Ozone uptake by an evergreen forest canopy: temporal variation and possible mechanisms

Patterns of ozone concentration ([O-3]), O-3 deposition velocity (nu(d)) an d O-3 flux (F-c) over an evergreen forest canopy are shown in relation to m easuring method, physiological activity of the trees, and lime of year. The gradient and eddy correlation methods were compared and showed similar die l nu(d) patterns. Daytime F-c was correlated with CO2 and water vapour flux es, while no correlation between [O-3] in the range 10-70 ppb (nl l(-1)) an d F(c)was seen in this study. F-c was primarily driven by stomatal conducta nce, reactions with surfaces, particles and gases, and not by [O-3]. On a m onthly basis, [O-3] was always highest in the afternoon while nu(d) was typ ically higher in the morning, resulting in an equal F-c over the day. Night -time F-c was more than half of the daytime O-3 flux. The data reveal the i mportance of emissions of nitric oxide and terpenes as O-3 removal factors in evergreen forest dominated by Norway spruce. (C) 2000 Elsevier Science L td. All rights reserved.