH* diffuses from the net generation zone to both sides.

Chemical transformation of &amp;lt;i&amp;gt;α&amp;lt;/i&amp;gt;-pinene-derived organosulfate via heterogeneous OH oxidation: implications for sources and environmental fates of atmospheric organosulfates

Abstract. Organosulfur compounds are found to be ubiquitous in atmospheric aerosols – a majority of which are expected to be organosulfates (OSs). Given the atmospheric abundance of OSs, and their potential to form a variety of reaction products upon aging, it is imperative to study the transformation kinetics and chemistry of OSs to better elucidate their atmospheric fates and impacts. In this work, we investigated the chemical transformation of an α-pinene-derived organosulfate (C10H17O5SNa, αpOS-249) through heterogeneous OH oxidation at a relative humidity of 50 % in an oxidation flow reactor (OFR). The aerosol-phase reaction products were characterized using high-performance liquid chromatography–electrospray ionization–high-resolution mass spectrometry and ion chromatography. By monitoring the decay rates of αpOS-249, the effective heterogeneous OH reaction rate was measured to be (6.72±0.55)×10-13 cm3 molecule−1 s−1. This infers an atmospheric lifetime of about 2 weeks at an average OH concentration of 1.5×106 molecules cm−3. Product analysis shows that OH oxidation of αpOS-249 can yield more oxygenated OSs with a nominal mass-to-charge ratio (m/z) at 247 (C10H15O5S−), 263 (C10H15O6S−), 265 (C10H17O6S−), 277 (C10H13O7S−), 279 (C10H15O7S−), and 281 (C10H17O7S−). The formation of fragmentation products, including both small OSs (C &lt;10) and inorganic sulfates, is found to be insignificant. These observations suggest that functionalization reactions are likely the dominant processes and that multigenerational oxidation possibly leads to formation of products with one or two hydroxyl and carbonyl functional groups adding to αpOS-249. Furthermore, all product ions except m/z=277 have been detected in laboratory-generated α-pinene-derived secondary organic aerosols as well as in atmospheric aerosols. Our results reveal that OSs freshly formed from the photochemical oxidation of α-pinene could react further to form OSs commonly detected in atmospheric aerosols through heterogeneous OH oxidation. Overall, this study provides more insights into the sources, transformation, and fate of atmospheric OSs.

Effect of CO2/H2O addition on laminar diffusion flame structure and soot formation of oxygen-enriched ethylene

Oxygen-enriched combustion technology has an important application prospect, in which Oxy-steam combustion is a new generation of potential carbon capture technology. In this paper, the effects of O2/CO2 and O2/H2O atmosphere on soot formation in laminar diffusion flame were studied. The flame image was taken by a color (CCD) camera in the visible band, the directional emission power in R and G bands was calibrated, and the two-dimensional distribution of temperature and soot volume fraction in the flame was reconstructed. The experimental results are compared with the numerical results, and the numerical results well predict the temperature of oxygen-enriched flame in O2/N2/CO2 atmosphere, but overestimate the soot volume fraction. The laminar diffusion flame of ethylene in two kinds of oxygen-rich atmosphere was numerically studied by using a step-by-step decoupling method to separate the density, transport, thermal and chemical effects of the diluent by introducing four virtual substances. The calculation results show that combustion in O2/H2O and O2/CO2 atmosphere reduces the flame peak temperature, but the effect on the flame centerline temperature distribution is different. The addition of H2O enhanced the concentration of OH through the reverse reaction rate of OH + H2=H + H2O, while the addition of CO2 decreased the concentration of OH.O2/CO2 combustion leads to the increase of flame height and radius, while in O2/H2O atmosphere, the flame height is shortened. Both CO2 and H2O have a good inhibitory effect on soot formation. CO2 in the oxidant affects soot nucleation and surface growth rate mainly by reducing flame temperature and H mole fraction, but not by promoting soot oxidation process. Water vapor also inhibits soot formation by increasing OH concentration through chemical effect.

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.

