Revealing the Contribution of Interfacial Processes to Atmospheric Oxidizing Capacity in Haze Chemistry

The atmospheric oxidizing capacity is the most important driving force for the chemical transformation of pollutants in the atmosphere. Traditionally, the atmospheric oxidizing capacity mainly depends on the concentration of O3 and other gaseous oxidants. However, the atmospheric oxidizing capacity based on gas-phase oxidation cannot accurately describe the explosive growth of secondary particulate matter under complex air pollution. From the chemical perspective, the atmospheric oxidizing capacity mainly comes from the activation of O2, which can be achieved in both gas-phase and interfacial processes. In the heterogeneous or multiphase formation pathways of secondary particulate matter, the enhancement of oxidizing capacity ascribed to the O2/H2O-involved interfacial oxidation and hydrolysis processes is an unrecognized source of atmospheric oxidizing capacity. Revealing the enhanced oxidizing capacity due to interfacial processes in high-concentration particulate matter environments and its contribution to the formation of secondary pollution are critical in understanding haze chemistry. The accurate evaluation of atmospheric oxidizing capacity ascribed to interfacial processes is also an important scientific basis for the implementation of PM2.5 and O3 collaborative control in China and around the world.

Heterogeneous interactions and transformations of dibasic esters with indoor relevant surfaces

Dibasic esters (DBEs) have recently become emerging indoor air pollutants due to their usage as a solvent for mixtures of paints and coatings. In this study, we explored the adsorption/desorption kinetics, heterogeneous interactions, and chemical transformations of dimethyl succinate (DMS, C6H10O4), a component of commercial dibasic ester solvent mixtures, on indoor relevant surfaces using transmission Fourier-transform infrared (FTIR) spectroscopy and high-resolution mass spectrometry (HRMS). Silica (SiO2) and rutile (TiO2) were used as proxies for window glass, and an active component in paint and self-cleaning surfaces, respectively. FTIR spectroscopy of these surfaces shows that DMS can interact with SiO2 and TiO2 through hydrogen bonding between the carbonyl oxygen and surface hydroxyl groups. The kinetics show fast adsorption of DMS onto these surfaces followed by slow desorption. Furthermore, new products formed observed on TiO2 surfaces in addition to molecularly adsorbed DMS. In particular, succinate (C5H7O) was observed binding to the surface in a bidentate chelating coordination mode as indicated by the appearance of νas(COO-) and νs(COO-) bands in the FTIR spectra. These absorption bands grow in intensity over time and the resulting product remains strongly adsorbed on the surface. The formation of adsorbed succinate is a result of a reaction with DMS on Lewis acid sites of the TiO2 surface. Overall, the slow desorption of these adsorbed species indicates that indoor surfaces can become long term reservoirs for dibasic esters and their surface products. Moreover, in the presence of ∼50% relative humidity, water displaces outer layers of adsorbed DMS on SiO2 and TiO2, while having no impact on the more strongly bound surface species.

Significantly Accelerated Photosensitized Formation of Atmospheric Sulfate at the Air-Water Interface of Microdroplets

The multiphase oxidation of sulfur dioxide (SO2) to form sulfate is a complex and important process in the atmosphere. While the conventional photosensitized reaction mainly explored in the bulk medium is reported to be one of the drivers to trigger atmospheric sulfate production, how this scheme functionalizes at the air-water interface (AWI) of aerosol remains an open question. Herein, employing an advanced size-controllable microdroplet-printing device, surface-enhanced Raman scattering (SERS) analysis, nanosecond transient adsorption spectrometer, and molecular level theoretical calculations, we revealed the previously overlooked interfacial role in photosensitized oxidation of SO2 in humic-like substance (HULIS) aerosol, where a 3-4 orders of magnitude increase in sulfate formation rate was speculated in cloud and aerosol relevant-sized particles relative to the conventional bulk-phase medium. The rapid formation of a battery of reactive oxygen species (ROS) comes from the accelerated electron transfer process at the AWI, where the excited triplet state of HULIS (3HULIS*) of the incomplete solvent cage can readily capture electrons from HSO3- in a way that is more efficient than that in the bulk medium fully blocked by water molecules. This phenomenon could be explained by the significantly reduced desolvation energy barrier required for reagents residing in the AWI region with an open solvent shell.

Uptake and reactivity of NO2 on the hydroxylated silica surface: A source of reactive oxygen species

We report state-of-the-art first-principles molecular dynamics results on the heterogeneous chemical uptake of NO2, a major anthropogenic pollutant, on the dry and wet hydroxylated surface of α-quartz, which is a significant component of silica-based catalysts and atmospheric dust aerosols. Our investigation spotlights an unexpected chemical pathway by which NO2 (i) can be adsorbed as HONO by deprotonation of interfacial silanols (i.e., -Si-OH group) on silica, (ii) can be barrierless converted to nitric acid, and (iii) can finally dissociated to surface bounded NO and hydroxyl gas phase radicals. This chemical pathway does not invoke any previously experimentally postulated NO2 dimerization, dimerization that is less likely to occur at low NO2 concentrations. Moreover, water significantly catalyzes the HONO formation and the dissociation of nitric acid into surface-bounded NO and OH radicals, while visible light adsorption can further promote these chemical transformations. This work highlights how water-restricted solvation regimes on common mineral substrates are likely to be a source of reactive oxygen species, and it offers a theoretical framework for further and desirable experimental efforts, aiming to better constrain trace gases/mineral interactions at different relative humidity conditions.

