Single-Particle Visualization of Water Uptake of Organic–Inorganic Aerosols with <i>In Situ</i> Transmission Electron Microscopy

Abstract Atmospheric aerosol water uptake properties influence cloud formation and precipitation, atmospheric chemistry, and global climate. Atmospheric aerosols often contain organic acids and inorganic hygroscopic salts, and their internal mixing influences water uptake properties in complex and nonintuitive ways. The heterogeneous nature of aerosol ensembles necessitates nanoscale visualization of aerosol mixing state and water uptake dynamics to uncover mechanistic details. Here we demonstrate that water uptake in phase separated phthalic acid (PTA)/sodium chloride (NaCl) aerosols proceeds by eutonic mixing and partial NaCl deliquescence, while subsequent drying creates core–shell particles containing newly formed sodium phthalate salts. Electron tomography and elemental mapping revealed that lab-made PTA/NaCl aerosols contained a cuboidal crystalline NaCl domain and amorphous PTA domain. Sodium extended beyond the NaCl into a ∼10 nm thick chemically distinct interphase between NaCl and the bulk PTA, consistent with formation of sodium phthalate via chloride depletion. Humidity controlled in situ TEM imaging revealed PTA/NaCl aerosols initially uptake water by adsorption and swelling of the PTA. Further increases in humidity caused partial NaCl deliquescence, hygroscopic droplet growth, and partial dissolution and morphology changes in the PTA. Dehumidification effected precipitation of an organic-rich solid shell followed by NaCl nanocrystal formation on the residual PTA core, yielding a distinctly different core–shell morphology compared to the as-made particles. Diffraction and elemental mapping confirm crystalline sodium acid phthalate forms following dehumidification. This work solidifies our understanding of water uptake and chemical reactions in mixed organic–inorganic atmospheric aerosols by uncovering multistep water uptake dynamics and new phase formation.