Distribution Of OH Research Articles

Enhanced CO2 hydrogenation to higher alcohols over K-Co promoted In2O3 catalysts

The use of CO2 to produce higher alcohols is a promising route to reduce emissions and make a profit. However, low activity of existing catalysts is a barrier for such processes to reach the industrial level. Here we report a novel K-Co promoted In2O3 catalyst for CO2 hydrogenation to higher alcohols. The K-Co promoted In2O3 with an optimum K and Co of 2.5 and 5.0 wt% achieves a high C2+OH space time yield of 169.6 g kgcat−1h−1 with the excellent C2+OH distribution of 87.4% in the total alcohols, while a very low space time yield of C2+OH (0.4 g kgcat−1h−1) is obtained over In2O3 promoted with 5 wt% Co. Characterization results reveal that, after reduction with H2 at 380 °C for 2 h, K-O-Co species is created for K-Co promoted In2O3 (2.5 wt% K and 5 wt% Co) sample which significantly reduces a number of weak H2 adsorption and strengthens the interaction of adsorbed H compared to 5 wt% Co promoted In2O3 sample containing a mixture of Co0 and CoO. The reduction of weak H2 adsorption and the improved interaction of adsorbed H with the catalyst surface retard the hydrogenation ability, allowing the CO insertion into adsorbed CxHy species prior to its hydrogenation to form hydrocarbon, resulting in a remarkable reduction of CH4 and higher hydrocarbon, and significant improvement of higher alcohols.

Numerical study on combustion performance of propane non-premixed mild in O2/CO2 atmosphere

ABSTRACT Moderate or intense low-oxygen dilution (MILD) combustion is one of novel clean combustion technologies. Propane MILD combustion in CO2/O2 atmosphere is investigated in a 20 KW MILD combustion furnace by numerical simulation. The results indicate propane MILD combustion is established and stable in O2/CO2 atmosphere instead of N2/O2 atmosphere, and O2/CO2 atmosphere leads to lower average temperature in the furnace, better temperature uniformity, as well as higher CO emission. The dilution concentration of CO2 affects the thermal and chemical performance in the propane MILD combustion, such as temperature distribution, propane pyrolysis, exothermic rate, free radical OH and CO distributions in the furnace. CO emission always keeps several thousand ppm in all CO2 cases, which indicates that CO emission and burnout are the key problem in propane MILD combustion in O2/CO2 atmosphere. It is concluded by analyzing these factors that in the range of 73%~85% of CO2 dilution concentration, the 79% Case presents best, which is closely related to the multiple effects of CO2 physical dilution, jet entrainment, jet penetration, and chemical reaction.

Experimental and Numerical Study on the Combustion Characteristics of a Laminar Non-Premixed Methane Jet Flame in Oxygen/Carbon Dioxide Coflow

The combustion characteristics of laminar non-premixed CH4 jet flame in an O2/CO2 coflows with different oxygen mole fractions were studied experimentally. The flame heights at different oxygen concentrations and fuel jet velocity were obtained. The experimental observation shows that the luminosity of the CH4 jet flame in O2/CO2 coflow is different from that of the flame in air stream. A two-dimensional numerical study of a laminar non-premixed CH4 jet flame in the O2/CO2 coflow with the O2 mole fraction of 0.35 was conducted to analyze the effects of CO2 dilution on the flame. The distribution of OH radicals in the flame was measured experimentally using planar laser-induced fluorescence (PLIF) to validate the computational method adopted in this work, and the computational and experimental results of the OH distributions showed good consistency at various fuel flow velocities. Three artificial species were created in the numerical experiment to analyze the effects of the chemical reactions, third-body collisions, and transport properties of CO2 on the height, width, and temperature distribution of the flame. The results showed that CO2 participation in chemical reactions exerts significant effects on the flame. However, the influences of the third-body effects and transport properties of CO2 on the jet flame are unremarkable. The global reaction pathways and distributions of important species in the laminar non-premixed CH4 jet flame were analyzed in detail to investigate the influence mechanisms of CO2 on the flame height and temperature. The entire flame can be divided into two oxidation parts, which separated by the boundary of the HCCO. The H, O, and OH concentrations and distributions in different parts of the flame were influenced by CO2 dilution, resulting in different flame heights and temperature distributions.

