Surface Concentration Of Species Research Articles

Studying Surface Chemistry of Mixed Conducting Perovskite Oxide Electrodes with Synchrotron-Based Soft X-rays.

A fundamental understanding of the electrochemical reactions and surface chemistry at the solid-gas interface in situ and operando is critical for electrode materials applied in electrochemical and catalytic applications. Here, the surface reactions and surface composition of a model of mixed ionic and electronic conducting (MIEC) perovskite oxide, (La0.8Sr0.2)0.95Cr0.5Fe0.5O3-δ (LSCrF8255), were investigated in situ using synchrotron-based near-ambient pressure (AP) X-ray photoelectron spectroscopy (XPS) and near-edge X-ray absorption fine-structure spectroscopy (NEXAFS). The measurements were conducted with a surface temperature of 500 °C under 1 mbar of dry oxygen and water vapor, to reflect the implementation of the materials for oxygen reduction/evolution and H2O electrolysis in the applications such as solid oxide fuel cell (SOFC) and electrolyzers. Our direct experimental results demonstrate that, rather than the transition metal (TM) cations, the surface lattice oxygen is the significant redox active species under both dry oxygen and water vapor environments. It was proven that the electron holes formed in dry oxygen have a strong oxygen character. Meanwhile, a relatively higher concentration of surface oxygen vacancies was observed on the sample measured in water vapor. We further showed that in water vapor, the adsorption and dissociation of H2O onto the perovskite surface were through forming hydroxyl groups. In addition, the concentration of Sr surface species was found to increase over time in dry oxygen due to Sr surface segregation, with the presence of oxygen holes on the surface serving as an additional driving force. Comparatively, less Sr contents were observed on the sample in water vapor, which could be due to the volatility of Sr(OH)2. A secondary phase was also observed, which exhibited an enrichment in B-site cations, particularly in Fe and relatively in Cr, and a deficiency in A-site cation, notably in La and relatively in Sr. The findings and methodology of this study allow for the quantification of surface defect chemistry and surface composition evolution, providing crucial understanding and design guidelines in the electrocatalytic activity and durability of electrodes for efficient conversions of energy and fuels.

Epoxidation of Light Olefin Mixtures with Hydrogen Peroxide on TS-1 Catalyst

The direct epoxidation of mixed ethene and propene feedstocks using hydrogen peroxide over a titanium silicalite (TS-1) catalyst was investigated within a continuous trickle bed reactor operating in laboratory scale. Methanol was employed as the reaction solvent. This study aimed to streamline the epoxidation process by obviating the need for prior separation of alkenes, thereby enhancing process efficiency. An extensive array of operational parameters was explored in a trickle bed reactor, encompassing experimental parameters such as temperature, total pressure, hydrogen peroxide concentration, liquid flow rate, and gas composition. In contrast to prior investigations involving separate ethene and propene epoxidation, this study revealed a reduction in epoxide selectivity. The principal by-products observed were methoxy species, formed through the interaction between the epoxide and methanol, resulting in a ring-opening reaction. The influence of water on this ring-opening process was negligible. Notably, the tunability of the system was demonstrated, highlighting low temperature and elevated partial ethene pressure as pivotal factors for augmentingthe epoxide selectivity. The findings suggest that binary olefin mixtures exhibit diminished selectivity but improved stability. This behavior is potentially linked to the olefin solubility in methanol, or alterations in the surface species concentrations, typically associated with catalyst activity variations. These insights offer a valuable foundation for understanding and optimizing the direct epoxidation of mixed ethene and propene feedstock.Graphical

Geochemical modelling of hydrogen wettability on Quartz: Implications for underground hydrogen storage in sandstone reservoirs

