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.

The role of aldehydes on sulfur based-new particle formation: a theoretical study

Aldehydes play a crucial role in the formation of atmospheric particles, attracting significant attention due to their environmental impact. However, the microscopic mechanisms underlying the formation of aldehyde-involved particles remain uncertain. In this study, through quantum chemical calculations and molecular dynamics (MD) simulations, we investigate the microscopic formation mechanisms of binary and ternary systems composed of three representative aldehydes, two sulfur-based acids, water, and two bases. Our research findings reveal that the most stable structures of acid-aldehyde clusters involve the connection of acids and aldehyde compounds through hydrogen bonds without involving proton transfer reactions, indicating relatively poor cluster stability. However, with the introduction of a third component, the stability of 18 clusters significantly increase. Among these, in ten systems, acids act as catalysts, facilitating reactions between aldehyde compounds and water or alkaline substances to generate glycols and amino alcohols. However, according to MD simulations conducted at 300 K, these acids readily dissociate from the resulting products. In the remaining eight systems, the most stable structural feature involves ion pairs formed by proton transfer reactions between acids and aldehyde compounds. These clusters exhibit remarkable thermodynamic stability. Furthermore, the acidity of the acid, the nature of nucleophilic agents, and the type of aldehyde all play significant roles in cluster stability and reactivity, and they have synergistic effects on the nucleation process. This study offers microscopic insights into the processes of new particle formation involving aldehydes, contributing to a deeper understanding of atmospheric chemistry at the molecular level.

Theoretical Insights into the Gas-Phase Oxidation of 3-Methyl-2-butene-1-thiol by the OH Radical: Thermochemical and Kinetic Analysis

3-Methyl-2-butene-1-thiol ((CH3)2C═CH-CH2-SH; MBT) is a recently identified volatile organosulfur compound emitted from Cannabis sativa and is purported to contribute to its skunky odor. To understand its environmental fate, hydroxyl radical (•OH)-mediated oxidation of MBT was conducted using high-level quantum chemical and theoretical kinetic calculations. Three stable conformers were identified for the title molecule. Abstraction and addition pathways are possible for the MBT + OH radical reaction, and thus, potential energy surfaces involving H-abstraction and •OH addition were computed at the CCSD(T)/aug-cc-pV(T+d)Z//M06-2X/aug-cc-pV(T+d)Z level of theory. The barrier height for the addition of the OH radical to a C atom of the alkene moiety, leading to the formation of a C-centered MBT-OH radical, was computed to be -4.1 kcal mol-1 below the energy of the starting MBT + OH radical-separated reactants. This reaction was found to be dominant compared to other site-specific H-abstraction and addition paths. The kinetics of all the site-specific abstraction and addition reactions associated with the most stable MBT + OH radical reaction were assessed using the MESMER kinetic code between 200 and 320 K. Further, we considered the contributions from two other conformers of MBT to the overall reaction of MBT + OH radical. The estimated global rate coefficient for the oxidation of MBT with respect to its reactions with the OH radical was found to be 6.1 × 10-11 cm3 molecule-1 s-1 at 298 K and 1 atm pressure. The thermodynamic parameters and atmospheric implications of the MBT + OH reaction are discussed.

Understanding the Formation and Growth of New Atmospheric Particles at the Molecular Level through Laboratory Molecular Beam Experiments

Atmospheric new particle formation (NPF), which exerts comprehensive implications for climate, air quality and human health, has received extensive attention. From molecule to cluster is the initial and most important stage of the nucleation process of atmospheric new particles. However, due to the complexity of the nucleation process and limitations of experimental characterization techniques, there is still a great uncertainty in understanding the nucleation mechanism at the molecular level. Laboratory-based molecular beam methods can experimentally implement the generation and growth of typical atmospheric gas-phase nucleation precursors to nanoscale clusters, characterize the key physical and chemical properties of clusters such as structure and composition, and obtain a series of their physicochemical parameters, including association rate coefficients, electron binding energy, pickup cross section and pickup probability and so on. These parameters can quantitatively illustrate the physicochemical properties of the cluster, and evaluate the effect of different gas phase nucleation precursors on the formation and growth of atmospheric new particles. We review the present literatures on atmospheric cluster formation and reaction employing the experimental method of laboratory molecular beam. The experimental apparatuses were classified and summarized from three aspects of cluster generation, growth and detection processes. Focus of this review is on the properties of nucleation clusters involving different precursor molecules of water, sulfuric acid, nitric acid and NxOy, respectively. We hope this review will provide a deep insight for effects of cluster physicochemical properties on nucleation, and reveal the formation and growth mechanism of atmospheric new particle at the molecular level.

Probing the dynamics and bottleneck of the key atmospheric SO2 oxidation reaction by the hydroxyl radical

SO2 (Sulfur dioxide) is the major precursor to the production of sulfuric acid (H2SO4), contributing to acid rain and atmospheric aerosols. Sulfuric acid formed from SO2 generates light-reflecting sulfate aerosol particles in the atmosphere. This property has prompted recent geoengineering proposals to inject sulfuric acid or its precursors into the Earth's atmosphere to increase the planetary albedo to counteract global warming. SO2 oxidation in the atmosphere by the hydroxyl radical HO to form HOSO2 is a key rate-limiting step in the mechanism for forming acid rain. However, the dynamics of the HO + SO2 → HOSO2 reaction and its slow rate in the atmosphere are poorly understood to date. Herein, we use photoelectron spectroscopy of cryogenically cooled HOSO2- anion to access the neutral HOSO2 radical near the transition state of the HO + SO2 reaction. Spectroscopic and dynamic calculations are conducted on the first ab initio-based full-dimensional potential energy surface to interpret the photoelectron spectra of HOSO2- and to probe the dynamics of the HO + SO2 reaction. In addition to the finding of a unique pre-reaction complex (HO⋯SO2) directly connected to the transition state, dynamic calculations reveal that the accessible phase space for the HO + SO2 → HOSO2 reaction is extremely narrow, forming a key reaction bottleneck and slowing the reaction rate in the atmosphere, despite the low reaction barrier. This study underlines the importance of understanding the full multidimensional potential energy surface to elucidate the dynamics of complex bimolecular reactions involving polyatomic reactants.

