Hydration of the Sulfuric Acid–Methylamine Complex and Implications for Aerosol Formation

Imaging of pH distribution inside individual microdroplet by stimulated Raman microscopy

Aerosol microdroplets as microreactors for many important atmospheric reactions are ubiquitous in the atmosphere. pH largely regulates the chemical processes within them; however, how pH and chemical species spatially distribute within an atmospheric microdroplet is still under intense debate. The challenge is to measure pH distribution within a tiny volume without affecting the chemical species distribution. We demonstrate a method based on stimulated Raman scattering microscopy to visualize the three-dimensional pH distribution inside single microdroplets of varying sizes. We find that the surface of all microdroplets is more acidic, and a monotonic trend of pH decreasing is observed in the 2.9-μm aerosol microdroplet from center to edge, which is well supported by molecular dynamics simulation. However, bigger cloud microdroplet differs from small aerosol for pH distribution. This size-dependent pH distribution in microdroplets can be related to the surface-to-volume ratio. This work presents noncontact measurement and chemical imaging of pH distribution in microdroplets, filling the gap in our understanding of spatial pH in atmospheric aerosol.

Theoretical Study of the Gas-Phase Hydrolysis of Formaldehyde to Produce Methanediol and Its Implication to New Particle Formation

Aldehydes were speculated to be important precursor species in new particle formation (NPF). The direct involvement of formaldehyde (CH₂O) in sulfuric acid and water nucleation is negligible; however, whether its atmospheric hydrolysate, methanediol (CH₂(OH)₂), which contains two hydroxyl groups, participates in NPF is not known. This work investigates both CH₂O hydrolysis and NPF from sulfuric acid and CH₂(OH)₂ with quantum chemistry calculations and atmospheric cluster dynamics modeling. Kinetic calculation shows that reaction rates of the gas-phase hydrolysis of CH₂O catalyzed by sulfuric acid are 11-15 orders of magnitude faster than those of the naked path at 253-298 K. Based on structures and the calculated formation Gibbs free energies, the interaction between sulfuric acid/its dimer/its trimer and CH₂(OH)₂ is thermodynamically favorable, and CH₂(OH)₂ forms hydrogen bonds with sulfuric acid/its dimer/its trimer via two hydroxyl groups to stabilize clusters. Our further cluster kinetic calculations suggested that the particle formation rates of the system are higher than those of the binary system of sulfuric acid and water at ambient low sulfuric acid concentrations and low relative humidity. In addition, the formation rate is found to present a negative temperature dependence because evaporation rate constants contribute significantly to it. However, cluster growth is essentially limited by the weak formation of the largest clusters, which implies that other stabilizing vapors are required for stable cluster formation and growth.

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.

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.

Hydration effect of selected atmospheric gases with finite water clusters: A quantum chemical investigation towards atmospheric implications

Water vapor in atmosphere is ubiquitous, and it varies according to geographical locations. Various toxic and non-toxic gases co-exist with water vapor/moisture in the atmosphere. This computational study addresses the fact that how those gases interact with water vapor. We have done quantum chemical density functional theory calculations to probe the interaction of certain gases with a finite number of water molecules in gas phase with various functionals/basis sets. An ensemble of 14 gas molecules comprising various diatomic, triatomic, and polyatomic gases have been chosen for the investigations. The intermolecular interactions are understood from the interaction energy, electrostatic potential, frontier molecular orbitals, energy gap, and natural bond orbital analyses. Furthermore, quantum molecular descriptors such as electronegativity, chemical potential, chemical hardness and electrophilicity index are calculated to have deep insight on chemical nature of the gas molecules. Additionally, we have done implicit solvent modelling using PCM, and the corresponding solvation energies have been calculated. Interestingly, all the calculations and analyses have projected the similar results that Cl₂, SO_2, and NH₃ have very high interaction with the water clusters. To mimic various altitudes (0 km, 5 km and 10 km) in the atmosphere, thermochemistry calculations have been carried out at different temperature and pressure values. The Gibbs free energies of formation suggest that the hydration of Cl₂ is higher followed by O₂, SO₂ and NH₃ at all altitudes. Remarkably, it is found that the formation of hydrated clusters of Cl₂ and O₂ with 4H₂O are thermodynamically favourable. On the other hand, SO₂ and NH₃ requires 5H₂O and 3H₂O to form thermodynamically favourable clusters. In summary, it is anticipated that this kind of extensive computational studies facilitate to understand the structural, electronic, chemical and thermochemical properties of hydrated atmospheric gases that leads to the formation of prenucleation clusters followed by atmospheric aerosols.