Renewable energy particular hydrogen can be optimum solution to replace fossil fuels to achieve net-zero carbon emission targets and metigate global warming. One challenge of the hydrogen industry is to safely and economically store hydrogen. To meet this requirement, underground geological storage is a potentially suitable solution to store hydrogen in large-scale and long-term manner. Wettabillity is an important formation parameter that can affect flow behaviors of stored gas, cycling efficiency, structural and residual trap capacity. However, current research is still scarce in geochemical reactions associated hydrogen wettability alteraction in subsurface sandstone reservoirs under in-situ conditions. To fill this knowledge gap, in this study, we performed surface complexation modelling to understand the wettability of hydrogen-brine-organic acid-quartz system. The surface potential of pure quartz at various temperatures and pressure was calculated to characterize disjoining pressure isotherm. Besides, surface species concentrations of quartz and organic stearic acid were predicted to quantify the electrostatic attraction between quartz and organic acid molecues thus the hydrogen wettability on quartz surface.The modelling results show that for pure quartz, increasing temperature and pressure has a negligible effect on disjoining pressure and hydrogen wettability on the pure quartz surface, which is different from the results reported by Iglauer et al., but is in line with the results reported by Hashemi et al. When organic stearic acid is added into system, increasing organic molecules concentration and pressure strengthens the electrostatic attraction of quartz-organic acid and leads to less hydrophilicity and more hydrogen-wetting. This result is consistent with previous contact angle measurements that increasing organic acid concentration increases the brine contact angle, thus the hydrogen wettability. Our work provides a framework to characterize hydrogen wettability on mineral surface using geochemical tools to better assess the feasibility of UHS in depleted sandstone reservoirs.

In English

The goal of this study is unraveling the specific features of non-stationary surface concentration distribution of electroactive and inactive species in a model electrochemical process with a preceding homogeneous first-order chemical reaction (CE mechanism). For this purpose, the exact analytical expressions for the non-stationary concentration distributions of electroactive and inactive species in the thin layer attached to a planar electrode are analyzed. The both cases of equal and unequal diffusion coefficients of species taking part in the preceding chemical reaction are considered. In the former case, the exact analytical expressions for the concentration distributions of electroactive and inactive species on a planar electrode are obtained. The peculiarities of the limiting cases of zero and infinite frequency of an applied alternating current for the both cases of equal and unequal diffusion coefficients of species are discussed. It is shown that there is a phase shift between AC and the surface concentration of species that changes under the action of this current. At low frequencies, the phase angle tends to p/2, whereas at high frequencies it decreases to p/4. The phase angle is the function of the two important measures, namely, the ratio of the Nernst diffusion layer thickness to the oscillation diffusion layer thickness, and the ratio of the Nernst diffusion layer thickness to the reaction layer one. It is shown that the phase angle depends on the diffusion coefficient of species in different manner for low and high values of the rate constants of the chemical reaction. At low values of these parameters, the phase angle shifts slightly to the range of high frequencies with an increase of diffusion coefficient. At the high rate constants, the phase angle decreases with frequency more slowly, and its dependence on diffusion coefficient is observed only at middle frequencies. The surface concentration of electroactive and inactive species decreases with an increase of frequency, but for the inactive species this process is faster than that for the electroactive species. The influence of the inactive species on the surface concentration of electroactive species decreases at high frequencies and at low rate constants of the preceding chemical reaction. The results obtained shed the light on complex dynamics at an electrode/electrolyte interface under non-stationary conditions.

Hydrogen wettability in carbonate reservoirs: Implication for underground hydrogen storage from geochemical perspective

Hydrogen has been considered as a promising renewable source to replace fossil fuels to meet energy demand and achieve net-zero carbon emission target. Underground hydrogen storage attracts more interest as it shows potential to store hydrogen at large-scale safely and economically. Meanwhile, wettability is one of the most important formation parameters which can affect hydrogen injection rate, reproduction efficiency and storage capacity. However, current knowledge is still very limited on how fluid-rock interactions affect formation wettability at in-situ conditions. In this study, we thus performed geochemical modelling to interpret our previous brine contact angle measurements of H2-brine-calcite system. The calcite surface potential at various temperatures, pressures and salinities was calculated to predict disjoining pressure. Moreover, the surface species concentrations of calcite and organic stearic acid were estimated to characterize calcite-organic acid electrostatic attractions and thus hydrogen wettability. The results of the study showed that increasing temperature increases the disjoining pressure on calcite surface, which intensifies the repulsion force of H2 against calcite and increases the hydrophilicity. Increasing salinity decreases the disjoining pressure, leading to more H2-wet and contact angle increment. Besides, increasing stearic acid concentration remarkably strengthens the adhesion force between calcite and organic acid, which leads to more hydrophobic and H2-wet. In general, the results from geochemical modelling are consistent with experimental observations that decreasing temperature and increasing salinity and organic acid concentration increase water contact angle. This work also demonstrates the importance of involving geochemical modelling on H2 wettability assessment during underground hydrogen storage.

