Microscopic mechanism of the interaction between water and formic acid-sodium chloride aerosol

Abstract

As the effect of aerosol on the atmosphere is related to its hygroscopic properties, it is desirable to know the microscopic mechanism for the interaction between water and the aerosol consisting of inorganic and organic ingredients. In this work, molecular dynamics method is employed to investigate the behavior of formic acid - water systems without or with sodium chloride (NaCl), which represent simplified typical atmospheric aerosols. It mainly focuses on the effects of temperature, humidity and NaCl concentration on the structure and the interfacial properties of the aerosol at different simulation times. The results show that when the simulation time is 2 ns, with the increase of temperature the binary aggregate containing 360 H2O displays the demixed structure 1 (water clusters adsorb on the acid aggregate), the mixed structure (water molecules dissolve part of the acid aggregate) and the demixed structure 2 (acid molecules uniformly wrap around the water core). The stability of formic acid particles increases with the increase of NaCl concentration. Hence, the ternary aggregate containing 20 NaCl presents three identical structures, while only the first two structures can be observed for the ternary aggregate with 40/80 NaCl. At the equilibrium state, all three structures can be found in each aggregate containing 360 H2O, while the difference in structure can only be observed at intermediate temperatures. Correspondingly, only the first two structures are evidenced for all aggregates containing 120 H2O at the time of 2 ns and the equilibrium state. Hydrogen atoms (H(CH)) of the formic acid at the surface of all aggregates point toward the gas phase. Moreover, the ratio of organic molecules to inorganic molecules at aerosol surface increases with the increase of temperature and the decrease of NaCl and water content. Finally, the hygroscopic properties of the studied aerosols are discussed.