A large fraction of secondary aerosol particles are liquid–liquid phase-separated with an organic shell and an inorganic core. This has the potential to regulate the hygroscopicity of such particles, with significant implications for their optical properties, reactivity, and lifetime. However, it is unclear how this phase separation affects the hygroscopic growth of the particles. Here, we showed a large variation in hygroscopic growth (e.g., 1.14–1.32 under a relative humidity (RH) of 90%) of particles from the forest and urban atmosphere, which had different average core–shell ratios. For this reason, a controlled laboratory experiment further quantifies the impact of the organic shell on particle growth with different RH values. Laboratory experiments demonstrated that (NH4)2SO4 particles with thicker secondary organic shells have a lower growth factor at an RH below 94%. Organic shells started to deliquesce first (RH > 50%) and the phase changes of sulfate cores from solid to liquid took place at an RH higher than 80% as deliquescence relative humidity of pure (NH4)2SO4. Our study provides the first direct evidence on an individual particle basis that hygroscopic growth behavior of phase-separated particles is dependent on the thickness of organic shells, highlighting the importance of organic coating in water uptake and possible heterogeneous reactions of the phase-separated particles.
Bibliographical noteFunding Information:
Cryo-EM characterization was conducted at the Center of Cryo-Electron Microscopy, Zhejiang University, with the assistance of L. Wu. We gratefully acknowledge Shujia Zhang to conduct part of the laboratory experiments. The work was funded by the National Natural Science Foundation of China (91844301 and 42075096) and Zhejiang Provincial Natural Science Foundation of China (LZ19D050001). All the data are presented in the paper.
© 2021 American Chemical Society
- liquid−liquid phase separation
- particle phase
ASJC Scopus subject areas
- Environmental Chemistry