Soot Aerosols as a Source for Ice Nucleating Particles in the Cirrus Regime—The Role of Soot Particle Properties.

A quantitative understanding of the aerosol-cloud interactions of soot particles, especially their ability to form ice, remains a key factor to reduce the uncertainties in the estimates of the net radiative forcing of black carbon [1]. However, previous studies have revealed significant spread, when reporting the ice nucleation ability of soot. This is partly caused by the diverse physico-chemical properties associated with morphologically complex soot aggregates.

These varying results are the primary motivation of this study, which aims to better understand the ice nucleation behavior and mechanism of soot particles in relation to the particle properties. In this study, we present a systematic laboratory-based investigation of the ice nucleation behavior of different soot types. Various commercial soot samples are used, including an amorphous industrial black carbon, a fullerene soot, and Lamp Black carbon. In addition, soot generated from a propane flame Combustion Aerosol Standard Generator (miniCAST, JING AG), is investigated. Such soot has previously been used for ice nucleation studies and is frequently taken as proxy for atmospheric soot particles [e.g. 2]. We tested the ice nucleation ability of the above soot samples on DMA (Differential Mobility Analyzer) size-selected aerosol over a temperature range from 253 K to 218 K, covering both the mixed-phase and cirrus cloud regime. The heterogeneous freezing ability of soot at low temperatures is especially important, as aircraft emissions are a direct source of combustion particles in the upper troposphere, where cirrus temperatures prevail.

The ice nucleation ability of soot is probed using the Horizontal Ice Nucleation Chamber (HINC, Lacher, Lohmann [3]), a Continuous Flow Diffusion Chamber. A suite of auxiliary measurements complements ice nucleation observations to characterize the physio-chemical properties of the tested aerosol particles that ultimately aid in determining their ice nucleation behaviour: Thermogravimetric Analysis (TGA) was performed over a temperature range between 0 to 1000 °C to estimate the proportion of volatile components associated with the different soot types. Nitrogen adsorption following the BET-method [4] was conducted to obtain specific surface area. Water adsorption isotherms of the soot particles are also collected to allow assessment of the hydrophilicity and the porosity of the samples. Size and aggregate morphology are investigated by a dedicated set of coupled DMA – CPMA (Centrifugal Particle Mass Analyzer) experiments to determine fractal dimension of the soot particles. Finally, Transmission Electron Microscopy studies, performed on size selected particles, using the Zurich Electron Microscopy Impactor, complement our aerosol characterization.

Our results reveal droplet activation for all soot types at RH_w > 100% at temperatures above 233 K with absence of any heterogeneous freezing. However, in the cirrus cloud regime, some soot types show significant heterogeneous freezing, depending on the aerosol size. These soot types are associated with relatively high porosity and water affinity, whereas those soot types with poor ice nucleation ability show considerably reduced water adsorption characteristics. We discuss our result in context of a pore condensation and freezing type ice nucleation mechanism [5] being responsible for the observed ice nucleation ability of the soot particles.

1. Bond, T.C., et al., Bounding the role of black carbon in the climate system: A scientific assessment. Journal of Geophysical Research-Atmospheres, 2013. 118(11): p. 5380-5552.
2. Crawford, I., et al., Studies of propane flame soot acting as heterogeneous ice nuclei in conjunction with single particle soot photometer measurements. Atmospheric Chemistry and Physics, 2011. 11(18): p. 9549-9561.
3. Lacher, L., et al., The Horizontal Ice Nucleation Chamber (HINC): INP measurements at conditions relevant for mixed-phase clouds at the High Altitude Research Station Jungfraujoch. Atmospheric Chemistry and Physics, 2017. 17(24): p. 15199-15224.
4. Brunauer, S., P.H. Emmett, and E. Teller, Adsorption of gases in multimolecular layers. Journal of the American Chemical Society, 1938. 60: p. 309-319.
5. Marcolli, C., Deposition nucleation viewed as homogeneous or immersion freezing in pores and cavities. Atmos. Chem. Phys., 2014. 14(4): p. 2071-2104.