Spontaneous And Stimulated Raman Spectroscopy Research On Dynamics And Thermodynamics Of The Hygroscopicity Of Magnesium Salt Aerosols

Aerosols are suspended particles in the atmosphere. Aerosol particles affect the climate, change the visibility and damage human health through the changing of size, phase state and morphology, which are determined by the hygroscopicities of aerosols. Thermodynamics and kinetics studies of aerosol hygroscopicities are the frontier of multi-disciplinary researches. The condensed phase mass transfer kinetics, ion-pair formation and molecular conformation of magnesium salts and mixed organic/ magnesium salts aerosols are researched by using the in situ spontaneous and stimulated Raman spectroscopy in this paper. The results are as follows:First, hygroscopicity and volatility of single magnesium acetate(Mg Ac2) aerosol particle at various relative humidities(RHs) are studied by a single-beam optical tweezers. Refractive indices(RIs) and radius of Mg Ac2 droplets are characterized by cavity enhanced Raman spectroscopy. The mass transfer in Mg Ac2 droplets are described by the ratios of half-time for changes in droplet size to the half-time for the change in RH(t1/2(radius)/t1/2(RH)) which provide a phenomenological guide as to the timeresponse of the droplet size and the changes in water flux. Due to the t1/2(radius)/t1/2(RH) water transfer in droplet at RH less than 50% is significantly impeded beacause of the formation of amorphous gel structure. Apparent diffusion constant of water molecule in gel state Mg Ac2 droplets are in the range of 6.43×10-17~1.51×10-16 m2·s-1. To compare volatilization of HAc in different systems, Mg Ac2 and sodium acetate(Na Ac) droplets are maintained at several different stable RHs during up to 86,000 s. At RH≈74%, insoluble magnesium hydroxide inclusions are formed in Mg Ac2 droplets due to the volatilization of HAc, and whispering gallery modes(WGMs) of Mg Ac2 droplets in the Raman spectrum quench after 50,000 s. In sharp contrast, after 86,000 s at RH≈70%, Na Ac droplets are in well-mixed liquid states, containing soluble sodium hydroxide. At this state, the RI of Na Ac droplet increases, and the quenching of WGMs is not observable.Secondly, the kinetics of water transport in the aerosol particle are reported by comparing Raman spectra with high spatial resolution collected from the surface and center of Mg SO4 droplet. The intensity ratio(I1021/I995) of Raman bands at 1021 and995 cm-1 which can be assigned to the polymeric chain and the contact ion pair structures, respectively, is used to characterize the gel state. The formation of gel state inhibits the mass transfer in the droplet, and leads to compositional differences between the surface and center of droplet. The delay of increasing of gel degree in the droplet centre is a characteristic of gel formation and mass transfer hinder. Apparent diffusion constant of gel state Mg SO4 aerosols with different radius is 2.55×10-16(for droplet with 10 mm radius) and 4.78×10-15 m2·s-1(for droplet with 25 mm radius), respectively. Optical tweezers are used to research how the waiting time(the time is equivalent to the time over which the particle is in RH 10%) influence the apparent diffusion constant. The apparent diffusion constant of a Mg SO4 droplet with different waiting time(from 30 min to 180 min) are in the range of 2.27×10-16 to 1.42×10-16 m2·s-1, respectively.Finally, we applied confocal-Raman technology combined with optical microscopy to investigate the relationship between the hygroscopic behavior and molecular interactions of mixed glycerol/Mg(NO3)2/water droplet. Raman spectra provide detail structural information about interactions between glycerol molecules and Mg2+ ions as well as glycerol and NO3- ions through electrostatic interaction and hydrogen bonding. The change of CH2 stretching band of glycerol molecules in mixed droplets suggests that the backbone structures of glycerol mainly transform from aa to gg in the dehumidifying processes, and the additional Mg2+ ions strongly influence the structure of glycerol molecules. Because the existence of glycerol suppresses the crystallization of Mg(NO3)2?6H2O in the dehumidifying processes, Mg(NO3)2 molecules in mixed droplets form amorphous state rather than crystallization of Mg(NO3)2?6H2O, when relative humidity is lower than 17.8%. Moreover, the molar ratio of NO3- to glycerol is higher in the center than that on the outer region in the mixed droplets.