Role Of Heterogeneous Reactions Research Articles

A CAPRAM Modeling Study on the Role of Heterogeneous Reactions on Dust in Tropospheric Chemistry

A CAPRAM Modeling Study on the Role of Heterogeneous Reactions on Dust in Tropospheric Chemistry

Direct observation for relative-humidity-dependent mixing states of submicron particles containing organic surfactants and inorganic salts

Abstract. Aerosol mixing state plays an important role in heterogeneous reactions and cloud condensation nuclei (CCN) activity. Organic surfactants could affect aerosol mixing state through bulk–surface partitioning. However, the mixing state of surfactant-containing particles remains unclear due to the lack of direct measurements. Here, direct characterizations of the mixing state for 20 kinds of submicron particles containing inorganic salts (NaCl and (NH4)2SO4) and atmospheric organic surfactants (organosulfates, organosulfonates, and dicarboxylic acids) were conducted upon relative humidity (RH) cycling by environmental scanning electron microscopy (ESEM). As the RH increased, the surfactant shells inhibited water diffusion being exposed to the inorganic core, leading to notably increased inorganic deliquescence RH (88.3 %–99.5 %) when compared with pure inorganic aerosol. Meanwhile, we directly observed an obvious Ostwald ripening process (that is, the growth of larger crystals at the expense of smaller ones) in 6 out of 10 NaCl–organic surfactant systems. As a result of water inhibition by the organic surfactant shell, Ostwald ripening in all systems occurred at RH above 90 %, which were higher than the reported RH range for pure NaCl measured at 27 ∘C (75 %–77 %). As RH decreased, eight systems underwent liquid–liquid-phase separation (LLPS) before efflorescence, showing a strong dependence on the organic molecular oxygen-to-carbon ratio (O:C). Quantitatively, LLPS was always observed when O:C≤0.43 and was never observed when O:C>∼0.57. Separation RH (SRH) of inorganic salt–organic surfactant mixtures generally followed the trend of (NH4)2SO4 < NaCl, which is consistent with their salting-out efficiencies reported in previous studies. Solid-phase separations were observed after efflorescence for systems without LLPS. Our results provide a unique insight into the consecutive mixing processes of the inorganic salt–organic surfactant particles, which would help improve our fundamental knowledge of model development on radiative effect.

State-specific slip boundary conditions in non-equilibrium gas flows: Theoretical models and their assessment

In this study, we develop and assess a new approach to modeling slip boundary conditions in gas mixtures with coupled state-to-state vibrational-chemical kinetics and surface physical and chemical processes: adsorption, desorption, vibrational energy transitions, and chemical reactions. Expressions for velocity slip, temperature jump, and mass fluxes of species are derived on the basis of the advanced kinetic boundary condition taking into account gain and loss of particles in surface processes; theoretical expressions for the mass fluxes obtained in the frame of various approaches are compared. The developed model is implemented to the fluid-dynamic solver for modeling dynamics and state-to-state air kinetics in the boundary layer near stagnation point. Several test cases corresponding to a various degree of gas rarefaction are considered. Recombination probabilities and effective reaction rates are calculated and compared to recent molecular-dynamic simulations; the proposed model yields the best agreement for the recombination rate coefficient. It is shown that temperature jump significantly affects fluid-dynamic parameters and surface heat flux; the role of heterogeneous reactions on the silica surface is weaker. In the surface heating, there is a competition between these two effects: whereas the temperature jump reduces the wall heat flux, surface reactions cause its increase, but to a lesser extent. It is concluded that the model proposed in this study describes self-consistently detailed vibrational kinetics, rarefaction effects, and surface reactions and can be applied both in continuum and slip flow regimes.

Investigating the hygroscopicities of calcium and magnesium salt particles aged with SO2 using surface plasmon resonance microscopy

The hygroscopicities of calcium and magnesium salts strongly affect the environment and climate, but the aging products of these salts at high relative humidities (RHs) are still poorly understood. In this study, surface plasmon resonance microscopy (SPRM) was used to determine the hygroscopic growth factors (GFs) of Ca(NO3)2 and Mg(NO3)2 separately or mixed with galactose at different mass ratios at different RHs before and after aging. For all particles, the measured GFs showed no indication of deliquescence across the range of RHs tested, and overall hygroscopicity was clearly lower after than before aging. The Ca(NO3)2 and Mg(NO3)2 GFs at 90 % RH were 1.80 and 1.66, respectively, before aging and 1.33 and 1.42, respectively, after 4 h aging, meaning aging decreased the GFs by 26.11 % and 14.46 %, respectively. Aging decreased the hygroscopicity because insoluble or sparingly soluble substances (CaSO3, CaSO4, MgSO3) formed and strongly changed the overall hygroscopicity. For bicomponent aerosols with different mass ratios, the GFs (calculated using the Zdanovskii-Stokes-Robinson method) of the other components except galactose at 90 % RH after 1 h aging were all lower, respectively, than the measured GFs of pure Ca(NO3)2 and Mg(NO3)2 after aging for 1 h, especially with the mass ratio of 1:2, their GFs have decreased by 14.63 % and 7.50 %, respectively. Subsequently, Ion chromatograms indicated that the peak area ratio of SO42− to NO3− ratios were higher for the aged bicomponent particles than aged single-component particles, possibly because adding galactose improved the gas–liquid state stability during drying after the aging process and therefore promoted nitrate consumption and sulfate formation. The results indicated that organic components may play important roles in heterogeneous reactions between trace gases and multicomponent aerosols and should be considered in evaluating the impacts on submicron aerosol composition of high atmospheric SO2 concentrations at high humidities.

