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.

Highlights

  • Black carbon (BC) particles associated with organics are called soot (Bond et al, 2013)

  • The authors suggested that these soot particles with small mesopores can be effective ice nucleating particles (INPs) and form ice crystals via pore condensation and freezing (PCF) at humidity conditions lower than 355 homogeneous freezing conditions in the cirrus cloud regime (Mahrt et al, 2018)

  • The H2SO4 coating generated in this way can be distributed more uniformly over the soot particle surface, compared to a nonuniform H2SO4 coating in this study

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Summary

Introduction

Black carbon (BC) particles associated with organics are called soot (Bond et al, 2013). Soot particles are of significance in both physical and chemical atmospheric processes in the atmosphere These carbonaceous aerosol particles can engage in cloud formation process and form hydrometeors, which affects their lifecycle in the atmosphere and is the source of 35 uncertainties to their overall climate impacts (Liu et al, 2020). Directly emitted by commercial aircrafts in the upper troposphere, are potential ice nucleating particles (INPs) at high altitudes where cirrus clouds usually form, and exert warming effects on climate (Liou, 1986). Cziczo and Froyd (2014) suggested that aviation 40 soot particles can engage in contrail evolution and cirrus formation by inducing ice crystal formation heterogeneously at aircraft cruise altitudes. In order to understand the climate impact of soot particles, it is essential to improve the knowledge about their ice 45 nucleation activities in the atmosphere

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