Correlation for Species Concentration on a Hypersonic Stagnation Point with Mass Injection

This paper presents a correlation to obtain the surface species concentration on a stagnation point with mass injection for hypersonic flow. The correlation can be used to determine surface recombination heat fluxes and oxidation levels for ablatives or actively cooled heat shields in thermochemical equilibrium flow. It explicitly expresses the wall species concentration as a function of freestream conditions, coolant properties, and the stagnation-point geometry, revealing the direct effect of the aforementioned parameters on the desired quantity. The method was compared against the numerically obtained self-similar solution and showed an accuracy of . The concentration depends on the boundary-layer edge pressure, temperature, and velocity gradient, as well as the wall temperature and injected mass flux. The molar mass and diffusion coefficient are further needed for scaling if the injected gas differs from the freestream gas.

In Situ Raman Spectroscopic Study of Species in Co-MnCeOx Catalysts under COPrOx Reaction Conditions

This work is focused on the study of Co-MnCeOx catalysts by in situ Raman spectroscopy, under COPrOx conditions, and aims to establish a correlation with the catalytic results of different catalytic formulations. Through XRD, the catalysts structures were explored, and the oxidation states and relative concentrations of surface species were analyzed by XPS. Catalysts were prepared with Co (10 wt %) and different Mn concentrations. The more active catalyst contains 2.5 wt % of Mn and Co/Ce ≈ 1 (CO conversion 99.5%, 150–180 °C). The Co-Mn2.5Ce catalyst is composed of Co3O4 spinel, Mn inserted in the CeO2 lattice, and probably MnOx dispersed, which remains stable up to 250 °C under reaction conditions. XRD results confirm the crystalline structure of Co3O4 spinel; however, there was no evidence of crystalline phases of MnOx oxides. A lower lattice parameter suggests the insertion of Mnn+ ions in the ceria framework. The catalytic surface is composed of Co3+-Co2+ belonging to Co3O4 and Ce4+ of ceria and Mn4+-Mn3+ species.

A Qualitative Numerical Study on Catalytic Hydrogenation of Nitrobenzene in Gas-Liquid Taylor Flow with Detailed Reaction Mechanism

While the number of computational studies considering two-phase flows in microfluidic systems with or without mass transfer is increasing, numerical studies incorporating chemical reactions are still rare. This study aims to simulate the catalytic hydrogenation of nitrobenzene in gas-liquid Taylor flow by combining interface-resolving numerical simulations of two-phase flow and mass transfer by a volume-of-fluid method with detailed modeling of the heterogeneous chemical reaction by software package DETCHEMTM. Practically relevant physical properties are utilized for hydrodynamic and mass transfer simulations in combination with a preliminary reaction mechanism based on density functional theory. Simulations of mass transfer are conducted using a predetermined velocity field and Taylor bubble shape. At the beginning of the simulation when liquid nitrobenzene is not saturated by hydrogen, axial profiles of surface species concentrations and reaction rates show local variations. As hydrogen dissolves in nitrobenzene, the concentration profiles of surface species at the wall become uniform, eventually reaching an equilibrium state. Neglecting the local variation in a short initial period will allow further simplification of modeling surface reactions within a Taylor flow.

High-Resolution X-ray Photoelectron Spectroscopy of an IrO2(110) Film on Ir(100).

