Ice nucleation imaged with X-ray spectro-microscopy

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, RH _i, 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 RH _i 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.

Fig. 1. Sketch of the INXCell configuration. X-rays were focused by a Fresnel zone plate through a temperature controlled order selecting aperture (OSA) with an integrated gas jet. Particles inside the INXCell were exposed to humidified air and cooled by the impinging gas jet from the OSA. The INXCell was mounted on a circuit board (light brown) that was connected to a lithographically fabricated temperature sensor on the sample substrate with a thickness of 40 nm. An optical microscope image of the sensor is outlined in pink. Transmitted X-rays were detected using a phosphor screen coupled with a photomultiplier tube (PMT). Further details are given in the text. When desired, a 40 nm aluminum layer was condensed onto the reverse side (upstream facing) of the membrane to increase the lateral thermal conductivity. The flows of humidified sample air and dry nitrogen is depicted. The sketch is not drawn to scale.

Fig. 2. Ice nucleation observations from ferrihydrite, mixed ferrihydrite and citric acid, and mixed K-feldspar and xanthan gum particles as a function of relative humidity with respect to ice, RH_i, and temperature, T. Deposition ice nucleation, immersion freezing and water uptake are indicated by different symbols given in the legend, where each are averages of repeat experiments. An example of individual data points are shown as small open blue diamonds for ferrihydrite deposition ice nucleation. Solid and hashed orange boxes indicate the modelled range for deposition and immersion freezing, respectively, from ferrihydrite. Solid and hashed light green boxes indicate deposition ice nucleation and immersion freezing, respectively, from feldspar. Black open boxes are the modelled range of homogeneous freezing. Grey filled diamonds are freezing due to hematite, crosses are freezing of K-feldspar particles with an electrical mobility diameter of 300 nm, and gray open diamonds are deposition ice nucleation of K-feldspar particles with diameters between 1–100 μm, respectively. The gray solid and dashed lines are a range of immersion freezing of K-feldspar in water droplets. The orange line is the expected glass transition temperature for aqueous xanthan gum solutions. The solid black line indicates water saturation, i.e. when the relative humidity with respect to water, RH_w = 100%. Dotted gray lines indicate decreasing RH_w by 10%. The black dashed line indicates homogeneous ice nucleation from a single water or aqueous solution droplet ∼ 10 μm in diameter.

Fig. 3. A water droplet imaged with a ferrihydrite particle immersed inside at 236 K. The image in (a) was taken at the iron pre-edge (700.0 eV) and the image in (b) was taken at the resonant energy (709.6 eV) corresponding to iron(iii). The blue and red outlines indicate where the water and ferrihydrite particle are, respectively. The scale bar in both images is 2 μm.

Fig. 4. Example of an INXCell experiment in which (a) water droplets formed, following by (b) ice formation (c) and crystal growth on dry deposited ferrihydrite particles. The temperature, T, of the sample is indicated above the STXM images. The coarse spatial resolution was necessary to quickly image the nucleated and growing ice crystal.

Fig. 5. Demonstration of ice nucleation and spectroscopic identification of ferrihydrite particles coated with citric acid. (a)–(c) A sequence of X-ray images at 280 eV showing the last instances of a sublimating ice crystal. The coarse resolution is necessary to quickly image the shrinking crystal. The blue and orange outline indicates the crystal boundaries. (d) An X-ray image at 288.6 eV showing the crystal boundaries, residual particles after sublimation and organic rich particles across the sample. The scale bar is 2 μm for all images. NEXAFS spectra were acquired first (f) at the carbon K-edge and then (e) the iron L_2,3-edges of a residual particle and a non-ice nucleating particle.

Fig. 6. Calculated heterogeneous ice nucleation rate coefficients, J_het, ice nucleation events, N_nuc, and ice particle production rates, P_ice, are shown. (a) J_het parameterizations and their certainty at 0.999 confidence are shown as the solid colored lines and shadings. The solid gray line is for immersion freezing due to feldspar from Alpert and Knopf. Modelled ice nucleation accounting for deposition ice nucleation, immersion freezing and homogeneous ice nucleation is shown in (b) and (c) from an aerosol population having ferrihydrite and non-ferrihydrite particles. Lognormal distributions with two modes were used with parameters N₁ = 285 cm⁻³, μ₁ = 0.05 μm, σ₁ = 1.2, N₂ = 15 cm⁻³, μ₂ = 0.9 μm and σ₂ = 0.5 for non-ferrihydrite particles and N₃ = 14 cm⁻³, μ₃ = 0.05 μm, σ₃ = 1.2, N₄ = 0.8 cm⁻³, μ₄ = 0.9 μm and σ₄ = 0.5 for ferrihydrite particles.