Spectroscopic Observation Of Physical Chemistry Processes In Optical Levitated Single Organic/Inorganic Aerosol Droplet

Investigation of thermodynamics and kinetics process of atmospheric aerosol is the frontier field of atmospheric aerosol research.In this thesis,optical levitation technique is adopted together with cavity enhanced stimulated Raman spectrograph,to make measurements of hygroscopicity,SVOC volatility,diffusion of water in ultra-viscous state,bulk-phase viscosity,liquid-liquid phase separation and heterogeneous nucleation process.We will present our progress by following emphasis:1.A new approach to determine vapour pressures and hygroscopicities of aqueousaerosols containing semi-volatile organic compoundsConcurrent measurements of particle size and composition?inferred from refractive index?are reported here using optical tweezers to isolate and probe individual aerosol droplets over extended timeframes.The measurements are shown to allow accurate retrievals of component vapor pressures and hygroscopic response through examining correlated variations in size and composition for binary droplets containing water and a single organic component.Measurements are reported for a homologous series of dicarboxylic acids,maleic acid,citric acid,glycerol,or 1,2,6-hexanetriol.An assessment of the inherent uncertainties in such measurements when measuring only particle size is provided to confirm the value of such a correlational approach.2.Liquid-Liquid phase separation in mixed organic/inorganic single aqueous aerosoldropletsDirect measurements of the phase separation relative humidity?RH?and morphology of aerosol particles consisting of liquid organic and aqueous inorganic domains are presented.Single droplets of mixed phase composition are captured in a gradient force optical trap,and the evolving size,refractive index?RI?,and morphology are characterized by cavity-enhanced Raman spectroscopy.Starting at a RH above the phase separation RH,the trapped particle is dried to lower RH and the transition to a phase-separated structure is inferred from distinct changes in the spectroscopic fingerprint.In particular,the phase separation RHs of droplets composed of aqueous solutions of polyethylene glycol?PEG-400?/ammonium sulfate and a mixture of C₆-diacids/ammonium sulfate are probed,inferring the RH from the RI of the droplet immediately prior to phase separation.The observed phase separation RHs occur at RH marginally higher?at most 4%?than reported in previous measurements made from studies of particles deposited on hydrophobic surfaces by brightfield imaging.Clear evidence for the formation of phase-separated droplets of core-shell morphology is observed,although partially engulfed structures can also be inferred to form.Transitions between the different spectroscopic signatures of phase separation suggest that fluctuations in morphology can occur.For droplets that are repeatedly cycled through the phase separation RH,the water activity at phase separation is found to be remarkably reproducible?within±0.0013?and is the same for the 1-phase to2-phase transition and the 2-phase to 1-phase transition.By contrast,larger variation between the water activities at phase separation is observed for different droplets?typically±0.02?.3.Slow water transport in ultra-viscous aerosol droplets at low relative humiditiesThe effect of gel formation on the mass transfer of water during evaporation or condensation from MgSO₄ droplets is studied using aerosol optical tweezers coupled with Raman spectroscopy.In particular,the kinetics of water transport during hydration and dehydration are followed for variable step changes in relative humidity and compared with previous measurements using different methodologies.Slow diffusion of water in the particle bulk is shown to limit water evaporation and condensation from the aerosol.Desorption of water continues over a long time at the very low RH region and this is validated with complementary studies made by FTIR-ATR and measurements of water adsorption isotherms.The observations can be rationalized when considering the possible phase transformation of the gel structure at very low RHs.Finally,the influence of the duration of the drying time?RH=10%?on the kinetics of condensation during hydration is investigated.Apparent diffusion coefficients of water molecules in the gel are obtained,showing little dependence on the water activity and droplet composition,and are consistent with the slow removal of water during drying from pores formed at the gel transition RH.On the other hand,influences between SVOC evaporation and mass transfer blocking under different conditions are vital for investigation of atmopheric aerosol chemistry.We acquire real-time radii and composition of the trapped aerosol droplet with the optical tweezer for the analysis of SVOC evaporation and mass transfer process.Bulk phase water diffusion constant derived from Kohlrausch-Williams-Watts exponential fit varies for 7orders of magnitude through the whole RH range,in which diffusion constant corresponds to RH=0%is about 2.88×10^-16 m² s^-1.This roughly matches the viscosity results which vary for 9 orders of magnitude through the whole RH range.Pure component vapour pressure from Maxwell equation ranges for around 2 orders of magnitudes,indicates SVOC evaporation is blocked by formation of amorphous state and mass transfer blocking.4.Micro-rheology and interparticle interactions in aerosols probed with optical tweezersWe will demonstrate that rheological measurements of the bulk and surface properties of aerosol particles can be made directly using optical tweezers,providing important insights into the phase behavior of materials in confined environments and the rate of molecular diffusion in viscous phases.In particular,we will show how the viscosity and surface tension of particles can be measured directly in the under-damped regime at low viscosity.In the over-damped regime,we will show that viscosity measurements can extend close to the glass transition,allowing measurements over an impressive dynamic range of12 orders of magnitude in relaxation timescale and viscosity.Indeed,prior to the coalescence event,we will show how the Brownian trajectories of trapped particles can yield important and unique insights into the interactions of aerosol particles.5.?NH₄?₂SO₄ heterogeneous nucleation of?NH₄?₂SO₄-glycerol system in its dynamicefflorescence progressUsing the Fourier transform infrared attenuated total reflection?FTIR-ATR?technique,we investigated the heterogeneous nucleation process of aqueous ammonium sulphate??NH₄?₂SO₄?binary droplets and?NH₄?₂SO₄/glycerol ternary droplets deposited on the ZnSe substrate with a mean diameter of particle size of 3?m.From the changing absorbance of?-NH₄⁺band at 1435 cm^-1,we determined the heterogeneous nucleation rates of crystalline?NH₄?₂SO₄ in the pure and mixed droplets,which were approximately on the same level.Variation of nucleation rate on composition,droplet substrate contact,relative humidity,size and temperature were estimated.The interfacial tension between an?NH₄?₂SO₄ critical nucleus and surrounding?NH₄?₂SO₄ solution was determined using classical nucleation theory and experimental data in this study.