Retrieving daytime distributions of O, H, OH, HO2, and chemical heating rate in the mesopause region from satellite observations of ozone and OH* volume emission: The evaluation of the importance of the reaction H + O3 → O2 + OH in the ozone balance

Based on data from the SABER (Sounding of the Atmosphere using Broadband Emission Radiometry) instrument onboard the TIMED (Thermosphere Ionosphere Mesosphere Energetics and Dynamics) satellite observations 2003–2015, we retrieved atomic oxygen and atomic hydrogen distributions in the mesopause region (∼77 km–100 km) for daytime conditions taking into account the reaction of the ozone with atomic hydrogen in the ozone balance equation (OBE). Subsequently we calculated daytime distributions of OH, HO2, and the corresponding total chemical heating rate (CHR) from seven exothermic chemical reactions. The novelty of our approach is that we consider, to the best of our knowledge, the reaction H + O3 → O2 + OH in the daytime OBE for the retrieving procedure for the first time. It is shown that neglecting this reaction has little effect on H distributions, but it leads to O and CHR underestimations (up to ∼ 35–40%) and HO2 and OH overestimations (up to ∼ 50–85%).

Fabrication of uniform and anti-corrosion layered double hydroxides film on Mg-Gd-Y-Zn-Zr alloy through solution pH tailoring

Second phase is generally beneficial to the mechanical properties of magnesium (Mg) alloys. As an electrochemically heterogeneous structure, it leads to the formation of uneven chemical conversion films which are detrimental to corrosion protection. To address this problem for multiphase Mg alloys, a simple hydrothermal method was reported herein for fabricating a layered double hydroxides (LDHs) film with a uniform structure and superior corrosion protection upon Mg-Gd-Y-Zn-Zr alloy through tailoring solution pH. Morphology characterization reveals that high solution pH facilitates the production of LDHs nanosheets with dense and fine features, which is attributed to the high density of hydroxy groups on substrate as nucleation sites for LDHs growth. In addition, the uniformity of films depended on solution pH. The highest pH (13.5) of this study drove even distribution of OH− that in turn circumvented the micro-galvanic effect between second phases and α-Mg matrix. Immersion and electrochemical tests confirmed that corrosion resistance of the sample fabricated at pH of 13.5 outperformed those of the counterparts fabricated at pH 10−12. It is postulated that the high compactness of the LDHs film and the self-healing ability of RE(OH)3 were the driving forces.

Experimental study on structure and blow-off characteristics of NH3/CH4 co-firing flames in a swirl combustor

Experimental study on the unconfined lean premixed NH3/CH4/air flames with three CH4 fractions of 0, 50%, and 100% stabilized on a bluff-body and swirl burner was investigated. The flow fields and instantaneous OH distributions were captured by synchronous PIV/OH-PLIF measurement. The macrostructure and critical mechanism of blow-off for the compact NH3/air flame at conditions far away and near blow-off, and the effect of CH4 addition on the topology and flame stabilization was discussed. Results show that the NH3/air flame possesses a poor lean blow-off limit Φb, and the NH3/air mixture can not be ignited when Uin is higher than 5 m/s. After adding 50% CH4 fuel, the laminar flame speed SL and the extinction strain rate Kext are approximately 3 times and 7 times that of the NH3/air flame with equivalence ratio Φ = 0.8, resulting in a more stable flame. And the NH3/air flame Φb = 0.78 with inlet bulk velocity Uin = 4 m/s can be extended wider by 16.6% to Φb = 0.65. The NH3 flame front has been locally extinguished due to excessive stretching, which is the key factor of triggering the overall blow-off. Stretching is mainly caused by shear strain. CH4 addition makes the flame front more wrinkled. Furthermore, the flame is strengthened due to the enhanced resistance to stretching by blending CH4 with NH3. But the root of the NH3/CH4/air flames with methane fractions of 50% and 100% also have excessive straining near the blow-off conditions, which induces flame blow-off. The increase of burning velocity and temperature by adding CH4 is also conducive to improving flame stability. The Inner Shear Layer (ISL) vortexes promote the uniform mixing of combustion products in the Inner Recirculation Zone (IRZ) and downstream combustion gas, which is conducive to the stability of CH4/air flame. For NH3/air flame, the ISL vortexes accelerate the flame blow-off by entraining the cold reactants into the IRZ.

