Above-canopy fluxes of ozone were measured from the 35 m PROPHET tower. The ozone sample inlet and sonic anemometer were located 33 m above ground and the air sample was transported to the detector via a 40 m length of 5/8-in. Teflon tubing. The residence time from sample inlet to the detector in the laboratory at the base of the tower was typically less than 25 s. Wind speed and direction (Wind Monitor-RE, R. M. Young Company, USA), pressure (Barometric Pressure Sensor Model 61201, R. M. Young Company, USA), temperature and relative humidity (MP100, Rotronics Instrument Corp, USA) are measured continuously at the top of the PROPHET tower (Carroll et al., 2001). An open-path infrared gas analyzer (IRGA) (Auble and Meyers, 1992) was co-located with the sonic anemometer (K- configuration, ATI, USA) to measure water and CO2 concentration (Pressley et al., 2005). Photosynthetic photon flux density (PPFD) (LI-190SZ, Li-Cor, USA) was measured on the adjacent Ameriflux tower (Schmid et al., 2003), which is located 132 m north–northeast of the PROPHET tower.</p> <p>Ozone was measured using the University of Michigan Multichannel Chemiluminescence Instrument (UMMCI), a custom-built chemiluminescence detector (e.g. Ridley, et al., 1992), illustrated in Fig. 1. The detector consists of a gold-plated 316 stainless-steel reaction vessel (RV, 17 cm3, maintained at 35 degrees C, design by B. A. Ridley, Ridley, et al., 1992), a red-sensitive Hamamatsu R1333 photomultiplier tube (PMT, operated at 5 degrees C), and zeroing volume (ZV, maintained at 100 degrees C) containing 0.5% Pd on Al ozone destruction catalyst (Degussa Metals Corp.).