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 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>235 K (homogeneous nucleation temperature, HNT), but IN is observed for compacted and larger-sized soot aggregates (>200 nm) well below the homogeneous freezing relative humidity (RHhom) for T< HNT, demonstrating PCF as the dominating mechanism for soot IN. We also observed that mechanically compacted soot particles can reach a higher particle activation fraction (AF) value for the same T and RH condition compared to the same aggregate size fresh soot particles. The results also reveal a clear size dependence for the IN activity of soot particles with the same degree of compaction, showing that compacted soot particles with large sizes (200 and 400 nm) are more active INPs and can convey the single importance of soot aggregate morphology for the IN ability. In order to understand the role of soot aggregate morphology for its IN activity, both fresh and compacted soot samples were characterized systematically using particle mass and size measurements, comparisons from TEM (transmission electron microscopy) images, soot porosity characteristics from argon (Ar) and nitrogen (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 IN activities, the enhanced IN abilities of compacted soot particles are attributed to decreasing mesopore width and increasing mesopore occurrence probability due to the compaction process.

Abstract. Soot particles are potential candidates for ice-nucleating particles in cirrus cloud formation, which is known to exert a net-warming effect on climate. Bare soot particles, generally hydrophobic and fractal ones, mainly exist near emission sources. Coated or internally mixed soot particles are more abundant in the atmosphere and have a higher probability of impacting cloud formation and climate. However, the ice nucleation ability of coated soot particles is not as well understood as that of freshly produced soot particles. In this laboratory study, two samples, a propane flame soot and a commercial carbon black, were used as atmospheric soot surrogates and coated with varying wt % of sulfuric acid (H2SO4). The ratio of coating material mass to the mass of bare soot particles was controlled and progressively increased from less than 5 wt % to over 100 wt %. Both bare and coated soot particle ice nucleation activities were investigated with a continuous-flow diffusion chamber operated at mixed-phase and cirrus cloud conditions. The mobility diameter and mass distribution of size-selected soot particles with/without H2SO4 coating were measured by a scanning mobility particle sizer and a centrifugal particle mass analyser running in parallel. The mixing state and morphology of soot particles were characterized by scanning electron microscopy and transmission electron microscopy. In addition, the evidence of the presence of H2SO4 on a coated soot particle surface is shown by energy-dispersive X-ray spectroscopy. Our study demonstrates that H2SO4 coatings suppress the ice nucleation activity of soot particles to varying degrees depending on the coating thickness, but in a non-linear fashion. Thin coatings causing pore filling in the soot aggregate inhibits pore condensation and freezing. Thick coatings promote particle ice activation via droplet homogeneous freezing. Overall, our findings reveal that H2SO4 coatings will suppress soot particle ice nucleation abilities in the cirrus cloud regime, having implications for the fate of soot particles with respect to cloud formation in the upper troposphere.

Aviation soot can affect contrail and cirrus cloud formation and impact climate. A product of incomplete combustion, soot particles are fractal and hydrophobic aggregates comprising carbonaceous spheres with complex physicochemical...