Abstract

Ice nucleation is one of the most uncertain microphysical processes, as it occurs in various ways and on many types of particles. To overcome this challenge, we present a heterogeneous ice nucleation study on deposition ice nucleation and immersion freezing in a novel cryogenic X-ray experiment with the capability to spectroscopically probe individual ice nucleating and non-ice nucleating particles. Mineral dust type particles composed of either ferrihydrite or feldspar were used and mixed with organic matter of either citric acid or xanthan gum. We observed in situ ice nucleation using scanning transmission X-ray microscopy (STXM) and identified unique organic carbon functionalities and iron oxidation state using near-edge X-ray absorption fine structure (NEXAFS) spectroscopy in the new in situ environmental ice cell, termed the ice nucleation X-ray cell (INXCell). Deposition ice nucleation of ferrihydrite occurred at a relative humidity with respect to ice, RHi, between ∼120–138% and temperatures, T ∼ 232 K. However, we also observed water uptake on ferrihydrite at the same T when deposition ice nucleation did not occur. Although, immersion freezing of ferrihydrite both in pure water droplets and in aqueous citric acid occurred at or slightly below conditions for homogeneous freezing, i.e. the effect of ferrihydrite particles acting as a heterogeneous ice nucleus for immersion freezing was small. Microcline K-rich feldspar mixed with xanthan gum was also used in INXCell experiments. Deposition ice nucleation occurred at conditions when xanthan gum was expected to be highly viscous (glassy). At less viscous conditions, immersion freezing was observed. We extended a model for heterogeneous and homogeneous ice nucleation, named the stochastic freezing model (SFM). It was used to quantify heterogeneous ice nucleation rate coefficients, mimic the competition between homogeneous ice nucleation; water uptake; deposition ice nucleation and immersion freezing, and predict the T and RHi at which ice was observed. The importance of ferrihydrite to act as a heterogeneous ice nucleating particle in the atmosphere using the SFM is discussed.

Highlights

  • We have aimed to investigate whether deposition ice nucleation, immersion freezing or homogeneous ice nucleation is favored over the other depending on the particle types present

  • Ice nucleation for ferrihydrite is not so efficient compared to hematite, its importance to atmospheric ice formation will be evaluated in a later section

  • Heterogeneous ice nucleation via deposition ice nucleation from the water vapor phase was in competition with water uptake onto ferrihydrite particles

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Summary

Introduction

The pair of membranes provide a window through which the sample volume of INXCell can be observed in situ with STXM/NEXAFS spectroscopy.[79,80,90] This allows the composition and chemical state of particles to be mapped in situ, as well as having the stability and control to image particles inside ice crystals and water droplets at saturation with respect to liquid or ice (RHw 1⁄4 100% or RHi 1⁄4 100%, respectively). Our new method and model makes it possible to investigate single ice nucleating particles in situ as a function of T, RHw, and RHi

Experimental
INXCell design and description
Ice nucleation and humidity calibration procedure
Particle preparation
 10À2 2  10À2 2  10À2
Observation of ice nucleation
Modelling homogeneous and heterogeneous ice nucleation
Atmospheric importance of ferrihydrite
Conclusions
Code availability
Full Text
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