Ice nucleation by black carbon particles in the cirrus regime: dominated by pore condensation and freezing or deposition ice nucleation?

Deposition ice nucleation (IN) is a heterogeneous pathway by which water vapor deposits directly onto a solid surface and forms ice. Deposition IN happens below water saturation. However, the pore condensation and freezing (PCF) mechanism offers another explanation to ice formation on porous particles at low ice supersaturation. A single black carbon (BC) aggregate consists of several primary particles, forming crevices between primary particles. Whether BC IN happens via deposition or PCF remains uncertain due to the fractal nature of BC particles.

We estimated aggregate surface area, morphology, and primary particle size distribution directly from scanning electron microscopy (SEM) images of size-selected (200 nm, 300 nm, and 400 nm) commercial BC particles. Correlations between surface area data obtained from SEM image estimation and traditional BET tests were explored. Several shape parameters were chosen to characterize particle morphology. The IN ability of aerosolized BC particles was determined with the Spectrometer for Ice Nuclei (SPIN) in the cirrus regime (-46 to -38°C). Particle number concentration and chemical composition were monitored online by a Condensation Particle Counter (CPC) and the Particle Analysis by Laser Mass Spectrometry (PALMS) instrument, respectively.

Preliminary experimental results suggest that larger (400 nm) BC particles are more fractal and branching compared with smaller (200-300 nm) particles. Larger, more fractal BC particles are superior ice nucleating particles (INP) when compared with smaller, more spherical ones. The primary particle size distribution of all samples peaks around 30-45 nm. To understand the relevance of the PCF mechanism with our experimental IN results, we established Young-Laplace equations for the potential liquid-vapor interfaces within inter-primary particle crevices and pores and inter-aggregate pores. Solutions of the Young-Laplace equation on a saddle surface was deducted. Whether ice nucleation happens via PCF mechanism or deposition still requires further investigation, since particle surface chemistry can also affect both ice formation pathways.