Modelling the effect of condensed-phase diffusion on the homogeneous nucleation of ice in ultra-viscous particles

Kathryn Fowler,David Topping,Paul Connolly

doi:10.5194/acp-20-683-2020

Abstract

Abstract. Computational models of cloud formation typically use homogeneous nucleation to predict the ice nucleated in supercooled water. However, the existence of ultra-viscous organic aerosol in the upper troposphere has offered alternative ice nucleation pathways, which have been observed in laboratory studies. The possible effects of aerosol viscosity on cloud microphysical properties have traditionally been interpreted from simple model simulations of an individual aerosol particle based on equilibration timescales. In this study, to gain insight into the formation of ice in an ensemble of ultra-viscous aerosol particles, we have developed the first cloud parcel model with bin microphysics to simulate condensed phase diffusion through each individual aerosol particle. Our findings demonstrate, for the first time, the complex relationship between the rate of ice formation and the viscosity of secondary organic aerosol, driven by two competing effects – which cannot be explained using existing modelling approaches. The first is inhibition of homogeneous ice nucleation below 200 K, due to restricted particle growth and low water volume. The second occurs at temperatures between 200 and 220 K, where water molecules are slightly more mobile, and a layer of water condenses on the outside of the particle, causing an increase in the number of frozen aerosol particles. Our new model provides a basis to better understand and simulate ice cloud formation on a larger scale, addressing a major source of uncertainty in climate modelling through the representation of microphysical cloud processes.

Highlights

Clouds in the upper troposphere play an important role in climatic processes, interacting with long-wave radiation leaving the Earth’s troposphere, controlling moisture entering the stratosphere and ensuring the overall stability of the Earth’s atmosphere (Fueglistaler et al, 2009)
Our findings suggest that viscous organic aerosols, such as α-pinene secondary organic aerosol (SOA), are expected to have no effect on the rate of homogeneous ice nucleation even at the highest limit of updraft velocities found in the tropical tropopause layer as shown in the lower left panel of Fig. 5
The model analysis presented in this paper suggests that there are two competing effects as a result of the immobility of water molecules through viscous α-pinene secondary organic aerosol particles

Summary

Introduction

Clouds in the upper troposphere play an important role in climatic processes, interacting with long-wave radiation leaving the Earth’s troposphere, controlling moisture entering the stratosphere and ensuring the overall stability of the Earth’s atmosphere (Fueglistaler et al, 2009). In situ studies of low-temperature (< 205 K) and subvisible cirrus clouds in the upper troposphere near to the tropical tropopause have found evidence of an order of magnitude fewer than expected ice crystal numbers compared to predictions from traditional homogeneous theory (ranging from 0.005 to 0.1 cm−3) and higher than expected supersaturations with respect to ice (Jensen et al, 2005; Krämer et al, 2009; Jensen et al, 2013, 2017; Spichtinger and Krämer, 2013) These observations suggested that the current understanding of ice formation mechanisms and methods of modelling the formation of low-temperature cirrus clouds are incorrect or incomplete (Peter et al, 2006; Krämer et al, 2009; Jensen et al, 2010). A number of different mechanisms have been proposed for the discrepancies observed between atmospheric observations of ice number densities and ice supersaturations with those predicted in model simula-

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