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Precipitation is mostly formed via the ice phase in mixed phase clouds, and ice clouds are very relevant for Earths’ climate. Freezing or prevention of freezing is common to everyday life, e.g. for food and drug storage, icing and de-icing, etc. However, the ice nucleation process is not well understood, since it occurs on the size scale of clusters of molecules and time scales of molecular fluctuations. In this study, we have taken a step toward nanoscale observation of particles that nucleate ice by developing a new ice nucleation instrument, referred as the INXcell, which couples an ice nucleation environmental cell to the scanning transmission X-ray microscope (STXM) at the Swiss Light Source. We employ near-edge X-ray absorption fine-structure spectroscopy (NEXAFS) to map in situ chemical composition of ice nucleating particles with 35 × 35 nm2 spatial resolution. The main technical challenge was control of temperature, T, and thus relative humidity, RH, while maintaining X-ray transparency. In the INXcell, X-rays are focused onto a sample through a temperature-controlled aperture, which was modified to host a jet of nitrogen cooled down to 170 K. The cold jet impinges on the back surface of a sample exposed to water vapor to control sample temperature and thus RH. We used our unique spectroscopic and ice nucleation capability and investigated the heterogeneous freezing ability of ferrihydrite particles with and without coatings of citric acid. Ferrihydrite is an amorphous or poorly crystalline iron oxyhydroxide abundant in mineral dust and is difficult to identify with conventional XRD analysis. We confirmed that ferrihydrite could nucleate ice via immersion freezing and deposition ice nucleation, depending on whether or not the particles first take up water, respectively. When coating ferrihydrite with citric acid, mimicking organic coatings that aerosol particles obtain throughout their atmospheric lifetime, we observed a reduction in the efficiency to nucleate ice following freezing point depression. Spectroscopic identification of the coated ferrihydrite structure emplyed the iron and carbon X-ray absorption L-edges and K-edge, respectively. We also investigated feldspar particles coated with xanthan gum, a surrogate for a highly ice active mineral with a highly viscous organic coating. We observed that deposition ice nucleation occurred only below the RH dependent glass transition of xanthan gum. Using a newly developed stochastic freezing model (SFM) based on solution water activity, we reproduced average conditions and data scatter of the RH and T at which ice formed. Additionally, we ran our model with atmospheric idealized air parcel trajectories and found overall that deposition ice nucleation was the dominant heterogeneous freezing mechanism. Homogeneous ice nucleation subsequent to water uptake out-performed immersion freezing.

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., Environmental Science: Atmospheres, 2 (3), ISSN:2634-3606

Ice nucleating particles are a minor fraction of tropospheric aerosol, yet they play a key role for cloud microphysical processes. One poorly understood process is the impact of atmospheric aging of aerosol particles on ice nucleation. Here we study the impact of cloud processing on the ice nucleation abilities of two physicochemically different aerosol particles by taking two model systems for atmospheric organic aerosol (OA), as well as coal fly ash (CFA) particles representing an inorganic aerosol type. The ice nucleation activity of the unprocessed particles is compared to aerosol particles that are first exposed to conditions mimicking trajectories though cirrus clouds (CC) and mixed-phase clouds (MPC) prior to testing their ice nucleation activity at temperatures below 243 K. We observed that unprocessed OA do not exhibit heterogeneous ice nucleation, requiring homogeneous freezing conditions of solution droplets to form ice. However, after CC processing raffinose particles showed heterogeneous ice nucleation activity at 218 K and a water saturation ratio of 0.68–0.82, reaching activated fractions of up to 0.3. This enhancement compared to unprocessed raffinose particles results from an increase in particle viscosity upon CC processing. We also present new results of unprocessed CFA particles exhibiting strong heterogeneous ice nucleation activity at temperatures below 235 K in the deposition and/or pore condensation and freezing mode. In contrast to the OA, the CFA show a decrease in ice nucleation activity after both MPC and CC processing. Furthermore, cloud processing and generating CFA particles from aqueous suspensions do not have the same effect on their ice nucleation ability. ISSN:0148-0227 ISSN:2169-897X