Interactions between Heterogeneous Uptake and Adsorption of Sulfur Dioxide and Acetaldehyde on Hematite

Heterogeneous reaction of NO2 with hematite: The effects of UV irradiation, O2 and relative humidity

Heterogeneous reactions on mineral dust surfaces are increasingly considered important in the removal of gaseous pollutants and the formation of secondary aerosols. Although the heterogeneous reaction of NO2 on the hematite surface has been investigated in many previous studies, little is known about the reaction of NO2 with hematite under ambient conditions. In this work, heterogeneous reactions of NO2 with hematite at 298 K were investigated via a coated-wall flow tube reactor and in situ diffuse reflectance Fourier transformed infrared spectroscopy (DRIFTS). The influence of UV illumination, relative humidity (RH) and O2 on the uptake coefficients and adsorption amount of NO2, as well as the nitrate formation on the hematite surface, has been analyzed comprehensively. UV irradiation shows a significant effect on the true uptake coefficient (γBET), which increases from 2.00 × 10-6 to 4.76 × 10-6 in the N2 stream and 1.32 × 10-6 to 4.07 × 10-6 in the air stream under dry conditions (∼0.3 % RH). RH (in the range of 0-67 %) exhibits an inhibitory effect on the adsorption of NO2 on the hematite surface because of the competition between NO2 and water molecules, that is, γBET and adsorption amount of NO2 decrease with an increase in RH under both the dark and light reaction conditions. Meanwhile, both the γBET and adsorption amount of NO2 on hematite decrease in the air stream compared to those in N2 conditions. In addition, the results from the DRIFTS experiments indicate that the presence of UV irradiation promotes the conversion of NO2 to nitrate and both the RH and O2 suppress the nitrate formation. From this research, the heterogeneous reactions between NO2 with mineral dust under ambient conditions will be better understood.

Multiphase Oxidation of Sulfur Dioxide in Aerosol Particles: Implications for Sulfate Formation in Polluted Environments

Atmospheric oxidation of sulfur dioxide (SO₂) forms sulfate-containing aerosol particles that impact air quality, climate, and human and ecosystem health. It is well-known that in-cloud oxidation of SO₂ frequently dominates over gas-phase oxidation on regional and global scales. Multiphase oxidation involving aerosol particles, fog, and cloud droplets has been generally thought to scale with liquid water content (LWC) so multiphase oxidation would be negligible for aerosol particles due to their low aerosol LWC. However, recent field evidence, particularly from East Asia, shows that fast sulfate formation prevails in cloud-free environments that are characterized by high aerosol loadings. By assuming that the kinetics of cloud water chemistry prevails for aerosol particles, most atmospheric models do not capture this phenomenon. Therefore, the field of aerosol SO₂ multiphase chemistry has blossomed in the past decade, with many oxidation processes proposed to bridge the difference between modeled and observed sulfate mass loadings. This review summarizes recent advances in the fundamental understanding of the aerosol multiphase oxidation of SO₂, with a focus on environmental conditions that affect the oxidation rate, experimental challenges, mechanisms and kinetics results for individual reaction pathways, and future research directions. Compared to dilute cloud water conditions, this paper highlights the differences that arise at the molecular level with the extremely high solute strengths present in aerosol particles.

Influence of relative humidity on SO2 oxidation by O3 and NO2 on the surface of TiO2 particles: Potential for formation of secondary sulfate aerosol

The heterogeneous reactions of SO₂/O₃ and SO₂/NO₂ with TiO₂ particles were studied as a function of relative humidities (RHs). An in situ microscopic Fourier transform infrared (micro-FTIR) spectrometer was used to monitor the reaction kinetics. Rapid conversion of SO₂ to sulfate occurs on the surface of TiO₂ particles in the presence of O₃ or NO₂, which is sensitive to RHs. For unreacted (fresh) particles, the uptake coefficients for SO₂ in initial stage are both obviously enhanced over four times with the increasing RH from ~4% to ~85%. Moreover, the uptake coefficient in the system of SO₂/O₃ is about 40% higher than that of SO₂/NO₂ on TiO₂ particles at the similar RH conditions. For TiO₂ after exposure to SO₂/O₃ or SO₂/NO₂ (sulfated) particles, the uptake coefficients for SO₂ in moisture absorption stage are all higher than that on fresh particles in initial stage at the similar RH, indicating rapid mixture gases adsorption with particle hygroscopic growth. The high production of the secondary sulfate for heterogeneous reaction of mixture gases on TiO₂ surface from arid region to humid region provides new insights for better understanding the severe haze under the humid condition.

