Discovering the Hygroscopic Behavior of Atmospheric Aerosols with Vapor-Phase Transmission Electron Microscopy

Abstract

Atmospheric aerosols play a large role in cloud formation, radiative forcing, and precipitation. Inorganic and organic aerosol particles can act as seeds for cloud droplet formation through water adsorption and hygroscopic growth. In this work, we utilize in situ transmission electron microscopy to investigate the atomic structure and water uptake properties of atmospheric aerosols with nanometer scale spatial resolution and single second time resolution. This work reveals complex nanoscale water uptake dynamics that have not been previously observed and stresses the need to consider atomic and nanoscale structure and chemistry in water uptake models for atmospheric aerosols. We first used vapor-phase transmission electron microscopy (VPTEM) to directly image nanoscale condensation dynamics of sessile water droplets in electric fields. This work established the effect of the imaging electron beam on the water vapor–liquid equilibrium during VPTEM imaging. Simulations showed that the electron beam generated electric fields of ~108 V/m, which was high enough to depress the water vapor pressure and promote rapid nucleation of nanosized water droplets. A mass-balance model showed that droplet growth was consistent with electric-field-induced condensation, while droplet evaporation was consistent with a radiolysis-induced mechanism. This work identifies several electron beam–sample interactions that impact condensation dynamics and provides the fundamental understanding for the study of more complex vapor–liquid equilibria study with VPTEM in the future.Next, we used in situ VPTEM imaging of individual sodium chloride (NaCl) nanoparticles to visualize water adsorption and aerosol particle dissolution prior to and during deliquescence. This work demonstrates that deliquescence of simple pure salt particles with sizes in the range of 100 nm to several microns is not a spontaneous phase transition, and instead involves a range of complex dissolution and water-condensation dynamics. Then, we imaged lab generated mixed sodium chloride (NaCl)–phthalic acid (PTA) aerosols with VPTEM. Comprehensive, high-resolution characterization demonstrated that the NaCl–PTA aerosols contained a single-crystal NaCl domain that was either fully or partially engulfed by an amorphous PTA shell. Humidity controlled in situ VPTEM revealed multistep water uptake dynamics that depended on the aerosol morphology. This work represents a first step in establishing a complete picture of how complex multicomponent aerosol particles take up water. Finally, we summarize the work in this dissertation and recommend future research avenues for VPTEM imaging of water uptake on atmospheric aerosols. The future work will focus on aged aerosol hygroscopic measurements and organic vapor condensation on aerosol particles, which extends the perspective of our study and the feasibility of VPTEM in aerosol research.