Binding of SO3 to Group 4 Transition Metal Oxide Nanoclusters

Transition metal oxide (TMO) clusters are being studied for their ability to absorb acid gases generated by energy production processes. The interaction of SO3, a byproduct of common industrial processes, with group 4 metal (Ti, Zr, and Hf) oxide nanoclusters, has been predicted using electronic structure methods. The calculations were done at the density functional theory (DFT) and correlated molecular orbital coupled cluster singles and doubles CCSD(T) theory levels. There is a reasonable agreement between the DFT/ωB97x-D energies with the CCSD(T) results. SO3 is predicted to strongly chemisorb to these clusters, as do NO2 and CO2. For SO3, these chemisorption processes favor binding to TMO clusters as SO42- sulfate in both the terminal and bridging configurations. It is predicted that SO3 fully extracts the bridging oxygen from the TMO lattice to form bridging SO42-. This is favorable because of the lower S-O bond dissociation energy of SO3, whereas other acid gases add across the bridging oxygen because of their higher A-O bond dissociation energy. SO3 is capable of physisorption as long as an exposed metal center is present in the lattice. If a metal center has a terminal oxo-group, then SO3 will prefer the SO42- configuration. An approximately linear relationship exists between the physisorption energy and proton affinity for rows 2 and 3 elements.

Solar-CO2 -to-Syngas Conversion Enabled by Precise Charge Transport Modulation

The rational design of the directional charge transfer channel represents an important strategy to finely tune the charge migration and separation in photocatalytic CO2 -to-fuel conversion. Despite the progress made in crafting high-performance photocatalysts, developing elegant photosystems with precisely modulated interfacial charge transfer feature remains a grand challenge. Here, a facile one-pot method is developed to achieve in situ self-assembly of Pd nanocrystals (NYs) on the transition metal chalcogenide (TMC) substrate with the aid of a non-conjugated insulating polymer, i.e., branched polyethylenimine (bPEI), for photoreduction of CO2 to syngas (CO/H2 ). The generic reducing capability of the abundant amine groups grafted on the molecular backbone of bPEI fosters the homogeneous growth of Pd NYs on the TMC framework. Intriguingly, the self-assembled TMCs@bPEI@Pd heterostructure with bi-directional spatial charge transport pathways exhibit significantly boosted photoactivity toward CO2 -to-syngas conversion under visible light irradiation, wherein bPEI serves as an efficient hole transfer mediator, and simultaneously Pd NYs act as an electron-withdrawing modulator for accelerating spatially vectorial charge separation. Furthermore, in-depth understanding of the in situ formed intermediates during the CO2 photoreduction process are exquisitely probed. This work provides a quintessential paradigm for in situ construction of multi-component heterojunction photosystem for solar-to-fuel energy conversion.

Heterogeneous Formation of Organonitrates (ON) and Nitroxy-Organosulfates (NOS) from Adsorbed α-Pinene-Derived Organosulfates (OS) on Mineral Surfaces

Organonitrates (ON) and nitroxy-organosulfates (NOS) are important components of secondary organic aerosols (SOAs). Gas-phase reactions of α-pinene (C₁₀H₁₆), a primary precursor for several ON compounds, are fairly well understood although formation pathways for NOS largely remain unknown. NOS formation may occur via reactions of ON and organic peroxides with sulfates as well as through radical-initiated photochemical processes. Despite the fact that organosulfates (OS) represent a significant portion of the organic aerosol mass, ON and NOS formation from OS is less understood, especially through nighttime heterogeneous and multiphase chemistry pathways. In the current study, surface reactions of adsorbed α-pinene-derived OS with nitrogen oxides on hematite and kaolinite surfaces, common components of mineral dust, have been investigated. α-Pinene reacts with sulfated mineral surfaces, forming a range of OS compounds on the surface. These OS compounds when adsorbed on mineral surfaces can further react with HNO₃ and NO₂, producing several ON and NOS compounds as well as several oxidation products. Overall, this study reveals the complexity of reactions of prevalent organic compounds leading to the formation of OS, ON, and NOS via heterogeneous and multiphase reaction pathways on mineral surfaces. It is also shown that this chemistry is mineralogy-specific.

Fabrication of Na0.4MnO2 Microrods for Room-Temperature Oxidation of Sulfurous Gases

Phase pure Na_0.4MnO₂ microrods crystallized in the orthorhombic symmetry were fabricated for the wet oxidation of H₂S and SO₂ gases at room temperature. The material was found highly effective for the mineralization of low concentrations of acidic gases. The material was fully regenerable after soaking in a basic H₂O₂ solution.

