Interaction between water and acetic acid-sodium halide aerosol: A molecular dynamics study

Hygroscopicity of atmospheric aerosols is closely related to their composition, but the detailed mechanism is still poorly understood. In this study, molecular dynamics studies on water adsorption around the pure acetic acid aerosol (a binary system) and around the acetic add-sodium halide aerosol (a ternary system) are conducted. Effects of sodium halide species and the simulation duration on the structural and the interfacial properties of the acetic acid particle under different water contents and thermodynamic temperatures are analyzed emphatically. It is found that, at the simulation duration of 2 ns, for the binary acetic add-water aggregate with a higher water content (360 H2O), three structures can be observed in turn with the increase of temperature, corresponding to the demixed structure 1 (water clusters are adsorbed on the surface of add aggregate), the mixed structure (acid cluster is partially dissolved) and the demixed structure 2 (acid molecules uniformly distribute on the surface of water nucleus), while at a lower water content (120 H2O), only the first two structures appear. What's more, the stability of organic particle is increased after the addition of sodium halide, and the order of enhancement effect is NaF > NaCl > NaBr > NaI, which becomes more obvious with the increase of water content and temperature. These effects lead to the appearance of only the demixed structure 1 for the ternary acetic acid-NaF-water aerosol, the demixed structure 1 and the mixed structure for the ternary acetic acid-NaCl/NaBr-water aerosol and the demixed structure 1, the mixed structure and the demixed structure 2 for the ternary acetic acid-NaI-water aerosol at higher water content (360 H2O). After reaching equilibrium, all three structures are presented for each system containing 360 H2O, and the significant difference in the structure of particles only occurs at intermediate temperatures. However, the morphology of acetic add molecules on the surface of particle is not influenced by the sodium halide and the simulation duration. Finally, the effect of sodium halide on the hygroscopicity of the organic aerosol is discussed. (C) 2016 Elsevier B.V. All rights reserved.