Self-Catalytic Reaction of SO3 and NH3 To Produce Sulfamic Acid and Its Implication to Atmospheric Particle Formation

The role of sulfur cycle in new particle formation: Cycloaddition reaction of SO3 to H2S

The chemistry of sulfur cycle contributes significantly to the atmospheric nucleation process, which is the first step of new particle formation (NPF). In the present study, cycloaddition reaction mechanism of sulfur trioxide (SO3) to hydrogen sulfide (H2S) which is a typical air pollutant and toxic gas detrimental to the environment were comprehensively investigate through theoretical calculations and Atmospheric Cluster Dynamic Code simulations. Gas-phase stability and nucleation potential of the product thiosulfuric acid (H2S2O3, TSA) were further analyzed to evaluate its atmospheric impact. Without any catalysts, the H2S + SO3 reaction is infeasible with a barrier of 24.2 kcal/mol. Atmospheric nucleation precursors formic acid (FA), sulfuric acid (SA), and water (H2O) could effectively lower the reaction barriers as catalysts, even to a barrierless reaction with the efficiency of cis-SA > trans-FA > trans-SA > H2O. Subsequently, the gas-phase stability of TSA was investigated. A hydrolysis reaction barrier of up to 61.4 kcal/mol alone with an endothermic isomerization reaction barrier of 5.1 kcal/mol under the catalytic effect of SA demonstrates the sufficient stability of TSA. Furthermore, topological and kinetic analysis were conducted to determine the nucleation potential of TSA. Atmospheric clusters formed by TSA and atmospheric nucleation precursors (SA, ammonia NH3, and dimethylamine DMA) were thermodynamically stable. Moreover, the gradually decreasing evaporation coefficients for TSA-base clusters, particularly for TSA-DMA, suggests that TSA may participate in NPF where the concentration of base molecules are relatively higher. The present new reaction mechanism may contributes to a better understanding of atmospheric sulfur cycle and NPF.

The reaction mechanism of SO3 with the multifunctional compound ethanolamine and its atmospheric implications

SO3 is an important reactive species in sulfur cycle and sulfuric acid formation processes and its reactions with some functional group substances, such as H2O, NH3, CH3OH, and organic and inorganic acids, have been extensively studied. However, its loss mechanism with multifunctional species is still lacking in detail. Herein, the reaction mechanism between SO3 and monoethanolamide (MEA) was investigated in the gas phase and on water droplets. The quantum chemical calculations indicate that the gas-phase reactions of SO3 with the OH and NH2 moieties of MEA hardly occur as their reaction energy barriers are up to 21.9-29.4 kcal mol-1. When a single water molecule is added into the SO3 + MEA reaction, it not only decreases the reaction barrier by at least 15.0 kcal mol-1 and thus enhances the rate obviously, but can also lead to the main product changing from HOCH2CH2NHSO3H to NH2CH2CH2OSO3H. The Born Oppenheimer molecular dynamics simulations on a water droplet show that the routes of the NH2CH2CH2OSO3-⋯H3O+ ion pair, HSO4- and HOCH2CH2NH3+ ions and zwitterionic formations of HOCH2CH2NH2+-SO3- and SO3--OCH2CH2NH3+ occur through a loop-structure route or chain reaction process, and can be finished within several picoseconds. Interestingly, the nucleation simulations show that the products of HOCH2CH2NHSO3H and NH2CH2CH2OSO3H have a potential ability to participate in the formation of new particles as they can form larger clusters with H2SO4, NH3 and H2O molecules within 20 ns. Thus, the present study will not only give new insight into the reaction between SO3 and multifunctional compounds, but also provide a new potential formation mechanism for particles resulting from the loss of SO3.

Direct Measurements of Covalently Bonded Sulfuric Anhydrides from Gas-Phase Reactions of SO3 with Acids under Ambient Conditions

Sulfur trioxide (SO3) is an important oxide of sulfur and a key intermediate in the formation of sulfuric acid (H2SO4, SA) in the Earth's atmosphere. This conversion to SA occurs rapidly due to the reaction of SO3 with a water dimer. However, gas-phase SO3 has been measured directly at concentrations that are comparable to that of SA under polluted mega-city conditions, indicating gaps in our current understanding of the sources and fates of SO3. Its reaction with atmospheric acids could be one such fate that can have significant implications for atmospheric chemistry. In the present investigation, laboratory experiments were conducted in a flow reactor to generate a range of previously uncharacterized condensable sulfur-containing reaction products by reacting SO3 with a set of atmospherically relevant inorganic and organic acids at room temperature and atmospheric pressure. Specifically, key inorganic acids known to be responsible for most ambient new particle formation events, iodic acid (HIO3, IA) and SA, are observed to react promptly with SO3 to form iodic sulfuric anhydride (IO3SO3H, ISA) and disulfuric acid (H2S2O7, DSA). Carboxylic sulfuric anhydrides (CSAs) were observed to form by the reaction of SO3 with C2 and C3 monocarboxylic (acetic and propanoic acid) and dicarboxylic (oxalic and malonic acid)-carboxylic acids. The formed products were detected by a nitrate-ion-based chemical ionization atmospheric pressure interface time-of-flight mass spectrometer (NO3--CI-APi-TOF; NO3--CIMS). Quantum chemical methods were used to compute the relevant SO3 reaction rate coefficients, probe the reaction mechanisms, and model the ionization chemistry inherent in the detection of the products by NO3--CIMS. Additionally, we use NO3--CIMS ambient data to report that significant concentrations of SO3 and its acid anhydride reaction products are present under polluted, marine and polar, and volcanic plume conditions. Considering that these regions are rich in the acid precursors studied here, the reported reactions need to be accounted for in the modeling of atmospheric new particle formation.

Engineering surface framework TiO6 single sites for unprecedented deep oxidative desulfurization

Catalytic oxidative desulfurization (ODS) using titanium silicate catalysts has emerged as an efficient technique for the complete removal of organosulfur compounds from automotive fuels. However, the precise control of highly accessible and stable-framework Ti active sites remains highly challenging. Here we reveal for the first time by using density functional theory calculations that framework hexa-coordinated Ti (TiO6) species of mesoporous titanium silicates are the most active sites for ODS and lead to a lower-energy pathway of ODS. A novel method to achieve highly accessible and homogeneously distributed framework TiO6 active single sites at the mesoporous surface has been developed. Such surface framework TiO6 species exhibit an exceptional ODS performance. A removal of 920 ppm of benzothiophene is achieved at 60°C in 60 min, which is 1.67 times that of the best catalyst reported so far. For bulky molecules such as 4,6-dimethyldibenzothiophene (DMDBT), it takes only 3 min to remove 500 ppm of DMDBT at 60°C with our catalyst, which is five times faster than that with the current best catalyst. Such a catalyst can be easily upscaled and could be used for concrete industrial application in the ODS of bulky organosulfur compounds with minimized energy consumption and high reaction efficiency.

Shapeshifting Nucleophiles HO-(NH3)n React with Methyl Chloride

The microsolvated anions HO-(NH3)n were found to induce new nucleophile NH2-(H2O)(NH3)n-1 via intramolecular proton transfer. Hence, the ion-molecule nucleophilic substitution (SN2) reaction between CH3Cl and these shapeshifting nucleophiles lead to both the HO- path and NH2- path, meaning that the respective attacking nucleophile is HO- or NH2-. The CCSD(T) level of calculation was performed to characterize the potential energy surfaces. Calculations indicate that the HO- species are lower in energy than the NH2- species, and the SN2 reaction barriers are lower for the HO- path than the NH2--path. Incremental solvation increases the barrier for both paths. Comparison between HO-(NH3)n and HOO-(NH3)n confirmed the existence of an α-effect under microsolvated conditions. Comparison between HO-(NH3)n and HO-(H2O)n indicated that the more polarized H2O stabilizes the nucleophiles more than NH3, and thus, the hydrated systems have higher SN2 reaction barriers. The aforementioned barrier changes can be explained by the differential stabilization of the nucleophile and HOMO levels upon solvation, thus affecting the HOMO-LUMO interaction between the nucleophile and substrate. For the same kind of nucleophilic attacking atom, O or N, the reaction barrier has a good linear correlation with the HOMO level of the nucleophiles. Hence, the HOMO level or the binding energy of microsolvated nucleophiles is a good indicator to evaluate the order of barrier heights. This work expands our understanding of the microsolvation effect on prototype SN2 reactions beyond the water solvent.

