The importance of soot-water contact angle in soot ice nucleation ability in the cirrus regime
Authors: Gao Kunfeng , Kanji Zamin 
Published: February 2023

Soot particles emitted by aircraft in the troposphere can serve as ice nucleating particles (INPs) in the cirrus regime, which can compete with natural cirrus formation by homogeneous freezing of solution droplets and influence cirrus cloudiness and microphysics. Soot particles show varying ice nucleation (IN) abilities, depending on the particle properties that regulate the IN pathway and effectiveness. Pore condensation and freezing (PCF) is an important pathway for soot IN, which first forms supercooled water in soot mesopores (2-50 nm width) via capillary condensation and then freezes below homogeneous nucleation temperature. Soot PCF shows dependence on the particle mesopore abundance and soot-water contact angle (&#952;) according to the Kelvin equation. However, the relative importance of &#952; and mesopore abundance in soot PCF has not been disentangled.In this study, the &#952; of organic-lean soot was changed after exposure to a high (20 ppmv) and a low (2 ppmv) O3 concentration condition to mimic a long- (~20 h) and short-term (~2 h) aging in the real atmosphere respectively, without changing the soot mesopore abundance. Secondary organic formation was avoided by establishing a volatile free experimental environment. The IN activities of both fresh and O3-aged soot particles, with aggregate size of 200 and 400 nm, were tested by the Horizontal Ice Nucleation Chamber (HINC) under cirrus cloud conditions for T&#8804;233 K. The size and mass of soot particles for IN experiments were also monitored. The &#952; of bulk samples exposed to the same O3 concentration conditions (20, 2 or 0 ppmv) as used for IN experiments was directly measured by the Sessile-drop method. The O3-aging effects on soot chemical composition, soot-water interaction ability and porosity were also characterized by using thermogravimetric analysis (TGA), dynamic vapor sorption (DVS) and N2 Brunauer&#8211;Emmett&#8211;Teller (BET) techniques, respectively.The adsorption of O3 onto soot particles was demonstrated by a particle mass increase after O3-exposure. The unchanged chemical functional group abundance and increased water-uptake ability at low relative humidity (RH) conditions after O3-aging implies that the soot-water interaction increase is likely due to direct O3-H2O binding rather than surface oxidation, which explains the &#952; decrease of O3-aged soot. Compared to unaged soot of the same size, a higher O3 exposure level leads to a larger decrease in soot-water &#952; and a more significant IN enhancement for O3-aged soot. However, O3-aged soot presents unchanged pore size distribution and particle size, irrespective of O3 exposure concentration. According to the Kelvin equation, the &#952; decrease induced by O3-aging can alone contribute to the IN enhancement of soot, given that the lower the &#952; is, the lower the RH condition required to trigger capillary condensation. In conclusion, this study highlights the single importance of &#952; in soot PCF in the cirrus regime, by modifying the soot-water &#952; through O3-aging while remaining the porosity.