COSMOGENIC14CO FOR ASSESSING THE OH-BASED SELF-CLEANING CAPACITY OF THE TROPOSPHERE

ABSTRACTAn application of radiocarbon (14C) in atmospheric chemistry is reviewed.14C produced by cosmic neutrons immediately forms14CO, which reacts with hydroxyl radicals (OH) to14CO2. By this the distribution and seasonality (the lifetime of14CO is ∼1 month) of the pivotal atmospheric oxidant OH can be established.14CO measurement is a complex but unique application which benefitted enormously from the realization of AMS, bearing in mind that14CO abundance is of the order of merely 10 molecules per cm3not only provides14CO an independent measure for the OH based self-cleansing capacity of the troposphere, but also enabled detection of14C production due to high energy solar protons in 1989. Although its production takes place throughout the atmosphere and does not have the character of a point source, transport processes in the atmosphere affect the distribution of14CO. Vertical mixing in the troposphere renders gradients in its production rate less critical, but considerable meridional gradients exist. One question has remained open, namely confirmation of calculated14C production by direct measurement. A new sampling method is proposed. The conclusions are a guide to future work on14CO in relation to OH and atmospheric transport.

Photodissociation Dynamics of Methyl Hydroperoxide at 193 nm: ATrajectory Surface-Hopping Study.

The photodissociation of methyl hydroperoxide (CH3OOH) at 193 nm has been studied using a direct dynamics trajectory surface-hopping (TSH) method. The potential energies, energy gradients, and nonadiabatic couplings are calculated on the fly at the MRCIS(6,7)/aug-cc-pVDZ level of theory. The hopping of a trajectory from one electronic state to another is decided on the basis of Tully's fewest switches algorithm. An analysis of the trajectories reveals that the cleavage of the weakest O-O bond leads to major products CH3O(2E) + OH(2Π), contributing about 72.7% of the overall product formation. This OH elimination was completed in the ground degenerate product state where both the ground singlet (S0) and first excited singlet (S1) states become degenerate. The O-H bond dissociation of CH3OOH is a minor channel contributing about 27.3% to product formation, resulting in products CH3OO + H. An inspection of the trajectories indicates that unlike the major channel OH elimination, the H-atom elimination channel makes a significant contribution (∼3% of the overall product formation) through the nonadiabatic pathway via conical intersection S1/S0 leading to ground-state products CH3OO(X 2A″) + H(2S) in addition to adiabatic dissociation in the first excited singlet state, S1, correlating to products CH3OO(1 2A') + H(2S). The computed translational energy of the majority of the OH products is found to be high, distributed in the range of 70 to 100 kcal/mol, indicating that the dissociation takes place on a strong repulsive potential energy surface. This finding is consistent with the nature of the experimentally derived translational energy distribution of OH with an average translational energy of 67 kcal/mol after the excitation of CH3OOH at 193 nm.

Methane/Ammonia Radical Formation during High Temperature Reactions in Swirl Burners