Mechanistic insight into the competition between interfacial and bulk reactions in microdroplets through N2O5 ammonolysis and hydrolysis

Reactive uptake of dinitrogen pentaoxide (N2O5) into aqueous aerosols is a major loss channel for NOx in the troposphere; however, a quantitative understanding of the uptake mechanism is lacking. Herein, a computational chemistry strategy is developed employing high-level quantum chemical methods; the method offers detailed molecular insight into the hydrolysis and ammonolysis mechanisms of N2O5 in microdroplets. Specifically, our calculations estimate the bulk and interfacial hydrolysis rates to be (2.3 ± 1.6) × 10-3 and (6.3 ± 4.2) × 10-7 ns-1, respectively, and ammonolysis competes with hydrolysis at NH3 concentrations above 1.9 × 10-4 mol L-1. The slow interfacial hydrolysis rate suggests that interfacial processes have negligible effect on the hydrolysis of N2O5 in liquid water. In contrast, N2O5 ammonolysis in liquid water is dominated by interfacial processes due to the high interfacial ammonolysis rate. Our findings and strategy are applicable to high-chemical complexity microdroplets.

Observation of the Water-HNSO2 Complex

Sulfamic acid (NH2SO3H, SFA) is supposed to play an important role in aerosol new particle formation (NPF) in the atmosphere, and its formation mainly arises from the SO3-NH3 reaction system in which weakly bonded donor-acceptor complexes such as SO3···NH3 and isomeric HNSO2···H2O have been proposed as the key intermediates. In this study, we reveal the first spectroscopic observation of HNSO2···H2O in two forms in a solid Ar matrix at 10 K. The major form consists of two intermolecular H bonds by forming a six-membered ring structure with a calculated dissociation energy of 7.6 kcal mol-1 at the CCSD(T)-F12a/aug-cc-pVTZ level of theory. The less stable form resembles SO3···H2O in containing a pure chalcogen bond (S···O) with a dissociation energy of 7.2 kcal mol-1. The characterization of HNSO2···H2O with matrix-isolation IR spectroscopy is supported by D- and 18O-isotope labeling and quantum chemical calculations.

Field observations of C2 and C3 organosulfates and insights into their formation mechanisms at a suburban site in Hong Kong

Organosulfates (OSs) are formed from volatile organic compounds (VOCs) and their oxidation products in the presence of sulfate particles. While OSs represent an important component in secondary organic aerosol, the knowledge of their formation driving force, mechanisms, and environmental impact remain inadequately understood. In this study, we report ambient observations of C2-3 oxygenated VOCs derived OSs (C2-3 OSs) at a suburban location of Hong Kong during autumn 2016. The C2-3 OSs, including glycolaldehyde sulfate (GS), hydroxyacetone sulfate (HAS), glycolic acid sulfate (GAS), and lactic acid sulfate (LAS), were quantified/semi-quantified using offline liquid chromatography-mass spectrometry analysis of aerosol filter samples. The average sum concentration of C2-3 OSs was 36 ng/m3. Correlation analysis revealed that sulfate, surface area, and liquid water content were important factors influencing C2-3 OS formation. Online measurement with an iodide High-Resolution Time-of-Flight Chemical-Ionization Mass Spectrometer (HR-ToF-CIMS) coupled with the Filter Inlet for Gases and AEROsols (FIGAERO) was also conducted to monitor C2-3 OSs, and their potential oxygenated VOC precursors in both gas- and particle-phase, and aerosol acidity tracer simultaneously. Our measurements support that glycolaldehyde/glyoxal, hydroxyacetone, glycolic acid/glyoxal, and lactic acid/methylglyoxal are likely precursors for GS, HAS, GAS, and LAS, respectively. Additionally, we found strong correlation between C2-3 OSs and H3S2O8-, a marker for aerosol acidity, providing field observational evidence for acid-catalyzed formation of small OSs. Based on both online and offline measurements, acid-catalyzed formation mechanisms in particle/aqueous phase are proposed. Specifically, the unique structure of adjacent carbonyl and hydroxyl groups in the C2-3 oxygenated VOC precursors can facilitate the formation of (1) a five-member ring intermediate via intramolecular hydrogen bond to react with sulfur trioxide through heterogenous reaction or (2) cyclic sulfate intermediate via particle-phase reaction with sulfuric acid to generate C2-3 OSs. These proposed mechanisms provide an alternative pathway for the liquid-phase production of C2-3 OSs.

Significant influence of water molecules on the SO3 + HCl reaction in the gas phase and at the air-water interface

The products resulting from the reactions between atmospheric acids and SO3 have a catalytic effect on the formation of new particles in aerosols. However, the SO3 + HCl reaction in the gas-phase and at the air-water interface has not been considered. Herein, this reaction was explored exhaustively by using high-level quantum chemical calculations and Born Oppenheimer molecular dynamics (BOMD) simulations. The quantum calculations show that the gas-phase reaction of SO3 + HCl is highly unlikely to occur under atmospheric conditions with a high energy barrier of 22.6 kcal mol-1. H2O and (H2O)2 play obvious catalytic roles in reducing the energy barrier of the SO3 + HCl reaction by over 18.2 kcal mol-1. The atmospheric lifetimes of SO3 show that the (H2O)2-assisted reaction dominates over the H2O-assisted reaction within the altitude range of 0-5 km, whereas the H2O-assisted reaction is more favorable within an altitude range of 10-50 km. BOMD simulations show that H2O-induced formation of the ClSO3-⋯H3O+ ion pair and HCl-assisted formation of the HSO4-⋯H3O+ ion pair were identified at the air-water interface. These routes followed a stepwise reaction mechanism and proceeded at a picosecond time scale. Interestingly, the formed ClSO3H in the gas phase has a tendency to aggregate with sulfuric acids, ammonias, and water molecules to form stable clusters within 40 ns simulation time, while the interfacial ClSO3- and H3O+ can attract H2SO4, NH3, and HNO3 for particle formation from the gas phase to the water surface. Thus, this work will not only help in understanding the SO3 + HCl reaction driven by water molecules in the gas-phase and at the air-water interface, but it will also provide some potential routes of aerosol formation from the reaction between SO3 and inorganic acids.

Atmospheric Chemistry of NH2SO3H in Polluted Areas: An Unexpected Isomerization of NH2SO3H in Acid-Polluted Regions

NH2SO3H is an effective nucleation agent for the formation of atmospheric aerosols and cloud particles. So, the ammonolysis of SO3 to form NH2SO3H without and with neutral (H2O) and basic (NH3) trace gases has been extensively investigated. However, the acidic trace gas X (X = H2SO4 and CH3SO3H)-assisted ammonolysis of SO3 is still up for debate. In this work, a comprehensive theoretical investigation of X-assisted ammonolysis of SO3 and its reverse reaction (the isomerization of NH2SO3H to form SO3-···NH3+) was carried out in the gas phase and at the air-water interface. The gas-phase results show that X-assisted isomerization of NH2SO3H to form SO3-···NH3+ is more energetically and kinetically favorable than its reverse reaction and the isomerization of NH2SO3H in the presence of H2O and NH3. Such unexpected findings revealed that gas-phase NH2SO3H is highly reactive in the presence of acidic trace gas in contrast to the high stability of NH2SO3H in neutral and basic conditions. At the air-water interface, the X-assisted isomerization reaction of NH2SO3H involves multiple water molecules. The loop structure of the reaction center (X···NH2SO3H···3H2O) promotes the transfer of protons in the water molecules to form the SO3-···NH3+ ion pair, which can then interact with several interfacial water molecules to form ammonium bisulfate. These interfacial reaction channels follow a stepwise mechanism and proceed at the picosecond time-scale. The findings of this study will contribute to a better understanding of the atmospheric behavior of NH2SO3H in polluted acidic trace gases.

