Measurements of ozone, sensible heat, and latent heat fluxes and plant physiological parameters were made at a northern mixed hardwood forest located at the University of Michigan Biological Station in northern Michigan from June 27 to September 28, 2002. These measurements were used to calculate total ozone flux and partitioning between stomatal and non-stomatal sinks. Total ozone flux varied diurnally with maximum values reaching 100 8mol m-2 h-1 at midday and minimums at or near zero at night. Mean daytime canopy conductance was 0.5 mol m-2 s-1. During daytime, non-stomatal ozone conductance accounted for as much as 66% of canopy conductance, with the non-stomatal sink representing 63% of the ozone flux. Stomatal conductance showed expected patterns of behaviour with respect to photosynthetic photon flux density (PPFD) and vapour pressure defecit (VPD). Non-stomatal conductance for ozone increased monotonically with increasing PPFD, increased with temperature (T) before falling off again at high T, and behaved similarly for VPD. Day-time non-stomatal ozone sinks are large and vary with time and environmental drivers, particularly PPFD and T. This information is crucial to deriving mechanistic models that can simulate ozone uptake by different vegetation types.
These measurements and post processing were repeated in 2003, 2004, and 2005 at varying time intervals.
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.
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.).