ABSTRACT
Atmospheric aerosols have a tremendous influence on visibility, climate, and human health. New particle formation (NPF) is a crucial source of atmospheric aerosols. At present, certain field observations and experiments have discovered the presence of 3-hydroxy-4,4-dimethylglutaric acid (HDMGA), which may participate in NPF events. However, the nucleation mechanism of HDMGA has not been clearly understood. In addition, dimethylamine (DMA) is an important precursor of nucleation. The nucleation mechanism involving HDMGA and DMA has not been studied. In this study, the most stable structures of (HDMGA)(H2O)n (n = 0-3) and (HDMGA)(DMA)(H2O)n (n = 0-3) were obtained by using M06-2X coupled with the 6-311++G(3df,3pd) basis set. The α-carboxyl group is directly attached to the amino group in all the most stable configurations. Proton transfer enhances the strength of a hydrogen bond, as well as promotes the generation of a global minimum structure. Temperature has a considerable influence on the distribution of isomers for (HDMGA)(DMA)(H2O)3 as compared to the other investigated clusters. The Gibbs free energy values reveal that most of the clusters can exist in NPF, except for (HDMGA)(H2O)1. The process of adding a cluster of (H2O)n more likely occurs in the atmosphere than gradually adding a single water molecule.
ABSTRACT
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.
ABSTRACT
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.
ABSTRACT
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.
ABSTRACT
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.