On the correlation between hygroscopic properties and chemical composition of cloud condensation nuclei obtained from the chemical aging of soot particles with O3 and SO2

Soot particles released in the atmosphere have long been investigated for their ability to affect the radiative forcing. Although freshly emitted soot particles are generally considered to yield only positive contributions to the radiative forcing, atmospheric aging can activate them into efficient cloud condensation or ice nuclei, which can trigger the formation of persistent clouds and ultimately provide a negative contribution to the radiative forcing. Depending on their residence time in the atmosphere, soot particles can undergo several physical and chemical aging processes that affect their chemical composition, particle size distribution and morphology, and ultimately their optical and hygroscopic properties. The impact of the physical-chemical aging on the properties of soot particles is still difficult to quantify, as well as their effect on the radiative forcing of the atmosphere. This work investigates the hygroscopic properties of chemically aged soot particles obtained from the combustion of aviation fuel, and in particular the interplay between aging mechanisms initiated by two widespread atmospheric oxidizers (O3 and SO2). Activation is measured in water supersaturation conditions using a cloud condensation nuclei counter. Once particle morphology and size distribution are taken into account, the hygroscopicity parameter κ is derived using κ-Köhler theory and correlated to the change of the chemical composition of the particles aged in a simulation chamber. While fresh soot particles are poor cloud condensation nuclei (κ < 10-4) and are not significantly affected by either O3 or SO2 at the timescale of the experiments, rapid activation is observed when they are simultaneously exposed to both oxidizers. Activated particles become efficient cloud condensation nuclei, comparable to the highly hygroscopic particulate matter typically found in the atmosphere (κ = 0.2-0.6 at RH = 20 %). Statistical analysis reveals a correlation between the activation and sulfur-containing ions detected on the chemically aged particles that are absent from the fresh particles.

Unexpected enhancement of sulfuric acid-driven new particle formation by alcoholic amines: The role of ion-induced nucleation

New particle formation (NPF) contributes more than half of the global aerosol. Diethanolamine (DEA) and methyldiethanolamine (MDEA) are the most common amines used to remove CO2 and H2S, which are lost to the atmosphere from CO2 chemical absorbers, livestock and consumer products and are involved in sulfuric acid (SA)-driven NPF. Ion-induced nucleation (IIN) is an important nucleation pathway for NPF. We investigated the role of IIN on DEA and MDEA enhancing SA-driven NPF using density functional method (DFT), molecular dynamics (MD) simulation and atmospheric cluster dynamics code (ACDC). The effects of SO42-, H3O+, NH4+, HSO4-, NO3-, ammonia, methylamine, dimethylamine, trimethylamine and water (W) on the nucleation of SA-DEA were further investigated. The enhancement ability of DEA is greater than that of dimethylamine (DMA) and MDEA. Participation in SA-based NPF is a removal pathway for DEA and MDEA. DEA-SA clusters are generated that not only aggregate DEA and SA molecules, but also increase further growth of atmospheric ions. The very low Gibbs formation free energy highlights the importance of ion-induced nucleation for SA-based NPF. The order of the ability of common atmospheric ions to increase the (SA)(DEA) cluster nucleation is SO42- > H3O+ > NH4+ > HSO4- > NO3-. The addition of 20 water molecules increases the (SA)(DEA)9 cluster from 1.882 nm to 2.053 nm, promoting SA-based NPF. The atmospheric ions accelerate the aggregation rate of the (SA)5(DEA)5 cluster within 15 ns?

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.

Toward Modeling the Growth of Large Atmospheric Sulfuric Acid-Ammonia Clusters

Studying large atmospheric molecular clusters is needed to understand the transition between clusters and aerosol particles. In this work, we studied the (SA)n(AM)n clusters with n up to 30 and the (SA)m(AM)m±2 clusters, with m = 6-20. The cluster configurations are sampled using the ABCluster program, and the cluster geometries and thermochemical parameters are calculated using GFN1-xTB. The cluster binding energies are calculated using B97-3c. We find that the addition of sulfuric acid is preferred to the addition of ammonia. The addition free energies were found to have large uncertainties, which could potentially be attributed to errors in the applied level of theory. Based on DLPNO-CCSD(T0)/aug-cc-pVTZ benchmarks of the binding energies of the large (SA)8-9(AM)10 and (SA)10(AM)10-11 clusters, we find that ωB97X-D3BJ with a large basis set is required to yield accurate binding and addition energies. However, based on recalculations of the single-point energy at r2SCAN-3c and ωB97X-D3BJ/6-311++G(3df,3pd), we show that the single-point energy contribution is not the primary source of error. We hypothesize that a larger source of error might be present in the form of insufficient configurational sampling. Finally, we train Δ machine learning model on (SA)n(AM)n clusters with n up to 5 and show that we can predict the binding energies of clusters up to sizes of (SA)30(AM)30 with a binding energy error below 0.6 %. This is an encouraging approach for accurately modeling the binding energies of large acid-base clusters in the future.

Nitric Acid and Organic Acids Suppress the Role of Methanesulfonic Acid in Atmospheric New Particle Formation

Multicomponent atmospheric molecular clusters, typically comprising a combination of acids and bases, play a pivotal role in our climate system and contribute to the perplexing uncertainties embedded in modern climate models. Our understanding of cluster formation is limited by the lack of studies on complex mixed-acid-mixed-base systems. Here, we investigate multicomponent clusters consisting of mixtures of several acid and base molecules: sulfuric acid (SA), methanesulfonic acid (MSA), nitric acid (NA), formic acid (FA), along with methylamine (MA), dimethylamine (DMA), and trimethylamine (TMA). We calculated the binding free energies of a comprehensive set of 252 mixed-acid-mixed-base clusters at the DLPNO-CCSD(T0)/aug-cc-pVTZ//ωB97X-D/6-31++G(d,p) level of theory. Combined with the existing datasets, we simulated the new particle formation (NPF) rates using the Atmospheric Cluster Dynamics Code (ACDC). We find that the presence of NA and FA had a substantial impact, increasing the NPF rate by 60% at realistic conditions. Intriguingly, we find that NA and FA suppress the role of MSA in NPF. These findings suggest that even high concentration of MSA has a limited impact on NPF in polluted regions with high FA and NA. We outline a method for generating a lookup table that could potentially be used in climate models that sufficiently incorporates all the required chemistry. By unraveling the molecular mechanisms of mixed-acid-mixed-base clusters, we get one step closer to comprehending their implications for our global climate system.

Kinetics of Sulfur Trioxide Reaction with Water Vapor to Form Atmospheric Sulfuric Acid

Although experimental methods can be used to obtain the quantitative kinetics of atmospheric reactions, experimental data are often limited to a narrow temperature range. The reaction of SO3 with water vapor is important for elucidating the formation of sulfuric acid in the atmosphere; however, the kinetics is uncertain at low temperatures. Here, we calculate rate constants for reactions of sulfur trioxide with two water molecules. We consider two mechanisms: the SO3···H2O + H2O reaction and the SO3 + (H2O)2 reaction. We find that beyond-CCSD(T) contributions to the barrier heights are very large, and multidimensional tunneling, unusually large anharmonicity of high-frequency modes, and torsional anharmonicity are important for obtaining quantitative kinetics. We find that at lower temperatures, the formation of the termolecular precursor complexes, which is often neglected, is rate-limiting compared to passage through the tight transition states. Our calculations show that the SO3···H2O + H2O mechanism is more important than the SO3 + (H2O)2 mechanism at 5-50 km altitudes. We find that the rate ratio between SO3···H2O + H2O and SO3 + (H2O)2 is greater than 20 at altitudes between 10 and 35 km, where the concentration of SO3 is very high.

