Five-year measurements of ozone fluxes to a Danish Norway spruce canopy

Ozone concentrations and fluxes have been measured continuously during 5 years (1996–2000) by the gradient method in a Norway spruce dominated forest stand in West Jutland, Denmark, planted in 1965. The method has been validated against other methodologies and a relatively good relationship was found. The data are analysed to quantify diurnal, seasonal and yearly fluxes, and non-stomatal and stomatal removal are estimated. Monthly means of climatic data are shown, and day and night values of the aerodynamic resistance, ra, viscous sub-layer resistance, rb, and the surface or canopy resistance, rc, are presented. The yearly ozone deposition is approximately 126 kg ha−1. The canopy ozone uptake is highest during the day and during the summer. This is interpreted as increased stomatal uptake and physical and chemical reactions. The daily means of ozone concentration and fluxes averaged over 5 years correlate, but the correlation is primarily based on two different uncoupled processes outside and inside the stomates: (1) The ozone destruction in the canopy occurring outside the stomates is much influenced by temperature, light and humidity, e.g. surface reactions, NO- and VOC-emissions. (2) The same factors have a strong influence on the stomatal opening, e.g. midday and night closure. Thus, looking at diurnal variations, the diurnal ozone concentration and ozone flux do not correlate at all during the growing season. The maximum diurnal difference for the ozone concentration is a factor 1.3 and the maximum diurnal difference for the ozone flux is a factor 3. From dawn to ca. 8:00 the ozone deposition increases and the ozone concentration decreases. The yearly stomatal uptake of ozone is estimated to minimum 21% of the total deposition, being highest in May–August (30–33%) and lowest in November–February (4–9%). The physiological ozone uptake per leaf area is estimated to 0.33 g ozone m−2 y−1.