Study The Hygroscopicity,Mass Transfer And Nucleation Of Aerosols By Infrared Spectroscopy

The studying of thermodynamics and kinetics of aerosol hygroscopicity is the forefront field of atmospheric aerosol researches.In this work,the author built up a pulse relative humidity?RH?controlling system and a simple expansion cloud chamber,combining with Vacuum FTIR spectrometer,to study thermodynamic hygroscopicity of typical aerosols,nucleation kinetics during aerosol efflorescence,water transfer kinetics in high viscous aerosols and our findings covered the following points:?1?The hygroscopic curves of sodium nitrate and ammonium nitrate under pulsed RH were found to be overlapped with those under equilibrate state,bulk phase limited water transfer was not observed.Which means complicated ion pairs formed in supersaturated sodium nitrate droplets would not significantly hinder water transfer.While under pulsed RH,gel structure formed below RH 40%in magnesium sulfate droplets slowed down the water diffusion,showed characters of water transfer limited process;for aged magnesium nitrate droplets,hydrolysis of magnesium ions under high vacuum increased the viscosity of droplets and then hindered water transfer.?2?The water diffusion coefficients in sucrose droplets under wide range of RH were measured.When RH decreased from 70%to 0.3%,the water diffusion coefficient decreased from 1.6×10^-12 m²s^-1 to 2.0×10^-16 m²s^-1.Under RH range of 70%to 30%,the measured water diffusion coefficients in new-made and aged magnesium nitrate droplets were in the magnitude of 10^-12 m²s^-1 and 10^-13 m²s^-1,respectively.?3?Studied the efflorescence nucleation kinetics of sodium nitrate,ammonium nitrate and ammonium chloride aqueous droplets,obtained the corresponding crystalline nucleation rates.The heterogeneous nucleation rate of sodium nitrate increased from 10⁸ to 10¹¹ m^-2s^-1,when RH decreased from 46%to 26%.The heterogeneous nucleation rate of ammonium nitrate increased from 10⁸ to 10¹¹ m^-2s^-1,when RH decreased from 35%to 15%.And the heterogeneous nucleation rate of ammonium chloride increased from 10⁶ to 10¹¹ m^-2s^-1,when RH decreased from 61%to 46%.By comparing nucleation rate results under different RH change velocity,the impact of RH change velocity on nucleation rate was analyzed.For magnesium chloride droplets,during dehydration,when the solution concentration reached water solute mole ratio of about 6,magnesium chloride experienced a liquid-solid phase transition,transformed into amorphous solid hydrated salt,then after slowly re-arrangement of ions and molecules,transformed into crystalline phase by solid-solid phase transition.?4?By using the mid-infrared extinction spectra of droplets,combining with the simulated results from MiePlot software,we observed the droplets formation and growth processes during adiabatic expansion of water vapor contained air.The size of generated droplets reached 2?m in about 0.1 s,and then continuously increased to 5?m in another 0.5s.The droplets formation rate was much higher than the homogeneous nucleation rate of water vapor under the same condition,possible reason was the nucleus in the air promoted vapor to translate into droplets,indicating that the particle number concentration is a key factor of cloud droplets concentration in the air.