Surface Confinement of Finite-Size Water Droplets for SO3 Hydrolysis Reaction Revealed by Molecular Dynamics Simulations Based on a Machine Learning Force Field

As an important source for sulfuric acid in the atmosphere, hydrolysis of sulfur trioxide (SO₃) takes place with water clusters of sizes from several molecules to several nanometers, resulting in various final products, including neutral (H₂SO₄)-(H₂O) clusters and ionic (HSO₄)^--(H₃O)⁺ clusters. The diverse products may be due to the ability of proton transfer and the formation of hydrated ions for water cluster of finite sizes, especially the sub-micrometer ones. However, the detailed molecular-level mechanism is still unclear due to the lack of available characterization and simulations tools. Here, we developed a quantum chemistry-level machine learning (ML) model to simulate the hydrolysis of SO₃ with water clusters of sizes up to nanometers. The simulation results demonstrate diverse reaction paths taking place between SO₃ and water clusters of different sizes. Generally, neutral (H₂SO₄)-(H₂O) clusters are preferred by water clusters of ultra-small size, and a loop structure-mediated mechanism with SO₃(H₂O)_n≤4 structures and a non-loop structure-mediated mechanism with structure relaxation are observed. As the water cluster size increases to (H₂O)₈, a (HSO₄)^--(H₃O)⁺ ion-pair product emerges; and the Eigen-Zundel ion conversion-like proton transfer mechanism takes place and stabilizes the ion pairs. As the water cluster sizes further increase beyond several nanometers ((H₂O)_n≥32), the (SO₄)^2-[(H₃O)⁺]₂ ion-pair product appears. The reason could be that the surface of these water clusters is large enough to screen Coulomb repulsion between two tri-coordinated ion-pair complexes. These findings would provide new perspectives for understanding SO₃ hydrolysis in the real atmosphere and sulfuric acid chemistry in atmospheric aerosols.

Multiple evaluations of atmospheric behavior between Criegee intermediates and HCHO: Gas-phase and air-water interface reaction

Given the high abundance of water in the atmosphere, the reaction of Criegee intermediates (CIs) with (H₂O)₂ is considered to be the predominant removal pathway for CIs. However, recent experimental findings reported that the reactions of CIs with organic acids and carbonyls are faster than expected. At the same time, the interface behavior between CIs and carbonyls has not been reported so far. Here, the gas-phase and air-water interface behavior between Criegee intermediates and HCHO were explored by adopting high-level quantum chemical calculations and Born-Oppenheimer molecular dynamics (BOMD) simulations. Quantum chemical calculations evidence that the gas-phase reactions of CIs + HCHO are submerged energy or low energy barriers processes. The rate ratios speculate that the HCHO could be not only a significant tropospheric scavenger of CIs, but also an inhibitor in the oxidizing ability of CIs on SO_x in dry and highly polluted areas with abundant HCHO concentration. The reactions of CH₂OO with HCHO at the droplet's surface follow a loop structure mechanism to produce i) SOZ (), ii) BHMP (HOCH₂OOCH₂OH), and iii) HMHP (HOCH₂OOH). Considering the harsh reaction conditions between CIs and HCHO at the interface (i.e., the two molecules must be sufficiently close to each other), the hydration of CIs is still their main atmospheric loss pathway. These results could help us get a better interpretation of the underlying CIs-aldehydes chemical processes in the global polluted urban atmospheres.

Nucleophilic substitution reactions of microsolvated hydroperoxide anion HOO-(NH3)n with methyl chloride and comparison between ammonia and water as the solvent

Similar to microhydrated hydroperoxide anion HOO^-(H₂O)_n, the HOO^-(NH₃)_n=1-3 anion can induce alternative nucleophiles by proton transfer (PT) from the solvent molecule NH₃. The PT-induced species NH₂^-(H₂O₂)(NH₃)_n-1 is higher in energy than HOO^-(NH₃)_n, obeying the proton affinity (PA) prediction that HOO^- has a higher PA than NH₂^-. The potential energy profile of HOO^-(NH₃)_n reacting with CH₃Cl shows that the transition states of the traditional HOO^--S_N2 pathway are ∼10 kcal mol^-1 lower in energy than those of the PT-induced NH₂^--S_N2 pathway, indicating the latter path is unlikely to compete. The differential solvation energy for reactants and transition states with incremental solvation increases the barrier height of both HOO^--/NH₂^--S_N2 pathways and makes the transition structures more product-like. For HOO^-(sol)_n + CH₃Cl → CH₃OOH + Cl^-(sol)_n reactions, the barrier heights for sol = H₂O are higher than those for sol = NH₃, because H₂O is more polar than NH₃, and the electrostatic interaction is strengthened, hence H₂O molecules stabilize the microsolvated nucleophiles more. In addition, because the H₂O molecule is a better proton donor than the NH₃ molecule, the PT-induced HO^-S_N2 pathway is more likely to compete with the HOO^-S_N2 pathway. The HOMO level of nucleophiles, which negatively correlates with the S_N2 barrier heights, is found to be a good descriptor to predict the S_N2 barrier height of a microsolvated system with the same attacking nucleophile. This work adds to our understanding of the differential solvent effect on the prototype ion-molecule S_N2 reactions.

Spontaneous emergence of enantioenriched chiral aldol reaction products from Achiral precursors in solution and origin of biological homochirality of sugars: a first-principles study

Experimental reports about observation of spontaneous mirror symmetry breaking and chiral amplification in stereoselective Mannich and aldol reactions, run under fully achiral initial conditions, have drawn a lot of attention, fuelled partly by the role these reactions could have played in chemical evolution as a cause for still puzzling observed homochirality of biomolecules, often considered a prerequisite for the origin of life. We have now revisited this still unresolved problem, using DFT computation of all combinatorially possible transition states and numerical solution of complete set of resulting coupled kinetic rate equations to model the aldol reaction rigorously "from the first principles" and without making any a priori assumptions. Spontaneous mirror symmetry breaking in this autocatalytic, reversible, closed and homogenous system is explained by a supercritical pitchfork bifurcation, occurring in concentrations of enantiomers due to time-delayed kinetic instability of racemic composition of reaction mixture, when reactants are initially provided in non-stoichiometric quantities. Same process, taking place under similar conditions in primordial "soup" of chemicals, might conceivably explain origin of biological homochirality of sugar molecules on early earth billions of years ago. Our results suggest that seemingly innocuous chemical reactions could exhibit unexpected and counter-intuitive emergent behaviour, when initial conditions are appropriately chosen. Chiral amplification in self-catalyzed aldol reaction occurs during approach of thermodynamic equilibrium in accord with principle of microscopic reversibility and second law of thermodynamics.

Formation mechanism of typical aromatic sulfuric anhydrides and their potential role in atmospheric nucleation process

Sulfuric anhydrides, generated from the cycloaddition reaction of SO₃ with carboxylic acids, have been revealed to be potential participants in the nucleation process of new particle formation (NPF). Hence the reaction mechanisms of typical aromatic acids (benzoic acid (BA), phenylacetic acid (PAA), phthalic acid (PA), isophthalic acid (mPA), and terephthalic acid (PTA)) with SO₃ to generate the corresponding aromatic sulfuric anhydrides were investigated by density functional theory calculations at the level of M06-2X/6-311++G(3df,3pd). As a result, these reactions were found to be feasible in the gas phase with barriers of 0.34, 0.30, 0.18, 0.08 and 0.12 kcal/mol to generate corresponding aromatic sulfuric anhydrides, respectively. The thermodynamic stabilities of clusters containing aromatic sulfuric anhydrides and atmospheric nucleation precursors (sulfuric acid, ammonia and dimethylamine) were further analyzed to identify the potential role of aromatic sulfuric anhydrides in NPF. As the thermodynamic stability of a cluster depends on both the number and strength of hydrogen bonds, the greater stability of the interactions between atmospheric nucleation precursors and aromatic sulfuric anhydrides than with aromatic acids make aromatic sulfuric anhydrides potential participators in the nucleation process of NPF. Moreover, compared with BA, the addition of a -CH₂- functional group in PAA has little influence on the reaction barrier with SO₃ but an inhibitive effect on the thermodynamic stability of clusters. The position of the two -COOH functional groups in PA, mPA and PTA does not have a consistent impact on the reaction barrier with SO₃ or the thermodynamic stability.

