Photochemical generation of reactive species upon irradiation of rainwater: Negligible photoactivity of dissolved organic matter

This paper focuses on the study of the photochemical activity of dissolved organic matter present in rainwater. Formation rates of the reactive species hydroxyl radical (OH(center dot)), singlet oxygen CO(2)) and dissolved organic matter triplet states ((3)DOM*) were determined by irradiation (UV-A) of wet-only rainwater samples collected in Turin (Italy) in the presence of specific scavengers (benzene, furfuryl alcohol and phenol, respectively). Photo-formation rates of OH(center dot) (approximate to 3.10(-11) M s(-1)) and (1)O(2) (approximate to 10(-14) M s(-1)) were lower (1 or 2 orders of magnitude) or largely lower (4 to 10 orders of magnitude) than those determined for fog and cloud samples in previous studies. (3)DOM* formation rate values were either negligible or quite low (approximate to 10(-12) M s(-1)) by comparison with those evaluated for surface water samples. Deduced steady-state [OH(center dot)] were in the same range as those reported for fog samples in the literature (8.7 . 10(-16) to 1.5 . 10(-15) M), while [(1)O(2)] was often several orders of magnitude lower and, therefore, could be considered as negligible. Nitrite (NO(2)(-)) constituted the main source of OH(center dot) (69 +/- 21 to 138 +/- 36%), and the deduced contribution of DOM was low or nil. All the results obtained in this study tend to demonstrate that DOM (including HUmic LIke Substances, HULIS) present in rainwater is poorly or not photoactive. Therefore, there could be considerable difference between rainwater DOM (HULIS included) and the organic matter present in surface waters, particularly the humic substances, as far as the photochemical activity is concerned. (C) 2010 Elsevier B.V. All rights reserved.