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

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 non-intuitive 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 phthalic acid (PTA)/sodium chloride (NaCl) aerosols proceeds by PTA swelling, NaCl deliquescence, and droplet growth, while subsequent drying creates core-shell particles containing newly formed sodium phthalate salts. Lab-made individual PTA/NaCl aerosols consisted of cuboidal crystalline NaCl domains phase separated from amorphous PTA domains. Sodium ions were observed in both NaCl and PTA domains, which were separated by a ~10 nm thick interphase with distinct composition and structure. Humidity controlled in-situ TEM imaging revealed PTA/NaCl aerosols initially uptake water by adsorption and swelling of the PTA. Further increases in humidity cause NaCl deliquescence, hygroscopic droplet growth, and partial dissolution and morphology changes in the PTA. Dehumidification initially causes precipitation of an organic-rich shell followed by a polycrystalline NaCl 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.