In natural aquatic systems, the most abundant 1 O 2 -reactive functional group are phenols with concentrations in terrestrial dissolved organic matter of 2 -4 mmol g -1 C. With typical dissolved organic carbon concentrations of 0.5 -3 mg L -1 for oceans and 0.5 -50 mg L -1 for lakes and rivers, formation specifically (0 to -30 ) and previously for overall photochemical O 2 consumption (-8 to -10 ). This comparison suggests that 1 O 2 formation might not be the dominant pathway for photochemical O 2 consumption in complex CDOM mixtures.