Phase State, Surface Tension, Water Activity, and Accommodation Coefficient of Water-Organic Clusters Near the Critical Size for Atmospheric New Particle Formation

Interactions between water and organic molecules in sub-4 nm clusters play a significant role in the formation and growth of secondary organic aerosol (SOA) particles. However, a complete understanding of the relevant water microphysics has not yet been achieved due to challenges in the experimental characterization of soft nuclei. Here, we use molecular dynamics simulations to study the phase-mixing states, surface tension, water activity, and water accommodation coefficient of organic-water clusters representative of freshly nucleated SOA particles. Our results reveal large deviations from the behavior expected based on continuum theories. In particular, the phase-mixing state has a strong dependence on cluster size; surface tension displays a minimum at a specific organic-water mass ratio (morg/mw ∼ 4.5 in this study) corresponding to a minimum inhibition of droplet nucleation associated with the Kelvin effect; and the water accommodation coefficient increases by a factor of 2 with nanocluster hygroscopic growth, in agreement with recent experimental studies. Overall, our results yield parametric relations for water microphysical properties in sub-4 nm clusters and provide insight into the role of water in the initial stages of SOA nucleation and growth.

Particle number size distributions and formation and growth rates of different new particle formation types of a megacity in China

To understand the contribution of new particle formation (NPF) events to ambient fine particle pollution, measurements of particle size distributions, trace gases and meteorological conditions, were conducted at a suburban site (NJU) from October to December 2016 and at an industrial site (NUIST) from September to November 2015 in Nanjing. According to the temporal evolution of the particle size distributions, three types NPF events were observed: typical NPF (Type A), moderate NPF events (Type B) and strong NPF (Type C) events. The favorable conditions for Type A events included low relative humidity, low concentration of pre-existing particles, and high solar radiation. The favorable conditions of Type B events were similar to Type A, except for a higher concentration of pre-existing particles. Type C events were more likely to happen with the higher relative humidity, lower solar radiation and continuous growth of pre-existing particle concentration. The formation rate of 3 nm (J₃) was the lowest for Type A events and highest for Type C events. In contrast, the growth rates of 10 nm and 40 nm particles were the highest for Type A, and lowest for Type C. Results show that NPF events with only higher J₃ would lead to the accumulation of nucleation mode particles. Sulfuric acid was important for the formation of particles but had little effect on the growth of particle size.

Neural network potential energy surfaces and dipole moment surfaces for SO2(H2O) and SO2(H2O)2 complexes

Full-dimensional, ab initio-based many-body potential energy surfaces and dipole moment surfaces constructed using the neural network method for SO2(H2O)n (n = 1,2) complexes are reported. The database of the SO2 1-body PES, SO2(H2O) 2-body PES and SO2(H2O)2 3-body PES consists of 11 952, 79 882 and 84 159 ab initio energies, respectively. All 1-body energies were calculated at the CCSD(T)/CBS(AVTZ:AVQZ) level and all 2,3-body energies were calculated at the DSD-PBEP86/AVTZ level. The database of DMSs is the same as that of PESs and all dipole moments were calculated at the MP2/AVTZ level. Harmonic frequencies and dissociation energies of SO2(H2O) and SO2(H2O)2 were calculated on these PESs and compared with ab initio results to examine the fidelity of these PESs.

Mechanistic understanding of rapid H2SO4-HNO3-NH3 nucleation in the upper troposphere

The upper troposphere (UT) nucleation is thought to be responsible for at least one-third of the global cloud condensation nuclei. Although NH₃ was considered to be extremely rare in the UT, recent studies show that NH₃ is convected aloft, promoting H₂SO₄-HNO₃-NH₃ rapid nucleation in the UT during the Asian monsoon. In this study, the roles of HNO₃, H₂SO₄ (SA), and NH₃ in the nucleation of SA-HNO₃-NH₃ were investigated by quantum chemical calculation and molecular dynamic (MD) simulations at the level of M06-2×/6-31 + G (d, p). The nucleation ability of SA-HNO₃-NH₃ is suppressed as the temperature increases in the UT. The results indicated that bisulfate (HSO₄^-), nitrate (NO₃^-), and ammonium (NH₄⁺) ionized from SA, HNO₃, and NH₃, respectively, can significantly enhance the nucleation ability of SA-HNO₃-NH₃. In addition, hydrated hydrogen ion (H₃O⁺) as well as sulfate ions (SO₄^2-) ionized by SA can also actively participate in the process of ion-induced nucleation. The results reveal that the enhancement effect of five ions on the SA-HNO₃-NH₃ nucleation can be ordered as follows: SO₄^2- > H₃O⁺ > HSO₄^- > NO₃^- > NH₄⁺. Many ion-induced nucleation pathways of SA-HNO₃-NH₃ with the Gibbs free energies of formation (ΔG) lower than -100 kcal mol^-1 were energetically favorable. HNO₃ and NH₃ can promote the nucleation of SA-HNO₃-NH₃ and water (W) molecules are also beneficial to promote the new particle formation (NPF) of SA-HNO₃-NH₃. Under the action of H-bonds and electrostatic interaction, ion-induced nucleation could lead to the rapid nucleation of H₂SO₄-HNO₃-NH₃ in the UT.

Clusterome: A Comprehensive Data Set of Atmospheric Molecular Clusters for Machine Learning Applications

Formation and growth of atmospheric molecular clusters into aerosol particles impact the global climate and contribute to the high uncertainty in modern climate models. Cluster formation is usually studied using quantum chemical methods, which quickly becomes computationally expensive when system sizes grow. In this work, we present a large database of ∼250k atmospheric relevant cluster structures, which can be applied for developing machine learning (ML) models. The database is used to train the ML model kernel ridge regression (KRR) with the FCHL19 representation. We test the ability of the model to extrapolate from smaller clusters to larger clusters, between different molecules, between equilibrium structures and out-of-equilibrium structures, and the transferability onto systems with new interactions. We show that KRR models can extrapolate to larger sizes and transfer acid and base interactions with mean absolute errors below 1 kcal/mol. We suggest introducing an iterative ML step in configurational sampling processes, which can reduce the computational expense. Such an approach would allow us to study significantly more cluster systems at higher accuracy than previously possible and thereby allow us to cover a much larger part of relevant atmospheric compounds.

Limited Role of Malonic Acid in Sulfuric Acid-Dimethylamine New Particle Formation

Aerosols play an important role in climate and air quality; however, the mechanisms behind aerosol particle formation in the atmosphere are poorly understood. Studies have identified sulfuric acid, water, oxidized organics, and ammonia/amines as key precursors for forming aerosol particles in the atmosphere. Theoretical and experimental investigations have indicated that other species, such as organic acids, may be involved in atmospheric nucleation and growth of freshly formed aerosol particles. Organic acids, such as dicarboxylic acids, which are abundant in the atmosphere, have been measured in ultrafine aerosol particles. These observations suggest that organic acids may contribute to new particle formation in the atmosphere but their role remains ambiguous. This study examines how malonic acid interacts with sulfuric acid and dimethylamine to form new particles at warm boundary layer conditions using experimental observations from a laminar flow reactor and quantum chemical calculations coupled with cluster dynamics simulations. Observations reveal that malonic acid does not contribute to the initial steps (formation of <1 nm diameter particle) of nucleation with sulfuric acid-dimethylamine. In addition, malonic acid was found to not participate in the subsequent growth of the freshly nucleated 1 nm particles from sulfuric acid-dimethylamine reactions to diameters of 2 nm.

