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

Abstract

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 the upper troposphere can potentially impact ice nucleation (IN) in cirrus clouds. Pore condensation and freezing (PCF) is an important ice formation pathway for soot particles in the cirrus regime, which requires the soot INP to have specific morphological properties, i.e., mesopore structures. In this study, the morphology and pore size distribution of two kinds of soot samples were modified by a physical agitation method without any chemical modification by which more compacted soot sample aggregates could be produced compared to the unmodified sample. The IN activities of both fresh and compacted soot particles with different sizes, 60, 100, 200 and 400gnm, 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>235gK (homogeneous nucleation temperature, HNT), but IN is observed for compacted and larger-sized soot aggregates (>200gnm) well below the homogeneous freezing relative humidity (RHhom) for T