Catalytic performance and reaction mechanisms of NO removal with NH3 at low and medium temperatures on Mn-W-Sb modified siderite catalysts

Iron-based catalysts have been explored for selective catalytic reduction (SCR) of NO due to environmentally benign characters and good SCR activity. Mn-W-Sb modified siderite catalysts were prepared by impregnation method based on siderite ore, and SCR performance of the catalysts was investigated. The catalysts were analyzed by X-ray diffraction, H₂-temperature-programmed reduction, Brunauer-Emmett-Teller, Thermogravimetry-derivative thermogravimetry and in-situ diffused reflectance infrared Fourier transform spectroscopy (DRIFTS). The modified siderite catalysts calcined at 450°C mainly consist of Fe₂O₃, and added Mn, W and Sb species are amorphous. 3Mn-5W-1.5Sb-siderite catalyst has a wide temperature window of 180-360°C and good N₂ selectivity at low temperatures. In-situ DRIFTS results show NH₄⁺, coordinated NH₃, NH₂, NO₃^- species (bidentate), NO₂^- species (nitro, nitro-nitrito, monodentate), and adsorbed NO₂ can be discovered on the surface of Mn-W-Sb modified siderite catalysts, and doping of Mn will enhance adsorbed NO₂ formation by synergistic catalysis with Fe³⁺. In addition, the addition of Sb can inhibit sulfates formation on the surface of the catalyst in the presence of SO₂ and H₂O. Time-dependent in-situ DRIFTS studies also indicate that both of Lewis and Brønsted acid sites play a role in SCR of NO by ammonia at low temperatures. The mechanism of NO removal on the 3Mn-5W-1.5Sb-siderite catalyst can be discovered as a combination of Eley-Rideal and Langmuir-Hinshelwood mechanisms with three reaction pathways. The mechanism of NO, oxidized by synergistic catalysis of Fe³⁺ and Mn^4+/3+ to form NO₂ among three pathways, reveals the reason of high NO_x conversion of the catalyst at medium and low temperatures.

Carbon consumption and regeneration of oxygen-containing functional groups on activated carbon for flue gas purification

High carbon consumption is an important factor restricting the wide application of activated carbon technology for flue gas purification. A fixed-bed reactor combined with a Fourier transform infrared (FTIR) spectrometer was used to explore the source of carbon consumption at various SO₂ concentrations and cyclic adsorption-regeneration times. The results demonstrate that carbon consumption originates from two sources and is mainly determined by the reaction of H₂SO₄ and C at high SO₂ concentrations and by the thermal decomposition of oxygen-containing functional groups at low SO₂ concentrations. An interesting observed phenomenon is that carbon consumption does not increase as the SO₂ concentration increases. The conversion mechanism reveals that carboxylic and anhydride groups are converted to phenol and quinone groups, which do not easily decompose with increasing SO₂ concentration. In the process of cyclic adsorption-regeneration, it is discovered that the carbon consumption in the first cycle is several times higher than that in the following cycles due to the decomposition of functional groups from the activated carbon itself. The regeneration mechanism of functional groups has been elucidated. The carboxylic acid and the phenolic hydroxyl on the surface of activated carbon are consumed in the regeneration process and formed again from the conversion of carbonyl groups in the next adsorption process under the roles of O₂ and H₂O. It is proposed that the functional groups are regenerated in the adsorption process rather than in the regeneration process.

Metal-organic framework-derived NaMxOy adsorbents for low-temperature SO2 removal

This work reported the fabrication of NaM_xO_y-type adsorbents from air calcination of (Na, M)-trimesate metal-organic frameworks. NaMn_xO_y (NMO) crystallized as disc-shaped microsheets, whereas NaCo_xO_y (NCO) crystallized as smooth microsheets with surface deposition of polyhedral nanoparticles. The oxides have a surface area of 1.90-2.56 m² g^-1. The synthesized adsorbents were studied for low-temperature SO₂ removal in breakthrough studies. The maximum adsorption capacity of 46.8 mg g^-1 was recorded for NMO at 70 °C. The adsorption capacity increased with the increasing temperature due to the chemisorptive nature of the adsorption process. The capacity increased with the increasing bed loading and decreasing flow rate due to the improved SO₂ retention time. The elemental mapping confirmed the uniform distribution of sulfur species over the oxide surface. X-ray diffraction showed the absence of metal sulfate nanoparticles in the SO₂-exposed samples. The X-ray photoelectron analysis confirmed the formation of surface sulfate and bisulfate. The formation of oxidized sulfur species was mediated by hydroxyl groups over NMO and lattice oxygen over NCO. Thus, the work demonstrated here is the first such report on the use of NaM_xO_y-type materials for SO₂ mineralization.

