How Porosity Influences the Heterogeneous Ice Nucleation Ability of Secondary Organic Aerosol Particles.

During processing in deep convective cloud systems, highly viscous or glassy secondary organic aerosol (SOA) particles can develop a porous structure through a process known as atmospheric freeze‐drying. This structural modification may enhance their heterogeneous ice nucleation ability under cirrus conditions through the pore condensation and freezing mechanism. Pristine, compact SOA particles, on the other hand, are recommended to be treated as ice‐inactive in models. This recommendation also applies to internally mixed particles, where a coating layer of secondary organic matter (SOM) deactivates the intrinsic ice nucleation ability of the core, which may be a mineral dust grain. Ice cloud‐processing may also improve the ice nucleation ability of such a composite particle by inducing structural changes in the coating layer, which can release active sites on the mineral surface. In this work, we investigated the change in the ice nucleation ability of pure SOA particles from the ozonolysis of α‐pinene and two types of internally mixed particles (zeolite and coal fly ash particles coated with SOM) after being subjected to the atmospheric freeze‐drying process simulated in an expansion cloud chamber. For pure α‐pinene SOA, we found only a slight improvement in the ice nucleation ability of the ice cloud‐processed, porous particles compared to their pristine, compact counterparts at 221 and 217 K. In contrast, the zeolite and coal fly ash particles, which were initially deactivated by the organic coating, became significantly more ice‐active after atmospheric freeze‐drying, emphasizing that such composite particles cannot be excluded from model simulations of heterogeneous ice formation. Plain Language Summary: Understanding how ice crystals form in the Earth's atmosphere is important for predicting climate using cloud models. It is believed that certain surface features on atmospheric aerosol particles, such as cracks and pores found on mineral dust grains, can effectively induce ice formation. Ice crystals are formed from small amounts of liquid water that have condensed and frozen in the pores of the particles. As a result, ice formation is less likely to occur on solid particles that lack such pores and have a smooth surface. One type of the latter are particles formed by the condensation of volatile, biogenic and anthropogenic organic compounds. In models of ice formation, such secondary organic aerosol particles are often neglected because of their compact, non‐porous shape. In our work, we simulated a pathway by which such particles can acquire a porous structure through cloud‐processing in the atmosphere, and investigated how this affects their ability to form ice. We also investigated mixed particle types in which the organic compounds condensed on a mineral surface, blocking the pores that allowed efficient ice formation. The aforementioned cloud‐processing partially restructured the organic material on the mineral surface, and the particles became more effective at forming ice again. Key Points: Regardless of particle morphology, pure secondary organic aerosol particles are inefficient at nucleating ice under cirrus conditionsA pristine coating layer of secondary organic matter deactivates efficient ice nucleating particles such as mineral dust and coal fly ashIce cloud‐processing changes the structure of the coating layer and partially restores the ice nucleation ability of such mixed particles [ABSTRACT FROM AUTHOR]