Influence of Lowering Soot-Water Contact Angle on Ice Nucleation of Ozone-Aged Soot

Organic-lean and organic-rich size-selected soot particles were exposed to a varying O-3 concentration, progressively decreasing the soot-water contact angle (theta) to study its impact on ice nucleation (IN). The IN ability of fresh and O-3-aged soot between 218 and 233 K was observed while monitoring the particle mass and size distributions. The properties of fresh and O-3-aged bulk organic-lean soot samples with a low and high O-3-adsorption were characterized for soot-water theta, chemical composition, functional groups, soot-water interaction ability and porosity. By retaining the soot porosity between aged and unaged samples, we demonstrate that a decrease in theta after O-3-aging enhances organic-lean soot IN via pore condensation and freezing. Fresh organic-rich soot exhibits suppressed homogeneous freezing, but after O-3-aging it freezes within uncertainty of the homogeneous freezing threshold of solution drops, because of increased hydrophilicity. Plain Language Summary This study investigates the ice formation ability of soot exposed to O-3 in the temperature range <-40degree celsius relevant to the cirrus cloud regime for the role of surface wettability in the ice formation ability of organic-lean and -rich soot. O-3-aged organic-lean soot is more effective at forming ice crystals than unaged soot, due to an increase in soot-water interaction ability promoting capillary condensation of water into porous structures that subsequently freeze. As the chemical functional group abundance of the organic-lean soot does not change after O-3-exposure, the increased soot-water interaction ability for the soot is attributed to O-3-H2O binding rather than surface oxidation. However, both water sorption isotherms and pore size distribution derived from N-2 sorption measurements confirm the O-3-adsorption does not change the porosity of organic-lean soot. This highlights the sole importance of hydrophilicity in pore water condensation prior to soot ice nucleation. O-3-aged organic-rich soot shows poor ice formation ability because of unavailability of porous structures to condense capillary water. This study implies that organic-lean soot will have an enhanced ice formation ability in the upper troposphere if exposed to O(3 )concentrations similar to that simulated in this study.