Uptake and hydration of sulfur dioxide on dry and wet hydroxylated silica surfaces: a computational study

We present a first-principles molecular dynamics study on the uptake and hydration of sulfur dioxide on the dry and wet fully hydroxylated surfaces of (0001) α-quartz, which are a proxy for suspended silica dust in the atmosphere. The average adsorption energy for SO₂ is about -10 kcal mol^-1 on both dry and wet surfaces. The adsorption is driven by hydrogen bond formation between SO₂ and the interfacial hydroxyl groups (on dry silica), or with water molecules (in the wet case). In the dry system, we report an additional electrostatic interaction between the interfacial hydroxyl oxygen and the sulfur atom, which further stabilizes the adsorbate. On dry silica, the interfacial hydroxyl group coordinates to SO₂ yielding a surface bound bisulfite (Si-SO₃H) complex. On the wet surface, SO₂ reacts with water forming bisulfite (HSO₃^-), and the latter remains solvated inside the adsorbed water layer. The hydration barrier for sulfur dioxide is 1 kcal mol^-1 and 3 kcal mol^-1 on dry and wet silica, respectively, while for the backward reaction (i.e., bisulfite to SO₂) the barrier is 6 kcal mol^-1 on both surfaces. The modest backward barrier rationalizes earlier experimental findings showing no SO₂ uptake on silica. These results underline the importance of the surface hydroxylation and/or adsorbed water layers for the SO₂ uptake and its hydration on silica. Moreover, the hydration to bisulfite may prevent direct SO₂ photochemistry and be an additional source of sulfate; this is especially relevant in atmospheres subject to a high level of suspended mineral dust, intense solar radiation and atmospheric oxidizers.

Photocatalytic Oxidation of SO2 by TiO2: Aerosol Formation and the Key Role of Gaseous Reactive Oxygen Species

Photocatalytic materials are proved to effectively eliminate gaseous pollutants and are widely used in the environment. However, as one of the rare experiments focusing on their influence on secondary aerosol formation generated in the gas phase (SA_g), our study demonstrated the high-yield SA_g formation in the photocatalysis process. In this study, the photodegradation of SO₂ by TiO₂ under various relative humidity (RH) conditions was deeply explored with multiple methods. Unexpectedly, H₂SO₄ aerosols (SA_g-H2SO4) in yields of 10.10-32.64% were observed under the studied RH conditions for the first time. Gaseous ^•OH and H₂O₂ generated from the oxidation of H₂O and reduction of O₂ by TiO₂ were directly detected in the photocatalysis process, and they were identified as the determining factor for SA_g-H2SO4 formation. The formation of SA_g-H2SO4 was also influenced by RH, the heterogeneous reaction of SO₂, and the uptake of H₂SO₄. The role of the released gaseous ^•OH and H₂O₂ on atmospheric chemistry was proved to be unignorable by adopting the obtained parameters into the real environment. These findings provided direct experimental evidence of secondary pollution in the photocatalysis process and are of great significance to the field of atmospheric environment and photocatalytic materials.

Battling Chemical Weapons with Zirconium Hydroxide Nanoparticle Sorbent: Impact of Environmental Contaminants on Sarin Sequestration and Decomposition

The promising reactive sorbent zirconium hydroxide (ZH) was challenged with common environmental contaminants (CO₂, SO₂, and NO₂) to determine the impact on chemical warfare agent decomposition. Several environmental adsorbates rapidly formed on the ZH surface through available hydroxyl species and coordinatively unsaturated zirconium sites. ZH decontamination effectiveness was determined using a suite of instrumentation including in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) to monitor sarin (GB) decomposition in real time and at ambient pressure. Surface products were characterized by ex situ X-ray photoelectron spectroscopy (XPS). The adsorption enthalpies, entropies, and bond lengths for environmental contaminants and GB decomposition products were estimated using density functional theory (DFT). Consistent with the XPS and DRIFTS results, DFT simulations predicted the relative stabilities of molecular adsorbates and reaction products in the following order: CO₂ < NO₂ < GB ≈ SO₂. Microbreakthrough capacity measurements on ZH showed a 7-fold increase in the sorption of NO₂ vs SO₂, which indicates differences in the surface reactivity of these species. GB decomposition was rapid on clean and CO₂-dosed ZH and showed reduced decomposition on SO₂- and NO₂-predosed samples. Despite these findings, the total GB sorption capacity of clean and predosed ZH was consistent across all samples. These data provide insight into the real-world use of ZH as a reactive sorbent for chemical decontamination applications.

How Relevant Is It to Use Mineral Proxies to Mimic the Atmospheric Reactivity of Natural Dust Samples? A Reactivity Study Using SO2 as Probe Molecule

The experimental investigation of heterogeneous atmospheric processes involving mineral aerosols is extensively performed in the literature using proxy materials. In this work we questioned the validity of using proxies such as Fe2O3, FeOOH, Al2O3, MgO, CaO, TiO2, MnO2, SiO2, and CaCO3 to represent the behavior of complex mixtures of minerals, such as natural desert and volcanic dusts. Five volcanic dusts and three desert dusts were compared to a number of metal oxides, commonly used in the literature to mimic the behavior of desert dusts in the ability to form sulfites and sulfates on the surface exposed to SO2 gas. First, all samples were aged at room temperature, atmospheric pressure, under controlled experimental conditions of 175 ppm SO2 for 1 h under 30% of relative humidity. Second, they were extracted with 1% formalin and analyzed by High-Performance Liquid Chromatography (HPLC) to quantify and compare the amount of sulfites and sulfates formed on their surfaces. It was evidenced that under the experimental conditions of this study neither one selected pure oxide nor a mixture of oxides can adequately typify the behavior of complex mixtures of natural minerals. Therefore, to evaluate the real-life impact of natural dust on atmospheric processes it is of vital importance to work directly with the natural samples, both to observe the real effects of desert and volcanic dusts and to evaluate the relevancy of proposed proxies.

