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[1] The photolysis of nitrous acid (HONO) in the early morning hours is an important source of OH radicals, the most important daytime oxidizing species. Although the importance of this mechanism has been recognized for many years, no accurate quantification of this OH source is available, and the role of HONO photolysis is often underestimated. We present measurements of HONO and its precursor NO 2 by Differential Optical Absorption Spectroscopy (DOAS) during the Berliner Ozonexperiment (BERLIOZ) field campaign in July/August 1998 at Pabstthum near Berlin, Germany. HONO concentrations, formation rates, and simultaneously measured HONO photolysis frequencies are used to calculate the total amount of OH formed by HONO photolysis during a full diurnal cycle. A comparison with the OH formation by photolysis of O 3 and HCHO and by the reaction of alkenes with ozone shows that HONO photolysis contributed up to 20% of the total OH formed in a 24 hour period during this campaign. In the morning hours, HONO photolysis was by far the most important OH source during BERLIOZ.