Total ozone deposition and sink distribution in a grass canopy

Presently, quantitative description of ozone deposition to vegetation is usually based on simple bulk resistance concepts (big-leaf models). They were developed and work satisfyingly well for closed canopies with a maximum of active leaves in the upper part (e.g. cereal or maize crops, forests). In such canopies the main part of ozone deposition occurs in the top layer where the ozone concentration is close to ambient. The gas exchange within the canopy and towards the soil is poorly represented in the models, but it is also not very important in these cases. In grass ecosystems, however, a large part of the (active) leaf area and thus of the potential ozone sink is often found in the lower part of the canopy. Therefore, the in-canopy exchange and the resulting ozone distribution are important for the deposition in the different layers and at the soil surface.

In order to study the distribution of ozone concentration and ozone sinks in a complex grass canopy, we performed a field experiment on an intensively managed grassland field in Switzerland during several weeks in May/June 2003 before and after the first cut. Total ecosystem fluxes of ozone and water vapor were measured by an eddy covariance system. A special profiling system was developed to allow continuous profile measurements of ozone, water vapor as well as temperature throughout the grass canopy with minimum disturbance effect (Jäggi et al., Atmospheric Environment, in press). Canopy height before the cut was between 40 and 80 cm with a maximum leaf area index of 6.8. Stomatal conductance of the two dominant plant species in the upper and lower half of the canopy were occasionally measured.

With the help of a diagnostic multi-layer resistance model adjusted to the measured quantities, the total ozone deposition flux was partitioned to the different canopy layers and deposition pathways (stomates, cuticles, soil). The derived results show that stomatal uptake is able to explain the total deposition flux during daytime to a large extent. The maximum sink strength is generally found in the lower half of the canopy. Combining the vertical concentration and sink profiles, effective resistance values for turbulent in-canopy transfer could be estimated.