Shou-Kui Miao

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

Oxalic acid (OA), one of the most common organic acids in the Earths atmosphere, is expected to enhance the nucleation and growth of nanoparticles containing sulfuric acid (SA) and water (W); however, the details about the hydration of OA–SA are poorly understood, especially for the larger clusters with more water molecules. We have investigated the structural characteristics and thermodynamics of these clusters using density functional theory at the PW91PW91/6-311++G(3df,3pd) level. The favorable free energies of formation and obvious concentrations of the OA–SA–Wn (n = 0–6) clusters at 298.15 K predict that oxalic acid can contribute to the aerosol nucleation process by binding to sulfuric acid and water until n = 6. There is strong temperature dependence for the complexes formation, and the energy order of these complexes is altered from 100 to 400 K, regardless of different cluster sizes or different isomers within the same cluster size. The lower temperature and higher relative humidity promote the formation of hydrates. Additionally, the investigation of acid dissociation predicts that several acid-dissociated models could coexist in the atmosphere, specifically when more water molecules are present. Fewer waters may be needed to cause the acid dissociation, as the relative acidity of the cluster increases, which plays a key role in forming relatively stable hydrated clusters of OA–SA. Finally, the Rayleigh scattering properties of OA–SA–Wn (n = 0–6) have been systematically investigated for the first time to further discuss its atmospheric implication.

Hydration of oxalic acid–ammonia complex: atmospheric implication and Rayleigh-scattering properties

A previous study of the binary system (H2C2O4)(NH3)n (n = 1–6) suggested that an oxalic acid–ammonia complex may participate in atmospheric aerosol formations. However, the mechanism of the hydration of these cores is poorly understood. In this study, the hydration of (H2C2O4)(NH3) and (H2C2O4)(NH3)2 cores with up to three water molecules is investigated with respect to different routes of formation. The results may improve understanding of the nucleation of clusters containing oxalic acid in the atmosphere. Acid dissociation is found to occur during the hydration process, leading to a HC2O4−/NH4+ ion pair. In contrast with the (H2C2O4)(NH3)2 core, water molecules appear to be unfavorable with regard to the formation of hydrates with a (H2C2O4)(NH3) core; additionally, temperature is found to affect the formation of clusters and the distributions of different isomers with the same size, but the impact of relative humidity on the hydrates seems insignificant, implying that the formation of these clusters may be more favorable under cold ambient conditions. The monohydrates and dihydrates of the (H2C2O4)(NH3)2 core may be relatively extensive in (H2C2O4)(NH3)m(H2O)n (m = 1–2, n = 1–3) clusters and may contribute to the atmospheric nucleation. Furthermore, this study presents a first attempt at determining the Rayleigh scattering properties of oxalic acid–ammonia–water pre-nucleation clusters; the results show that adding a water molecule could effectively increase the Rayleigh scattering intensity, but a single ammonia molecule may be able to generate a larger increase in the Rayleigh light scattering intensity than a water molecule. This may also indicate that clusters containing oxalic acid and ammonia show high Rayleigh light scattering intensities, but the more ammonia molecules there are in clusters, the higher the Rayleigh light scattering intensity and the greater the contribution to the extinction properties.

Properties and Atmospheric Implication of Methylamine–Sulfuric Acid–Water Clusters

The presence of amines can increase aerosol formation rates. Most studies have been devoted to dimethylamine as the representative of amine; however, there have been a few works devoted to methylamine. In this study, theoretical calculations are performed on CH3NH2(H2SO4)m(H2O)n (m = 0-3, n = 0-3) clusters. In addition to the structures and energetics, we focused on determining the following characteristics: (1) the growth mechanism, (2) the hydrate distributions and the influences of humidity and temperature, (3) Rayleigh scattering properties. We explored the cluster growth mechanism from a thermodynamics aspect by calculating the Gibbs free energy of adding a water or sulfuric acid molecule step by step at three atmospherically relevant temperatures. The relative ease of the reaction at each step is discussed. From the analysis of hydrate distributions, we find that CH3NH2(H2SO4)(H2O)2, CH3NH2(H2SO4)2, and CH3NH2(H2SO4)3 are most likely to exist in the atmosphere. The general trend of hydration in all cases is more extensive with the growing relative humidity (RH), whereas the distributions do not significantly change with the temperature. Analysis of the Rayleigh scattering properties showed that both H2SO4 and H2O molecules could increase the Rayleigh scattering intensities and isotropic mean polarizabilities, with greater influence by the sulfuric acid molecules. This work sheds light on the mechanism for further research on new particle formation (NPF) containing methylamine in the atmosphere.

