Photochemical Reactivity of Organic Matter and its Size Fractions
The photochemical reactions that occur in natural water bodies play an important role in many biogeochemical processes, such as global carbon cycling and the fate of organic contaminants. Photochemically active compounds found in these systems include nitrate, nitrite, and dissolved organic matter (DOM). DOM is a major constituent in the water matrix and thus its photochemical reactions are central to understanding the photochemistry of natural waters. DOM is colored (typically yellow to brown) and absorbs light in the ultraviolet and visible range. Formation of excited states following DOM absorption leads to (sensitized) production of reactive intermediates, such as singlet oxygen, organic peroxyl radicals, hydrogen peroxide, hydroxyl radical, and excited triplet states. As DOM is a complex, heterogeneous material, a complete mechanistic understanding of these species’ production has been difficult to obtain. The aim of this chapter is to develop a framework for understanding the mechanism of sensitized reactive intermediate production from DOM, with special attention to the role of DOM molecular size.