Ozone dry deposition to vegetated surfaces is an important process because it contributes substantially to the removal of tropospheric ozone, but also because it may affect the physiological functioning of plant species and reduce the yield of agricultural crops. Yet the physiological, physical and physico-chemical processes determining the magnitude of deposition and their relative contribution are not well understood. Previous studies reported different contributions of stomatal uptake and other competing sinks in the canopy layer, like e.g. uptake by soil, outer foliage surfaces, and titration by NO emitted from the soil. Additionally, the effect of surface wetness on the ozone deposition represents an important factor which is still unclear.
The present investigation is focused on the role of stomatal conductance and leaf wetness in controlling ozone deposition to a triticale cereal field in middle Europe (Bellheim, Southern Germany, 49.18 N, 8.28 E, 127 m a.s.l.). Measurements reported here were performed in June and July 1995 during the senescence period of the triticale canopy with a continuous decrease in active leaf area. Eddy correlation (and profile) systems were used for the continuous measurement of momentum, energy and ozone fluxes above the canopy. The flux values were corrected for the effects of sensor misalignment, correlated density fluctuations and high frequency attenuation. The stomatal resistance of the various leaf levels was determined by water exchange measurements with a porometric chamber. The vertical leaf area distribution and other plant characteristics were measured several times during the campaign. Various environmental parameters were also monitored, in particular the rain intensity, surface wetness (by electric resistance grids) within as well as on top of the canopy, and canopy temperature (by thermal radiation measurements).
The effective surface resistance for ozone deposition (Rc) was determined from the observed ozone flux and ambient concentration together with estimated values of the aerodynamic (Ra) and quasi-laminar boundary layer (Rb) resistance. Rc was compared to the canopy-level stomatal resistance (Rstom), which was either upscaled from the leaf-level porometric measurements or inferred from the latent heat flux and leaf temperature observations. Between early and late senescence, the ozone deposition was continuously decreasing in good correlation to the decrease in active leaf area and the corresponding value for Rstom. Under dry conditions mainly occurring during daytime, Rc and Rstom showed similar values however with a considerable scatter. This indicates a substantial contribution of the stomatal uptake to the total ozone deposition. Some rain events and common dew formation during the night led to frequently wet leaf and soil surfaces. Under such conditions, the observation of a representative stomatal resistance was generally not possible. However, a significant decrease in Rc compared to dry conditions indicate the existence of a strong non-stomatal deposition pathway, which is related to surface wetness. It might be due to homogenous reaction with NO, which is favourably emitted from wet soil, or due to heterogeneous decomposition processes which remain to be characterized.