Atmospheric Chemistry of c-C5HF7 and c-C5F8: Temperature-Dependent OH Reaction Rate Coefficients, Degradation Products, Infrared Spectra, and Global Warming Potentials.

c-C5HF7 (1H-heptafluorocyclopentene) and c-C5F8 (perfluorocyclopentene) are potent greenhouse gases presently used as replacement compounds in Si etching. A thorough understanding of their potential impact on climate and air quality necessitates studies of their atmospheric reactivity, radiative properties, and atmospheric degradation pathways. The predominant atmospheric removal process for these compounds is expected to be via reaction with the OH radical. In this study, rate coefficients, k, for the gas-phase reaction of the OH radical with c-C5HF7 and c-C5F8 were measured over a range of temperatures (242-370 K) and pressures (50-100 Torr, He) using a pulsed laser photolysis-laser-induced fluorescence technique. In addition, a complementary relative rate technique, employing multiple reference compounds, was used to study the reactions between 273 and 372 K at 100 Torr (He) total pressure. Reaction rate coefficients were found to be independent of pressure over this range of conditions with k1(296 K) = (4.59 ± 0.10) × 10-14 cm3 molecule-1 s-1 and k1(T) = (4.00 ± 0.40) × 10-13 exp(-(631 ± 30)/T) cm3 molecule-1 s-1 for c-C5HF7 and k2(296 K) = (4.90 ± 0.14) × 10-14 cm3 molecule-1 s-1 and k2(T) = (3.59 ± 0.4) × 10-13 exp(-(591 ± 25)/T) cm3 molecule-1 s-1 for c-C5F8. Stable end-products were measured following the OH radical-initiated degradation of c-C5HF7 and c-C5F8 in the presence of O2. F(O)CCF2CF2CF2CH(O), CF2O, and CO2 were observed as the major end-products in the oxidation of c-C5HF7 with molar yields of 0.64, 1.27, and 0.53, respectively. For c-C5F8, F(O)CCF2CF2CF2CF(O), CF2O, and CO2 were observed with molar yields of 0.66, 0.63, and 0.43, respectively. The total carbon mass balance in both systems was 1.0 ± 0.15. The high yield of a C5-dicarbonyl end-product is consistent with a ring opening at the carbon-carbon double bond site for both c-C5HF7 and c-C5F8. A comparison of the present kinetic and degradation product results with previously published studies is presented. A rate coefficient upper limit for the gas-phase reaction of O3 with c-C5HF7 and c-C5F8 of 1 × 10-21 cm3 molecule-1 s-1 was measured as part of this work. Atmospheric lifetimes for c-C5HF7 and c-C5F8 are estimated to be 252 and 236 days, respectively. Infrared absorption spectra of c-C5HF7 and c-C5F8 were also measured and found to agree, to within 5%, with results from previous studies. The well-mixed and lifetime adjusted radiative efficiencies (RE, W m-2 ppb-1) and 100 year time horizon global warming potential (GWP) for c-C5HF7 are 0.35, 0.24, and 46.7 and for c-C5F8 are 0.38, 0.25, and 46.2, respectively.

Inorganic Sulfur Species Formed upon Heterogeneous OH Oxidation of Organosulfates: A Case Study of Methyl Sulfate

Recent studies reveal that organosulfates at the particle surface can be oxidized by gas-phase OH radicals with significant rates. Inorganic sulfur species, such as the bisulfate ion (HSO4–) and sulfate ion (SO42–), can be formed upon these heterogeneous oxidation processes through the formation and subsequent reactions of sulfate radical anions (SO4•–) in the particle phase. However, the amount of inorganic sulfur species produced in these heterogeneous oxidation reactions is not known. We investigate the heterogeneous OH oxidation of sodium methyl sulfate (CH3SO4Na), the smallest organosulfate detected in atmospheric particles, using an oxidation flow reactor at a relative humidity of 75%. We quantify the kinetics by measuring the decay of CH3SO4Na and the amount of HSO4– and SO42– formed upon oxidation using ion chromatography. Kinetic measurements determine the heterogeneous OH reaction rate to be (5.72 ± 0.14) × 10–13 cm3 molecule–1 s–1, with an effective OH uptake coefficient, γeff, of 0.31 ± 0.06. The molar yield of inorganic sulfur species, defined as the total number of moles of HSO4– and SO42– formed per mole of CH3SO4Na consumed upon oxidation, is found to be significant and has an average value of 0.62 ± 0.18 upon oxidation. A kinetic model is developed to describe the kinetics and inorganic sulfur species formation upon oxidation. Model simulations suggest that CH3SO4Na tends to decompose rapidly into formaldehyde and SO4•–, and the reaction of SO4•– with CH3SO4Na plays a significant role in both governing the kinetics and the formation of inorganic sulfur species.