Current and future machine learning approaches for modeling atmospheric cluster formation

The formation of strongly bound atmospheric molecular clusters is the first step towards forming new aerosol particles. Recent advances in the application of machine learning models open an enormous opportunity for complementing expensive quantum chemical calculations with efficient machine learning predictions. In this Perspective, we present how data-driven approaches can be applied to accelerate cluster configurational sampling, thereby greatly increasing the number of chemically relevant systems that can be covered.

Role of Methanesulfonic Acid in Sulfuric Acid-Amine and Ammonia New Particle Formation

Aerosol nucleation accounts for over half of all seed particles for cloud droplet formation. In the atmosphere, sulfuric acid (SA) nucleates with ammonia, amines, oxidized organics, and many more compounds to form particles. Studies have also shown that methanesulfonic acid (MSA) nucleates independently with amines and ammonia. MSA and SA are produced simultaneously via dimethyl sulfide oxidation in the marine atmosphere. However, limited knowledge exists on how MSA and SA nucleate together in the presence of various atmospherically relevant base compounds, which is critical to predicting marine nucleation rates accurately. This work provides experimental evidence that SA and MSA react to form particles with amines and that the SA-MSA-base nucleation has different reaction pathways than SA-base nucleation. Specifically, the formation of the SA-MSA heterodimer creates more energetically favorable pathways for SA-MSA-methylamine nucleation and an enhancement of nucleation rates. However, SA-trimethylamine nucleation is suppressed by MSA, likely due to the steric hindrance of the MSA and trimethylamine. These results display the importance of including nucleation reactions between SA, MSA, and various amines to predict particle nucleation rates in the marine atmosphere.

Clusteromics V: Organic Enhanced Atmospheric Cluster Formation

Formic acid (FA) is a prominent candidate for organic enhanced nucleation due to its high abundance and stabilizing effect on smaller clusters. Its role in new particle formation is studied through the use of state-of-the-art quantum chemical methods on the cluster systems (acid)_1-2(FA)₁(base)_1-2 with the acids being sulfuric acid (SA)/methanesulfonic acid (MSA) and the bases consisting of ammonia (A), methylamine (MA), dimethylamine (DMA), trimethylamine (TMA), and ethylenediamine (EDA). A funneling approach is used to determine the cluster structures with initial configurations generated through the ABCluster program, followed by semiempirical PM7 and ωB97X-D/6-31++G(d,p) calculations. The final binding free energy is calculated at the DLPNO-CCSD(T₀)/aug-cc-pVTZ//ωB97X-D/6-31++G(d,p) level of theory using the quasi-harmonic approximation. Cluster dynamics simulations show that FA has a minuscule or negligible effect on the MSA-FA-base systems as well as most of the SA-FA-base systems. The SA-FA-DMA cluster system shows the highest influence from FA with an enhancement of 21%, compared to its non-FA counterpart.

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.

Review of the influencing factors of secondary organic aerosol formation and aging mechanism based on photochemical smog chamber simulation methods

The formation and aging mechanism of secondary organic aerosol (SOA) and its influencing factors have attracted increasing attention in recent years because of their effects on climate change, atmospheric quality and human health. However, there are still large errors between air quality model simulation results and field observations. The currently undetected components during the formation and aging of SOA due to the limitation of current monitoring techniques and the interactions among multiple SOA formation influencing factors might be the main reasons for the differences. In this paper, we present a detailed review of the complex dynamic physical and chemical processes and the corresponding influencing factors involved in SOA formation and aging. And all these results were mainly based the studies of photochemical smog chamber simulation. Although the properties of precursor volatile organic compounds (VOCs), oxidants (such as OH radicals), and atmospheric environmental factors (such as NO_x, SO₂, NH₃, light intensity, temperature, humidity and seed aerosols) jointly influence the products and yield of SOA, the nucleation and vapor pressure of these products were found to be the most fundamental aspects when interpreting the dynamics of the SOA formation and aging process. The development of techniques for measuring intermediate species in SOA generation processes and the study of SOA generation and aging mechanism in complex systems should be important topics of future SOA research.

Measurement of atmospheric nanoparticles: Bridging the gap between gas-phase molecules and larger particles

Atmospheric nanoparticles are crucial components contributing to fine particulate matter (PM_2.5), and therefore have significant effects on visibility, climate, and human health. Due to the unique role of atmospheric nanoparticles during the evolution process from gas-phase molecules to larger particles, a number of sophisticated experimental techniques have been developed and employed for online monitoring and characterization of the physical and chemical properties of atmospheric nanoparticles, helping us to better understand the formation and growth of new particles. In this paper, we firstly review these state-of-the-art techniques for investigating the formation and growth of atmospheric nanoparticles (e.g., the gas-phase precursor species, molecular clusters, physicochemical properties, and chemical composition). Secondly, we present findings from recent field studies on the formation and growth of atmospheric nanoparticles, utilizing several advanced techniques. Furthermore, perspectives are proposed for technique development and improvements in measuring atmospheric nanoparticles.

Unexpected hygroscopic behaviors of individual sub-50 nm NaNO3 nanoparticles observed by in situ atomic force microscopy

Hygroscopicity is one of the most important physicochemical properties of salt nanoparticles, greatly influencing the environment, climate and human health. However, the hygroscopic properties of salt nanoparticles are poorly understood owing to the great challenges of the preparation, preservation and in situ characterization. Here we show the unexpected shape- and size-dependent hygroscopic behaviors of NaNO₃ nanoparticles prepared from molten salts using in situ environment-controlled atomic force microscopy. During the humidifying process, the angular and round sub-50 nm NaNO₃ particles display anisotropic and isotropic water adsorption behaviors, respectively. The sub-10 nm NaNO₃ nanoparticles abnormally shrink and disappear. The growth factors of the NaNO₃ nanoparticles are highly sensitive to their sizes and shapes, and quite different from those of NaNO₃ microparticles. These findings show that the hygroscopic behaviors of salt nanoparticles may not be comprehensively described by the traditional growth factors, and open up a new pathway to study the hygroscopic behaviors of salt nanoparticles.

