Structures And Properties Of Methyamine-Containing Atmospheric Nucleation Clusters

Aerosol is defined as a relatively stable suspension system forming with liquid or solid particles uniformly dispersing in the atmosphere, with 0.003 to 100 μm particle size range. Because of its hygroscopicity, its different effects to sunlight including absorption, scattering and reflection, and containing kinds of toxic organics such as polycyclic aromatic hydrocarbons, aerosol has serious influence on the visibility and climate of atmosphere and human health. The mechanism of new particle formation (NPF) of atmospheric aerosol has become the vital problem to be solved in the word, especially in our country.Sulfuric acid and water are generally considered to play a key role in the process of the atmospheric NPF. But binary homogeneous nucleation of sulfuric acid and water by itself can not explain the rate of NPF observed, particularly in the lower regions of the troposphere, indicating that other ternary species might also be involved in nucleation. Amines are common atmospheric species. The global emission of methylamine is about 83±26 Gg N a-1 compared to 33 ± 19 Gg N a-1 for dimethylamine. However, most experimental and computational studies have been devoted to dimethylamine, while there is no systematic work devoted to methylamine. In this work, we studied the structures and the properties of CH3NH2(H2O)n (n=1-7) and CH3NH2(H2SO4)m(H2O)n (m=0-3, n=0-3) clusters. The main contents and conclusions are summarized as follows:For CH3NH2(H2O)n (n=1-7) clusters, we firstly used the Basin-Hopping (BH) structure search algorithm and density functional theory (DFT) to get the stable structures. And then we had studied the stability of the structures and atmospheric relevance. The results are as follows:(1)The stable structures of clusters consisting of one methylamine molecule and up to seven waters arise from a fusion of tetrameric or pentameric rings; (2) The strengths of the H2N…H-O hydrogen bonds of the global minima increase as the sizes of clusters increase except n=5 where there is a slight fluctuation; (3) The methylamine hydration is difficult to form and grow at lower troposphere and they are likely to exist in the upper troposphere where the temperature is relatively low.For CH3NH2(H2SO4)m(H2O)n (m=0-3, n=0-3) clusters, we had studied the structures, energies, the mechanism of nucleation, the distribution of hydrate and optical properties. The results are as follows:(1) The configurations of the clusters are corresponding with the relative quantity of sulfuric acid in the clusters. When the number of sulfuric acid is relative small, water molecules located between the acid and base molecules, while with a large number of sulfuric acid in the cluster, the acid and base molecules contact directly and water molecules located at the edge of the clusters. This is mainly due to the dilution effect of water molecules:(2) 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, respectively. The degree of reactions each step are discussed. Our work provides a reliable reference for further investigation on the mechanism of atmospheric NPF containing methylamine molecule; (3) CH3NH2(H2SO4)(H2O)2, CH3NH2(H2SO4)2 and CH3NH2(H2SO4)3 are most likely to exist in 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; (4) Both H2SO4 and H2O molecules could increase the Rayleigh scattering intensities and isotropic mean polarizabilities, with greater influence by the sulfuric acid molecules.There are three innovations in this present work:(1) We firstly propose that the most stable structures of methylamine hydrate arise from a fusion of tetrameric or pentameric rings, and further we explained this form may make the structures more stable;(2) We studied the change of intermolecular force -H2N…H-O hydrogen bonds with the change of cluster sizes in multi-molecular systems;(3) We compared the growth by adding a water and sulfuric acid molecule step by step to explore the growth mechanism of clusters in the atmosphere.