Soot PCF: pore condensation and freezing framework for soot aggregates

Abstract

Abstract. Atmospheric ice formation in cirrus clouds is often initiated by aerosol particles that act as ice-nucleating particles. The aerosol–cloud interactions of soot and associated feedbacks remain uncertain, in part because a coherent understanding of the ice nucleation mechanism and activity of soot has not yet emerged. Here, we provide a new framework that predicts 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 aerosol species such as mineral dust. During PCF, water is taken up into pores of the soot aggregates by capillary condensation. At cirrus temperatures, the pore water can freeze homogeneously and subsequently grow into a macroscopic ice crystal. In the soot-PCF framework presented here, the relative humidity conditions required for these steps are derived for different pore types as a function of temperature. The pore types considered here encompass n-membered ring pores that form between n individual spheres within the same layer of primary particles as well as pores in the form of inner cavities that form between two layers of primary particles. We treat soot primary particles as perfect spheres and use the contact angle between soot and water (θsw), the primary particle diameter (Dpp), and the degree of primary particle overlap (overlap coefficient, Cov) to characterize pore properties. We find that three-membered and four-membered ring pores are of the right size for PCF, assuming primary particle sizes typical of atmospheric soot particles. For these pore types, we derive equations that describe the conditions for all three steps of soot PCF, namely capillary condensation, ice nucleation, and ice growth. Since at typical cirrus conditions homogeneous ice nucleation can be considered immediate as soon as the water volume within the pore is large enough to host a critical ice embryo, soot PCF becomes limited by either capillary condensation or ice crystal growth. We use the soot-PCF framework to derive a new equation to parameterize ice formation on soot particles via PCF, based on soot properties that are routinely measured, including the primary particle size, overlap, and the fractal dimension. These properties, along with the number of primary particles making up an aggregate and the contact angle between water and soot, constrain the parameterization. Applying the new parameterization to previously reported laboratory data of ice formation on soot particles provides direct evidence that ice nucleation on soot aggregates takes place via PCF. We conclude that this new framework clarifies the ice formation mechanism on soot particles in cirrus conditions and provides a new perspective to represent ice formation on soot in climate models.

Ice nucleation activities of soot particles internally mixed with sulphuric acid at cirrus cloud conditions

“…This shows coherence with the results at warm temperatures (T > HNT) and can be explained by the H2SO4 pore filling effect. Homogeneous freezing of supercooled water in pores is an important step in a PCF activation process (Marcolli et al, 2021;Marcolli, 2014). According to , the homogeneous freezing rate depends on the liquid water activity.…”

“…Considering its solubility in water and strong hygroscopic ability, H2SO4 coating very likely enhances soot particle surface wettability, i.e. a lower soot-water contact angle (Mahrt et al, 2020b), which plays an important role in PCF (David et al, 2020;Marcolli et al, 2021). In addition, the coating process can alter soot particle morphology and influence the availability of potential pores for PCF at cirrus cloud conditions.…”

Enhanced soot particle ice nucleation ability induced by aggregate compaction and densification

“…6, 400 nm compacted FW200 soot particles are more effective INPs than the fresh sample as the compacted particles required a lower RH to reach the same AF value at T < 233 K. We ascribe this to an enrichment in small mesopores for compacted FW200 soot particles. After agitation, macropores (> 50 nm) and larger mesopores (> 20 nm) among fresh soot aggregate might be compacted into smaller mesopores which induce inverse Kelvin effect and lead to capillary condensation at RH conditions well below water saturation condition thereby promoting PCF activation, according to soot-PCF framework (Marcolli et al, 2021). Given that physical agitation is the only modification to soot samples, the enhancement of IN ability must result exclusively from the changes in morphology (resulting in mesopore enrichment) induced by soot aggregate compaction.…”

“…There are also differences between FW200 and PR90 IN activities. Firstly, PR90 soot particles are less active INPs compared to FW200 as shown that the same size fresh or compacted PR90 soot particles require a higher RH than that of FW200 soot with the same compaction level to reach the same AF value at the same T. According to the PCF mechanism (Marcolli, 2014;Marcolli et al, 2021), this probably results from the differences in soot sample properties, including sample PSD and watersoot contact angle, both of which are determinators for the PCF mechanism. Thus, PR90 soot PCF process occurs at higher saturation conditions and requires low T if PR90 soot is originally less porous than FW200 and/or of a lower surface wettability.…”

Aerosol–cloud interactions: the representation of heterogeneous ice activation in cloud models

“…During immersion freezing, ice nucleates over a wider temperature range compared with homogeneous freezing of pure water droplets (Peckhaus et al, 2016;Tarn et al, 2018). Heterogeneous freezing curves become even broader when a mixture of different INP types is investigated.…”