Exploration of Photon Fluxes in Illuminated Water Ice

The photochemistry of impurities in ice can impact snow and ice composition as well as meltwater and air quality. For some trapped impurities in/on ice, exposure to sunlight can trigger direct photodegradation to form new stable products (e.g., formaldehyde and NOx) and/or reactive intermediates such as hydroxyl radical. The rates of some direct photoreactions are the subject of debate, due in part to the complex nature of ice. Solutes can be present in at least three distinct locations: (1) in a quasi-liquid layer (QLL) at the air-ice interface, (2) in liquid-like regions (LLR) within the solid ice, and (3) within the bulk ice matrix. While the rate of direct photodegradation of impurities depends on the photon flux, little is known about how the flux varies in different ice reservoirs. This complicates the interpretation of direct photodegradation kinetics. To address this issue, we are quantifying the photon fluxes in the different reservoirs using 2-nitrobenzaldehyde actinometry. We have found that in some cases the photon flux in an ice sample depends upon how the sample was prepared (e.g., quickly frozen in liquid nitrogen versus slowly frozen on a cold plate), likely a result of differences in ice morphology altering the extent of internal reflections of the incident light. Based on our results so far, for ice pellets of similar geometry but different preparation method, differences in photon flux can be up to a factor of 3. Overall, these results highlight the importance of measuring photon fluxes in all sample types in ice photochemical studies.