Photochemistry of Solutes in Different Locations in/on Ice. Part I: Visualizing Solute Locations

Solutes in snow and ice are present in at least 3 different types of locations: at the air-ice interface (sometimes called the quasi-liquid layer or QLL), in other liquid-like regions (LLRs) within the ice, and in the matrix of the bulk ice. Some reports in the literature show that direct photodegradation of solutes is enhanced in the QLL relative to in solution, while other reports show that solutes in whole ice samples behave as if they were in supercooled aqueous solution. Our overall goal is to understand where solutes reside in laboratory ice samples and whether their photochemistry depends upon location. In this first portion of our work we will describe our efforts to address the first component of this goal, i.e., the location of solutes.

Using micro-CT (computed tomography) we can create a 3-dimensional image of a frozen sample, identifying areas of air, water ice, and concentrated solute. Slower freezing methods tend to produce samples with larger air bubbles and concentrated solute inclusions. Interestingly, areas of concentrated solute are often adjacent to internal air bubbles, providing an air-ice interface that is not at the top surface of the sample. We also see some concentrated solute in internal veins or at the ice-air interface at the top of the sample. These areas have some liquid-like behavior, often forming thin layers and moving within the frozen sample in a thermal gradient (but still below 273 K). In contrast, for samples prepared by flash-freezing solution in liquid nitrogen, the solutes are nearly invisible to the micro-CT technique, suggesting they are present in very small domains, possibly dispersed throughout the sample.