Recent studies have demonstrated that ammonia is an emerging energy vector for the distribution of hydrogen from stranded sources. However, there are still many unknown parameters that need to be understood before ammonia can be a substantial substitute in fuelling current power generation systems. Therefore, current attempts have mainly utilised ammonia as a substitute for natural gas (mainly composed of methane) to mitigate the carbon footprint of the latter. Co-firing of ammonia/methane is likely to occur in the transition of replacing carbonaceous fuels with zero-carbo options. Hence, a better understanding of the combustion performance, flame features, and radical formation of ammonia/methane blends is required to address the challenges that these fuel combinations will bring. This study involves an experimental approach in combination with numerical modelling to elucidate the changes in radical formation across ammonia/methane flames at various concentrations. Radicals such as OH*, CH*, NH*, and NH2* are characterised via chemiluminescence whilst OH, CH, NH, and NH2 are described via RANS κ-ω SST complex chemistry modelling. The results show a clear progression of radicals across flames, with higher ammonia fraction blends showing flames with more retreated shape distribution of CH* and NH* radicals in combination with more spread distribution of OH*. Simultaneously, equivalence ratio is a key parameter in defining the flame features, especially for production of NH2*. Since NH2* distribution is dependent on the equivalence ratio, CFD modelling was conducted at a constant equivalence ratio to enable the comparison between different blends. The results denote the good qualitative resemblance between models and chemiluminescence experiments, whilst it was recognised that for ammonia/methane blends the combined use of OH, CH, and NH2 radicals is essential for defining the heat release rate of these flames.

Mixture formation and combustion process analysis of an innovative diesel piston bowl design through the synergetic application of numerical and optical techniques

The optimization of diesel engine piston bowl geometries has a crucial role in improving the near-wall flame evolution for better air/fuel mixing and soot reduction. With these aims, an innovative piston bowl for a light-duty diesel engine was designed, coupling both a sharp-stepped lip and radial bumps in the inner bowl rim. The impact of the proposed design was investigated through both 3D-CFD and single-cylinder optical engine. The numerical model, featuring a calibrated spray model and a detailed combustion mechanism, was used to analyse the non-reactive air/fuel mixing and the combustion processes. Results highlighted a reduced jet-to-jet interaction and better air/fuel mixing for the innovative bowl with respect to a conventional re-entrant design, thus enabling faster combustion process after the end of main injection. Numerical results in terms of flame’s kinematic and oxidation process were compared with the combustion image velocimetry (CIV) and OH* chemiluminescence from the optical engine, showing higher velocity near the radial bumps, and faster flame recirculation towards the piston center. Moreover, both experiments and simulations showed a more intense OH distribution in the radial-bumps region and above the step during the first stage of the combustion process, thanks to the enhanced air/fuel mixing.

Depth resolution of transmission ERDA for H in Al under nuclear-elastically enhanced recoiling of H by 8 MeV He

Using stacked samples of Al foil and H-containing resin film, we have carried out elastic recoil detection analysis with transmission layout (T-ERDA) to investigate the depth resolution in the measurements of H distribution in Al. For narrow and wide acceptance conditions of the detector, the depth resolutions of 1.5–4.9 μm at several depths in Al of 50 and 80 μm thicknesses have been determined for incidence of 8 MeV 4He. While the main factor to degrade the depth resolution is the energy straggling of recoil H for narrow acceptance conditions, it is the extended low-energy side of the H spectrum for wide acceptance conditions. The knowledge obtained in this work is useful for analysis of 3D images of H distribution measured by T-ERDA, for example, future analysis of minerals or natural glass samples to determine abundances and distributions of water or OH in the samples.

CFD Study and Experimental Validation of a Dual Fuel Engine: Effect of Engine Speed

Dual fuel engines induce benefits in terms of pollutant emissions of PM and NOx together with carbon dioxide reduction and being powered by natural gas (mainly methane) characterized by a low C/H ratio. Therefore, using natural gas (NG) in diesel engines can be a viable solution to reevaluate this type of engine and to prevent its disappearance from the automotive market, as it is a well-established technology in both energy and transportation fields. It is characterized by high performance and reliability. Nevertheless, further improvements are needed in terms of the optimization of combustion development, a more efficient oxidation, and a more efficient exploitation of gaseous fuel energy. To this aim, in this work, a CFD numerical methodology is described to simulate the processes that characterize combustion in a light-duty diesel engine in dual fuel mode by analyzing the effects of the changes in engine speed on the interaction between fluid-dynamics and chemistry as well as when the diesel/natural gas ratio changes at constant injected diesel amount. With the aid of experimental data obtained at the engine test bench on an optically accessible research engine, models of a 3D code, i.e., KIVA-3V, were validated. The ability to view images of OH distribution inside the cylinder allowed us to better model the complex combustion phenomenon of two fuels with very different burning characteristics. The numerical results also defined the importance of this free radical that characterizes the areas with the greatest combustion activity.