Interaction of oxalic acid with dimethylamine 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.

On the properties and atmospheric implication of amine-hydrated clusters

Amines have been recognized as important precursor species in the formation of new atmospheric particles. Although dimethylamine–water clusters have been the focus of a large number of theoretical studies during the last few years, some information regarding these clusters, such as the influence of temperature, the analysis of their weak interactions, and their Rayleigh scattering properties, is still lacking. In this study, the equilibrium geometric structures and thermodynamics of (CH3)2NH(H2O)n (n = 1–6) clusters were systematically investigated using density functional theory (PW91PW91) coupled with the 6-311++G(3df,3pd) basis set. To determine the most stable isomer and the order of the different isomers, single-point calculations were executed using a two-point extrapolation method in conjunction with the complete basis set for all isomers. The optimized structures show that the addition of a fifth water molecule changes the most stable configuration from a quasi-planar ring structure to a cage-like configuration. Electron density analysis shows that the interactions of these complexes are mainly medium hydrogen bonds. The dependence on temperature of the conformational population and the Gibbs free energies of the (CH3)2NH(H2O)n (n = 1–6) clusters were determined with respect to temperature (200–300 K). A weak dependence on temperature was found for the formation of (CH3)2NH(H2O)n (n = 1–6) clusters. Dimethylamine–water clusters are favorable at low temperatures, but these clusters may be difficult to form because of the combined effect of Gibbs free energies with small negative values and the low relative concentration of dimethylamine in various atmospheric conditions, and this implies that dimethylamine–water clusters are difficult to form spontaneously in the atmosphere. Finally, the Rayleigh scattering properties of (CH3)2NH(H2O)n (n = 1–6) have been investigated systematically for the first time.

π-Hydrogen Bonding of Aromatics on the Surface of Aerosols: Insights from Ab Initio and Molecular Dynamics Simulation

Molecular level insight into the interaction between volatile organic compounds (VOCs) and aerosols is crucial for improvement of atmospheric chemistry models. In this paper, the interaction between adsorbed toluene, one of the most significant VOCs in the urban atmosphere, and the aqueous surface of aerosols was studied by means of combined molecular dynamics simulations and ab initio quantum chemistry calculations. It is revealed that toluene can be stably adsorbed on the surface of aqueous droplets via hydroxyl-π hydrogen bonding between the H atoms of the water molecules and the C atoms in the aromatic ring. Further, significant modifications on the electrostatic potential map and frontier molecular orbital are induced by the solvation effect of surface water molecules, which would affect the reactivity and pathway of the atmospheric photooxidation of toluene. This study demonstrates that the surface interactions should be taken into consideration in the atmospheric chemical models on oxidation of aromatics.

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.

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. In this study, the clusters of (H2SO4)m(CH3NHCH3)n (m = 1–2, n = 1–3) are studied using the basin-hopping method coupled with density functional theory (DFT). Considering the high flexibility and complexity of a hydrogen bonding environment, the temperature dependence of the conformational population and the relative population fraction of the clusters are investigated. Moreover, the electron density is analyzed to identify the different types of intra-cluster interactions. The results indicate that the ratio between acid and base is very important for the cluster formation. The main interaction type changes from hydrogen bonding to a weak attraction as the number of bases increase. When the number of dimethylamine molecules is less than or equal to that of the sulfuric acid molecules as the most abundant clusters in the atmosphere, we tentatively suggest that the cluster contains less than two dimethylamine molecules because the critical clusters contain two or fewer sulfuric acid molecules. This means that the sulfuric acid–dimethylamine system can only form three main small clusters in the real atmosphere. Thus, other substances, such as water or organic acids, may be involved to promote the growth of clusters, and they may also affect the nucleation. This work predicts the possible forms of dimethylamine with sulfuric acid when participating in nucleation in a theoretical approach, and provides a reliable reference for the research on the nucleation mechanism containing dimethylamine in the atmosphere.