Photoinduced Adsorption and Oxidation of SO2 on Anatase TiO2(101)

Adsorption of molecules is a fundamental step in all heterogeneous catalytic reactions. Nevertheless, the basic mechanism by which photon-mediated adsorption processes occur on solid surfaces is poorly understood, mainly because they involve excited catalyst states that complicate the analysis. Here we demonstrate a method by which density functional theory (DFT) can be used to quantify photoinduced adsorption processes on transition metal oxides and reveal the fundamental nature of these reactions. Specifically, the photoadsorption of SO₂ on TiO₂(101) has been investigated by using a combination of DFT and in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). The combined experimental and theoretical approach gives a detailed description of the photocatalytic desulfurization process on TiO₂, in which sulfate forms as a stable surface product that is known to poison the catalytic surface. This work identifies surface-SO₄^2- as the sulfate species responsible for the surface poisoning and shows how this product can be obtained from a stepwise oxidation of SO₂ on TiO₂(101). Initially, the molecule binds to a lattice O₂^- ion through a photomediated adsorption process and forms surface sulfite, which is subsequently oxidized into surface-SO₄^2- by transfer of a neutral oxygen from an adsorbed O₂ molecule. The work further explains how the infrared spectra associated with this oxidation product change during interactions with water and surface hydroxyl groups, which can be used as fingerprints for the surface reactions. The approach outlined here can be generalized to other photo- and electrocatalytic transition metal oxide systems.

Formation of sulfur trioxide during the SCR of NO with NH3 over a V2O5/TiO2 catalyst

The oxidation of sulfur dioxide (SO2) to sulfur trioxide (SO3) is an undesirable reaction that occurs during the selective catalytic reduction (SCR) of nitrogen oxides (NO x ) with ammonia (NH3), which is a process applied to purify flue gas from coal-fired power plants. The objectives of this work were to establish the fundamental kinetics of SO3 formation over a V2O5/TiO2 catalyst and to illustrate the formation mechanism of SO3 in the presence of NO x , H2O and NH3. A fixed-bed reactor was combined with a Fourier transform infrared (FTIR) spectrometer and a Pentol SO3 analyser to test the outlet concentrations of the multiple components. The results showed that the rate of SO2 oxidation was zero-order in O2, 0.77-order in SO2 and -0.19-order in SO3 and that the apparent activation energy for SO2 oxidation was 74.3 kJ mol-1 over the range of studied conditions. Based on in situ diffuse reflectance infrared Fourier transform (in situ DRIFT) spectroscopy, X-ray photoelectron spectroscopy (XPS) and temperature programmed desorption (TPD) tests, the SO3 formation process is described here in detail. The adsorbed SO2 was oxidized by V2O5 to produce adsorbed SO3 in the form of bridge tridentate sulfate, and the adsorbed SO3 was desorbed to the gas phase. NO x promoted the oxidation of the adsorbed SO2 due to the promotion of the conversion of low-valent vanadium to high-valent vanadium. In addition, the desorption of the adsorbed SO3 was inhibited by H2O or NH3 due to the conversion of tridentate sulfate to the more stable bidentate sulfate or ammonium bisulfate. Finally, the mechanism of the influence of NO x , H2O and NH3 on the formation of gaseous SO3 was proposed.

Sulfur Dioxide Degradation by Composite Photocatalysts Prepared by Recycled Fine Aggregates and Nanoscale Titanium Dioxide

To alleviate the heavy burden on landfilling, construction and demolition wastes (C&DWs) are recycled and reused as aggregates in cementitious materials. However, the inherent characteristics of recycled fine aggregates (RFA), such as the high crushing index and high-water absorption, magnify the reusing difficulty. Nevertheless, attributing to the high porosity and high level of calcium hydroxides existing in the old mortar, RFA is featured with a high specific surface area and a high alkalinity. These features are useful to augment the total photo-degradation of SO2 by nano-TiO2 (NT) intermixed mortar, leading RFA to be an excellent potential carrier to load nano-TiO2 and prepare the composite photocatalyst. Hence, this study proposed to load NT onto the surface of RFAs and river sands (RSs) (the control) by the soaking method, preparing composite photocatalysts denoted as NT@RFA and NT@RS, respectively. The prepared composite photocatalysts were then utilized as sands in photocatalytic mortar to evaluate for SO2 degradation. Experiments identified a 50% higher amount of NT was loaded onto the surface of FRA relative to the control. This higher loading amount plus higher alkalinity ultimately translated into a higher photocatalytic activity. In addition, the mortar containing NT@RFA exhibited 46.3% higher physiochemical absorption and 23.9% higher photocatalytic activity than that containing NT@RS. In addition, the durability, embodied by the reuse and anti-abrasive properties, of NT@RFA exceeded that of NT@RS. The overall findings reveal that the NT@RFA not only garners beneficial effect from the high porosity but also generates positive effect from the high alkalinity. Though a number of studies deal with building materials with NT, this study is the first to load NT onto RFA and prepare composite photocatalysts which were then used as fine aggregates in building materials. Consequently, this study proves the potential high-added-value reusability of RFA in green construction materials and provides a low-cost, high-efficiency approach to degrade atmospheric SO2.

