Ice nucleation abilities of soot particles determined with the Horizontal Ice Nucleation Chamber

Abstract

Abstract. Ice nucleation by different types of soot particles is systematically investigated over the temperature range from 218 to 253 K relevant for both mixed-phase (MPCs) and cirrus clouds. Soot types were selected to represent a range of physicochemical properties associated with combustion particles. Their ice nucleation ability was determined as a function of particle size using relative humidity (RH) scans in the Horizontal Ice Nucleation Chamber (HINC). We complement our ice nucleation results by a suite of particle characterization measurements, including determination of particle surface area, fractal dimension, temperature-dependent mass loss (ML), water vapor sorption and inferred porosity measurements. Independent of particle size, all soot types reveal absence of ice nucleation below and at water saturation in the MPC regime (T>235 K). In the cirrus regime (T≤235 K), soot types show different freezing behavior depending on particle size and soot type, but the freezing is closely linked to the soot particle properties. Specifically, our results suggest that if soot aggregates contain mesopores (pore diameters of 2–50 nm) and have sufficiently low water–soot contact angles, they show ice nucleation activity and can contribute to ice formation in the cirrus regime at RH well below homogeneous freezing of solution droplets. We attribute the observed ice nucleation to a pore condensation and freezing (PCF) mechanism. Nevertheless, soot particles without cavities of the right size and/or too-high contact angles nucleate ice only at or well above the RH required for homogeneous freezing conditions of solution droplets. Thus, our results imply that soot particles able to nucleate ice via PCF could impact the microphysical properties of ice clouds.

The Impact of Cloud Processing on the Ice Nucleation Abilities of Soot Particles at Cirrus Temperatures

“…Cloud processing and ice nucleation experiments were performed on size‐selected soot aggregates using two coupled horizontal ice nucleation chambers (HINC; Lacher et al., ; Mahrt et al., ). Figure gives an overview of the experimental setup used within this study, which can be segregated into four main parts.…”

“…The soot used here is identical to the mCASTblack sample characterized in more detail by Mahrt et al. (, C:O ratio of ≈0.21 and average primary particle diameter, nm). The particle size was chosen based on our previous ice nucleation experiments (Mahrt et al., ).…”

“…(, C:O ratio of ≈0.21 and average primary particle diameter, nm). The particle size was chosen based on our previous ice nucleation experiments (Mahrt et al., ). The theoretical amount of double‐charged (700 nm) particles for 400 nm diameter size‐selected particles is approximately 15% of the particles being sampled (Wiedensohler, ).…”

“…() and Mahrt et al. () and is based on the design detailed in Kanji and Abbatt (). In brief, two ice coated horizontally oriented parallel plates are individually cooled, so that a linear T profile between the two plates is created, establishing a RH profile across the chamber.…”

“…Soot particles have recently been shown to form ice at temperatures relevant for cirrus cloud formation via the PCF mechanism (Mahrt et al., ; Nichman et al., ; Ullrich et al., ), which was found to be strongly dependent on the physicochemical particle properties, namely, hydrophilicity (soot‐water contact angle) as well as the availability of pores of the right size (Mahrt et al., ). Previous studies reported a compaction of fractal soot aggregates following condensation and evaporation cycles (Bhandari et al, ; Chamberlain et al., ; Colbeck et al., ; Huang et al., ; Ma et al., ; Tritscher et al., ; Zuberi et al., ) and upon ice formation on the soot aggregates (China et al., ).…”