The Emission and Chemical Transformation of Biogenic Volatile Organic Compounds (ECHO) project investigated formation, transport, and photochemistry of biogenic organic compounds in a deciduous forest. Within the project, filterradiometers were used to measure photolysis frequencies j(O¹ D) (O₃ + hv → O₂+ O(¹ D)) and j(NO₂) (NO₂ + hv → NO + O(³ P)) at different levels of a tower set up in a small clearing in a forest at Jülich, Germany. The tower served as a measurement platform for instruments in two photochemistry field campaigns during the summer seasons of 2002 and 2003. Filterradiometer measurements were made throughout a complete vegetation cycle to give an extended characterization of the field site UV radiation conditions. At fully developed foliage, 3–7% of above-forest values of j(O¹ D) and j(NO₂) were obtained close to ground at the tower clearing, dependent on external conditions. In the course of the vegetation period, these ratios remained fairly constant. After fall of leaves, photolysis frequencies increased to about 33% of above-forest values. Profile measurements in undisturbed forest showed a strong decrease of j(NO₂) by about 90% within 5 m below canopy top (≈33 m) and a slowed decrease to typically 2% at ground level. At the tower clearing the decrease was more gradual reaching 50% of above-canopy values approximately 10 m below canopy top under overcast conditions. With regard to photochemical conversion of biogenic compounds we estimated that degradation of the major reactive compound isoprene within the forest by OH and O₃ was insignificant (<10%) if residence times were less than about 1 hour.