Ice nucleation activities of soot particles internally mixed with sulphuric acid at cirrus cloud conditions

Abstract

Abstract. Soot particles are important candidates for ice nucleating particles (INPs) in cirrus cloud formation which is known to exert a warming effect on climate. Bare soot particles, generally hydrophobic and fractal, mainly exist near emission sources. Coated or internally mixed soot particles are more abundant in the atmosphere and have a higher probability to impact cloud formation and climate. However, the ice nucleation ability of coated soot particles is not as well understood as that of freshly produced soot particles. In this study, two samples, a propane (C3H8) flame soot and a commercial carbon black were coated with varying wt % of sulphuric acid (H2SO4). The ratio of coating material mass to the mass of bare soot particle was controlled and progressively increased from less than 5 wt % to over 100 wt %. Both bare and coated soot particle ice nucleation activities were investigated with a continuous flow diffusion chamber operated at mixed-phase and cirrus cloud conditions. The mobility size and mass distribution of size selected soot particles with/without H2SO4 coating were measured by a scanning mobility particle sizer (SMPS) and a centrifugal particle mass analyser (CPMA) running in parallel. The mixing state and morphology of soot particles were characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). In addition, the evidence for the presence of H2SO4 on coated soot particle surface is shown by Energy Dispersive X-ray spectroscopy (EDX). Our study demonstrates that H2SO4 coatings suppress the ice nucleation activity of soot particles to varying degrees depending on the coating thickness, but in a non-linear fashion. Thin coatings causing pore filling in the soot-aggregate inhibits pore condensation and freezing (PCF). Thick coatings promote particle ice activation via droplet homogeneous freezing. Overall, our findings reveal that H2SO4 coatings will suppress soot particle ice nucleation abilities in the cirrus cloud regime, having implications for the fate of soot particles with respect to cloud formation in the upper troposphere.

Laboratory studies of ice nucleation onto bare and internally mixed soot–sulfuric acid particles

“…Data availability. The data presented in this publication are available at https://doi.org/10.3929/ethz-b-000498786 (Gao et al, 2022b, last access: 19 April 2022.…”

A Parameterization of Cirrus Cloud Formation: Revisiting Competing Ice Nucleation

“…Soot particles are generally poor INPs, especially when coated with aqueous sulfuric acid solutions (Gao et al, 2021). Large (exceeding about 200 nm in size) soot particle aggregates form ice more efficiently after being processed in either mixed-phase clouds or contrails (Mahrt et al, 2020).…”