Assessing ignitions of explosive gas mixtures by low-energetic electrical discharges using OH-LIF and 1D-simulations

ABSTRACT Spark ignition by low-energetic electrical discharges in combustible fuel/air mixtures is a significant safety risk in various industries. In the present paper, OH-LIF measurements and numerical simulations of the ignition and early flame propagation of hydrogen/air, propane/air and ethene/air mixtures are reported. The experiments, carried out at four energy levels close to the respective minimum ignition energy (MIE), show that higher energy input leads to a wider flame radius, which is in agreement with one-dimensional numerical simulations. Further, the evaluation of the radial OH distribution reveals the highly stochastic nature of this process if the introduced energy is near the respective MIE. By utilizing the combined methodical approach of experiments and simulations, the key factors impairing the experimental repeatability are identified. The numerical results indicate at lower energies a longer time delay between source time and ignition, during which loss processes and perturbations may lead to extinction. One important influencing factor is the three-dimensional flow induced by the discharge, effectively increasing the experimental scatter. The presented work facilitates a detailed view on the complex physiochemical mechanisms dominating ignitions of explosive gas mixtures by low-energetic electrical discharges.

Observational Evidence for a Thick Disk of Dark Molecular Gas in the Outer Galaxy

Abstract We present the serendipitous discovery of an extremely broad (ΔV LSR ∼ 150 km s−1), faint (T mb < 10 mK), and ubiquitous 1667 and 1665 MHz ground-state thermal OH emission toward the second quadrant of the outer Galaxy (R gal > 8 kpc) with the Green Bank Telescope. Originally discovered in 2015, we describe the redundant experimental, observational, and data quality tests of this result over the last five years. The longitude–velocity distribution of the emission unambiguously suggests large-scale Galactic structure. We observe a smooth distribution of OH in radial velocity that is morphologically similar to the H i radial velocity distribution in the outer Galaxy, showing that molecular gas is significantly more extended in the outer Galaxy than previously expected. Our results imply the existence of a thick (−200 pc < z < 200 pc) disk of diffuse ( n H 2 ∼ 5 × 10−3 cm−3) molecular gas in the outer Galaxy previously undetected in all-sky 12CO surveys.

OH mid-infrared emission as a diagnostic of H2O UV photodissociation

Context. Water is an important molecule in interstellar and circumstellar environments. Previous observations of mid-infrared (IR) rotational lines of OH toward star-forming regions suggest that OH emission may be used to probe the photodissociation of water. Aims. Our goal is to propose a method to quantify H2O photodissociation and measure the local ultraviolet (UV) flux from observations of mid-IR OH lines. Methods. Cross sections for the photodissociation of H2O resolving individual electronic, vibrational, and rotational states of the OH fragment are collected. The state distribution of nascent OH following H2O photodissociation is computed for various astrophysically relevant UV radiation fields (e.g., a single Lyα line or a broadband spectrum). These distributions are incorporated in a new molecular excitation code called GROSBETA, which includes radiative pumping, collisional (de)excitation, and prompt emission (i.e., following the production of OH in excited states). The influence of the photodissociation rate of H2O, the spectral shape of the UV radiation field, the density, the temperature of the gas, and the strength of the IR background radiation field on the integrated line intensities are studied in detail. As a test case, our model is compared to Spitzer-IRS observations at the tip of the HH 211 bow-shock. Results. The OH rotational line intensities in the range 9–16 μm, covering rotational transitions with Nup = 18–45, are proportional to the column density of H2O photodissociated per second by photons in the range 114–143 nm (denoted as ΦB̃) and do not depend on other local properties such as the IR radiation field, the density, or the kinetic temperature. Provided an independent measurement of the column density of water is available, the strength of the local UV radiation field can be deduced with good accuracy, regardless of the exact shape of the UV field. In contrast, OH lines at longer far-IR wavelengths are primarily produced by IR radiative pumping and collisions, depending on the chemical pumping rate defined as 𝒟B̃ = ΦB̃/N(OH) and on the local physical conditions (nH, TK, IR radiation field). Our model successfully reproduces the OH mid-IR lines in the 10–16 μm range observed toward the tip of the HH 211 bow-shock and shows that the jet shock irradiates its surroundings, exposing H2O to a UV photon flux that is about 5 × 103 times larger than the standard interstellar radiation field. We also find that chemical pumping by the reaction H2 + O may supplement the excitation of lines in the range 16–30 μm, suggesting that these lines could also be used to measure the two-body formation rates of OH. Conclusions. The mid-IR lines of OH constitute a powerful diagnostic for inferring the photodissociation rate of water and thus the UV field that water is exposed to. Future JWST-MIRI observations will be able to map the photodestruction rate of H2O in various dense (nH ≳ 106 cm−3) and irradiated environments and provide robust estimates of the local UV radiation field.

