Photochemistry of solutes in/on ice: reaction rate dependence on sample orientation and photon flux

Abstract:

Particularly in polar regions, photochemical reactions in snowpacks can be an important mechanism for transforming organic and inorganic compounds. Chemicals within snow and ice are found in three different compartments: distributed in the bulk ice, concentrated in liquid-like regions (LLRs) within the ice matrix (such as at grain boundaries), or present in quasi-liquid layers (QLLs) at the air-ice interface. While some previous work suggested reaction rates may vary in these different compartments, our preliminary experiments found similar reaction rates in all three compartments, as well as in aqueous solution. Previous work also suggested reaction rate constants may be independent of photon flux under certain illumination conditions. Here, we extend our investigations to measure reaction rate constants in ice samples with different orientations to the illumination source, which our work thus far suggests may impact the measured rate constants.

Polycyclic aromatic hydrocarbons (PAHs) are common pollutants in snow and ice. We first prepared aqueous solutions of a single PAH. We then froze these samples using various methods previously shown to segregate the solute into known locations in the ice matrix. With simulated polar sunlight, we illuminated these samples and measured photon flux (using 2-nitrobenzaldehyde as a chemical actinometer) and photodecay of the PAH. Using this information, we normalized the rate of PAH loss to the photon flux and calculated the rate constants for PAH photodegradation under various freezing conditions, photon fluxes, and sample orientations. We will report on the impact of these variables on PAH photodegradation as well as the effect of varying the photon flux.