SURFACE WETNESS ENHANCES OZONE DEPOSITION TO A PASTURE CANOPY

A mass balance approach was used to estimate the deposition of ozone t o a grass-clover canopy enclosed in open-top chambers. Three different concentrations of ozone were used. Deposition measurements were made before and after spraying the canopy with water. It was found that the wet canopy absorbed more ozone than the dry canopy. After spraying, t he vapour pressure deficit (VPD) decreased in the chambers. Thus, an i ncrease in stomatal conductance was expected, which would lead to an i ncrease in ozone uptake by the plants. The change in VPD was, however, of a magnitude which is unlikely to account for more than part of the change in deposition velocity for ozone. It is concluded that the sur face wetness as such accounted for a substantial part of the increase in ozone deposition after spraying.