High-resolution X-ray photoelectron spectroscopy (XPS) and density functional theory (DFT) were used to characterize IrO2(110) films on Ir(100) with stoichiometric as well as OH-rich terminations. Core-level Ir 4f and O 1s peaks were identified for the undercoordinated Ir and O atoms and bridging and on-top OH groups at the IrO2(110) surfaces. Peak assignments were validated by comparison of the core-level shifts determined experimentally with those computed using DFT, quantitative analysis of the concentrations of surface species, and the measured variation of the Ir 4f peak intensities with photoelectron kinetic energy. We show that exposure of the IrO2(110) surface to O2 near room temperature produces a large quantity of on-top OH groups because of reaction of background H2 with the surface. The peak assignments made in this study can serve as a foundation for future experiments designed to utilize XPS to uncover atomic-level details of the surface chemistry of IrO2(110).

Polarization- and Wavelength-Dependent Shell-Isolated-Nanoparticle-Enhanced Sum-Frequency Generation with High Sensitivity.

Sum-frequency generation (SFG) spectroscopy is a highly versatile tool for surface analysis. Improving the SFG intensity per molecule is important for observing low concentrations of surface species and intermediates in dynamic systems. Herein, Shell-Isolated-Nanoparticle-Enhanced SFG (SHINE-SFG) was used to probe a model substrate. The model substrate, p-mercaptobenzonitrile adsorbed on a Au film with SHINs deposited on top, provided an enhancement factor of up to 10^{5}. Through wavelength- and polarization-dependent SHINE-SFG spectroscopy, the majority of the signal enhancement was found to come from both plasmon enhanced emission and chemical enhancement mechanisms. A new enhancement regime, i.e., the nonlinear coupling of SHINE-SFG with difference frequency generation, was also identified. This novel mechanism provides insight into the enhancement of nonlinear coherent spectroscopies and a possible strategy for the rational design of enhancing substrates utilizing coupling processes.

Free convective heat transfer in Jeffrey fluid with suspended nanoparticles and Cattaneo–Christov heat flux

Free convection flow of Jeffrey nanofluid past a vertical plate with sinusoidal variations of surface temperature and species concentration is presented. The study of heat transfer and nanofluid transport has been done by employing Cattaneo–Christov heat flux model and Buongiorno model, respectively. Equations governing the flow are non-dimensionalized using appropriate transformations. Furthermore, the method of local similarity and local non-similarity is used to reduce the equations into non-linear coupled system of equations which are then solved by homotopy analysis method. The obtained results are validated by comparing with the existing results available in the literature. The numerical results are found to be in good agreement. The effects of varying the physical parameters such as Deborah Number, Prandtl number, Schmidt number, thermophoresis parameter, Brownian motion parameter and buoyancy ratio parameter are obtained and presented graphically. The effect of sinusoidal variation of surface temperature and species concentration on the skin friction coefficient, Nusselt number and Sherwood number is also shown. Velocity for Jeffrey nanofluid is more than the Newtonian nanofluid while temperature and nanoparticle concentration for Jeffrey nanofluid is less than the Newtonian nanofluid. Raising value of thermal relaxation times leads to an increase in the heat transfer coefficient. It is observed that temperature of Cattaneo–Christov heat flux model is less than that in classical Fourier’s model away from the vertical wall. These types of boundary layer flow problems are found in vertical film solar energy collector, grain storage, transportation and power generation, thermal insulation, gas production, petroleum resources, geothermal reservoirs.

Surface charge properties of variable charge soils influenced by environmental factors

The variable charge soils that have been developed in tropical and subtropical zones are unique soils compared with the constant charge soils that have been developed in Europe and North America, especially the sensitivity of the particle surface charges to environmental factors in the variable charge soils. In this study, to avoid the interference of side reactions, experiments were performed using a fast potentiometric acid-base titration under non-equilibrium condition, and simulation by the surface complexation model to investigate the effects of environmental factors, including the impact of CO2 (in nitrogen/in air), the impact of different ionic strengths, the impact of dry-wet rotation (different soil-to-liquid (S/L) ratios), and the impact of seasonal temperature variation (different reaction temperatures), on the surface acidity constants and on the protonation/deprotonation processes in a typical variable charge soil collected in southern China. The titration results demonstrate that the observed measured apparent pH when surface charge equal to zero (pHQ=0) were significantly negative shifted in air, and negative correlations were proven between the S/L ratios, the reaction temperatures and the observed pHQ=0 values. The simulation results from the fast titration prove that changing the S/L ratios modifies the surface species concentration (≡SOH, ≡SOH2+, and ≡SO−), further influencing the modification of the surface charge properties of the soil. Meanwhile, an increase in temperature not only increases the amount of the surface species concentration but also causes an enlargement of the protonation/deprotonation processing range, which may increase the acid-base buffer capacity of soils. Taken together, these results can help us to further understand the natural soil acidification processes occurring in variable charge soils, especially in the southern areas of China.