Interaction of Glutamic Acid/Protonated Glutamic Acid with Amide and Water Molecules: A Theoretical Study

Amino acids are important nitrogen-containing compounds and organic carbon components that exist widely in the atmosphere. The formation of atmospheric aerosols is affected by their interactions with amides. The dimers formed by glutamic acid (Glu) or protonated glutamic acid (Glu⁺) with three kinds of amide molecules (formamide FA, acetamide AA, urea U) and the hydrated clusters formed by Glu or Glu⁺, U molecules along with one to six water molecules were systematically studied at the M06-2X/6-311++G(3df,3pd) level. U is predicted to form a more stable structure with Glu/Glu⁺ than FA and AA by thermodynamics. If the concentration ratio of FA to U is less than 10⁴, U will play a critical role in NPF. The degree of hydration in Glu⁺-mU-nW is higher than that of Glu-mU-nW (m = 0, 1; n = 0-6) clusters. Notably, Glu contributes more to the Rayleigh scattering properties than glutaric acid and sulfuric acid, and thus may lead to the destruction of atmospheric visibility. This study is helpful to better understand the properties of organic aerosols containing amino acids or amides.

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.

Hydrogen-Bond Topology Is More Important Than Acid/Base Strength in Atmospheric Prenucleation Clusters

We explored the hypothesis that on the nanoscale level, acids and bases might exhibit different behavior than in bulk solution. Our study system consisted of sulfuric acid, formic acid, ammonia, and water. We calculated highly accurate Domain-based Local pair-Natural Orbital- Coupled-Cluster/Complete Basis Set (DLPNO-CCSD(T)/CBS) energies on DFT geometries and used the resulting Gibbs free energies for cluster formation to compute the overall equilibrium constants for every possible cluster. The equilibrium constants combined with the initial monomer concentrations were used to predict the formation of clusters at the top and the bottom of the troposphere. Our results show that formic acid is as effective as ammonia at forming clusters with sulfuric acid and water. The structure of formic acid is uniquely suited to form hydrogen bonds with sulfuric acid. Additionally, it can partner with water to form bridges from one side of sulfuric acid to the other, hence demonstrating that hydrogen bonding topology is more important than acid/base strength in these atmospheric prenucleation clusters.

Monomers of Glycine and Serine Have a Limited Ability to Hydrate in the Atmosphere

The role of atmospheric aerosols on climate change is one of the biggest uncertainties in most global climate models. Organic aerosols have been identified as potential cloud condensation nuclei (CCN), and amino acids are organic molecules that could serve as CCN. Amino acids make up a significant portion of the total organic material in the atmosphere, and herein we present a systematic study of hydration for two of the most common atmospheric amino acids, glycine and serine. We compute DLPNO/CCSD(T)//M08-HX/MG3S thermodynamic properties and atmospheric concentrations of Gly(H₂O)_n and Ser(H₂O)_n, where n = 1-5. We predict that serine-water clusters have higher concentrations at n = 1 and 5, while glycine-water clusters have higher concentrations at n = 2-4. However, both glycine and serine are inferred to exist primarily in their nonhydrated monomer forms in the absence of other species such as sulfuric acid.

Determination of the amine-catalyzed SO3 hydrolysis mechanism in the gas phase and at the air-water interface

New particle formation (NPF) involving amines in the atmosphere is considered an aggregation process, during which stable molecular clusters are formed from amines and sulfuric acid via hydrogen bond interaction. In this work, ab initio dynamics simulations of ammonium bisulfate formation from a series of amines, SO₃, and H₂O molecules were carried out in the gas phase and at the air-water interface. The results show that reactions between amines and hydrated SO₃ molecules in the gas phase are barrierless or nearly barrierless processes. The reaction rate is related to the basicity of gas-phase amines-the stronger the basicity, the faster the reaction. Furthermore, SO₃ hydrolysis catalyzed by amines occurs simultaneously with H₂SO₄-amine cluster formation. At the air-water interface, reactions between amines and SO₃ involve multiple water molecules. The reaction center's ring structure (amine-SO₃-nH₂O) promotes the transfer of protons in the water molecules. The formed ammonium cation (-RNH₃⁺) and the bisulfate anion (HSO₄^-) are present and stable by means of hydrogen bond interaction. The cluster formation mechanism provides new insights into NPF involving amines, which may play an important role in the formation of aerosols in some heavily polluted areas - e.g., those with a high amine concentration.

Theoretical study of the hydration effects on alkylamine and alkanolamine clusters and the atmospheric implication

Amines are important atmospheric nucleation precursors in polluted areas. However, specific roles of various amines in enhancing the stability of pre-nucleation clusters are poorly understood. Herein, different roles of trimethylamine (TMA) and monoethanolamine (MEA) in the formation of sulfuric acid (SA)-based pre-nucleation clusters were investigated. The hydration effects of up to four water (W) molecules on the interaction of the acid-base pairs of (TMA)(SA) and (MEA)(SA) were computationally investigated at the M06-2X/6-311++G (3df, 3pd) level of theory. Results show that the formation thermodynamics of key intermediate clusters are different with amines. Besides, MEA-enhanced formation of pre-nucleation clusters plays an important role in environments with high humidity while TMA may be the dominant nucleation precursors in dry conditions. The concentration of dominant MEA-containing pre-nucleation cluster is at least three orders of magnitude higher than that of TMA and dimethylamine near emission sources at conditions of T = 298.15 K and RH = 60%, indicating that alkanolamine may play an important role in atmospheric nucleation. Furthermore, the hydration of MEA is easier than that of alkylamines. Our results put forward the need to distinguish the performance of different types of amine under specific conditions to better model the new particle formation events in highly polluted areas. Besides, this study indicates that alkanolamines such as MEA are important participator in new particle formation.

Enhancing Potential of Trimethylamine Oxide on Atmospheric Particle Formation

The role of an oxidation product of trimethylamine, trimethylamine oxide, in atmospheric particle formation is studied using quantum chemical methods and cluster formation simulations. Molecular-level cluster formation mechanisms are resolved, and theoretical results on particle formation are confirmed with mass spectrometer measurements. Trimethylamine oxide is capable of forming only one hydrogen bond with sulfuric acid, but unlike amines, trimethylamine oxide can form stable clusters via ion–dipole interactions. That is because of its zwitterionic structure, which causes a high dipole moment. Cluster growth occurs close to the acid:base ratio of 1:1, which is the same as for other monoprotic bases. Enhancement potential of trimethylamine oxide in particle formation is much higher than that of dimethylamine, but lower compared to guanidine. Therefore, at relatively low concentrations and high temperatures, guanidine and trimethylamine oxide may dominate particle formation events over amines.

Hydration of Atmospheric Molecular Clusters: A New Method for Systematic Configurational Sampling

We present a new systematic configurational sampling algorithm for investigating the potential energy surface of hydrated atmospheric molecular clusters. The algorithm is based on creating a Fibonacci sphere around each atom in the cluster and adding water molecules to each point in nine different orientations. For the sampling of water molecules to existing hydrogen bonds, the cluster is displaced along the hydrogen bond, and a water molecule is placed in between in three different orientations. Generated redundant structures are eliminated based on minimizing the root-mean-square distance of different conformers. Initially, the clusters are sampled using the semiempirical PM6 method and subsequently using density functional theory (M06-2X and ωB97X-D) with the 6-31++G(d,p) basis set. Applying the developed algorithm, we study the hydration of sulfuric acid with up to 15 water molecules. We find that the addition of the first four water molecules "saturate" the sulfuric acid molecule and that they are more thermodynamically favorable than the addition of water molecules 5-15. Using the large generated set of conformers, we assess the performance of approximate methods (ωB97X-D, M06-2X, PW91, and PW6B95-D3) in calculating the binding energies and assigning the global minimum conformation compared to high level CCSD(T)-F12a/VDZ-F12 reference calculations. The tested DFT functionals systematically overestimate the binding energies compared to coupled cluster calculations, and we find that this deficiency can be corrected by a simple scaling factor.

Interaction of oxalic acid with methylamine and its atmospheric implications

Oxalic acid, which is one of the most common dicarboxylic acids, is expected to be an important component of atmospheric aerosols. However, the contribution of oxalic acid to the generation of new particles is still poorly understood. In this study, the structural characteristics and thermodynamics of (C2H2O4)(CH3NH2) n (n = 1-4) were investigated at the PW91PW91/6-311++G(3df,3pd) level of theory. We found that clusters formed by oxalic acid and methylamine are relatively stable, and the more the atoms participating in the formation of a ring-like structure, the more stable is the cluster. In addition, via the analysis of atmospheric relevance, it can be revealed that clusters of (C2H2O4)(CH3NH2) n (n = 1-4) have a noteworthy concentration in the atmosphere, which indicates that these clusters could be participating in new particle formation. Moreover, by comparison with (H2C2O4)(NH3) n (n = 1-6) species, it can be seen that oxalic acid is more readily bound to methylamine than to ammonia, which promotes nucleation or new particle formation. Finally, the Rayleigh scattering properties of clusters of (C2H2O4)(CH3NH2) n (n = 1-4) were investigated for the first time to determine their atmospheric implications.

Mechanistic and kinetic study on the catalytic hydrolysis of COS in small clusters of sulfuric acid

The catalytic hydrolysis of carbonyl sulfide (COS) and the effect of small clusters of H₂O and H₂SO₄ have been studied by theoretical calculations. The addition of H₂SO₄ could increase the enthalpy change (ΔH<0) and decrease relative energy of products (relative energy<0), resulting in hydrolysis reaction changed from an endothermic reaction to an exothermic reaction. Further, H₂SO₄ decreases the energy barrier by 5.25 kcal/mol, and it enhances the catalytic hydrolysis through the hydrogen transfer effect. The (COS + H₂SO₄-H₂O) reaction has the lowest energy barrier of 29.97 kcal/mol. Although an excess addition of H₂O and H₂SO₄ increases the energy barrier, decreases the catalytic hydrolysis, which is consistent with experimental observations. The order of the energy barriers for the three reactions from low to high are as follows: COS + H₂SO₄-H₂O < COS + H₂O + H₂SO₄-H₂O < COS + H₂O+(H₂SO₄)₂. Kinetic simulations show that the addition of H₂SO₄ can increase the reaction rate constants. Consequently, adding an appropriate amount of sulfuric acid promotes the catalytic hydrolysis of COS both kinetically and thermodynamically.

Uptake of water by an acid–base nanoparticle: theoretical and experimental studies of the methanesulfonic acid–methylamine system

Uptake of water by nanoparticles composed by methanesulfonic acid and methylamine using a combination of theoretical calculations and laboratory experiments.

Hydration of the methanesulfonate–ammonia/amine complex and its atmospheric implications

Methanesulfonate (MSA⁻), found in substantial concentrations in the atmosphere, is expected to enhance aerosol nucleation and the growth of nanoparticles, but the details of methanesulfonate clusters are poorly understood. In this study, MSA⁻ was chosen along with ammonia (NH₃) or three common amines and water (H₂O) to discuss the roles of ternary homogeneous nucleation and ion-induced nucleation in aerosol formation. We studied the structural characteristics and thermodynamics of the clusters using density functional theory at the PW91PW91/6-311++G(3df,3pd) level. The analysis of noncovalent interactions predicts that the amines can form more stable clusters with MSA⁻ than NH₃, in agreement with the results from structures and thermodynamics; however, the enhancement in stability for amines is not large enough to overcome the difference in the concentrations of NH₃ and amines under typical atmospheric conditions. In addition, the favorable free energies of formation for the (MSA⁻)(NH₃/amines)(H₂O)_n (n = 0–3) clusters at 298.15 K show that MSA⁻ could contribute to the aerosol nucleation process with binding NH₃/amines and H₂O up to n = 3. There are strong temperature and humidity dependences for the formation of complexes; higher humidity and temperature promote the formation of larger hydrates. Finally, for the (MSA⁻)(NH₃/amines)(H₂O)_n clusters, the evaporation rates were determined to further investigate the atmospheric implications.

Methanesulfonate (MSA⁻), found in substantial concentrations in the atmosphere, is expected to enhance aerosol nucleation and the growth of nanoparticles, but the details of methanesulfonate clusters are poorly understood.

ArbAlign: A Tool for Optimal Alignment of Arbitrarily Ordered Isomers Using the Kuhn-Munkres Algorithm

When assessing the similarity between two isomers whose atoms are ordered identically, one typically translates and rotates their Cartesian coordinates for best alignment and computes the pairwise root-mean-square distance (RMSD). However, if the atoms are ordered differently or the molecular axes are switched, it is necessary to find the best ordering of the atoms and check for optimal axes before calculating a meaningful pairwise RMSD. The factorial scaling of finding the best ordering by looking at all permutations is too expensive for any system with more than ten atoms. We report use of the Kuhn-Munkres matching algorithm to reduce the cost of finding the best ordering from factorial to polynomial scaling. That allows the application of this scheme to any arbitrary system efficiently. Its performance is demonstrated for a range of molecular clusters as well as rigid systems. The largely standalone tool is freely available for download and distribution under the GNU General Public License v3.0 (GNU_GPL_v3) agreement. An online implementation is also provided via a web server ( http://www.arbalign.org ) for convenient use.

Formation of atmospheric molecular clusters consisting of sulfuric acid and C8H12O6 tricarboxylic acid

We present the structures and thermochemical properties of (MBTCA)₁₋₃(H₂SO₄)₁₋₄ atmospheric molecular clusters.

Basis set convergence of the binding energies of strongly hydrogen-bonded atmospheric clusters

We present the first binding energy benchmark set at the CBS limit of strongly hydrogen bonded atmospheric molecular clusters.

Effects of Global and Local Anharmonicities on the Thermodynamic Properties of Sulfuric Acid Monohydrate

We use state-of-the-art electronic structure calculation methods and large basis sets to obtain reliable values for the thermodynamic properties of sulfuric acid monohydrate and study the effects of vibrational anharmonicity on these properties. We distinguish between two forms of vibrational anharmonicity: local anharmonicity, which refers to the anharmonicity of the vibrational modes of a given cluster conformer, and global anharmonicity, which originates from accounting for the presence of different conformers in the first place. In our most accurate approach, we solve the nuclear Schrödinger equation variationally for the intermolecular large-amplitude motions, thus quantum-mechanically accounting for the presence of higher-energy conformers for both reactants and products, while using the standard vibrational perturbational approach for the other vibrational modes. This results in a value of -11.0 kJ/mol for the reaction Gibbs free energy at 298.15 K. When standard vibrational perturbational approaches are employed, the effects of local anharmonicity depend heavily on the choice of the electronic structure calculation basis set. In fact, better results can often be achieved by combining a simple harmonic treatment for the vibrational partition function with a statistical mechanical accounting of global anharmonicity. Thus, we recommend that future studies that intend to include anharmonicity start by accounting for the presence of higher-energy conformers and only then consider whether local anharmonicity calculations are feasible and necessary.

