Formation of highly porous aerosol particles by atmospheric freeze-drying in ice clouds

Significance Aerosols cycling through clouds affect particle morphological and chemical properties, thus modifying aerosol effects on cloud microphysics and climate. Previous studies have focused on aerosol processing in warm clouds via aqueous-phase reactions. Here we investigate the physical modifications of aerosols following processing within ice clouds using a unique laboratory setup that simulates ice cloud processes. The processed particles have a porous structure due to phase separation upon freezing, subsequent glass transition, and ice sublimation. Such modified aerosols can be better ice and cloud condensation nuclei and scatter less light. These changes have implications for aerosol–cloud interactions and optical properties of aerosols in the vicinity of clouds. The cycling of atmospheric aerosols through clouds can change their chemical and physical properties and thus modify how aerosols affect cloud microphysics and, subsequently, precipitation and climate. Current knowledge about aerosol processing by clouds is rather limited to chemical reactions within water droplets in warm low-altitude clouds. However, in cold high-altitude cirrus clouds and anvils of high convective clouds in the tropics and midlatitudes, humidified aerosols freeze to form ice, which upon exposure to subsaturation conditions with respect to ice can sublimate, leaving behind residual modified aerosols. This freeze-drying process can occur in various types of clouds. Here we simulate an atmospheric freeze-drying cycle of aerosols in laboratory experiments using proxies for atmospheric aerosols. We find that aerosols that contain organic material that undergo such a process can form highly porous aerosol particles with a larger diameter and a lower density than the initial homogeneous aerosol. We attribute this morphology change to phase separation upon freezing followed by a glass transition of the organic material that can preserve a porous structure after ice sublimation. A porous structure may explain the previously observed enhancement in ice nucleation efficiency of glassy organic particles. We find that highly porous aerosol particles scatter solar light less efficiently than nonporous aerosol particles. Using a combination of satellite and radiosonde data, we show that highly porous aerosol formation can readily occur in highly convective clouds, which are widespread in the tropics and midlatitudes. These observations may have implications for subsequent cloud formation cycles and aerosol albedo near cloud edges.