Impacts of Cloud-Processing on Ice Nucleation of Soot Particles Internally Mixed With Sulfate and Organics

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 (H2SO4) 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 H2SO4 coating and/or enriched organics occupy the pore volume, inhibition of PCF is observed for the cloud-processed soot. Notably, a thick H2SO4 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. Plain Language Summary We study the effect of contrail-processing (an ice activation process) on the ice nucleation (IN) ability of soot particles containing variable organic and sulfate content in the cirrus cloud regime. The results demonstrate that an ice activation process leads to increased soot particle compactness. Only uncoated and organic-lean soot particles show enhanced IN ability after contrail formation due to mesopore availability increase. However, an enrichment in organic content and/or a H2SO4 coating may decrease the mesopore availability and inhibit such an IN enhancement from contrail-processing, although contrail-processing promotes the particle homogeneous freezing. In addition, a thick H2SO4 coating may result in soot-aggregate compaction and promote the IN of organic-lean soot particles if the thick coating caused compaction increases the particle mesopore availability. In brief, the mesopore availability change induced by contrail-processing can be viewed as a predictor for soot IN ability change.