[1] Daytime concentrations of HONO, NO x , OH and photolysis frequencies were measured during the ECHO field campaign in a mixed deciduous forest near Jülich, West-Germany, in summer 2003. Midday measurements show clear evidence for a large, yet unexplained daytime source of HONO ($500 pptv/h), which represents an important net source of OH radicals due to ongoing HONO photolysis. The evidence for a large HONO daytime source is for the first time completely constrained by measured parameters, needed to determine the daytime budget of HONO. The large contribution of 33% to the primary OH production during noon at the top of the forest canopy suggests that the unexplained source of HONO could have an important impact on the photochemical transformation of biogenically emitted volatile organic compounds (VOCs) by OH into partly oxidized VOCs and secondary organic aerosols during their release from forest regions into the troposphere. Citation: Kleffmann, J., T. Gavriloaiei, A. Hofzumahaus, F. Holland, R. Koppmann, L. Rupp, E. Schlosser, M. Siese, and A. Wahner (2005), Daytime formation of nitrous acid: A major source of OH radicals in a forest, Geophys. Res. Lett., 32, L05818,

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The aquatic chemistry of iron involves many environmental aspects because dissolved iron is present as ferrous and ferric ion and the equilibrium between these two forms depends on many natural or antropogenic factors. This paper aims to provide a brief introduction to chemistry of iron in an oligotrophic lake, a new topic in this domain. A number of 28 sampling were taken from Catamarasti Lake, Botosani (Romania). The main physico-chemical properties showed that the acidity, dissolved oxygen, electrical conductivity and saturation satisfied the water quality criteria for this ecosystem. The results for dissolved iron species indicated that there is no strictly correlation with the variation of oxygen content. Thus, for the samples from NE shore, the contents of ferrous iron are bigger than of ferric iron, for the samples from SW shore the contents are almost equal, because of the low contents of oxygen. For the samples from the middle and from downstream of the lake, the contents of ferric iron are bigger than of ferrous iron, due to the oxygen content recorded.