Mechanistic insights into the kinetic compensation effects during the gasification of biochar: Effects of the partial pressure of H2O

This study has focused on the kinetic compensation effects (KCE) during the gasification of biochar in 15% H2O-Ar and 2% H2O-Ar. The biochar samples were characterised by FT-Raman and X-ray photoelectron spectroscopies. We found that the extent of the KCE of char gasification and formation of CO, CO2, and H2 was higher during the gasification in 15% H2O-Ar than that in 2% H2O-Ar. The relative ratio of ID/(I(Gr+Vl+Vr) was higher in 15% H2O-Ar than that in 2% H2O-Ar, which reflects that the relative concentration of large aromatic rings is higher during the gasification in 15% H2O-Ar than that in 2% H2O-Ar. The partial pressure of H2O affected the relative concentration of O-containing surface species [C(O)] on the biochar external surface and inside the pores of the char matrix. The concentration of these O-containing species [C(O)] on the char surface led to different relative rates of CO and CO2 release, resulting in lower CO/CO2 ratios (at lower char conversions) during the gasification in 15% H2O-Ar than those in 2% H2O-Ar. The lower relative ratio of ID/(I(Gr+Vl+Vr) and lower extent of the kinetic compensation effects revealed that the relative concentration of active sites (with similar properties), which were created on the activation of aromatic rings, was lower for the gasification in 2% H2O-Ar than that in 15% H2O-Ar.

Effect of Zinc Oxide Modification by Indium Oxide on Microstructure, Adsorbed Surface Species, and Sensitivity to CO

Additives in semiconductor metal oxides are commonly used to improve sensing behavior of gas sensors. Due to complicated effects of additives on the materials microstructure, adsorption sites and reactivity to target gases the sensing mechanism with modified metal oxides is a matter of thorough research. Herein, we establish the promoting effect of nanocrystalline zinc oxide modification by 1 – 7 at.% of indium on the sensitivity to CO gas due to improved nanostructure dispersion and concentration of active sites. The sensing materials were synthesized via an aqueous coprecipitation route. Materials composition, particle size and BET area were evaluated using X-ray diffraction, nitrogen adsorption isotherms, high-resolution electron microscopy techniques and EDX-mapping. Surface species of chemisorbed oxygen, OH-groups and acid sites were characterized by probe molecule techniques and infrared spectroscopy. Zinc oxide particle size was found to decrease and the BET area to increase with the amount of indium. The additive was observed in the form of amorphous indium oxide segregated on agglomerated crystalline ZnO nanoparticles. The concentration of surface species was found to be higher on In-modified zinc oxide. With the increase of indium content, the sensor response of ZnO(In) to CO was improved. Using in situ infrared spectroscopy, it was shown that oxidation of CO molecules was enhanced on the modified zinc oxide surface. The effect of modifier was attributed to promotion of surface OH-groups and enhancement of CO oxidation on the segregated indium ions, as suggested by DFT in previous work.

Experimental Framework for Evaluation of the Thermodynamic and Kinetic Parameters of Metal-Oxides for Solar Thermochemical Fuel Production

