| The presence of snow can greatly affect the composition of the overlying atmosphere. Furthermore, understanding the chemistry of compounds in a snowpack provides insight into the cycling and fate of pollutants in the environment. In this work, we focus on the photochemical production and reactions of singlet molecular oxygen (O2(1Deltag), hence written 1O2*) on ice. While reactions of 1O2* are important in surface waters and atmospheric drops, neither the concentrations nor the chemical behavior of 1O 2* on ice have yet been determined. In illuminated samples, we found concentrations of 1O2* could be greater than 10,000 times higher on ice than in identical, but liquid solution. This enhanced concentration is a result of freeze-concentration, in which solutes/impurities are excluded to liquid-like regions (LLRs) at the surfaces and grain boundaries of, or are trapped in inclusions within, forming ice crystals. For 1O2*, which is a reactive intermediate formed after an organic chromophore (i.e., sensitizer) absorbs light and subsequently transfers energy to dissolved oxygen (O2), freeze-concentration results in elevated concentrations of sensitizers in the liquid-like regions. However, the dominant sink (water) remains essentially unchanged. Thus, 1O 2* kinetics on ice are greatly enhanced relative to liquid solutions. If the total solute concentration of a pre-frozen solution is sufficiently high (greater than ∼1 mM) and an ice sample is above the eutectic temperature of the mixture, we find that freezing-point depression can very accurately describe the observed enhancement of 1O2* in liquid-like regions. Finally, we found that for water-soluble pollutants, such as furans or aromatic amino acids, 1O2* is likely an important oxidant, perhaps rivaling the importance of hydroxyl radical for certain classes of pollutants. |
