1 NATURE AND ABUNDANCE OF ORGANIC 1 RADICALS IN NATURAL ORGANIC MATTER - 2 EFFECT OF pH AND IRRADIATION 3 Andrea Paul *† , Reinhard Stösser ‡ , Andrea Zehl ‡ , Elke Zwirnmann † , Rolf D. Vogt ₣ , and Christian E.W. 4 Steinberg § 5 † Leibniz Institute of Freshwater Ecology and Inland Fisheries, Müggelseedamm 301, D-12561 Berlin, 6 Germany, ‡ Humboldt University of Berlin, Institute of Chemistry, Brook-Taylor-Str. 2, D-12489 Berlin, 7 Germany, ₣ Department of Chemistry, University of Oslo, P.O.Box 1033 Blindern, N-0315 Oslo, 8 Norway, § Humboldt University of Berlin, ₣ Institute of Biology, Arboretum, Späthstr. 80/81, D-12437 9 Berlin, Germany 10 a.paul@.igb-berlin.de, Reinhard.Stoesser@rz.hu-berlin.de, Andrea.Zehl@rz.hu-berlin.de, 11 zwirnmann@igb-berlin.de, r.d.vogt@kjemi.uio.no, christian_ew_steinberg@web.de 12 *) Corresponding author phone: +49-30-64181-745; fax: +49-30-64181-682; e-mail: a.paul@igb- 13 berlin.de. 14 Dissolved natural organic matter (NOM) plays an essential role in freshwater geochemical and 15 biochemical processes. A major property, its redox behavior, can be attributed to the chinone building 16 blocks, which can form stable radicals. However, electron paramagnetic resonance (EPR) data 17 indicating free radicals on solid NOM are sparse. Here we present EPR-spectra of 23 NOM from 18 European surface waters isolated by reverse osmosis. The organic radical concentrations of NOM 19 ranged from 5ｯ10 15 to 1.84ｯ10 17 spins g -1 , and g-values from 2.0031 to 2.0045. Number and type of 20 organic radicals in solid NOM are significantly influenced by the pH of raw water. EPR-experiments 21