Hydrogen bond networks of dimethylsulfoxide (DMSO) pentamer

Understanding of clusters of dimethylsulfoxide (DMSO) is important in several applications in Chemistry. Despite its importance, very few studies of DMSO clusters, (DMSO)_n, have been reported in comparison to systems such as water clusters or methanol clusters. In order to provide further understanding of DMSO clusters, we investigated the structures and non-covalent interactions of the (DMSO)_n, n=5. Therefore, the potential energy surface (PES) of the DMSO pentamer has been examined using classical molecular dynamics. The structures generated using classical molecular dynamics are further optimized at the PW6B95D3/aug-cc-pVDZ level of theory. To comprehend the non-covalent bondings in the DMSO pentamer, we carried out a quantum theory of atoms in molecule (QTAIM) analysis. In addition, the effects of temperature on the structural stability is investigated between 20 and 500K. It comes out that seven different kind of non-covalent bondings can be found in DMSO pentamers.

Computational chemistry of cluster: Understanding the mechanism of atmospheric new particle formation at the molecular level

New particle formation (NPF), which exerts significant influence over human health and global climate, has been a hot topic and rapidly expands field of research in the environmental and atmospheric chemistry recent years. Generally, NPF contains two processes: formation of critical nucleus and further growth of the nucleus. However, due to the complexity of the atmospheric nucleation, which is a multicomponent process, formation of critical clusters as well as their growth is still connected to large uncertainties. Detection limits of instruments in measuring specific gaseous aerosol precursors and chemical compositions at the molecular level call for computational studies. Computational chemistry could effectively compensate the deficiency of laboratory experiments as well as observations and predict the nucleation mechanisms. We review the present theoretical literatures that discuss nucleation mechanism of atmospheric clusters. Focus of this review is on different nucleation systems involving sulfur-containing species, nitrogen-containing species and iodine-containing species. We hope this review will provide a deep insight for the molecular interaction of nucleation precursors and reveal nucleation mechanism at the molecular level.

Towards fully ab initio simulation of atmospheric aerosol nucleation

Atmospheric aerosol nucleation contributes to approximately half of the worldwide cloud condensation nuclei. Despite the importance of climate, detailed nucleation mechanisms are still poorly understood. Understanding aerosol nucleation dynamics is hindered by the nonreactivity of force fields (FFs) and high computational costs due to the rare event nature of aerosol nucleation. Developing reactive FFs for nucleation systems is even more challenging than developing covalently bonded materials because of the wide size range and high dimensional characteristics of noncovalent hydrogen bonding bridging clusters. Here, we propose a general workflow that is also applicable to other systems to train an accurate reactive FF based on a deep neural network (DNN) and further bridge DNN-FF-based molecular dynamics (MD) with a cluster kinetics model based on Poisson distributions of reactive events to overcome the high computational costs of direct MD. We found that previously reported acid-base formation rates tend to be significantly underestimated, especially in polluted environments, emphasizing that acid-base nucleation observed in multiple environments should be revisited.

Discovery of a Missing Link: First Observation of the HONO-Water Complex

The still unexplained daytime HONO concentration in the Earth's atmosphere and the impact of water on the HONO chemistry have been a mystery for decades. Several pathways and many modeling methods have failed to reproduce the atmospheric measurements. We reveal in this study the first spectroscopic observation and characterization of the complex of HONO with water observed through its rotational signature. Under the experimental conditions, HONO-water is stable, particularly straightforward to form, and features intense absorption signals. This could explain both the influence of water on the HONO chemistry and the missing HONO sources, as well as the missing contribution of many other molecules of atmospheric relevance that skew the accuracy of field measurements and the full account of partitioning species in the atmosphere.

OH Group Orientation Leads to Organosulfate Formation at the Liquid Aerosol Surface

Organosulfates (OSs) are well-known and ubiquitous constituents of atmospheric aerosol particles and have been used as secondary organic aerosol markers in many field studies. Hence, it is imperative to understand the formation of OS species in the atmosphere. Recently, hydroxy acids (HAs) and hydroxy acid sulfates have been extensively detected in the atmospheric environment. However, the reaction mechanism of HAs to form OSs is much less understood. In this work, we have mainly investigated the reaction of typical α-HAs, including glycolic acid (GA) and lactic acid (LA), and SO₃ at the liquid aerosol surface using quantum chemistry calculations and Born-Oppenheimer molecular dynamics simulations. The OH group orientation of α-HAs at the air-water interface is found to exert a significant impact on the formation of OSs. The OH group pointing to the gas phase is obviously beneficial to the formation of OSs. Two key factors are discovered important to the reaction of α-HAs adsorbed on the liquid surface with SO₃: (a) the exposure position of the active site to the gas phase and (b) the reactivity of the exposed site to the attracted SO₃ molecule. Moreover, we found that the air-water interface exerts a significant influence on the physicochemical behaviors of GA and LA, especially on their OH group orientation, and thus leads to their different properties for the SO₃ colliding reaction. The presented reaction mechanism provides a new feasible pathway for the production of OSs at the liquid aerosol surface, which may have important impacts on the formation of organic aerosols.

Conformational energies of microsolvated Na+ clusters with protic and aprotic solvents from GFNn-xTB methods

Performance of contemporary tight-binding semiempirical GFNn-xTB methods for the conformational energies of singly charged sodium clusters Na⁺ (S)_n (n = 4-8) with 3 protic and 8 aprotic solvents is examined against the reference RI-MP2/CBS method. The median Pearson correlation coefficients of ρ = 0.84 (GFN2-xTB) and ρ = 0.82 (GFN1-xTB) do not give the clear preference to any tested approach. GFN1-xTB method demonstrates more stable performance than its GFN2-xTB successor with the average mean absolute errors (MAEs)/mean signed errors (MSEs) of 1.2/0.2 and 2.3/1.6 kcal mol^-1 , respectively. Conformational energies produced by the computationally efficient DFT functional PBE and double-ζ basis set complemented with -D3(BJ) dispersion correction are suitable for the preliminary sampling (median ρ = 0.93), but should be used with a caution for the calculations of the average ensemble properties (MAE/MSE = 1.7/1.1 kcal mol^-1 ). Higher-ranking PBE0-D3(BJ) and ωB97M-V with triple-ζ basis sets yield significantly lower MAEs/MSEs of 0.55/0.20 and 0.51/0.23 kcal mol^-1 , respectively.

Probing autoxidation of oleic acid at air-water interface: A neglected and significant pathway for secondary organic aerosols formation

Fatty acids have been proposed to be a potential source of precursors for SOAs, but the autoxidation process was neglected in the oxidation studies. Here, the autoxidation of oleic acid was explored using microdroplet mass spectrometry. Bulk solution, concentration and solvent composition experiments provided direct evidences for that the autoxidation occurred at or near the air-water interface. The kinetic data showed an acceleration at this interface and was comparable to ozonation, indicating that autoxidation is an important pathway for SOAs formation. In addition, intermediates/products were captured and identified using tandem mass spectrometry, spin-trapping and quenched agents. The autoxidation mechanism was divided into addition intermediates (AIs) and Criegee intermediates (CIs) pathways mediated by hydroxyl radicals (OH). The CI chemistry which is ubiquitous in gas phase was observed at the air-water interface, and this leaded to the mass/volume loss of aerosols. Inversely, the AI chemistry caused the increase of mass, density and hygroscopicity of aerosols. AI chemistry was dominated compared to CI chemistry, but varied by concerning aerosol sizes, ultraviolet light (UV) and charge. Moreover, the MS approach of selectively probing the interfacial substances at the scale of sub-seconds opens new opportunities to study heterogeneous chemistry in atmosphere.

