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 in 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 clouds 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 was observed for compacted and larger size soot aggregates (> 200 nm) well below homogeneous freezing relative humidity (RHhom) at 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 agitation degree, 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.
Highlights
Black carbon (BC) particles are estimated as the second-most important forcing for climate warming only after CO2 (Ramanathan and Carmichael, 2008; Bond et al, 2013)
Soot aggregate morphological properties were characterized both online and offline for ice nucleation (IN) results interpretation, including effective density of size selected soot particles, soot aggregate microscopic transmission electron microscopy (TEM) images, bulk sample soot-water interaction ability tested by dynamic vapor sorption (DVS) measurements, as well as soot sample pore size distribution (PSD) analysis based on Ar, N2 and water sorption measurements
DVS measurements show that soot-water contact angle does not change after soot compaction, demonstrating the single importance of soot particle PSD 770 in its ice nucleation process via Pore condensation and freezing (PCF) mechanism
Summary
Black carbon (BC) particles are estimated as the second-most important forcing for climate warming only after CO2 (Ramanathan and Carmichael, 2008; Bond et al, 2013). BC particles can influence the radiation balance in the atmosphere directly by scattering or absorbing shortwave radiation and indirectly by acting as cloud condensation nuclei (CCN) or ice nucleating particles (INPs) in the atmosphere to form water droplets or ice crystals (Bond et al, 2013; Jacobson, 2004), thereby 35 changing cloud properties. McGraw et al (2020) suggested that BC particles influence cirrus cloud formation by acting as INPs and competing with the homogeneous freezing of aerosol solution droplets, and exhibit a large uncertainty in their global net radiative forcing on climate. As aviation emissions emit BC with a great amount of water vapor in contrail plumes at high altitude and cold temperature (T) conditions, aviation soot particles can be activated as ice crystals, which potentially regulates cirrus cloud coverage in aviation corridors. Homogeneous freezing, requiring low T and high RH, i.e
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
