Influence of dissolved organic carbon on photochemically mediated cycling of hydrogen peroxide in rainwater

The influence of sunlight and dissolved organic carbon (DOC) on the photochemically mediated cycling of hydrogen peroxide (H 2 O 2 ) was investigated in rainwater samples collected in Wilmington, North Carolina USA. Upon exposure to simulated sunlight 14 of 19 authentic rainwater samples exhibited significant decreases in H 2 O 2 . The concentration of hydrogen peroxide did not change significantly in organic-free synthetic rainwater spiked with H 2 O 2 in the light or in dark controls suggesting that the loss was not due to direct photolysis or dark mediated reactions. There was a significant correlation between pseudo-first order rate constants of H 2 O 2 decay and initial H 2 O 2 concentrations. There was also a significant correlation between the rate constant and the abundance of DOC suggesting that rainwater organic carbon plays an important role during photolytic decay either via direct reaction or indirectly through production of peroxide reactive species or scavenging of peroxide generating radicals. Several rain samples exhibited an initial increase in H 2 O 2 during the first 2 h of irradiation. These increases were generally small and most likely do not represent a significant input of peroxide in precipitation. The photo-induced destruction of H 2 O 2 is important because it may partly explain the late afternoon decrease of peroxide concentrations observed in earlier field studies and the substantial under saturation (<10%) of this oxidant in rainwater compared with gas phase concentrations.