The reaction of Criegee intermediates with formamide and its implication to atmospheric aerosols

The reactions of Criegee intermediates (CIs) play an important role in the formation of secondary organic aerosols that have negative effect on visibility, human health, and global climate. New particle formation (NPF) can contribute to more than half of the aerosols in terms of their number concentration. Here, the reactions of CIs with formamide (FA) in the gas-phase and at the air/water interface were investigated using quantum chemistry calculation and Born-Oppenheimer molecular dynamic simulations. The results show that the reaction mechanism of CIs with FA is similar to that with formic acid, and the formation of hydroperoxymethyl formimidate (P4) is the most favorable pathway both in the gas-phase and at the air/water interface. Moreover, the potential contribution of the products to NPF was also evaluated by means of the molecular dynamic simulations. The results indicate that the product (P4) can participate in the SA-based NPF and water molecules are beneficial to enhance the NPF. The exploration will provide insight into the reaction of CIs with amide and the effect of the Criegee chemistry on the atmospheric aerosols.

Gas-phase catalytic hydration of I2O5 in the polluted coastal regions: Reaction mechanisms and atmospheric implications

Marine aerosols play an important role in the global aerosol system. In polluted coastal regions, ultra-fine particles have been recognized to be related to iodine-containing species and is more serious due to the impact of atmospheric pollutants. Many previous studies have identified iodine pentoxide (I₂O₅, IP) to be the key species in new particles formation (NPF) in marine regions, but the role of IP in the polluted coastal atmosphere is far to be fully understood. Considering the high humidity and concentrations of pollutants in the polluted coastal regions, the gas-phase hydration of IP catalyzed by sulfuric acid (SA), nitric acid (NA), dimethylamine (DMA), and ammonia (A) have been investigated at DLPNO-CCSD(T)//ωB97X-D/aug-cc-pVTZ + aug-cc-pVTZ-PP with ECP28MDF (for iodine) level of theory. The results show that the hydration of IP involves a significant energy barrier of 22.33 kcal/mol, while the pollutants SA, NA, DMA, and A all could catalyze the hydration of IP. Especially, with SA and DMA as catalysts, the hydration reactions of IP present extremely low barriers and high rate constants. It is suggested that IP is unstable under the catalysis of SA and DMA to generate iodic acid, which is the key component in NPF in marine regions. Thus, the catalytic hydration of IP is very likely to trigger the formation of iodine-containing particles. Our research provides a clear picture of the catalytic hydration of IP as well as theoretical guidance for NPF in the polluted coastal atmosphere.

Possible atmospheric source of NH2SO3H: the hydrolysis of HNSO2 in the presence of neutral, basic, and acidic catalysts

NH₂SO₃H can directly participate in H₂SO₄-(CH₃)₂NH-based cluster formation, and thereby substantially enhance the cluster formation rate. Herein, the reaction mechanisms and kinetics for the formation of NH₂SO₃H from the hydrolysis of HNSO₂ without and with neutral (H₂O, (H₂O)₂, and (H₂O)₃), basic (NH₃ and CH₃NH₂), and acidic (HCOOH, H₂SO₄, H₂SO₄⋯H₂O, and (H₂SO₄)₂) catalysts were studied theoretically at the CCSD(T)-F12/cc-pVDZ-F12//M06-2X/6-311+G(2df,2pd) level. The calculated results showed that neutral, basic, and acidic catalysts decrease the energy barrier by over 18.1 kcal mol^-1; meanwhile, the product formation of NH₂SO₃H was more strongly bonded to neutral, basic, and acidic catalysts than to the reactants HNSO₂ and H₂O. This reveals that the reported neutral, basic, and acidic catalysts promote the formation of NH₂SO₃H from the hydrolysis of HNSO₂ both kinetically and thermodynamically. Kinetic calculations using the master equation showed that (H₂O)₂ (100% RH) dominate over the other catalysts within the range of 0-10 km altitudes and 230-320 K with its rate ratio larger by at least 2.98 times, whereas HCOOH (3.2 × 10⁹ molecules cm^-3) is the most favorable catalysts at 15 km altitude in the troposphere. Overall, the present results will provide a definitive example that neutral, basic, and acidic catalysts have important influences on atmospheric reactions.

A theoretical study on the formation mechanism of carboxylic sulfuric anhydride and its potential role in new particle formation

New particle formation (NPF) is the major source of atmospheric aerosol particles. However, the chemical species involved and the exact mechanism are still unclear. Cycloaddition reaction of SO₃ to carboxylic acids bas been identified as a possible formation mechanism of carboxylic sulfuric anhydrides which may be involved in NPF. Herein, energy profiles for forming diaterpenylic acetate sulfuric anhydride (DTASA) through cycloaddition of SO₃ to diaterpenylic acid acetate (DTAA) and the potential role of DTASA in NPF were studied through computational methods combined with atmospheric cluster dynamics code (ACDC). Gas phase reaction barriers for the two carboxyl groups of DTAA are 0.4 and 0.6 kcal mol⁻¹, respectively, illustrating a feasible formation mechanism for DTASA. According to thermodynamical analysis and dynamical simulations, atmospheric clusters containing DTASA and atmospheric nucleation precursors sulfuric acid (SA), ammonia (NH₃) and dimethylamine (DMA) possess both thermodynamically and dynamically higher stabilities than those of DTAA-contained clusters. Furthermore, DTASA–NH₃ and DTASA–DMA are more stable than SA–NH₃ and SA–DMA, enabling DTASA, even carboxylic sulfuric anhydrides, to become potential participants in the atmospheric NPF process which may hence promote a better understanding of NPF.

Organic acids could improve their nucleation ability through the cycloaddition reaction of SO₃ to generate corresponding carboxylic sulfuric anhydrides which may play a potential role in the atmospheric new particle formation.

Aqueous-Microdroplet-Driven Abiotic Synthesis of Ribonucleotides

Phosphorylation for ribonucleotide formation is a critical step in the origin of life but has had limited success due to the thermodynamic and kinetic constraints in aqueous media. Here, we report that the production of ribonucleotides from ribonucleosides in the presence of monopotassium phosphate (KH₂PO₄) spontaneously proceeded in aqueous microdroplets under ambient conditions and without using a catalyst. A full set of ribonucleotides including adenosine monophosphate (AMP), guanosine monophosphate (GMP), uridine monophosphate (UMP), and cytidine monophosphate (CMP) were generated on the scale of a few milliseconds. The aqueous microdroplets could transfer the ribonucleotides to oligoribonucleotides and showed mutual compatibility for individual phosphorylation. Conditions established the dependence of the conversion ratio on the droplet size and suggested that the condensation reactions occurred at or near the microdroplets' surface. This aqueous microdroplet approach also provides a route for elucidating phosphorylation chemistry in the prebiotic era.

Theoretical study of the reaction mechanism between Criegee intermediates and hydroxyl radicals in the presence of ammonia and amine

Criegee intermediates (CIs), formed in the ozonolysis process of unsaturated hydrocarbons, play an important role in the formation of OH radicals, sulfuric acid, and aerosols. In this study, quantum chemical calculations were carried out to investigate the mechanism for the reaction of Criegee intermediates [involving CH₂OO, CH₃CHOO and (CH₃)₂COO] with OH radicals at the level of CCSD(T)/jun-cc-pVTZ//M06-2X/6-311 + G(2d,2p). A third component, such as water, ammonia, or amines, was introduced to the reaction of CIs with OH to evaluate their catalytic effect. The results show that the OH addition is the favorable channel among four channels involving cis-H abstraction, trans-H abstraction and O abstraction. The third component has a positively catalytic effect on the trans-H abstraction and O abstraction pathways. Moreover, for the trans-H abstraction of CH₃CHOO and (CH₃)₂COO with OH, ammonia and amine exhibit more effectively catalytic ability than water. Furthermore, Born-Oppenheimer molecular dynamic simulation results show that the addition of third component to CIs and hydrogen abstraction from the third component by OH occur simultaneously.