Effect of inorganic-to-organic mass ratio on the heterogeneous OH reaction rates of erythritol: implications for atmospheric chemical stability of 2-methyltetrols

Abstract. The 2-methyltetrols have been widely chosen as chemical tracers for isoprene-derived secondary organic aerosols. While they are often assumed to be relatively unreactive, a laboratory study reported that pure erythritol particles (an analog of 2-methyltetrols) can be heterogeneously oxidized by gas-phase OH radicals at a significant rate. This might question the efficacy of these compounds as tracers in aerosol source-apportionment studies. Additional uncertainty could arise as organic compounds and inorganic salts often coexist in atmospheric particles. To gain more insights into the chemical stability of 2-methyltetrols in atmospheric particles, this study investigates the heterogeneous OH oxidation of pure erythritol particles and particles containing erythritol and ammonium sulfate (AS) at different dry inorganic-to-organic mass ratios (IOR) in an aerosol flow tube reactor at a high relative humidity of 85 %. The same reaction products are formed upon heterogenous OH oxidation of erythritol and erythritol–AS particles, suggesting that the reaction pathways are not strongly affected by the presence and amount of AS. On the other hand, the effective OH uptake coefficient, γeff, is found to decrease by about a factor of ∼20 from 0.45±0.025 to 0.02±0.001 when the relative abundance of AS increases and the IOR increases from 0.0 to 5.0. One likely explanation is the presence of dissolved ions slows down the reaction rates by decreasing the surface concentration of erythritol and reducing the frequency of collision between erythritol and gas-phase OH radicals at the particle surface. Hence, the heterogeneous OH reactivity of erythritol and likely 2-methyltetrols in atmospheric particles would be slower than previously thought when the salts are present. Given 2-methyltetrols often coexist with a significant amount of AS in many environments, where ambient IOR can vary from ∼1.89 to ∼250, our kinetic data would suggest that 2-methyltetrols in atmospheric particles are likely chemically stable against heterogeneous OH oxidation under humid conditions.

Determination of the JP10 + OH → Product Reaction Rate with Measured Fuel Concentrations in Shock Tube Experiments.

The overall reaction rate for JP10 + OH → products was measured directly via laser absorption of OH in shock tube experiments from 931 to 1308 K and 0.94 to 1.44 atm. The JP10 concentration of test gas mixtures was measured in the shock tube for several experiments using a 3.39 μm laser fuel diagnostic. The measured JP10 concentrations indicated fuel losses due to adsorption of 11-31% compared to values calculated manometrically from mixture preparation. OH was generated via rapid thermal decomposition of tert-butyl hydroperoxide behind reflected shock waves, and post-shock OH profiles were measured via laser absorption at 308.6 nm. The measured OH profiles were fit with a chemical kinetic model for JP10 chemistry to determine the overall JP10 + OH reaction rate. A recommendation is made for the JP10 + OH overall reaction rate over the temperature range explored in this study as k1 (931-1308 K) = 1.622 × 1014 exp(-1826/T [K]) ± 12%. To the authors' knowledge, these data are the first direct measurements of the overall reaction rate for JP10 + OH.

Hydroxyl radical scavenging by solid mineral surfaces in oxidative treatment systems: Rate constants and implications

Advanced oxidation treatment processes used in various applications to treat contaminated soil, water, and groundwater involve powerful radical intermediates, including hydroxyl radicals (•OH). Inefficiency in •OH-driven treatment systems involves scavenging reactions where •OH react with non-target species in the aqueous and solid phases. Here, •OH were generated in iron (Fe)- and UV-activated hydrogen peroxide (Fe-AHP, UV-AHP) systems where the loss of rhodamine B served as a quantitative metric for •OH activity. Kinetic analysis methods were developed to estimate the specific •OH surface scavenging rate constant (k≡S). In the Fe-AHP system, k≡S for silica (2.85 × 106 1/m2 × s) and alumina (3.92 × 106 1/m2 × s) were similar. In the UV-AHP system, estimates of k≡S for silica (4.50 × 106 1/m2 × s) and alumina (7.45 × 106 1/m2 × s) were higher. k≡S for montmorillonite (MMT) in the UV-AHP system was ≤4.22 × 105 1/m2 × s. Overall, k≡S,silica ∼ k≡S, alumina > k≡S,MMT indicating k≡S is mineral specific. Radical scavenging was dominated by surface scavenging at 10–50 g/L silica, alumina, or MMT, in both Fe-AHP and UV-AHP systems. The experimentally-derived surface •OH scavenging rate constants were extended to in-situ chemical oxidation (ISCO) treatment conditions to contrast •OH reaction rates with contaminant and aqueous phase reactants found in aquifer systems. •OH reaction was dominated by solid surfaces comprised of silica, alumina, and montmorillonite minerals relative to •OH reaction with trichloroethylene, the target compound, and H2O2, a well-documented radical scavenger. These results indicate that solid mineral surfaces play a key role in limiting the degradation rate of contaminants found in soil and groundwater, and the overall treatment efficiency in ISCO systems. The aggressive •OH scavenging measured was partially attributed to the relative abundance of scavenging sites on mineral surfaces.