Photochemical Production of Singlet Oxygen from Dissolved Organic Matter in Ice

Dissolved natural organic matter (DOM) is a ubiquitous component of natural waters and an important photosensitizer. A variety of reactive oxygen species (ROS) are known to be produced from DOM photochemistry, including singlet oxygen, O-1(2). Recently, it has been determined that humic-like substances and unknown organic chromophores are significant contributors to sunlight absorption in snowpack; however, DOM photochemistry in snow/ice has received little attention in the literature. We recently showed that DOM plays an important role in indirect photolysis processes in ice, producing ROS and leading to the efficient photodegradation of a probe hydrophobic organic pollutant, aldrin.(1) ROS scavenger experiments indicated that O-1(2) played a significant role in the indirect photodegradation of aldrin. Here we quantitatively examine O-1(2) photochemically produced from DOM in frozen and liquid aqueous solutions. Steady-state O-1(2) production is enhanced up to nearly 1000 times in frozen DOM samples compared to liquid samples. O-1(2) production is dependent on the concentration of DOM, but the nature of the DOM source (terrestrial vs microbial) does not have a significant effect on O-1(2) production in liquid or frozen samples, with different source types producing similar steady-state concentrations of O-1(2). The temperature of frozen samples also has a significant effect on steady-state O-1(2) production in the range of 228-262 K, with colder samples producing more steady-state O-1(2). The large enhancement in O-1(2) in frozen samples suggests that it may play a significant role in the photochemical processes that occur in snow and ice, and DOM could be a significant, but to date poorly understood, oxidant source in snow and ice.