A pH dependent sulfate formation mechanism caused by hypochlorous acid in the marine atmosphere

Secondary sulfate plays a crucial role in forming marine aerosol, which in turn is an important source of natural aerosol at a global level. Recent experimental studies suggest that oxidation of S(IV) compounds, in practice dissolved sulfur dioxide, to sulfate (S(VI)) by hypochloric acid could be one of the most significant pathways for sulfate formation in marine areas. However, the exact mechanism responsible for this process remains unknown. Using high-level quantum chemical calculations, we studied the reaction between dissolved sulfur dioxide and hypochloric acid. We account for the dominant protonation states of reactants in the pH range 3.0-9.0. We also consider possible catalytic effects of species such as H₂O. Our results show that sulfate formation in HOCl+HOSO₂^- and HOCl+SO₃^2- reactions relevant to acidic and nearly neutral conditions can occur either through previously proposed Cl⁺ transfer or through a novel HO⁺ transfer mechanism. In alkaline conditions, where the dominant reactants are OCl^- and SO₃^2-, an O atom transfer mechanism proposed in previous experimental studies may be more important than Cl⁺ transfer. Catalysis by common cloud-water species is found to lower barriers of Cl⁺ transfer mechanisms substantially. Nevertheless, we find that the dominant S(IV) + HOCl reaction mechanism for the full studied pH range is HO⁺ transfer from HOCl to SO₃^2-, which leads directly to sulfate formation without ClSO₃^- intermediates. The rate-limiting barrier of this reaction is low, leading to an essentially diffusion-controlled reaction rate. S(IV) lifetimes due to this reaction decrease with increasing pH due to the increasing fractional population of SO₃^2-. Especially in neutral and alkaline conditions, depletion of HOCl by the reaction is so rapid that S(IV) oxidation will be controlled mainly by mass transfer of gas-phase HOCl to the liquid phase. The mechanism proposed here may help to explain marine sulfate sources missing from current atmospheric models.

Catalytic sulfate formation mechanism influenced by important constituents of cloud water via the reaction of SO2 oxidized by hypobromic acid in marine areas

Comprehensive investigations of the possible formation pathways of sulfate, the main composition of atmospheric aerosol in marine areas, continue to challenge atmospheric chemists. As one of the most important oxidation routes of S(iv) contributing to sulfate formation, the reaction process of S(iv) oxidized by hypobromic acid, which is ubiquitous with the gas-phase mixing ratios of ∼310 ppt and has a well-known oxidative capacity, has attracted wide attention. However, little information is available about the detailed reaction mechanism. Especially, due to the abundant species in cloud water, the potential effect of these compositions on these reaction processes and the corresponding effect mechanism are also uncertain. Using high-level quantum chemical calculations, we theoretically elucidate the two-step mechanism of Br+ transfer proposed by experiment through the verification of the key BrSO3- intermediate formation and subsequent hydrolysis reaction or the uncovered reaction of BrSO3- intermediate with OH-. Further, the novel and more competitive mechanisms (OH+ or O atom transfer pathways) that have not been considered in previous studies, leading to sulfate formation directly, have been found. Furthermore, it should be mentioned that we revealed the effect mechanism of constituents catalyzed in cloud water, especially the important H2O-catalyzed mechanism. In addition, all the above pathways follow this catalytic mechanism. This finding indicates a linkage between the complex nature of the atmospheric constituents and related atmospheric reaction, as well as the enhanced occurrence of atmospheric secondary sulfate formation in the atmosphere. Hence, this exploration of sulfate formation related to hypobromic acid could provide a better understanding about the sources of sulfate in marine areas.

Water Microdroplets Allow Spontaneously Abiotic Production of Peptides

The chemistry of abiotic synthesis of peptides in the context of their prebiotic origins is a continuing challenge that arises from thermodynamic and kinetic constraints in aqueous media. Here we reported a strategy of microdroplets' mass spectrometry for peptide bonds formed from pure amino acids or a mixture in the presence of phosphoric acids in aqueous microdroplets. In contrast to bulk experiments, the condensation reactions proceed spontaneously under ambient conditions. The microdroplet gave a negative free-energy change (ΔG ∼ -1.1 kcal/mol), and product yields of ∼75% were obtained at the scale of a few milliseconds. Experiments in which nebulization gas pressure and external charge were varied established dependence of peptide production on the droplet size that has a high surface-to-volume ratio. It is concluded that the condensation reactions occurred at or near the air-water interfaces of microdroplets. This aqueous microdroplets approach also provides a route for chemistry synthesis in the prebiotic era.

Catalytic effect of water and formic acid on the reaction of carbonyl sulfide with dimethyl amine under tropospheric conditions

CCSD(T)/aug-cc-pVTZ//M06-2X/aug-cc-pVTZ calculations were performed on the addition of amines [i.e. ammonia (NH₃), methyl amine (MA), and dimethyl amine (DMA)] to carbonyl sulfide (OCS), followed by transfer of the amine H-atom to either the S-atom or O-atom of OCS, assisted by a single water (H₂O) or a formic acid (FA) molecule, leading to the formation of the corresponding carbamothioic S- or O acids. For the OCS + NH₃ and OCS + MA reactions with or without the H₂O or FA, very high barriers were observed, making these reactions unfeasible. Interestingly, the barrier heights for the OCS + DMA reaction, involving H-atom transfer to either the S-atom or O-atom of OCS and assisted by a FA, were found to be -4.2 kcal mol^-1 and -3.9 kcal mol^-1, respectively, relative to those of the separated reactants. The barrier height values suggest that FA lowers the reaction barriers by ∼28.4 kcal mol^-1 and ∼35.9 kcal mol^-1 compared to the OCS + DMA reaction without the catalyst. Rate coefficient calculations were performed on the OCS + DMA reaction both without a catalyst, and assisted by a H₂O and a FA molecule using canonical variational transition state theory and small curvature tunneling at the temperatures between 200 and 300 K. The rate data show that the OCS + DMA + FA reaction proceeds through H-atom transfer to the S-atom of OCS, which was found to be ∼10³-10¹¹ and 10³-10¹⁰ times faster than the OCS + DMA and OCS + DMA + H₂O reactions, respectively, in the studied temperature range. For the same temperature range, the rate of the OCS + DMA + FA reaction was found to be ∼10⁸-10¹⁶ and 10³-10¹² times faster than the OCS + DMA and OCS + DMA + H₂O reactions in which H-atom transfer to the O-atom of OCS occurred. This suggests that the OCS + DMA reaction that is assisted by FA is more efficient than the H₂O assisted reaction. In addition, the rate of the OCS + DMA + FA reaction was found to be ∼10¹⁰ times slower than the OCS + ˙OH reaction at 298 K. This clarifies that the OCS + DMA + FA reaction may be feasible for the atmospheric removal of OCS under night-time forest fire conditions when the OCS and DMA concentrations are high and the ˙OH concentration is low.

Theoretical investigation of the relative impacts of water and ammonia on the tropospheric conversion of N2O5 to HNO3

Reaction of ammonia with N₂O₅, without and with the assistance of water, in the troposphere has been investigated by electronic structure and chemical kinetic calculations. The whole process has been compared against the hydrolysis reaction, uncatalyzed as well as water and ammonia catalyzed. A comparative study between hydrolysis and ammonolysis based on relative rates has been extensively carried out. The analysis reveals that ammonolysis has negligible practical atmospheric implication compared to hydrolysis. The former could have a significant contribution in tropospheric HNO₃ formation only at 0 km altitude under two conditions: either on a local scale, where ammonia concentration could reach around a thousand times its global average value, or under very low humidity and at a lower temperature. Relative rate studies also suggest that the catalytic effect of both ammonia and water is negligibly small in determining the atmospheric fate of N₂O₅via gas phase hydrolysis and ammonolysis.

