Study On The Hygroscopicity Of Aerosol By Using Molecular Dynamics Simulation And к-K?hler Theory

Haze is an atmospheric phenomenon caused by a series of complex physical and chemical reactions of a large number of fine particles(such as PM2.5)suspended in the atmosphere for a long time,the essence of haze is the stable aerosol.With the acceleration of industrialization and urbanization of our country in recent years,serious haze events with long duration and wide coverage have occurred many times in various parts of the country.With the emphasis on haze prevention and control and the advancement of related research,the formation mechanism of haze has gradually become one of the hotspots in this field.In this work,the interaction of components in haze aerosol and the hygroscopicity of aerosol system were studied by means of molecular dynamics simulation and related theoretical models such as κ-Kohler theory.The main research contents are as follows:(1)Molecular dynamics simulation was used to study the distribution and interaction of components in an aerosol system consisting of(NH4)2SO4,pinonic acid and water.It is found that the components in the aerosol system show obvious layered distribution in space.Water molecules converge under surface tension to form spherical droplets.NH4+ ions and SO42-ions are close to each other and concentrated in the central region of droplets due to strong electrostatic interaction.Pinonic acid molecules are mainly spread on the outer surface of droplets,and hydrophilic hydroxyl and carboxyl groups point to the inner side of droplets,while hydrophobic alkyl groups point to the outer side The effect of pinonic acid on surface tension of droplets was studied.The results show that pinonic acid has an obvious inhibitory effect on the surface tension of droplets,and the larger the aerosol system is,the more obvious the inhibitory effect is.Finally,the Szyszkowski empirical equation is used to calculate the surface tension of the aerosol system.The results show that the simulation results are in good agreement with the calculation of the empirical equation.(2)The hygroscopicity of aerosol system was studied theoretically by using K-Kohler theory.For the single component system,the higher the relative humidity is,the stronger the hygroscopicity of the system is,and the hygroscopicity of the inorganic system is generally stronger than that of the organic system.At the same time,it was found that the addition of organic compounds could inhibit the hygroscopicity of inorganic systems to a certain extent.After that,the effect of particle size on the hygroscopicity of aerosols was studied.It was found that the increase of particle size contributed to the improvement of the hygroscopicity of the aerosol system.Finally,the hygroscopicity of the aerosol system is studied.The results show that the content of each component in the aerosol system is constantly changing,and the hygroscopicity of the system is also increasing during the process of the change of air quality from good to bad.The influence of the content of each component in the aerosol system on the hygroscopicity of the system was studied by using K-Kohler theory.It was found that the hygroscopicity of the system would be improved to the greatest extent by increasing the content of inorganic substances and decreasing the content of organic substances.The effects of different components on the hygroscopicity were studied,the results showed that(NH4)2SO4 had the most obvious effect on the hygroscopicity.(3)Based on the Szyszkowski empirical equation and the surface film theory,the deviation of surface tension caused by organic matter in K-Kohler theory is corrected.The calculation of two empirical formulas shows that the addition of organic compounds can significantly inhibit the surface tension of droplets.Finally,the Kohler curves drawn by using the surface tension parameters of the main phase water and the Kohler curves drawn by using two empirical formulas to modify the surface tension parameters are compared.The results show that the difference of the surface tension terms will make the final hygroscopicity growth curve deviate to a certain extent,and the calculated results after correcting the surface tension terms are closer to the experimental values.