Massive Assessment of the Binding Energies of Atmospheric Molecular Clusters

Quantum chemical studies of the formation and growth of atmospheric molecular clusters are important for understanding aerosol particle formation. However, the search for the lowest free-energy cluster configuration is extremely time consuming. This makes high-level benchmark data sets extremely valuable in the quest for the global minimum as it allows the identification of cost-efficient computational methodologies, as well as the development of high-level machine learning (ML) models. Herein, we present a highly versatile quantum chemical data set comprising a total of 11 749 (acid)_1-2(base)_1-2 cluster configurations, containing up to 44 atoms. Utilizing the LUMI supercomputer, we calculated highly accurate PNO-CCSD(F12*)(T)/cc-pVDZ-F12 binding energies of the full set of cluster configurations leading to an unprecedented data set both in regard to sheer size and with respect to the level of theory. We employ the constructed benchmark set to assess the performance of various semiempirical and density functional theory methods. In particular, we find that the r²-SCAN-3c method shows excellent performance across the data set related to both accuracy and CPU time, making it a promising method to employ during cluster configurational sampling. Furthermore, applying the data sets, we construct ML models based on Δ-learning and provide recommendations for future application of ML in cluster configurational sampling.

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.

The driving effects of common atmospheric molecules for formation of prenucleation clusters: the case of sulfuric acid, formic acid, nitric acid, ammonia, and dimethyl amine

How secondary aerosols form is critical as aerosols' impact on Earth's climate is one of the main sources of uncertainty for understanding global warming. The beginning stages for formation of prenucleation complexes, that lead to larger aerosols, are difficult to decipher experimentally. We present a computational chemistry study of the interactions between three different acid molecules and two different bases. By combining a comprehensive search routine covering many thousands of configurations at the semiempirical level with high level quantum chemical calculations of approximately 1000 clusters for every possible combination of clusters containing a sulfuric acid molecule, a formic acid molecule, a nitric acid molecule, an ammonia molecule, a dimethylamine molecule, and 0-5 water molecules, we have completed an exhaustive search of the DLPNO-CCSD(T)/CBS//ωB97X-D/6-31++G** Gibbs free energy surface for this system. We find that the detailed geometries of each minimum free energy cluster are often more important than traditional acid or base strength. Addition of a water molecule to a dry cluster can enhance stabilization, and we find that the (SA)(NA)(A)(DMA)(W) cluster has special stability. Equilibrium calculations of SA, FA, NA, A, DMA, and water using our quantum chemical ΔG° values for cluster formation and realistic estimates of the concentrations of these monomers in the atmosphere reveals that nitric acid can drive early stages of particle formation just as efficiently as sulfuric acid. Our results lead us to believe that particle formation in the atmosphere results from the combination of many different molecules that are able to form highly stable complexes with acid molecules such as SA, NA, and FA.

Contribution of Methanesulfonic Acid to the Formation of Molecular Clusters in the Marine Atmosphere

Because of the lack of long-term measurements, new particle formation (NPF) in the marine atmosphere remains puzzling. Using quantum chemical methods, this study elucidates the cluster formation and further growth of sulfuric acid-methanesulfonic acid-dimethylamine (SA-MSA-DMA) clusters, relevant to NPF in the marine atmosphere. The cluster structures and thermochemical parameters of (SA)_n(MSA)_m(DMA)_l (n + m ≤ 4 and l ≤ 4) systems are calculated using density functional theory at the ωB97X-D/6-31++G(d,p) level of theory, and the single-point energies are calculated using high-level DLPNO-CCSD(T₀)/aug-cc-pVTZ calculations. The calculated thermochemistry is used as input to the Atmospheric Cluster Dynamics Code (ACDC) to gain insight into the cluster dynamics. At ambient conditions (298.15 K, 1 atm), we find that the distribution of outgrowing clusters primarily consists of SA and DMA, with a minor contribution from the mixed SA-MSA-DMA clusters. At lower temperature (278.15 K, 1 atm) the distribution broadens, and clusters containing one or more MSA molecules emerge. These findings show that in the cold marine atmosphere MSA likely participates in atmospheric NPF.

Sulfuric acid-dimethylamine particle formation enhanced by functional organic acids: an integrated experimental and theoretical study

Atmospheric new particle formation (NPF), which has been observed globally in clean and polluted environments, is an important source of boundary-layer aerosol particles and cloud condensation nuclei, but the fundamental mechanisms leading to multi-component aerosol formation have not been well understood. Here, we use experiments and quantum chemical calculations to better understand the involvement of carboxylic acids in initial NPF from gas phase mixtures of carboxylic acid, sulfuric acid (SA), dimethylamine, and water. A turbulent flow tube coupled to an ultrafine condensation particle counter with particle size magnifier has been set up to measure NPF. Experimental results show that pyruvic acid (PA), succinic acid (SUA), and malic acid (MA) can enhance sulfuric acid-dimethylamine nucleation in the order PA < SUA < MA with a greater enhancement observed at lower SA concentrations. Computational results indicate that the carboxylic and hydroxyl groups are related to the enhancement. This experiment-theory study shows the formation of multi-component aerosol particles and the role of the organic functional group, which may aid in understanding the role of organics in aerosol nucleation and growth in polluted areas, and help to choose organic molecules of specific structures for simulation.

Clusteromics IV: The Role of Nitric Acid in Atmospheric Cluster Formation

Nitric acid (NA) has previously been shown to affect atmospheric new particle formation; however, its role still remains highly uncertain. Through the employment of state-of-the-art quantum chemical methods, we study the (acid)_1-2(base)_1-2 and (acid)₃(base)₂ clusters containing at least one nitric acid (NA) and sulfuric acid (SA) or methanesulfonic acid (MSA) with bases ammonia (A), methylamine (MA), dimethylamine (DMA), trimethylamine (TMA), and ethylenediamine (EDA). The initial cluster configurations are generated using the ABCluster program. PM7 and ωB97X-D/6-31++G(d,p) calculations are used to reduce the number of relevant configurations. The thermochemical parameters are calculated at the ωB97X-D/6-31++G(d,p) level of theory with the quasi-harmonic approximation, and the final single-point energies are calculated with high-level DLPNO-CCSD(T₀)/aug-cc-pVTZ calculations. The enhancing effect from the presence of nitric acid on cluster formation is studied using the calculated thermochemical data and cluster dynamics simulations. We find that when NA is in excess compared with the other acids, it has a substantial enhancing effect on the cluster formation potential.

Amino Acids Compete with Ammonia in Sulfuric Acid-Based Atmospheric Aerosol Prenucleation: The Case of Glycine and Serine

We present a computational investigation of the sulfuric acid, glycine, serine, ammonia, and water system to understand if this system can form prenucleation clusters, which are precursors to larger aerosols in the atmosphere. We have performed a comprehensive configurational search of all possible clusters in this system, starting with the four different monomers and zero to five waters. Accurate Gibbs free energies of formation have been calculated with the DLPNO-CCSD(T)/complete basis set (CBS) method on ωb97xd/6-31++G** geometries. For the dry dimers of sulfuric acid, the weakest base, serine, is found to form the most stable complex, which is a consequence of the strong di-ionic complex formed between the bisulfate ion and the protonated serine cation. For the dry dimers without sulfuric acid, the glycine-serine complex is more stable than the glycine-ammonia or serine-ammonia complexes, stemming from the detailed structure and not related to base strength. For the larger complexes, sulfuric acid deprotonates and the proton is shifted to glycine, serine, or ammonia. The two amino acids and ammonia are almost interchangeable and there is no easy way to predict which molecule will be protonated without the calculated results. Assuming reasonable starting concentrations and a closed system of sulfuric acid, glycine, serine, ammonia, and five waters, we predict the concentrations of all possible complexes at two temperatures spanning the troposphere. The most negative ΔG° values are a function of the detailed molecular interactions of these clusters. These details are more important than the base strength of ammonia, glycine, and serine.