Nitrate-Enhanced Oxidation of SO2 on Mineral Dust: A Vital Role of a Proton

Heterogeneous oxidation of SO₂ on mineral dust is a significant source of sulfate in the atmosphere. Given that a large fraction of nitrate is deposited on the mineral aerosols, the determination of the effect of nitrate on the SO₂ oxidation on mineral dust and its in-depth mechanism are much desired. In this work, we report nitrate-enhanced SO₂ oxidation on authentic mineral dust. By comparing the SO₂ uptake behaviors on Arizona test dust (ATD, a typical proxy of mineral dust) with or without nitrate, we found that although nitrate hinders the initial SO₂ uptake, it substantially accelerates SO₂ uptake and oxidation after a pronounced induction period. In other words, a hindering-then-accelerating feature in the SO₂ uptake profile was observed on nitrate-containing ATD (N-ATD) particles. In addition, HONO was released in the accelerating period as the reduction product of nitrate. The accumulation of protons (H⁺) from SO₂ oxidation during the induction period plays a key role in the acceleration of SO₂ oxidation. Our work suggests that the nitrate-participating SO₂ oxidation on mineral dust can be one of the important contributions of the sulfate source in the atmosphere.

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.

Molecular Insights into NO-Promoted Sulfate Formation on Model TiO2 Nanoparticles with Different Exposed Facets

In this study, molecular insights into NO-promoted SO₂ oxidation on model TiO₂ with well-defined (001), (010), and (101) facets are investigated in the laboratory. The adsorbed SO₂ is significantly promoted to transform into sulfate by the coexisting NO on the three facets. The appearance of active oxygen species indicates an active oxygen species-initiated NO oxidation. The resulting NO₂ acts as an oxidant to convert adsorbed sulfite on hydroxyls to sulfate species. The (101) facet presents the best performance in the NO-promoted sulfate formation possibly owing to its desirable structure to accommodate SO₃^2-, NO₂, and molecular water. The uptake coefficient (γ_t) of SO₂ increases by the coexistence of NO on the (001) facet at 0% RH and is relative humidity (RH) dependent, which first decreases from 0% RH to 32% RH but then increases in the range of 32%-80% RH. The probable explanation is the combined contribution of the blocking effect of water and the hydration of SO₂. The finding in this study provides insight into the possibility of its occurrence on common mineral dusts and requires further investigation.

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.

The influence of relative humidity on the heterogeneous oxidation of sulfur dioxide by ozone on calcium carbonate particles

Heterogeneous reactions of SO₂ and O₃ with CaCO₃ particles were investigated at a series of relative humidity (RH, 1% to 90%) and 298K using a diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). The uptake coefficients of SO₂ on CaCO₃ at different RHs were obtained for the first time. Our results proved that high RH could substantially promote the formation of sulfate, for which the highest concentration (80% RH and reaction time of 200min) and highest formation rate in stable stage (85% RH) were 14 times and 43 times that at 1% RH, respectively. The surface products, increment of concentration and formation rate of sulfate changed with RH which were due to the surface adsorbed water (SAW) on the particles. SAW could increase the reactive sites on the particles and thus accelerate the conversion of SO₂ into sulfite, and sulfite could be oxidized rapidly. Liquid-like water layers formed on the particle surface could enhance the ion mobility and promote the aggregation of CaSO₄ hydrates, which could expose more reactive sites and result in additional adsorption of SO₂. Piecewise equations of uptake coefficient with RH were given and could be referred by model simulation. The results are of importance in understanding the explosive growth of sulfate during severe haze episodes accompanied with high RH.

Heterogeneous Reaction of SO2 on Manganese Oxides: the Effect of Crystal Structure and Relative Humidity

Manganese oxides from anthropogenic sources can promote the formation of sulfate through catalytic oxidation of SO₂. In this study, the kinetics of SO₂ reactions on MnO₂ with different morphologies (α, β, γ and δ) was investigated using flow tube reactor and in situ Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS). Under dry conditions, the reactivity towards SO₂ uptake was highest on δ-MnO₂ but lowest on β-MnO₂, with a geometric uptake coefficient (γ_obs) of (2.42 ± 0.13) ×10^–2 and a corrected uptake coefficient (γ_c) of (1.48 ± 0.21) ×10⁻⁶ for the former while γ_obs of (3.35 ± 0.43) ×10⁻³ and γ_c of (7.46 ± 2.97) ×10⁻⁷ for the latter. Under wet conditions, the presence of water altered the chemical form of sulfate and was in favor for the heterogeneous oxidation of SO₂. The maximum sulfate formation rate was reached at 25% RH and 45% for δ-MnO₂ and γ-MnO₂, respectively, possibly due to their different crystal structures. The results suggest that morphologies and RH are important factors influencing the heterogeneous reaction of SO₂ on mineral aerosols, and that aqueous oxidation process involving transition metals of Mn might be a potential important pathway for SO₂ oxidation in the atmosphere.

