Quantification of singlet oxygen and hydroxyl radicals upon UV irradiation of surface water

We measured the formation rate and the steady-state concentration of hydroxyl radicals and of singlet oxygen upon irradiation of lake water. There is controversy about the importance of singlet oxygen in the environmental photochemistry, but here we show that the steady-state concentration of 1O2 under irradiation can be higher by about two orders of magnitude compared to the hydroxyl radical. The higher occurrence of singlet oxygen in surface waters is mainly due to a higher rate of formation, because the transformation rate constants of 1O2 (collision with the solvent) and of ·OH (reaction with dissolved compounds) are comparable.