Ambient acidic ultrafine particles in different land-use areas in two representative Chinese cities

The adverse effects of acidic ultrafine particles (AUFPs) have been widely recognized in scientific communities. However, a handful of studies successfully acquired the concentrations of AUFPs in the atmosphere. To explore the AUFPs pollution, six extensive measurements were for the first time conducted in the roadside, urban and rural areas in Hong Kong, and the urban area in Shanghai between 2017 and 2020. The concentrations of AUFPs and UFPs, and the proportions of AUFPs in UFPs were obtained. The concentration of UFPs was the highest at the roadside site, followed by the urban site and the rural site, while the proportion of AUFPs in UFPs showed a contrary trend. The difference, on one hand, indicated the potential transformation of AUFPs from non-acidic UFPs during the transport and aging of air masses, and on the other hand, suggested the minor contribution of anthropogenic sources to the emission of AUFPs. In addition, the urban area in Hong Kong suffered from heavier pollution of UFPs and AUFPs than that in Shanghai. As for size distribution, the proportion of AUFPs in UFPs peaked in the size range of 35-50 nm and 50-75 nm in roadside and urban area, respectively. In rural area, the peak was observed in the size range of 5-10 nm, which might indicate the stimulation of new particle formation with the AUFPs as seeds. Furthermore, in the urban areas of Hong Kong and Shanghai, no significant difference was found for the geometric mean diameters of UFPs and AUFPs (p > 0.05). At last, the sulfuric acid proxy was positively correlated with the proportions of AUFPs in UFPs but not well correlated with the AUFPs levels. The results suggested the important roles of interaction between sulfuric acid vapor and non-acidic UFPs in AUFPs formation. Due to the significant reduction of sulfur dioxide in China during the last decade, the pollution of AUFPs in urban areas was alleviated.

The striking effect of vertical mixing in the planetary boundary layer on new particle formation in the Yangtze River Delta

New particle formation (NPF) induces a sharp increase in ultrafine particle number concentrations and potentially acts as an important source of cloud condensation nuclei (CCN). As the densely populated area of China, the Yangtze River Delta (YRD) region shows a high frequency of observed NPF events at the ground level, especially in spring. Although recent observational studies suggested a possible connection between NPF at the higher altitudes and ground level, the role played by vertical mixing, particularly in the planetary boundary layer (PBL) is not fully understood. Here we integrate measurements in Nanjing on 15-20 April 2018, and the NPF-explicit Weather Research and Forecast coupled with chemistry (WRF-Chem) model simulations to better understand the governing mechanisms of the NPF and CCN. Our results indicate that newly formed particles at the boundary layer top could be transported downward by vertical mixing as the PBL develops. A numerical sensitivity simulation created by eliminating aerosol vertical mixing suppresses both the downward transport of particles formed at a higher altitude and the dilution of particles at the ground level. The resulting higher Fuchs surface area at the ground level, together with the lack of downward transport, yields a sharp weakening of NPF strength and delayed start of NPF therein. The aerosol vertical mixing, therefore, leads to a more than double increase of surface CN_10-40 and a one third decrease of boundary layer top CN_10-40. Additionally, the continuous growth of nucleated ultrafine particles at the boundary layer top is strongly steered by the upward transport of condensable gases, with close to half increase of particle number concentrations in Aitken mode and CCN at a supersaturation rate of 0.75%. The findings may bridge the gap in understanding the complex interaction between PBL dynamics and NPF events, reducing the uncertainty in assessing the climate impact of aerosols.

Experimental and Theoretical Study on the Enhancement of Alkanolamines on Sulfuric Acid Nucleation

Alkanolamines such as monoethanolamine (MEA), diethanolamine (DEA), and triethanolamine (TEA) are extensively used for CO₂ capture and consumer products. Despite their prevalence in industrial applications, the fate of alkanolamines in the atmosphere remains relatively unknown. One likely reaction pathway for these chemicals in the atmosphere is new particle formation with sulfuric acid. Here, we present the first experimental results showing the formation of sulfuric acid dimers enhanced by MEA, DEA, and TEA from the measurement of molecular clusters. This study examines the nucleation reactions of MEA, DEA, and TEA with sulfuric acid in a clean, laminar flow reactor. The chemical compositions and concentrations of the freshly nucleated clusters were analyzed using a custom-built atmospheric pressure chemical ionization long time-of-flight mass spectrometer known as the Pittsburgh Cluster CIMS. Quantum chemical calculations and kinetic modeling of sulfuric acid-MEA/DEA/TEA clusters were also performed to determine relative cluster stabilities between these sulfuric acid-base systems. Experimental results indicate that MEA, DEA, and TEA at the part per trillion by volume (pptv) concentrations can enhance sulfuric acid dimer formation rates but to a lesser extent than dimethylamine (DMA). Thus, MEA, DEA, and TEA will potentially play an important role in new particle formation in industrial cities where these alkanolamines are emitted.

Infrared Spectroscopy of Stepwise Hydration Motifs of Sulfur Dioxide

Experimental characterization of microscopic events and behaviors of SO₂-H₂O interactions is crucial to understanding SO₂ atmospheric chemistry but has been proven to be very challenging due to the difficulty in size selection. Here, size-dependent development of SO₂ hydrate structure and cluster growth in the SO₂(H₂O)_n (n = 1-16) complexes was probed by infrared spectroscopy based on threshold photoionization using a tunable vacuum ultraviolet free electron laser. Spectral changes with cluster size demonstrate that the sandwich structure initially formed at n = 1 develops into cycle structures with the sulfur and oxygen atoms in a two-dimensional plane (n = 2 and 3) and then into three-dimensional cage structures (n ≥ 4). SO₂ is favorably bound to the surface of larger water clusters. These stepwise features of SO₂ hydration on various sized water clusters contribute to understanding the reactive sites and electrophilicity of SO₂ on cloud droplets, which may have important atmospheric implications for studying the SO₂-containing aerosol systems.