Impact of adsorbed nitrate on the heterogeneous conversion of SO2 on α-Fe2O3 in the absence and presence of simulated solar irradiation

Adsorbed nitrate is ubiquitous in the atmosphere, and it can undergo photolysis to produce oxidizing active radicals. Nitrate photolysis may be coupled with the oxidation conversions of atmospheric gaseous pollutants. However, the processes involved remain poorly understood. In this study, the impact of adsorbed nitrate on the heterogeneous oxidation of SO₂ on α-Fe₂O₃ was investigated in the absence and presence of simulated solar irradiation by using in situ Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS). The results indicate that for α-Fe₂O₃ particles with no adsorbed nitrate, the formation of adsorbed sulfate on humid particles is stronger than that on dry particles. Meanwhile, light can also promote the heterogeneous conversion of SO₂ and the formation of sulfate on dry particles because α-Fe₂O₃ is a typical photocatalyst. However, the heterogeneous conversion of SO₂ on humid α-Fe₂O₃ particles is somewhat suppressed under light, suggesting the occurrence of photoinduced reductive dissolution. For the heterogeneous conversion of SO₂ on α-Fe₂O₃ particles with adsorbed nitrate, the formation of sulfate on humid particles is still higher than that on dry particles. For the dry α-Fe₂O₃ particles with adsorbed nitrate, light promotes the formation of adsorbed sulfate. For the humid α-Fe₂O₃ particles with adsorbed nitrate, the heterogeneous conversion of SO₂ under light is stronger than that under no light, indicating that the photolysis of adsorbed nitrate is coupled with the oxidation of SO₂ and the formation of sulfate. The consumption of adsorbed nitrate and the formation of adsorbed N₂O₄ are observed during the introduction of SO₂. A possible mechanism for the impact of adsorbed nitrate on the heterogeneous conversion of SO₂ on α-Fe₂O₃ particles is proposed, and atmospheric implications based on these results are discussed.

The influence of temperature on the heterogeneous uptake of SO2 on hematite particles

Despite the increased awareness of heterogeneous reactions of SO₂ on mineral particles, the knowledge of how temperature influences the product species and kinetic parameters remain a crucially important part in atmospheric research. Here, we reported the formation of sulfur-containing species on hematite particles under various temperature and humidity conditions by mean of in-situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and ion chromatography (IC). High temperature is helpful in the ionization of H₂SO₃, making sulfite compounds occupy a great share among total products. The whole reaction could be divided into three stages according to the formation rate of hydroxyl groups. High temperature brings about more activated SO₂ and then results in the increased uptake coefficients with increasing temperature in the initial reaction stage. On the contrary, moisture absorption on particles is inhibited by high temperature, leading to the decreased uptake coefficients with increasing temperature in the latter two stages. Observed enthalpy and entropy, as well as activation energy values for relevant reactions were calculated. Overall, the product specie and reaction rate vary with temperature and humidity conditions, as well as reaction stages. This work broadens the knowledge of heterogeneous reactions on mineral dust influenced by temperature, and consequently provides important opportunities for atmospheric model improvements.

Effect of Acid on Surface Hydroxyl Groups on Kaolinite and Montmorillonite

Mineral dust aerosol participates in heterogeneous chemistry in the atmosphere. In particular, the hydroxyl groups on the surface of aluminosilicate clay minerals are important for heterogeneous atmospheric processes. These functional groups may be altered by acidic processing during atmospheric transport. In this study, we exposed kaolinite (KGa-1b) and montmorillonite (STx-1b) to aqueous sulfuric acid and then rinsed the soluble reactants and products off in order to explore changes to functional groups on the mineral surface. To quantify the changes due to acid treatment of edge hydroxyl groups, we use 19F magic angle spinning nuclear magnetic resonance spectroscopy and a probe molecule, 3,3,3-trifluoropropyldimethylchlorosilane. We find that the edge hydroxyl groups (OH) increase in both number and density with acid treatment. Chemical reactions in the atmosphere may be impacted by the increase in OH at the mineral edge.