The two-step metal oxide redox cycle is a promising and thermodynamically attractive means of solar fuel production. In this work, we describe the development of a high-temperature tubular reactor in which the fundamental thermodynamic and kinetic behavior of thermochemical materials can be readily assessed. This reactor system is capable of operating at temperatures up to 1873 K, total pressures ranging from vacuum to ambient, and oxygen partial pressures (pO2) as low as 10−29 atm. Compared to off-the-shelf systems like thermogravimetric analyzers or indirect conductivity-based measurement systems, this system has three inherent benefits: (1) the flexibility to control the sample morphology (e.g., powder, packed bed, reticulated porous ceramic, or pellet), (2) the potential for a well-developed and characterized flow, and (3) the ability to readily customize the system on demand (e.g., easy integration with a steam generator to control and operate at very low pO2). The reactor system and experimental methods were validated by performing isothermal relaxation experiments with undoped ceria, wherein the sample environment was rapidly altered by stepwise changes in the delivered H2O vapor concentration, and comparing measured oxygen nonstoichiometries with accepted data available in the literature. Data were measured at temperatures from 1173 to 1473 K and pO2 from 4.54 × 10−18 to 1.02 × 10−9 atm. The measured equilibrium data displayed strong agreement with the literature and the expected trends were preserved. Kinetic data were extracted by first transforming reactant concentrations measured downstream of the reaction zone using a tanks-in-series mixing model to account for gas dispersion. Next, a mechanistic kinetic model distinguishing surface and bulk species concentrations was fit to the data to extract pertinent thermodynamic and kinetic parameters. The model assumed a two-step reaction mechanism mediated by the formation of an intermediate hydroxyl species on the surface. Activation energies and defect formation enthalpies and entropies for the forward and reverse reactions were found to be in good agreement with previous modeling efforts, providing further validation of the use of this system to explore thermodynamic and kinetic behavior of emerging thermochemical materials.

The early stages of FeCO3 scale formation kinetics in CO2 corrosion

In a carbon dioxide (CO2) corrosive environment, when the product of the local concentrations of iron (Fe2+) and carbonate (CO32−) ions exceed the solubility limit, the precipitation of iron carbonate (FeCO3) on the internal walls of carbon steel pipelines can significantly reduce the corrosion rate. In the following work, static glass cell corrosion experiments were conducted to understand the precipitation behavior in the early stages of FeCO3 development and the factors favorable to protective film formation. The corrosion and precipitation rates were followed using a combination of mass gain, mass loss, electrochemistry and surface analysis techniques. At the conditions studied, the protectiveness and the rate of film formation was observed to vary significantly. The results indicated that the early stages of FeCO3 precipitation consists of a complex simultaneous nucleation and growth process. FeCO3 precipitation is shown to be dependent on surface species concentrations which can be significantly different to that of the bulk solution. Additionally, the role of crystal surface coverage and the blocking of actively corroding sites on the steel surface is examined and is shown to play a critical role in reducing precipitation kinetics at low levels of bulk super-saturation.

Oxygen reduction reaction on Pt(1 1 1), Pt(2 2 1), and Ni/Au1Pt3(2 2 1) surfaces: Probing scaling relationships of reaction energetics and interfacial composition

In this work, we investigate the oxygen reduction reaction (ORR) on Pt(1 1 1), Pt(2 2 1), and Ni/Au1Pt3(2 2 1) surfaces using periodic density functional theory (DFT) calculations augmented with kinetic Monte Carlo (KMC) simulations. In the DFT calculations, both a uniform electric field and an implicit solvent were employed to quantify the changes in the ORR energetics. Based on the DFT values and the reaction mechanism of ORR, KMC simulations were performed to predict the current densities and the surface populations as a function of the electrode potential on the different catalyst surfaces. These results were compared to an ORR model with energetics approximated using scaling relationships. The two approaches for obtaining energetic parameters yield different KMC-predicted polarization curves. An analysis of the surface species concentrations on Pt(1 1 1), Pt(2 2 1), and Ni/Au1Pt3(2 2 1) indicates dramatic concentration variations at different potentials. Direct neighbor-neighbor interactions on the surface were also tested, but they were found to only moderately influence the surface species concentrations, and they led to imperceptible changes in the predicted current densities. Overall, we predict that an alloy surface of Ni/Au1Pt3(2 2 1) can significantly shift the onset potential of the polarization curve to higher potentials, providing guidance for future electrocatalyst design.

