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. Soot particles, acting as ice nucleating particles(INPs), can contribute to cirrus cloud formation, which has an important influence on climate. Aviation activities emitting soot particles into theupper troposphere can potentially impact ice nucleation (IN) in cirrusclouds. Pore condensation and freezing (PCF) is an important ice formationpathway for soot particles in the cirrus regime, which requires the soot INPto have specific morphological properties, i.e., mesopore structures. In this study, the morphology and pore size distribution of two kinds of sootsamples were modified by a physical agitation method without any chemicalmodification by which more compacted soot sample aggregates could be produced compared to the unmodified sample. The IN activities of both freshand compacted soot particles with different sizes, 60, 100, 200 and 400 nm,were systematically tested by the Horizontal Ice Nucleation Chamber (HINC)under mixed-phase and cirrus-cloud-relevant temperatures (T). Our results show that soot particles are unable to form ice crystals at T&gt;235 K (homogeneous nucleation temperature, HNT), but IN is observed for compacted and larger-sized soot aggregates (&gt;200 nm) well below the homogeneous freezing relative humidity (RHhom) for T&lt; HNT,demonstrating PCF as the dominating mechanism for soot IN. We also observedthat mechanically compacted soot particles can reach a higher particleactivation fraction (AF) value for the same T and RH condition compared to the same aggregate size fresh soot particles. The results also reveal aclear size dependence for the IN activity of soot particles with the samedegree of compaction, showing that compacted soot particles with large sizes(200 and 400 nm) are more active INPs and can convey the single importanceof soot aggregate morphology for the IN ability. In order to understand therole of soot aggregate morphology for its IN activity, both fresh andcompacted soot samples were characterized systematically using particle massand size measurements, comparisons from TEM (transmission electronmicroscopy) images, soot porosity characteristics from argon (Ar) andnitrogen (N2) physisorption measurements, as well as soot–water interaction results from DVS (dynamic vapor sorption) measurements.Considering the soot particle physical properties along with its INactivities, the enhanced IN abilities of compacted soot particles areattributed to decreasing mesopore width and increasing mesopore occurrenceprobability due to the compaction process.

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