Dual-Functional Ice/Water Interface Allows High-Yield Formation of Al13 with Low Energy

Keggin Al13[(AlO4Al12(OH)24(H2O)12]7+ is a well-known efficient coagulant widely used in water purification and treatment. The production of high-content Al13 is still costly, and it is therefore essential to develop new eco-efficient and low energy demand synthesis methods for practical application. Solid–water interfaces play a dominant role in heterogeneous reactions, which can be utilized in various processes through rationally designed interfaces. Here, we designed an Al3+ and OH– interface by using NaOH-doped ice as a solid and AlCl3 solution as liquid for Al13 production. This system provides a natural and renewable solid–water interface through ice melting, while simultaneously minimizing external energy input and maintaining an effective heterogeneous interface reaction for production of high-quality Al13 species. The dual-function of ice/water interface enables the formation of Al13 with a high yield of up to 90% compared to initial total Al. Moreover, global areas with cold winter climate condi...

Relative Order of Sulfuric Acid, Bisulfate, Hydronium, and Cations at the Air-Water Interface.

Sulfuric acid (H2SO4), bisulfate (HSO4(-)), and sulfate (SO4(2-)) are among the most abundant species in tropospheric and stratospheric aerosols due to high levels of atmospheric SO2 emitted from biomass burning and volcanic eruptions. The air/aqueous interfaces of sulfuric acid and bisulfate solutions play key roles in heterogeneous reactions, acid rain, radiative balance, and polar stratospheric cloud nucleation. Molecular-level knowledge about the interfacial distribution of these inorganic species and their perturbation of water organization facilitates a better understanding of the reactivity and growth of atmospheric aerosols and of the aerosol surface charge, thus shedding light on topics of air pollution, climate change, and thundercloud electrification. Here, the air/aqueous interface of NaHSO4, NH4HSO4, and Mg(HSO4)2 salt solutions as well as H2SO4 and HCl acid solutions are investigated by means of vibrational sum frequency generation (VSFG) and heterodyne-detected (HD) VSFG spectroscopy. VSFG spectra of all acid solutions show higher SFG response in the OH-bonded region relative to neat water, with 1.1 M H2SO4 being more enhanced than 1.1 M HCl. In addition, VSFG spectra of bisulfate salt solutions highly resemble that of the dilute H2SO4 solution (0.26 M) at a comparable pH. HD-VSFG (Im χ((2))) spectra of acid and bisulfate salt solutions further reveal that hydrogen-bonded water molecules are oriented preferentially toward the bulk liquid phase. General agreement between Im χ((2)) spectra of 1.1 M H2SO4 and 1.1 M HCl acid solutions indicate that HSO4(-) ions have a similar surface preference as that of chloride (Cl(-)) ions. By comparing the direction and magnitude of the electric fields arising from the interfacial ion distributions and the concentration of each species, the most reasonable relative surface preference that can be deduced from a simplified model follows the order H3O(+) > HSO4(-) > Na(+), NH4(+), Mg(2+) > SO4(2-). Interestingly, contrary to some other near-neutral salt solution interfaces (e.g., chlorides and nitrates), cation-specific effects are here overshadowed by hydronium ions.

ChemInform Abstract: Hydrocarbon Pyrolysis with a Methane Focus: A Review on the Catalytic Effect and the Coke Production

AbstractReview: 122 refs.

Modeling wintertime particulate matter formation in central California

A wintertime episode during the 2000 California Regional PM Air Quality Study (CRPAQS) was simulated with the air quality model CMAQ–MADRID. Model performance was evaluated with 24-h average measurements available from CRPAQS. Modeled organic matter (OM) was dominated by emissions, which were probably significantly under-represented, especially in urban areas. In one urban area, modeled daytime nitrate concentrations were low and evening concentrations were high. This diurnal profile was not explained by the partition of nitrate between the gas and particle phases, because gaseous nitric acid concentrations were low compared to PM nitrate. Both measured and simulated nitrate concentrations aloft were lower than at the surface at two tower locations during this episode. Heterogeneous reactions involving NO 3 and N 2O 5 accounted for significant nitrate production in the model, resulting in a nighttime peak. The sensitivity of PM nitrate to precursor emissions varied with time and space. Nitrate formation was on average sensitive to NO x emissions. However, for some periods at urban locations, reductions in NO x caused the contrary response of nitrate increases. Nitrate was only weakly sensitive to reductions in anthropogenic VOC emissions. Nitrate formation tended to be insensitive to the availability of ammonia at locations with high nitrate, although the spatial extent of the nitrate plume was reduced when ammonia was reduced. Reductions in PM emissions caused OM to decrease, but had no effect on nitrate despite the role of heterogeneous reactions. A control strategy that focuses on NO x and PM emissions would be effective on average, but reductions in VOC and NH 3 emissions would also be beneficial for certain times and locations.

