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

Patterns of ozone concentration ([O3]), O3 deposition velocity (vd) and O3 flux (Fc) over an evergreen forest canopy are shown in relation to measuring method, physiological activity of the trees, and time of year. The gradient and eddy correlation methods were compared and showed similar diel vd patterns. Daytime Fc was correlated with CO2 and water vapour fluxes, while no correlation between [O3] in the range 10-70 ppb (nl l?1) and Fc was seen in this study. Fc was primarily driven by stomatal conductance, reactions with surfaces, particles and gases, and not by [O3]. On a monthly basis, [O3] was always highest in the afternoon while vd was typically higher in the morning, resulting in an equal Fc over the day. Night-time Fc was more than half of the daytime O3 flux. The data reveal the importance of emissions of nitric oxide and terpenes as O3 removal factors in evergreen forest dominated by Norway spruce.