The Photochemical Reaction Between Phenol and an Excited Triplet State is Enhanced in/on Ice Compared to in Solution

Photochemical reactions appear to play an important role in the processing of organic compounds in/on snow and ice. These reactions likely control the fate and lifetimes of organic compounds and influence the emissions of volatile organic compounds (VOC) from snow and ice. Triplet excited states of organic compounds (3C*) can be important aqueous oxidants in atmospheric drops and in surface waters, but little is known of this class of oxidants in frozen samples. When exposed to light, 3, 4-dimethoxybenzaldehyde (DMB), a product from biomass combustion, is excited to its triplet state (3DMB*), which can either return to the ground state, react with oxygen to form singlet oxygen, or oxidize organics, such as phenol. In this study, we examine the photochemical loss of phenol due to reaction with 3DMB* in laboratory ice samples as a function of pH, temperature, total solute, and oxygen concentration. Our results show that the rate of phenol loss due to 3DMB* is significantly enhanced in ice compared to the equivalent liquid sample at the same photon flux. This suggests that reactions of triplet excited states in ice can be significant sinks for organics, and perhaps reactive inorganic species, in snow and ice.