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

Abstract

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.

Laboratory studies of ice nucleation onto bare and internally mixed soot–sulfuric acid particles

“…The understanding of the lower size threshold for soot ice formation is important, given the strong particle size dependence of (soot) ice nucleation and that the Aitken mode dominates size distribution from aviation soot emissions and other hightemperature combustion sources. A recent study reported that small-sized (< 100 nm) soot particles, even as active as uncoated FW200 in this study, nucleate ice homogeneously for T < HNT (Gao et al, 2022a), implying that H 2 SO 4 -coated soot particles of a size smaller than 100 nm require homogeneous freezing conditions to form ice in the cirrus cloud regime. Moreover, this study contributes to the climate impact evaluation of soot emissions.…”

The dependence of soot particle ice nucleation ability on its volatile content

“…Fresh 400 nm PR90 particles freeze homogeneously at 233 K but nucleate ice far below Shom conditions via the PCF mechanism for T ≤ 228 K as containing suitable mesopores to PCF processes. 16 For denuded 400 nm PR90 particles, the onset Si decreases by more than 0.12 at 233 K, compared to the fresh sample. However, the IN enhancement becomes smaller and the onset Si values of PR90 soot are within error bars at lower T (≤ 228 K).…”

“…In this study, we modified the mathematical approach for retrieving PSD based on DVS isotherms developed by Wheeler 48 and Shkol'nikov and Sidorova 49 and a high order polynomial fitting method was used to calculate a smoother desorption isotherm function of RHw. For detailed formulation, we refer the reader to Mahrt et al 15 and Gao et al 16 TEM: Size-selected mBlack and mBrown particles (400 nm) with and without thermal denuding were also sampled for transmission electron microscopy (TEM, JEOL-1400+, JEOL Ltd., Tokyo, Japan, operated at 120 kV) analysis.…”

“…3d and A8, the decrease in onset Si of 200 nm PR90 particles due to thermal denuding is less pronounced than the case for 400 nm particles at the same T. This is because small size PR90 particles should have less pore volume and less active sites scaling to the surface area. 16…”

“…11,12 Porosity characteristics and surface wettability are viewed as crucial and decisive properties for soot particle IN via PCF. [13][14][15][16] PCF favours both mesopore (2 nm < 𝐷𝐷 p (pore diameter) < 50 nm) 17 structures and a low soot-water contact angle to firstly induce supercooled water condensation in the pore. [18][19][20] An ice crystal can form via homogeneous freezing of supercooled pore water when the temperature (T) is lower than homogeneous nucleation temperature (HNT = 235 K) or by immersion freezing at T > 235 K if active sites are present within the pore and if the pore size is large enough to host an ice germ at the nucleation temperature.…”