THE CONDUCTANCE OF A MAIZE CROP AND THE UNDERLYING SOIL TO OZONE UNDER VARIOUS ENVIRONMENTAL-CONDITIONS

Flux measurements of ozone and water vapour employing the eddy correlation technique were used to determine the surface conductance and canopy conductance to ozone. In the surface conductance to ozone, all surfaces at which ozone is destroyed and the transport process to these surfaces are included. The canopy conductance to ozone represents the ozone uptake of transpiring plant parts. The surface conductance to ozone of the maize crop and the underlying soil was generally larger than the canopy conductance to ozone. This means that beside the uptake by stomata, there was another important ozone sink. Under wet soil surface conditions, the surface conductance and the canopy conductance to ozone coincided. This indicates that the resistance of wet soil and the remaining plant parts (cuticle) to ozone was much larger than the stomatal or soil resistance. On the other hand, under dry soil conditions the conductances differ, largely caused by a variation in the transport process to the soil. The transport of ozone to soil increased with increasing friction velocity (u*) and decreased with increasing atmospheric stability. leaf area index (LAI) or crop height (h). These effects for midday (unstable) conditions were parameterized with an ''in-crop'' aerodynamic resistance, r(inc) in a very straightforward way; r(inc) = 13.9 LAI h/u* + 67 (cc. = 0.77). If the ozone flux in air pollution models is described with a simple resistance model (Big Leaf model). the extra destruction at the soil should be modelled using an ''in-crop'' aerodynamic resistance. For these measurements the ozone flux to the soil was 0-65% of the total ozone flux measured above the crop. Under wet soil conditions, this was less than 20%; under dry soil conditions. this was 30-65%.