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Abstract. Atmospheric ice formation in cirrus clouds is often initiated by aerosolparticles that act as ice-nucleating particles. The aerosol–cloudinteractions of soot and associated feedbacks remain uncertain, in partbecause a coherent understanding of the ice nucleation mechanism andactivity of soot has not yet emerged. Here, we provide a new framework thatpredicts ice formation on soot particles via pore condensation and freezing(PCF) that, unlike previous approaches, considers soot particle properties,capturing their vastly different pore properties compared to other aerosolspecies such as mineral dust. During PCF, water is taken up into pores of thesoot aggregates by capillary condensation. At cirrus temperatures, the porewater can freeze homogeneously and subsequently grow into a macroscopic icecrystal. In the soot-PCF framework presented here, the relative humidityconditions required for these steps are derived for different pore types asa function of temperature. The pore types considered here encompassn-membered ring pores that form between n individual spheres within the samelayer of primary particles as well as pores in the form of inner cavitiesthat form between two layers of primary particles. We treat soot primaryparticles as perfect spheres and use the contact angle between soot andwater (θsw), the primary particle diameter (Dpp), and thedegree of primary particle overlap (overlap coefficient, Cov) tocharacterize pore properties. We find that three-membered and four-memberedring pores are of the right size for PCF, assuming primary particle sizestypical of atmospheric soot particles. For these pore types, we deriveequations that describe the conditions for all three steps of soot PCF,namely capillary condensation, ice nucleation, and ice growth. Since attypical cirrus conditions homogeneous ice nucleation can be consideredimmediate as soon as the water volume within the pore is large enough tohost a critical ice embryo, soot PCF becomes limited by either capillarycondensation or ice crystal growth. We use the soot-PCF framework to derivea new equation to parameterize ice formation on soot particles via PCF,based on soot properties that are routinely measured, including the primaryparticle size, overlap, and the fractal dimension. These properties, alongwith the number of primary particles making up an aggregate and the contactangle between water and soot, constrain the parameterization. Applying thenew parameterization to previously reported laboratory data of ice formationon soot particles provides direct evidence that ice nucleation on sootaggregates takes place via PCF. We conclude that this new frameworkclarifies the ice formation mechanism on soot particles in cirrus conditionsand provides a new perspective to represent ice formation on soot in climatemodels.

Soot PCF: pore condensation and freezing framework for soot aggregates