Amine-Enhanced Methanesulfonic Acid-Driven Nucleation: Predictive Model and Cluster Formation Mechanism

Atmospheric amines are considered to be an effective enhancer for methanesulfonic acid (MSA)-driven nucleation. However, out of the 195 detected atmospheric amines, the enhancing potential (EP) has so far only been studied for five amines. This severely hinders the understanding of the contribution of amines to MSA-driven nucleation. Herein, a two-step procedure was employed to probe the EP of various amines on MSA-driven nucleation. Initially, the formation free energies (ΔG) of 50 MSA-amine dimer clusters were calculated. Based on the calculated ΔG values, a robust quantitative structure-activity relationship (QSAR) model was built and utilized to predict the ΔG values of the remaining 145 amines. The QSAR model identified two guanidino-containing compounds as the potentially strongest enhancer for MSA-driven nucleation. Second, the EP of guanidino-containing compounds was studied by employing larger clusters and selecting guanidine (Gud) as a representative. The results indicate that Gud indeed has the strongest EP. The Gud-MSA system presents a unique clustering mechanism, proceeding via the initial formation of the (Gud)₁(MSA)₁ cluster, and subsequently by cluster collisions with either a (Gud)₁(MSA)₁ or (Gud)₂(MSA)₂ cluster. The developed QSAR model and the identification of amines with the strongest EP provide a foundation for comprehensively evaluating the contribution of atmospheric amines to MSA-driven nucleation.

Particle number size distribution and new particle formation in Xiamen, the coastal city of Southeast China in wintertime

New particle formation (NPF) has a great impact on regional and global climate, air quality and human health. This study uses a Scanning Mobility Particle Sizer (SMPS) for simultaneous measurement of particle number size distribution (PNSD) in wintertime to investigate NPF in the coastal city of Xiamen. The mean particle number concentration, surface area concentration and volume concentration were 7.25 × 10³ cm^-3, 152.54 μm² cm^-3, and 4.03 μm³ cm^-3, respectively. Particle number concentration was mainly influenced by the nucleation mode and the Aitken mode, whereas the main contributor to particle surface area concentration and volume concentration was accumulation mode particles. The frequency of NPF events occurred was around 41.4% in December 2019. The typical growth rates of new formed particles were 1.41-2.54 nm h^-1, and the observed formation rates were 0.49-1.43 cm^-3 s^-1. A comparative analysis of conditions between event and non-event days was performed. The results emphasized that air temperature, UV radiation and relative humidity were the most decisive meteorological factors, and NPF events usually occurred under clean atmospheric conditions with low PM concentrations. Although condensation sink was high when NPF event occurred, the level of SO₂ and O₃ concentration was also high.

Estimation of the nucleation barrier in a multicomponent system with intermolecular potential

The formation of a "critical nucleus" prior to phase change is a crucial step for new particle formation (NPF) in the atmosphere. However, the nucleation occurring below ∼1 nm is hard to observe directly. As an effective alternative, theoretical nucleation models have been widely studied. An energy barrier is involved in the nucleation and is the fundamental factor for the nucleation model. Typical atmospheric nucleation agents such as H₂SO₄, H₂O and NH₃ are dipole molecules, whose intermolecular interactions are non-ignorable. Herein, a dipole-dipole potential model is adopted to determine the interaction between molecules instead of the traditional hard sphere model, and graph theory is used to describe the structure of the cluster and the cluster-molecule interaction. The nucleation barriers (ΔE_b) of H₂SO₄-H₂SO₄, H₂SO₄-H₂O, H₂SO₄-NH₃ and H₂SO₄-H₂O-NH₃ are derived and compared to each other. In the presence of H₂O and NH₃, the ΔE_b value is decreased by 17-28% compared to that in the pure H₂SO₄ nucleation system. NH₃ is identified to be a key factor for ternary nucleation based on an orthogonal test. Atmospheric concentrations of H₂SO₄, H₂O and NH₃ are considered to investigate the influence of [H₂O + NH₃]/[H₂SO₄] on ΔE_b and the related effective collision coefficient (α). The α value in the ternary nucleation system reaches the range of (2.5-25) × 10^-5, which is 3-4 orders of magnitude higher than that in the pure H₂SO₄ system. Due to a significant enhancement of α, NH₃ and H₂O should be focused on in future aerosol particle estimation and control.

Formation of Liquid Film in Heterogeneous Condensation of Water Vapor: Effects of Solid-Fluid Interaction and Sulfuric Acid Component

Understanding the phenomenon of filmwise condensation on solid surfaces is vital for industrial processes such as air pollutant control and desalination. In this work, we study the formation of condensed liquid films via molecular dynamics simulations, and the effects of solid-fluid interactions and the sulfuric acid component are given major attention. Water is chosen as the fluid, while the solid-fluid interaction is modified to characterize different solid surfaces. The results show that as the solid-fluid interaction decreases, the solid surface transforms from a completely wetting surface to a partially wetting surface, and the film formation process shows significant differences. The condensed liquid on the completely wetting surface forms small liquid films, which merge to form a complete film covering the surface. With the enhancement of solid-fluid interaction, the condensation rate increases first and then remains virtually invariant, resulting in a film formation time that decreases first and then maintains constant. The condensed liquid on the partially wetting surfaces appears as nanodroplets, and the coalescence between nanodroplets leads to the formation of the liquid film. It is found that the stronger the solid-fluid interaction, the more the coalesced droplets tend to be pinned at nucleation sites, the easier it is to form a liquid film, and the shorter the time required for droplet merging. The sulfuric acid component accelerates liquid film formation on both completely wetting and partially wetting surfaces, but the effect of sulfuric acid is more significant on partially wetting surfaces. The 5% molar fraction of sulfuric acid reduces the nucleation time by 72% and increases the condensation rate by 137% under partial wetting, while the same amount of sulfuric acid only increases the nucleation rate by 6% on the completely wetting surface.

Atmospheric Sulfuric Acid Dimer Formation in a Polluted Environment

New particle formation (NPF) contributes significantly to atmospheric particle number concentrations and cloud condensation nuclei (CCN). In sulfur-rich environments, field measurements have shown that sulfuric acid dimer formation is likely the critical step in NPF. We investigated the dimer formation process based upon the measured sulfuric acid monomer and dimer concentrations, along with previously reported amine concentrations in a sulfur-rich atmosphere (Atlanta, USA). The average sulfuric acid concentration was in the range of 1.7 × 10⁷-1.4 × 10⁸ cm^-3 and the corresponding neutral dimer concentrations were 4.1 × 10⁵-5.0 × 10⁶ cm^-3 and 2.6 × 10⁵-2.7 × 10⁶ cm^-3 after sub-collision and collision ion-induced clustering (IIC) corrections, respectively. Two previously proposed acid-base mechanisms (namely AA and AB) were employed to respectively estimate the evaporation rates of the dimers and the acid-amine complexes. The results show evaporation rates of 0.1-1.3 s^-1 for the dimers based on the simultaneously measured average concentrations of the total amines, much higher than those (1.2-13.1 s^-1) for the acid-amine complexes. This indicates that the mechanism for dimer formation is likely AA through the formation of more volatile dimers in the initial step of the cluster formation.