SO2 Emissions in China - Their Network and Hierarchical Structures

SO₂ emissions lead to various harmful effects on environment and human health. The SO₂ emission in China has significant contribution to the global SO₂ emission, so it is necessary to employ various methods to study SO₂ emissions in China with great details in order to lay the foundation for policymaking to improve environmental conditions in China. Network analysis is used to analyze the SO₂ emissions from power generation, industrial, residential and transportation sectors in China for 2008 and 2010, which are recently available from 1744 ground surface monitoring stations. The results show that the SO₂ emissions from power generation sector were highly individualized as small-sized clusters, the SO₂ emissions from industrial sector underwent an integration process with a large cluster contained 1674 places covering all industrial areas in China, the SO₂ emissions from residential sector was not impacted by time, and the SO₂ emissions from transportation sector underwent significant integration. Hierarchical structure is obtained by further combining SO₂ emissions from all four sectors and is potentially useful to find out similar patterns of SO₂ emissions, which can provide information on understanding the mechanisms of SO₂ pollution and on designing different environmental measure to combat SO₂ emissions.

A survey of photogeochemistry

The participation of sunlight in the natural chemistry of the earth is presented as a unique field of study, from historical observations to prospects for future inquiry. A compilation of known reactions shows the extent of light-driven interactions between naturally occurring components of land, air, and water, and provides the backdrop for an outline of the mechanisms of these phenomena. Catalyzed reactions, uncatalyzed reactions, direct processes, and indirect processes all operate in natural photochemical transformations, many of which are analogous to well-known biological reactions. By overlaying photochemistry and surface geochemistry, complementary approaches can be adopted to identify natural photochemical reactions and discern their significance in the environment.

Atmospheric chemistry of bioaerosols: heterogeneous and multiphase reactions with atmospheric oxidants and other trace gases

Once airborne, biologically-derived aerosol particles are prone to reaction with various atmospheric oxidants such as OH, NO₃, and O₃.

Advances in analytical techniques and instrumentation have now established methods for detecting, quantifying, and identifying the chemical and microbial constituents of particulate matter in the atmosphere. For example, recent cryo-TEM studies of sea spray have identified whole bacteria and viruses ejected from ocean seawater into air. A focal point of this perspective is directed towards the reactivity of aerosol particles of biological origin with oxidants (OH, NO₃, and O₃) present in the atmosphere. Complementary information on the reactivity of aerosol particles is obtained from field investigations and laboratory studies. Laboratory studies of different types of biologically-derived particles offer important information related to their impacts on the local and global environment. These studies can also unravel a range of different chemistries and reactivity afforded by the complexity and diversity of the chemical make-up of these particles. Laboratory experiments as the ones reviewed herein can elucidate the chemistry of biological aerosols.

Sulfur Dioxide Capture by Heterogeneous Oxidation on Hydroxylated Manganese Dioxide

Here we demonstrate that sulfur dioxide (SO2) is efficiently captured via heterogeneous oxidation into sulfate on the surface of hydroxylated manganese dioxide (MnO2). Lab-scale activity tests in a fluidized bed reactor showed that the removal efficiency for a simulated flue gas containing 5000 mg·Nm(-3) SO2 could reach nearly 100% with a GHSV (gas hourly space velocity) of 10000 h(-1). The mechanism was investigated using a combination of experimental characterizations and theoretical calculations. It was found that formation of surface bound sulfate proceeds via association of SO2 with terminal hydroxyls. Both H2O and O2 are essential for the generation of reactive terminal hydroxyls, and the indirect role of O2 in heterogeneous SO2 oxidation at low temperature was also revealed. We propose that the high reactivity of terminal hydroxyls is attributed to the proper surface configuration of MnO2 to adsorb water with degenerate energies for associative and dissociative states, and maintain rapid proton dynamics. Viability analyses suggest that the desulfurization method that is based on such a direct oxidation reaction at the gas/solid interface represents a promising approach for SO2 capture.

How should the completeness and quality of curated nanomaterial data be evaluated?

Nanotechnology is of increasing significance. Curation of nanomaterial data into electronic databases offers opportunities to better understand and predict nanomaterials' behaviour. This supports innovation in, and regulation of, nanotechnology. It is commonly understood that curated data need to be sufficiently complete and of sufficient quality to serve their intended purpose. However, assessing data completeness and quality is non-trivial in general and is arguably especially difficult in the nanoscience area, given its highly multidisciplinary nature. The current article, part of the Nanomaterial Data Curation Initiative series, addresses how to assess the completeness and quality of (curated) nanomaterial data. In order to address this key challenge, a variety of related issues are discussed: the meaning and importance of data completeness and quality, existing approaches to their assessment and the key challenges associated with evaluating the completeness and quality of curated nanomaterial data. Considerations which are specific to the nanoscience area and lessons which can be learned from other relevant scientific disciplines are considered. Hence, the scope of this discussion ranges from physicochemical characterisation requirements for nanomaterials and interference of nanomaterials with nanotoxicology assays to broader issues such as minimum information checklists, toxicology data quality schemes and computational approaches that facilitate evaluation of the completeness and quality of (curated) data. This discussion is informed by a literature review and a survey of key nanomaterial data curation stakeholders. Finally, drawing upon this discussion, recommendations are presented concerning the central question: how should the completeness and quality of curated nanomaterial data be evaluated?