Composition of Ultrafine Particles in Urban Beijing: Measurement Using a Thermal Desorption Chemical Ionization Mass Spectrometer

Ultrafine particles (UFPs) dominate the particle number population in the urban atmosphere and revealing their chemical composition is important. The thermal desorption chemical ionization mass spectrometer (TDCIMS) can semicontinuously measure UFP composition at the molecular level. We modified a TDCIMS and deployed it in urban Beijing. Radioactive materials in the TDCIMS for aerosol charging and chemical ionization were replaced by soft X-ray ionizers so that it can be operated in countries with tight regulations on radioactive materials. Protonated N-methyl-2-pyrrolidone ions were used as the positive reagent ion, which selectively detects ammonia and low-molecular weight-aliphatic amines and amides vaporized from the particle phase. With superoxide as the negative reagent ion, a wide range of inorganic and organic compounds were observed, including nitrate, sulfate, aliphatic acids with carbon numbers up to 18, and highly oxygenated CHO, CHON, and CHOS compounds. The latter two can be attributed to parent ions or the decomposition products of organonitrates and organosulfates/organosulfonates, respectively. Components from both primary emissions and secondary formation of UFPs were identified. Compared to the UFPs measured at forest and marine sites, those in urban Beijing contain more nitrogen-containing and sulfur-containing compounds. These observations illustrate unique features of the UFPs in the urban environment and provide insights into their origins.

Propionamide participating in H2SO4-based new particle formation: a theory study

Propionamide (PA), an important pollutant emitted into the atmosphere from a variety of sources, is abundant in many areas worldwide, and could be involved in new particle formation (NPF). In this study, the enhancement of the H₂SO₄ (SA)-based NPF by PA was evaluated through investigating the formation mechanism of (PA)_m(SA)_n (m = 0–3 and n = 0–3) clusters using computational chemistry and kinetics modeling. Our study proved that the formation of all the PA-containing clusters is thermodynamically favorable. Furthermore, the Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> O group in PA plays an important role in the clusters with more PA than SA, and the basicity of bases exerts a greater influence with an increasing amount of SA. We demonstrate that although the enhancing potential of PA is lower than that of the strongest enhancers of SA-based NPF such as methylamine (MA) and dimethylamine (DMA), PA can enhance the SA-based NPF at the parts per billion (ppb) level, which is typical for concentrations of C₃-amides in, for example, urban Shanghai (China). The monomer evaporation is the dominant degradation pathway for the (PA)_m(SA)_n clusters, which differs from that of the SA–DMA system. The formation rate of PA-containing clusters is comparable to the rate coefficients for PA oxidation by hydroxyl (OH) radicals, indicating that participating in the SA-based NPF is a crucial sink for PA.

Propionamide (PA), an important pollutant emitted into the atmosphere from a variety of sources, is abundant in many areas worldwide, and could be involved in new particle formation (NPF).

Unprecedented Ambient Sulfur Trioxide (SO3) Detection: Possible Formation Mechanism and Atmospheric Implications

Sulfur trioxide (SO₃) is a crucial compound for atmospheric sulfuric acid (H₂SO₄) formation, acid rain formation, and other atmospheric physicochemical processes. During the daytime, SO₃ is mainly produced from the photo-oxidation of SO₂ by OH radicals. However, the sources of SO₃ during the early morning and night, when OH radicals are scarce, are not fully understood. We report results from two field measurements in urban Beijing during winter and summer 2019, using a nitrate-CI-APi-LTOF (chemical ionization-atmospheric pressure interface-long-time-of-flight) mass spectrometer to detect atmospheric SO₃ and H₂SO₄. Our results show the level of SO₃ was higher during the winter than during the summer, with high SO₃ levels observed especially during the early morning (∼05:00 to ∼08:30) and night (∼18:00 to ∼05:00 the next day). On the basis of analysis of SO₂, NO _x , black carbon, traffic flow, and atmospheric ions, we suggest SO₃ could be formed from the catalytic oxidation of SO₂ on the surface of traffic-related black carbon. This previously unidentified SO₃ source results in significant H₂SO₄ formation in the early morning and thus promotes sub-2.5 nm particle formation. These findings will help in understanding urban SO₃ and formulating policies to mitigate secondary particle formation in Chinese megacities.

Reactions of Criegee Intermediates are Enhanced by Hydrogen-Atom Relay Through Molecular Design

We report a type of highly efficient double hydrogen atom transfer (DHAT) reaction. The reactivities of 3-aminopropanol and 2-aminoethanol towards Criegee intermediates (syn- and anti-CH₃ CHOO) were found to be much higher than those of n-propanol and propylamine. Quantum chemistry calculation has confirmed that the main mechanism of these very rapid reactions is DHAT, in which the nucleophilic attack of the NH₂ group is catalyzed by the OH group which acts as a bridge of HAT. Typical gas-phase DHAT reactions are termolecular reactions involving two hydrogen bonding molecules; these reactions are typically slow due to the substantial entropy reduction of bringing three molecules together. Putting the reactive and catalytic groups in one molecule circumvents the problem of entropy reduction and allows us to observe the DHAT reactions even at low reactant concentrations. This idea can be applied to improve theoretical predictions for atmospherically relevant DHAT reactions.

Speeding up Viedma Deracemization through Water-catalyzed and Reactant Self-catalyzed Racemization

Viedma deracemization is based on solution phase racemization, dissolution of racemic or scalemic conglomerates and crystal growth through autocatalytic cluster formation. With rate limiting racemization, its acceleration by appropriate catalysts may result in speeding up deracemization. A conglomerate-forming chiral compound may principally racemize directly, or via reverse of its formation reaction. For a hydrazine derivative, we investigated available racemization pathways in presence of pyrrolidine or thiourea amine as base catalysts: via Mannich or aza-Michael reaction steps and their reverse, or by enolization. Racemization by enolization was computationally found to dominate, both under water-free conditions and in presence of water, involving a multitude of different pathways. Faster racemization in presence of water resulted indeed in more rapid deracemization, when the base was pyrrolidine. Under water-free conditions, the role of water as enolization catalyst is assumed by chiral hydrazine itself - in autocatalytic racemization and in which both reactant and product are catalysts.

Formation Mechanisms of Iodine-Ammonia Clusters in Polluted Coastal Areas Unveiled by Thermodynamics and Kinetic Simulations

It has been revealed that iodine species play important roles in atmospheric new particle formations (NPFs) in pristine coastal areas. However, it is unclear whether other atmospheric species, such as NH₃, for which the levels in coastal areas of China are >2.5 × 10¹⁰ molecules·cm^-3 are involved in the NPFs of iodine species, although NH₃ has been proved to promote particle formation of H₂SO₄. Via high-level quantum chemical calculations and atmospheric cluster dynamic code simulations, this study unveiled new mechanisms of nucleation, in which NH₃ mediates the formation of iodine particles by assisting hydrolysis of I₂O₅ or reacting with HIO₃. The simulated formation rates of iodine-ammonia clusters via the new mechanisms are much higher than those simulated via sequential addition of HIO₃ with subsequent release of H₂O, under the condition that NH₃ concentrations are higher than 10¹⁰ molecules·cm^-3. The new mechanisms can well explain the observed cluster formation rates at a coastal site in Zhejiang of China. The findings not only expand the current understandings of the role of NH₃ in NPFs but also highlight the importance of monitoring and evaluating NPFs via the iodine-ammonia cluster pathway in the coastal areas of China and other regions worldwide.