Reaction of SO3 with HONO2 and Implications for Sulfur Partitioning in the Atmosphere

Sulfur trioxide is a critical intermediate for the sulfur cycle and the formation of sulfuric acid in the atmosphere. The traditional view is that sulfur trioxide is removed by water vapor in the troposphere. However, the concentration of water vapor decreases significantly with increasing altitude, leading to longer atmospheric lifetimes of sulfur trioxide. Here, we utilize a dual-level strategy that combines transition state theory calculated at the W2X//DF-CCSD(T)-F12b/jun'-cc-pVDZ level, with variational transition state theory with small-curvature tunneling from direct dynamics calculations at the M08-HX/MG3S level. We also report the pressure-dependent rate constants calculated using the system-specific quantum Rice-Ramsperger-Kassel (SS-QRRK) theory. The present findings show that falloff effects in the SO₃ + HONO₂ reaction are pronounced below 1 bar. The SO₃ + HONO₂ reaction can be a potential removal reaction for SO₃ in the stratosphere and for HONO₂ in the troposphere, because the reaction can potentially compete well with the SO₃ + 2H₂O reaction between 25 and 35 km, as well as the OH + HONO₂ reaction. The present findings also suggest an unexpected new product from the SO₃ + HONO₂ reaction, which, although very short-lived, would have broad implications for understanding the partitioning of sulfur in the stratosphere and the potential for the SO₃ reaction with organic acids to generate organosulfates without the need for heterogeneous chemistry.

Contribution of reaction of atmospheric amine with sulfuric acid to mixing particle formation from clay mineral

During dust storm, mineral particle is frequently observed to be mixed with anthropogenic pollutants (APs) and forms mixing particle which arises more complex influences on regional climate than unmixed mineral particle. Even though mixing particle formation mechanism received significant attention recently, most studies focused on the heterogeneous reaction of inorganic APs on single composition of mineral. Here, the heterogeneous reaction mechanism of amine (a proxy of organic APs) with sulfuric acid (SA) on kaolinite (Kao, a proxy of mineral dust), and its contribution to mixing particle formation are investigated under variable atmospheric conditions. Two heterogeneous reactions of Kao-SA-amine and Kao-H₂O-SA-amine in absence/presence of water were comparably investigated using combined theoretical and experimental methods, respectively. The contribution from such two heterogeneous reactions to mixing particle formation was evaluated, respectively, exploring the effect of methyl groups (1-3 -CH₃), relative humidity (RH) (11-100%) and temperature (220-298.15 K). Water was observed to play a significant role in promoting heterogeneous reaction of amines with SA on Kao surface, reducing formation energy of mixing particle containing ammonium salt converted by SA. Moreover, the promotion effect from water is enhanced with the increasing RH and the decreasing temperature. For methylamine and dimethylamine containing 1-2 -CH₃, the heterogeneous reaction of Kao-H₂O-SA-amine contributes more to mixing particle formation. However, for trimethylamine containing 3 -CH₃, the heterogeneous reaction of Kao-SA-amine is the dominant source to mixing particle formation. For mixing particle generated from the above two heterogeneous reactions, ammoniums salts are supposed to be predominant components which is of strong hygroscopicity and further leads to significant influence on climate by altering radiative forcing of mixed particle and participating in the cloud condensation nuclei and ice nuclei.

Influence of Aerosol Chemical Composition on Condensation Sink Efficiency and New Particle Formation in Beijing

Relatively high concentrations of preexisting particles, acting as a condensation sink (CS) of gaseous precursors, have been thought to suppress the occurrence of new particle formation (NPF) in urban environments, yet NPF still occurs frequently. Here, we aim to understand the factors promoting and inhibiting NPF events in urban Beijing by combining one-year-long measurements of particle number size distributions and PM_2.5 chemical composition. Our results show that indeed the CS is an important factor controlling the occurrence of NPF events, with its chemical composition affecting the efficiency of the background particles in removing gaseous H₂SO₄ (effectiveness of the CS) driving NPF. During our observation period, the CS was found to be more effective for ammonium nitrate-rich (NH₄NO₃-rich) fine particles. On non-NPF event days, particles acting as CS contained a larger fraction of NH₄NO₃ compared to NPF event days under comparable CS levels. In particular, in the CS range from 0.02 to 0.03 s^-1, the nitrate fraction was 17% on NPF event days and 26% on non-NPF event days. Overall, our results highlight the importance of considering the chemical composition of preexisting particles when estimating the CS and their role in inhibiting NPF events, especially in urban environments.

Clusteromics III: Acid Synergy in Sulfuric Acid-Methanesulfonic Acid-Base Cluster Formation

Acid-base molecular clusters are an important stage in atmospheric new particle formation. While such clusters are most likely multicomponent in nature, there are very few reports on clusters consisting of multiple acid molecules and multiple base molecules. By applying state-of-the-art quantum chemical methods, we herein study electrically neutral (SA)₁(MSA)₁(base)_0-2 clusters with base = ammonia (A), methylamine (MA), dimethylamine (DMA), trimethylamine (TMA) and ethylenediamine (EDA). The cluster structures are obtained using a funneling approach employing the ABCluster program, semiempirical PM7 calculations and ωB97X-D/6-31++G(d,p) calculations. The final binding free energies are calculated at the DLPNO-CCSD(T₀)/aug-cc-pVTZ//ωB97X-D/6-31++G(d,p) level of theory using the quasi-harmonic approximation. Based on the calculated cluster geometries and thermochemistry (at 298.15 K and 1 atm), we find that the mixed (SA)₁(MSA)₁(base)_1-2 clusters more resemble the (SA)₂(base)_1-2 clusters compared to the (MSA)₂(base)_1-2 clusters. Hence, some of the steric hindrance and lack of hydrogen bond capacity previously observed in the (MSA)₂(base)_1-2 clusters is diminished in the corresponding (SA)₁(MSA)₁(base)_1-2 clusters. Cluster kinetics simulations reveal that the presence of an MSA molecule in the clusters enhances the cluster formation potential by up to a factor of 20. We find that the SA-MSA-DMA clusters have the highest cluster formation potential, and thus, this system should be further extended to larger sizes in future studies.

Microscopic Insights Into the Formation of Methanesulfonic Acid–Methylamine–Ammonia Particles Under Acid-Rich Conditions

Understanding the microscopic mechanisms of new particle formation under acid-rich conditions is of significance in atmospheric science. Using quantum chemistry calculations, we investigated the microscopic formation mechanism of methanesulfonic acid (MSA)–methylamine (MA)–ammonia (NH3) clusters. We focused on the binary (MSA)2n-(MA)n and ternary (MSA)3n-(MA)n-(NH3)n, (n = 1–4) systems which contain more acid than base molecules. We found that the lowest-energy isomers in each system possess considerable thermodynamic and dynamic stabilities. In studied cluster structures, all bases are protonated, and they form stable ion pairs with MSA, which contribute to the charge transfer and the stability of clusters. MA and NH3 have a synergistic effect on NPF under acid-rich conditions, and the role of NH3 becomes more remarkable as cluster size increases. The excess of MSA molecules does not only enhance the stability of clusters, but provides potential sites for further growth.

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.