Accelerated diffusion of chain carriers and kinetic features of heterogeneous processes in gas-phase chain reactions

In gas-phase combustion processes, the regeneration of free atoms and radicals in chain propagation reactions enhances the diffusion flux of these species from the flame zone. In flame propagation in tubular reactors and in filtration combustion, this effect facilitates the access of chain carriers to the surface even at atmospheric pressure, increases the role of heterogeneous reactions (primarily chain termination), and enhances heat removal due to heterogeneous recombination.

Heterogeneous Uptake Kinetics of Volatile Organic Compounds on Oxide Surfaces Using a Knudsen Cell Reactor: Adsorption of Acetic Acid, Formaldehyde, and Methanol on α-Fe2O3, α-Al2O3, and SiO2

The role of heterogeneous reactions on particulate matter present in the Earth's atmosphere remains an important question in tropospheric chemistry. It has been proposed in several modeling studies that mineral dust may provide reactive surfaces for trace atmospheric gases. Laboratory studies can provide some answers concerning the kinetics of these reactions, so that heterogeneous chemistry can be quantitatively assessed in atmospheric chemistry models. In this study, the heterogeneous uptake kinetics of several volatile organic compounds (VOCs) on oxide surfaces have been measured with a Knudsen cell reactor at 295 K. In particular, the heterogeneous uptake of acetic acid, methanol, and formaldehyde on α-Fe2O3, α-Al2O2, and SiO2 has been investigated. These VOCs are representative of some of the different types of oxygenated organics found in the atmosphere. The oxide particles used in this study are models for mineral dust found in the Earth's atmosphere. Initial uptake coefficients, γ0, have been extracted from the Knudsen cell data. The uptake kinetics have been measured as a function of sample surface area to ensure that realistic surface areas are used in the calculation of the uptake coefficient. Heterogeneous reaction rates are then compared to homogeneous reactions rates for gas-phase reactions involving acetic acid, methanol, and formaldehyde. From this comparison, the possible atmospheric implications of heterogeneous reactions involving these oxygenated organics are discussed.

Fourier transform infrared measurement of the formation of nitrogen compounds on sodium chloride particles exposed to the ambient air in the Arctic

The role of heterogeneous reactions on sea salts in the formation of particle nitrogen compounds in the Arctic during polar sunrise was studied. A diffuse reflection type Fourier transform infrared spectrometer was used to measure surface products on sodium chloride (NaCl) particles exposed to the ambient air at Alert, Northwest Territories, Canada (82.5°N, 62.3°W). The accumulation of nitrogen compounds on NaCl was observed in both winter and spring, while the rate in winter was greater. The important role of ambient N2O5 as well as HNO3 in the formation of nitrogen compounds on NaCl is deduced. The nitrogen compounds observed might be mainly composed of nitrate ion whose electronic states were different from those of nitrate ion in water solution and/or crystal. Its reactivity, including its possible photolysis to NO2−, is noted as to a potential formation pathway of particle nitrite in spring.

Contribution to the ozone trend of heterogeneous reactions of ClONO2 on the sulfate aerosol layer

A photochemical two‐dimensional model including a grid‐point temperature distribution calculated from long‐term circulation statistics has been used to assess the relative role of heterogeneous reactions of chlorine nitrate on background sulfate aerosols. The sulfuric acid weight percentage is calculated explicitly as a function of temperature and water vapor content. It is shown that the contribution of these reactions to the ozone trend in the 1980‧s accounts for about only 10% the ozone loss due to N2O5 + H2O during the Northern Hemisphere winter months at mid‐latitude and between 50–200 mb. The weight of these reactions, however, may substantially increase in the presence of larger aerosol loading. The reaction ClONO2 + HCl on sulfate aerosols is found to be of comparable importance with respect to ClONO2 + H2O, and it is found to represent a significant source of active chlorine in the Antarctic spring.

The role of heterogeneous reactions during the oxidative coupling of methane over Li/MgO catalysts

The role of heterogeneous and homogeneous reactions during the oxidative coupling of methane over Li/MgO was investigated by carrying out experiments at reduced pressure (60 Pa). The conclusion is that the radical reactions occurring in the gas phase play an essential role in the C2+ selectivity of the process. Our reaction model of consecutive reactions is confirmed. The selectivity of the active catalyst is mainly due to very high rates of methyl radical production which couple in the gas phase.