Impacts of Cloud‐Processing on Ice Nucleation of Soot Particles Internally Mixed With Sulfate and Organics
Kunfeng Gao, Zamin A. Kanji
Abstract
Abstract Soot particles are one of the major aerosol components (in particle number concentration) in the troposphere and can impact cirrus formation. Depending on their physicochemical properties, soot particles can nucleate ice in the cirrus regime via pore condensation and freezing (PCF) at a lower ice saturation threshold than that of the homogeneous freezing of solution droplets. Soot aerosol undergoing multiple cloud cycles can change its porosity and surface wettability, thus modulating its PCF ability. In this study, organic‐rich and ‐lean size‐selected propane flame soot particles were coated with different thicknesses of sulfuric acid (H 2 SO 4 ) and exposed to thermodynamic conditions under 228 K and relative humidity with respect to water 104% to simulate contrail formation. Ice‐activated soot particles from the mimicked contrail were sublimated before being tested for ice nucleation (IN) at T ≤ 233 K. Additionally, soot particle size and mass distribution as well as morphology were characterized. The results demonstrate that the increase in mesopore availability induced by cloud‐processing plays a key role in regulating soot IN. All cloud‐processed soot become more compacted, however, only if more PCF relevant mesopores are generated, can the compacted soot exhibit promoted IN. If a H 2 SO 4 coating and/or enriched organics occupy the pore volume, inhibition of PCF is observed for the cloud‐processed soot. Notably, a thick H 2 SO 4 coating enhances the PCF of 400 nm organic‐lean soot particles because the partially engulfed but collapsed soot‐core may have new mesopores produced by compaction. Therefore, the mesopore abundance plays the key role in evaluating aged soot IN ability.