Water Promotes the Oxidation of SO2 by O2 over Carbonaceous Aerosols

Severe haze episodes typically occur with concurrent high relative humidity. Here, the vital role of water in promoting the oxidation of SO₂ by O₂ on carbonaceous soot surfaces was identified at the atomic level by first-principles calculations. Water molecules can dissociate into surface hydroxyl groups through a self-catalyzed process under ambient conditions. The surface hydroxyl groups, acting as facilitators, can significantly accelerate the conversion of SO₂ to SO₃ (precursor of particulate sulfate) over soot aerosols by reducing the reaction barriers. Specifically, the hydroxyl groups activate the reactants and stabilize the transition states and products through hydrogen-bonding interactions, making the reactions both thermodynamically and kinetically more favorable at room temperature. The findings indicate that atmospheric humidity plays an important role in enhancing the atmospheric oxidation capacity, thus exacerbating SO₂ oxidation and severe haze development. Also, this study unravels a mechanism of surface hydroxyl-assisted O₂ and H₂O dissociation over metal-free carbocatalysts under normal conditions.

Under the radar - Exceptionally high environmental concentrations of the high production volume chemical sulfamic acid in the urban water cycle

Elevated concentrations of sulfamate, the anion of sulfamic acid, were found in surface waters and finished drinking water in Germany with concentrations up to 580 μg/L and 140 μg/L, respectively. Wastewater treatment plant (WWTP) effluent was identified as the dominant source of sulfamate in the urban water cycle, as sulfamate concentrations correlated positively (0.77 > r < 0.99) with concentrations of the wastewater tracer carbamazepine in samples from different waterbodies. Ozonation and activated sludge experiments proved that sulfamate can be formed from chemical and biological degradation of various precursors. Molar sulfamate yields were highly compound-specific and ranged from 2% to 56%. However, the transformation of precursors to sulfamate in WWTPs and wastewater-impacted waterbodies was found to be quantitatively irrelevant, since concentrations of sulfamate in these compartments are already high, presumably due to its primary use as an acidic cleaning agent. Sulfamate concentrations in the influent and effluent of studied WWTPs ranged from 520 μg/L to 1900 μg/L and from 490 μg/L to 1600 μg/L, respectively. Laboratory batch experiments were performed to assess the recalcitrance of sulfamate for chemical oxidation. In combination with the results from sampling conducted at full-scale waterworks, it was shown that common drinking water treatment techniques, including ozonation and filtration with activated carbon, are not capable to remove sulfamate. The results of biodegradation tests and from the analysis of samples taken at four bank filtration sites indicate that sulfamate is attenuated in the sediment/water interface of aquatic systems and during aquifer passage under aerobic and anaerobic conditions. Sulfamate concentrations decreased by between 62% and 99% during aquifer passage at the bank filtration sites. Considering the few data on short term ecotoxicity, about 30% of the presented sulfamate levels in ground and surface water samples did exceed the predicted no-effect concentration (PNEC) of sulfamate, and thus effects of sulfamate on the aquatic ecosystem of wastewater-impacted waterbodies in Germany cannot be excluded so far. Toxicological estimations suggest that no risk to human health is expected by concentrations of sulfamate typically encountered in tap water.

NWPEsSe: An Adaptive-Learning Global Optimization Algorithm for Nanosized Cluster Systems

Global optimization constitutes an important and fundamental problem in theoretical studies in many chemical fields, such as catalysis, materials, or separations problems. In this paper, a novel algorithm has been developed for the global optimization of large systems including neat and ligated clusters in the gas phase and supported clusters in periodic boundary conditions. The method is based on an updated artificial bee colony (ABC) algorithm method, that allows for adaptive-learning during the search process. The new algorithm is tested against four classes of systems of diverse chemical nature: gas phase Au₅₅, ligated Au₈²⁺, Au₈ supported on graphene oxide and defected rutile, and a large cluster assembly [Co₆Te₈(PEt₃)₆][C₆₀]_n, with sizes ranging between 1 and 3 nm and containing up to 1300 atoms. Reliable global minima (GMs) are obtained for all cases, either confirming published data or reporting new lower energy structures. The algorithm and interface to other codes in the form of an independent program, Northwest Potential Energy Search Engine (NWPEsSe), is freely available, and it provides a powerful and efficient approach for global optimization of nanosized cluster systems.

Evolution of Condensable Fine Particle Size Distribution in Simulated Flue Gas by External Regulation for Growth Enhancement

Condensation fine particles (CFPs) from coal-fired flue gas harm humans and the environment after being emitted into the atmosphere. Given their small size (<0.1 μm), difficulty arises in efficiently removing CFPs by wet electrostatic precipitators and mist eliminators. In this work, a laboratory apparatus was used to study the CFP growth under simulated power plant conditions. Four methods were independently investigated to increase the particle size: addition of ammonia, addition of fly ash, decreasing temperature, and applying an electrical discharge. Results demonstrated that the CFP size distribution possessed a unimodal structure with peak at 0.05 μm. At increased ammonia concentration from 10 to 30 ppm, the peak of growth factor shifted rightward and increased from 1.21 to 1.35 and the range of growth factor >1 was significantly broadened due to joint action of multiple mechanisms. Fly ash acted as the core, and CFPs adhered to the ash surface when forming ash-salt droplets. Cooling flue gas could also enhance the CFP growth due to vapor condensation. At decreased temperature from 45 to 30 °C, the median diameter of CFPs increased by 15%. Finally, the growth and agglomeration of CFPs can be further enhanced when an external electrical field was utilized. The size range of growth factor >1 can be broadened, and the peak growth factor significantly increased at 8 kV applied voltage. The research findings provide valuable guidance for effectively improving the CFP removal efficiency by external regulation for growth enhancement.

Influence of atmospheric conditions on sulfuric acid-dimethylamine-ammonia-based new particle formation

A recent quantitative measurement of rates of new particle formation (NPF) in urban Shanghai showed that the high rates of NPF can be largely attributed to the sulfuric acid (SA)-dimethylamine (DMA) nucleation due to relatively high DMA concentration in urban atmosphere (Yao et al., Science. 2018, 361, 278). In certain atmospheric conditions, the release of DMA is accompanied with the emission of high concentration of ammonia. As a result, the ammonia (A) may participate in SA-DMA-based NPF. However, the main sources of DMA and A can be different, thereby leading to different mechanism for the SA-DMA-A-based nucleation under different atmospheric conditions. Near industrial sources with relatively high DMA concentration of 10⁸ molecules cm^-3, the contribution of binary SA-DMA nucleation to cluster formation is 61% at 278 K, representing a dominant pathway for NPF. However, in the region not too close to major source of DMA emission, e.g., near agriculture farmland, the routes involving ternary SA-DMA-A nucleation make a 64% contribution at 278 K with DMA concentration of 10⁷ molecules cm^-3, showing that A has marked impact on the cluster formation. Under such a condition, we predict that coexisting DMA and A could be detected in the process of NPF. Moreover, at winter temperatures or at higher altitudes, our calculations suggest that the clustering of initial clusters likely involve ternary SA-DMA-A clusters rather than binary SA-DMA clusters. These new insights may be helpful to analyze and predict atmospheric-condition-dependent NFP in either urban or rural regions and/or in different season of the year.

A Colorimetric Artificial Olfactory System for Airborne Improvised Explosive Identification

The detection of ultralow or nonvolatile target analytes remains a significant challenge for artificial olfactory systems even after decades of development, which severely limits their widespread application. To overcome this challenge, an artificial olfactory system based on a colorimetric hydrogel array is constructed for the first time as a universal representative. As an effective extension of conventional artificial olfactory systems that integrates the merits of its predecessors, the proposed system accurately mimics olfactory mucosa and specific odorant binding proteins using hydrogels endowed with specific colorimetric reagents for the detection of hypochlorite, chlorate, perchlorate, urea, and nitrate. Therefore, the proposed system is capable of detecting and discriminating between these five airborne improvised explosive microparticulates with a detection limit as low as 39.4 pg. Additionally, the system demonstrates good reusability over ten cycles, rapid response time of ≈0.2 s, and excellent discrimination properties, despite significant variation. This proof-of-concept study on colorimetric artificial olfactory systems yields a novel strategy for the direct and discriminative detection of nonvolatile airborne microparticulates.