The effects of acetaldehyde, glyoxal and acetic acid on the heterogeneous reaction of nitrogen dioxide on gamma-alumina

Heterogeneous reactions of nitrogen oxides on the surface of aluminium oxide result in the formation of adsorbed nitrite and nitrate. However, little is known about the effects of other species on these heterogeneous reactions and their products. In this study, diffuse reflectance infrared spectroscopy (DRIFTS) was used to analyze the process of the heterogeneous reaction of NO2 on the surface of aluminium oxide particles in the presence of pre-adsorbed organic species (acetaldehyde, glyoxal and acetic acid) at 298 K and reveal the influence of these organic species on the formation of adsorbed nitrite and nitrate. It was found that the pre-adsorption of organic species (acetaldehyde, glyoxal and acetic acid) on γ-Al2O3 could suppress the formation of nitrate to different extents. Under the same experimental conditions, the suppression of the formation of nitrate by the pre-adsorption of acetic acid is much stronger than that by pre-adsorption of acetaldehyde and glyoxal, indicating that the influence of acetic acid on the heterogeneous reaction of NO2 is different from that of acetaldehyde and glyoxal. Surface nitrite is formed and identified to be an intermediate product. For the heterogeneous reaction of NO2 on the surface of γ-Al2O3 with and without the pre-adsorption of acetaldehyde and glyoxal, it is firstly formed and then gradually disappears as the reaction proceeds, but for the reaction with the pre-adsorption of acetic acid, it is the final main product besides nitrate. This indicates that the pre-adsorption of acetic acid would promote the formation of nitrite, while the others would not change the trend of the formation of nitrite. The possible influence mechanisms of the pre-adsorption of acetaldehyde, glyoxal and acetic acid on the heterogeneous conversion of NO2 on γ-Al2O3 are proposed and atmospheric implications based on these results are discussed.

Exploring the mineral-water interface: reduction and reaction kinetics of single hematite (α-Fe2O3) nanoparticles

Here we show that particle impact chronoamperometry allows the quantitative electrochemical characterization of individual mineral nanoparticles with adequate proton concentrations. Through this approach, we extract the kinetics and thermodynamics of the reductive dissolution of single hematite (α-Fe₂O₃) nanoparticles.

In spite of their natural and technological importance, the intrinsic electrochemical properties of hematite (α-Fe₂O₃) nanoparticles are not well understood. In particular, particle agglomeration, the presence of surface impurities, and/or inadequate proton concentrations are major obstacles to uncover the fundamental redox activities of minerals in solution. These are particularly problematic when samples are characterized in common electrochemical analyses such as cyclic voltammetry in which nanoparticles are immobilized on a stationary electrode. In this work, the intrinsic reaction kinetics and thermodynamics of individual hematite nanoparticles are investigated by particle impact chronoamperometry. The particle radius derived from the integrated area of spikes recorded in a chronoamperogram is in excellent agreement with electron microscopy results, indicating that the method provides a quantitative analysis of the reduction of the nanoparticles to the ferrous ion. A key finding is that the suspended individual nanoparticles undergo electrochemical reduction at potentials much more positive than those immobilized on a stationary electrode. The critical importance of the solid/water interface on nanoparticle activity is further illustrated by a kinetic model. It is found that the first electron transfer process is the rate determining step of the reductive dissolution of hematite nanoparticles, while the overall process is strongly affected by the interfacial proton concentration. This article highlights the effects of the interfacial proton and ferrous ion concentrations on the reductive dissolution of hematite nanoparticles and provides a highly effective method that can be readily applied to study a wide range of other mineral nanoparticles.