Relationship between the molecular structure of V2O5/TiO2 catalysts and the reactivity of SO2 oxidation

The VO site was demonstrated to play a critical role in the SO₂ oxidation over V₂O₅/TiO₂ catalysts.

Synergistic formation of sulfate and ammonium resulting from reaction between SO2 and NH3 on typical mineral dust

A synergistic effect between SO₂ and NH₃ on typical mineral dust.

Nano titania aided clustering and adhesion of beneficial bacteria to plant roots to enhance crop growth and stress management

A novel use of Titania nanoparticles as agents in the nano interface interaction between a beneficial plant growth promoting bacterium (Bacillus amyloliquefaciens UCMB5113) and oilseed rape plants (Brassica napus) for protection against the fungal pathogen Alternaria brassicae is presented. Two different TiO₂ nanoparticle material were produced by the Sol-Gel approach, one using the patented Captigel method and the other one applying TiBALDH precursor. The particles were characterized by transmission electron microscopy, thermogravimetric analysis, X-ray diffraction, dynamic light scattering and nano particle tracking analysis. Scanning electron microscopy showed that the bacterium was living in clusters on the roots and the combined energy-dispersive X-ray spectroscopy analysis revealed that titanium was present in these cluster formations. Confocal laser scanning microscopy further demonstrated an increased bacterial colonization of Arabidopsis thaliana roots and a semi-quantitative microscopic assay confirmed an increased bacterial adhesion to the roots. An increased amount of adhered bacteria was further confirmed by quantitative fluorescence measurements. The degree of infection by the fungus was measured and quantified by real-time-qPCR. Results showed that Titania nanoparticles increased adhesion of beneficial bacteria on to the roots of oilseed rape and protected the plants against infection.

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

Sulfur dioxide and organic aldehydes in the atmosphere are ubiquitous and often correlated with mineral dust aerosols. Heterogeneous uptake and adsorption of one of these species on mineral aerosols can potentially change the properties of the particles and further affect the subsequent heterogeneous reactions of the other species on the coating particles. In this study, the interactions between heterogeneous uptake and adsorption of sulfur dioxide and acetaldehyde on hematite are investigated by using in situ diffuse-reflectance infrared Fourier-transform spectroscopy (DRIFTS) at room temperature. It is found that the preadsorption of SO2 on α-Fe2O3 can significantly hinder the subsequent heterogeneous oxidation of CH3CHO to acetate, while the preadsorption of CH3CHO significantly suppresses the heterogeneous reaction of large amounts of SO2 on the surface of α-Fe2O3 and has a little influence on the uptake of small amount of SO2. The heterogeneous reactions of SO2 on α-Fe2O3 preadsorbed by CH3CHO change the existing acetate on the particle surface into chemisorbed acetic acid, for the enhancement of surface acidity after the uptake of SO2. During these processes, different surface hydroxyl groups showed different reactivities. Atmospheric implications of this study are discussed.

Photochemistry of nitrate chemisorbed on various metal oxide surfaces

Atmospheric aerosols are known to provide an important surface for gas–solid interfaces that can lead to heterogeneous reactions impacting tropospheric chemistry.

Heterogeneous uptake and adsorption of gas-phase formic acid on oxide and clay particle surfaces: the roles of surface hydroxyl groups and adsorbed water in formic acid adsorption and the impact of formic acid adsorption on water uptake

Organic acids in the atmosphere are ubiquitous and are often correlated with mineral dust aerosol. Heterogeneous chemistry and the uptake of organic acids on mineral dust particles can potentially alter the properties of the particle. In this study, heterogeneous uptake and reaction of formic acid, HCOOH, the most abundant carboxylic acid present in the atmosphere, on oxide and clays of the most abundant elements, Si and Al, present in the Earth's crust are investigated under dry and humid conditions. In particular, quantitative adsorption measurements using a Quartz Crystal Microbalance (QCM) coupled with spectroscopic studies using Attenuated Total Reflection Fourier Transform Infrared (ATR-FTIR) spectroscopy are combined to allow for both quantification of the amount of uptake and identification of distinct adsorbed species formed on silica, alumina, and kaolinite particle surfaces at 298 K. These oxides and clay particles show significant differences in the extent and speciation of adsorbed HCOOH due to inherent differences in surface -OH group reactivity. Adsorbed water, controlled by relative humidity, can increase the irreversible uptake of formic acid. Interestingly, the resulting layer of adsorbed formate on the particle surface decreases the particle hydrophilicity thereby decreasing the amount of water taken up by the surface as measured by QCM. Atmospheric implications of this study are discussed.