Experimental study of flame evolution, frequency and oscillation characteristics of steam diluted micro-mixing hydrogen flame

Effects of steam dilution on flame evolution, frequency and oscillation characteristics were investigated using an optically accessible micro-mixing combustor under the lean operating condition (equivalence ratio 0.4 to 0.9). The flame profile was visualized by 5 kHz OH planar laser-induced fluorescence (PLIF), meanwhile the structural zoning analysis, frequency spectrum and dynamic mode decomposition (DMD) methods were used to investigate flame instability. The effects on frequency-shift and temporal-spatial heat-release distribution characteristics were demonstrated. Results indicated that heat-release frequency got blue-shift with the increase of hydrogen equivalence ratio while flame length gradually extended, though the second harmonic frequency only occurred in the flame arm zone. The steam content influenced the heat-release obviously, and the obvious periodic oscillation existed in the dilution ratio of 25%. Furthermore, the increasing or decreasing steam content will make the OH radical concentration and distribution change significantly, meanwhile the intensity and location of oscillation zone were also be influenced. The mode decomposition analysis revealed the growth of oscillation zone was closely related to the flame arm zone (FAZ) and experienced the transition to the flame tail zone (FTZ) with the increase of equivalence ratio.

Experimental detection of liquid-phase OH radical decay originating from atmospheric-pressure plasma exposure

Breaking-through methods for experimental observation on short-lived reactive species in the liquid phase, generated at the interface of atmospheric-pressure plasmas (APPs), can contribute greatly to an understanding of the reaction processes. A newly developed high-speed liquid flow interface in helium plasma transports APP-generated liquid-phase OH radical (·OH) by advection, enabling observation of rapid APP-generated ·OH decay within approximately 0.5 ms for the first time, to our best knowledge. This experimental detection and the deduced quantification of the rapid ·OH decay suggests a surface localization of ·OH, and thus can be an important finding to characterize the inhomogeneous ·OH distribution.

Single-shot 3D imaging of hydroxyl radicals in the vicinity of a gliding arc discharge

Plasma-related studies in gas phase are challenging to carry out due to plasma’s transient and unpredictable behavior, excessive luminosity emission, 3D complexity and aggressive chemistry and physiochemical interactions that are easily affected by external probing. Laser-induced fluorescence is a robust technique for non-intrusive investigations of plasma-produced species. In this letter, we present 3D distributions of ground state hydroxyl radicals (OH) radicals in the vicinity of a glow-type gliding arc plasma. Such radical distributions are captured instantaneously in one single camera acquisition by combining structured laser illumination and a lock-in based imaging analysis method called FRAME. The interference of plasma emission is automatically subtracted by the FRAME technique. In addition, the orientation of the plasma discharge can be reconstructed from the 3D data matrix, which can then be used to calculate 2D distributions of ground state OH radicals in a plane perpendicular to the orientation of the plasma channel. Our results indicate that OH distributions around a gliding arc are strongly affected by gas dynamics. We believe that the ability to instantaneously capture 3D transient molecular distributions in a plasma discharge, with minimal plasma emission interference, will have a strong impact on the plasma community for in situ investigations of plasma-induced chemistry and physics.