A detailed spectroscopic analysis of the growth of oxy-carbon species on the surface of Pt/Al2O3 during propane oxidation

The growth of oxygenated carbonaceous (oxy-carbon) species on the surface of Pt/Al2O3 during total oxidation of propane is analyzed in detail—including their composition, their location on the catalyst surface, their reactivity, and their role in the propane oxidation mechanism—by in situ diffuse reflectance infrared Fourier-transform spectroscopy (DRIFTS). Platinum nanoparticles catalyze the transformation of propane into many different oxy-carbon surface species, including acetate, enolate, aliphatic ester, and acetone, which spillover and grow on the Al2O3 support. There is no correlation between the concentration of oxy-carbon surface species and the rate of CO2 production in the gas-phase, which indicates that these species are inert spectators in the propane oxidation mechanism. Temperature-programmed oxidation of the oxy-carbon surface species reveals that enolate, aliphatic ester, and acetone species are removed from the surface by combustion at similar temperatures with an activation barrier of 112kJ/mol, whereas acetate species are removed at higher temperatures with an activation barrier of 147kJ/mol. Both the formation and combustion of oxy-carbon surface species occur in pathways that are parallel to, and orders-of-magnitude slower than, the main pathway to CO2 production.

Impact of crop field burning and mountains on heavy haze in the North China Plain: a case study

Abstract. With the provincial statistical data and crop field burning (CFB) activities captured by Moderate Resolution Imaging Spectroradiometer (MODIS), we extracted a detailed CFB emission inventory in the North China Plain (NCP). The WRF-CHEM model was applied to investigate the impact of CFB on air pollution during the period from 6 to 12 October 2014, corresponding to a heavy haze incident with high concentrations of PM2.5 (particulate matter with aerodynamic diameter less than 2.5 µm). The WRF-CHEM model generally performed well in simulating the surface species concentrations of PM2.5, O3 and NO2 compared to the observations; in addition, it reasonably reproduced the observed temporal variations of wind speed, wind direction and planetary boundary layer height (PBLH). It was found that the CFB that occurred in southern NCP (SNCP) had a significant effect on PM2.5 concentrations locally, causing a maximum of 34 % PM2.5 increase. Under continuous southerly wind conditions, the CFB pollution plume went through a long-range transport to northern NCP (NNCP; with several mega cities, including Beijing, the capital city of China), where few CFBs occurred, resulting in a maximum of 32 % PM2.5 increase. As a result, the heavy haze in Beijing was enhanced by the CFB, which occurred in SNCP. Mountains also play significant roles in enhancing the PM2.5 pollution in NNCP through the blocking effect. The mountains blocked and redirected the airflows, causing the pollutant accumulations along the foothills of mountains. This study suggests that the prohibition of CFB should be strict not only in or around Beijing, but also on the ulterior crop growth areas of SNCP. PM2.5 emissions in SNCP should be significantly limited in order to reduce the occurrences of heavy haze events in the NNCP region.

Rate equations for sodium catalyzed quartz dissolution

Quartz dissolution rate data were fit to an equation that predicts the dissolution flux (J, mol/m2sec) as a function of temperature (T, K), sodium concentration (mNa+, molal), and hydrogen ion activity (aH+). J=0.64610-748002.303R1T+6.6410-880002.303R1TmNa+0.331aH+0.441The same data fit equally well to an equation that expresses the rate as a function of temperature, sodium concentration, and hydroxide ion activity (aOH-). J=0.64610-748002.303R1T+1120010-716002.303R1TmNa+0.326aOH-0.441These equations are more convenient to use than those given by Bickmore et al. (2008) because rates can be predicted without the implementation of a surface speciation model. They predict that at 25°C quartz dissolves more than 200 times faster in seawater than in pure water.These two equations fit the data just as well as five other equations from Bickmore et al. (2008) that are based on surface species concentrations. All of these rate equations contain information about the reaction mechanism(s) for quartz dissolution but that information is ambiguous because the independent variables used to develop the equations are correlated. This means that rate equations alone cannot be used to infer the dissolution mechanism. Existing surface complexation, surface charge, terrace-ledge-kink, and Lewis acid-base models must be modified and amalgamated in order to develop a reliable model of the reaction mechanism(s).