Strong Hydrogen Bonded Molecular Interactions between Atmospheric Diamines and Sulfuric Acid

We investigate the molecular interaction between methyl-substituted N,N,N',N'-ethylenediamines, propane-1,3-diamine, butane-1,4-diamine, and sulfuric acid using computational methods. Molecular structure of the diamines and their dimer clusters with sulfuric acid is studied using three density functional theory methods (PW91, M06-2X, and ωB97X-D) with the 6-31++G(d,p) basis set. A high level explicitly correlated CCSD(T)-F12a/VDZ-F12 method is used to obtain accurate binding energies. The reaction Gibbs free energies are evaluated and compared with values for reactions involving ammonia and atmospherically relevant monoamines (methylamine, dimethylamine, and trimethylamine). We find that the complex formation between sulfuric acid and the studied diamines provides similar or more favorable reaction free energies than dimethylamine. Diamines that contain one or more secondary amino groups are found to stabilize sulfuric acid complexes more efficiently. Elongating the carbon backbone from ethylenediamine to propane-1,3-diamine or butane-1,4-diamine further stabilizes the complex formation with sulfuric acid by up to 4.3 kcal/mol. Dimethyl-substituted butane-1,4-diamine yields a staggering formation free energy of -19.1 kcal/mol for the clustering with sulfuric acid, indicating that such diamines could potentially be a key species in the initial step in the formation of new particles. For studying larger clusters consisting of a diamine molecule with up to four sulfuric acid molecules, we benchmark and utilize a domain local pair natural orbital coupled cluster (DLPNO-CCSD(T)) method. We find that a single diamine is capable of efficiently stabilizing sulfuric acid clusters with up to four acid molecules, whereas monoamines such as dimethylamine are capable of stabilizing at most 2-3 sulfuric acid molecules.

Characterization of the nucleation precursor (H2SO4–(CH3)2NH) complex: intra-cluster interactions and atmospheric relevance

Amines have been proposed to participate in the nucleation process, but the electron density analysis and the determination of a temperature dependence of the clusters are still lacking.

Hydrolysis of Sulfur Dioxide in Small Clusters of Sulfuric Acid: Mechanistic and Kinetic Study

The deposition and hydrolysis reaction of SO2 + H2O in small clusters of sulfuric acid and water are studied by theoretical calculations of the molecular clusters SO2-(H2SO4)n-(H2O)m (m = 1,2; n = 1,2). Sulfuric acid exhibits a dramatic catalytic effect on the hydrolysis reaction of SO2 as it lowers the energy barrier by over 20 kcal/mol. The reaction with monohydrated sulfuric acid (SO2 + H2O + H2SO4 - H2O) has the lowest energy barrier of 3.83 kcal/mol, in which the cluster H2SO4-(H2O)2 forms initially at the entrance channel. The energy barriers for the three hydrolysis reactions are in the order SO2 + (H2SO4)-H2O > SO2 + (H2SO4)2-H2O > SO2 + H2SO4-H2O. Furthermore, sulfurous acid is more strongly bonded to the hydrated sulfuric acid (or dimer) clusters than the corresponding reactant (monohydrated SO2). Consequently, sulfuric acid promotes the hydrolysis of SO2 both kinetically and thermodynamically. Kinetics simulations have been performed to study the importance of these reactions in the reduction of atmospheric SO2. The results will give a new insight on how the pre-existing aerosols catalyze the hydrolysis of SO2, leading to the formation and growth of new particles.

Hydration of a sulfuric acid–oxalic acid complex: acid dissociation and its atmospheric implication

We have investigated structural characteristics and thermodynamics of the hydration of a sulfuric acid–oxalic acid complex using density functional theory to gain insight into the ternary nucleation and its atmospheric implication.

New particle formation and growth from methanesulfonic acid, trimethylamine and water

Water participates in the formation of initial clusters to enhance particle formation from methanesulfonic acid and trimethylamine.

Rayleigh light scattering properties of atmospheric molecular clusters consisting of sulfuric acid and bases

The Rayleigh light scattering properties of (H₂SO₄)_a(NH₃)_b and (H₂SO₄)_a((CH₃)₂NH)_b atmospheric molecular clusters have been investigated using a response theory approach.