A three-dimensional-model inversion of methyl chloroform to constrain the atmospheric oxidative capacity

Abstract. Variations in the atmospheric oxidative capacity, largely determined by variations in the hydroxyl radical (OH), form a key uncertainty in many greenhouse and other pollutant budgets, such as that of methane (CH4). Methyl chloroform (MCF) is an often-adopted tracer to indirectly put observational constraints on large-scale variations in OH. We investigated the budget of MCF in a 4DVAR inversion using the atmospheric transport model TM5, for the period 1998–2018, with the objective to derive information on large-scale, interannual variations in atmospheric OH concentrations. While our main inversion did not fully converge, we did derive interannual variations in the global oxidation of MCF that bring simulated mole fractions of MCF within 1 %–2 % of the assimilated observations from the NOAA-GMD surface network at most sites. Additionally, the posterior simulations better reproduce aircraft observations used for independent validation compared to the prior simulations. The derived OH variations showed robustness with respect to the prior MCF emissions and the prior OH distribution over the 1998 to 2008 period. Although we find a rapid 8 % increase in global mean OH concentrations between 2010 and 2012 that quickly declines afterwards, the derived interannual variations were typically small (< 3 %/yr), with no significant long-term trend in global mean OH concentrations. The inverse system found strong adjustments to the latitudinal distribution of OH, relative to widely used prior distributions, with systematic increases in tropical and decreases in extra-tropical OH concentrations (both up to 30 %). These spatial adjustments were driven by intrahemispheric biases in simulated MCF mole fractions, which have not been identified in previous studies. Given the large amplitude of these adjustments, which exceeds spread between literature estimates, and a residual bias in the MCF intrahemispheric gradients, we suggest a reversal in the extratropical ocean sink of MCF in response to declining atmospheric MCF abundance (as hypothesized in Wennberg et al., 2004). This ocean source provides a more realistic explanation for the biases, possibly complementary to adjustments in the OH distribution. We identified significant added value in the use of a 3D transport model, since it implicitly accounts for variable transport and optimizes the observed spatial gradients of MCF, which is not possible in simpler models. However, we also found a trade-off in computational expense and convergence problems. Despite these convergence problems, the derived OH variations do result in an improved match with MCF observations relative to an interannually repeating prior for OH. Therefore, we consider that variations in OH derived from MCF inversions with 3D models can add value to budget studies of long-lived gases like CH4.

Tunable OH– Transport and Alkaline Stability by Imidazolium-Based Groups of Poly(2,6-dimethyl-1,4-phenylene oxide) Anion Exchange Membranes: A Molecular Dynamics Simulation

Imidazolium-based groups are promising organic cations in anion exchange membrane (AEM) materials. To investigate the effect of the imidazolium structure on OH– transport and alkaline stability of AEMs, we performed molecular dynamics simulation studies on hydrated poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) AEMs with imidazoliums modified by various substituents and/or alkyl pendent/spacer chains. Imidazoliums with the methyl or phenyl substituent at the C2, C4, and C5 sites or alkyl pendant chains at the N3 site show a steric effect on the distribution of water and OH– around imidazoliums, which inhibits the OH– transport but increases the alkaline stability of AEMs. By introducing alkyl spacer chains, the enhanced hydration structure of imidazolium promotes OH– transport, but the weakened steric effect of PPO on imidazolium decreases alkaline stability. We elucidate that the PPO AEMs modified by 1,2,4,5-tetramethylimidazolium and alkyl spacer chains with six or eight aliphatic carbons show good balance between OH– transport and alkaline stability of AEMs. Moreover, the complete hydration shells of both imidazolium and OH– enhance the OH– transport efficiency and decrease the possibility of imidazolium degradation with the hydration level more than six. Our work provides a design principle of imidazolium-based AEMs in fuel cell applications.