Ice Nucleation Imaged In Situ with X-ray Spectro-Microscopy

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.