Journal of Atmospheric Chemistry 31: 205–225, 1998. © 1998 Kluwer Academic Publishers. Printed in the Netherlands. 205 Highly Time Resolved Measurements of OH during POPCORN Using Laser-Induced Fluorescence Spectroscopy F. HOLLAND, U. ASCHMUTAT, M. HEßLING, A. HOFZUMAHAUS and D. H. EHHALT Forschungszentrum Jülich, Inst. für Atmosphärische Chemie, Jülich, Germany (Received: 24 October 1996; in final form: 10 July 1997) Abstract. Tropospheric hydroxyl radical (OH) concentrations were measured by laser-induced fluo- rescence (LIF) during the POPCORN field campaign in August 1994 at a rural site in the North East of Germany. Ambient air spectra were recorded by tuning the laser wavelength over a spectral region covering the Q 11 (3), Q 21 (3), and P 11 (1) rotational transitions of the (0-0) band in the A-X system of OH around 308 nm. The observed spectra clearly identify the OH radical in the atmosphere. Besides the OH absorption lines there was no sign of any other narrow-band spectral structure nearby demonstrating the high specificity of the method. For OH measurements with a typical time resolution of 60–100 seconds per data point the laser wavelength was tuned repetitively over small spectral intervals covering the peak position of the P 11 (1) OH-line and background positions. A total of 2300 measurements were recorded including diurnal cycles of OH with more than 300 data points. The OH as well as the LIF background signal data will be presented. In a first analysis the background signal will be characterized and the correlation between OH and the ozone photolysis frequency will be derived. Key words: OH instrument, laser-induced fluorescence, OH measurements, photochemistry. 1. Introduction This paper reports measurements of tropospheric OH radical concentrations made during the POPCORN field campaign with an instrument based on the detection of laser-induced fluorescence photons of OH excited at 308 nm and low pressure. The applicability of this highly sensitive and very selective technique for the direct measurement of ambient OH in the troposphere has already been demonstrated by different groups (Stevens et al., 1994; Brune et al., 1995; Hard et al., 1995; Holland et al., 1995). Data obtained so far by this technique, however, are still sparse and have shown a relative low time resolution only. First tests of the newly developed LIF instrument at the KFA Jülich (Holland et al., 1995) demonstrated its high de- tection sensitivity and its capability to identify atmospheric OH spectroscopically, but at that time no diurnal profiles were measured. The deployment of the KFA LIF instrument during POPCORN therefore served several purposes: