The ozone transfer between atmosphere and vegetation. A study on Scots pine in the field

This thesis analysed the controlling processes of the transfer of ozone at the air-plant interface. Improvement in the knowledge of these processes benefits the prediction of both atmospheric removal of O3 and its impact on vegetation. This study was based on the measurement and analysis of multi-year field measurements of O3 flux to Scots pine (Pinus sylvestris L.) foliage with a shoot-scale gas-exchange enclosure system. In addition, the analyses made use of simultaneous CO2 and H2O exchange, canopy-scale O3, CO2 and H2O exchange, foliage surface wetness, and environmental variables. All data was gathered at the SMEAR measuring station (southern Finland). Enclosure gas-exchange techniques can be applied to the measure of O3 gas-exchange in the field with consideration of artefacts arising from O3 reactivity and low concentration. The O3 wall-loss was corrected with information from simultaneous and continuous measurements, and was included the mass balance formulation of O3 concentration inside the chamber. The flux of ozone to the Scots pine foliage was generated in equal proportions by stomatal and non-stomatal controlled processes. The temporal pattern of total flux was an overlap of the patterns of biological activity and presence of wetness in the environment. The stomatal uptake was estimated from models of stomatal behaviours and the non-stomatal portion of the flux was analysed further. The observed moisture enhancement was related to the presence of liquid films on the foliage surface. Theoretical simulation of the diffusive transfer at one-stoma-scale showed that O3 scavenging reactions located before or near the stomatal aperture can prevent or diminish the diffusion of ozone towards the intercellular air space of the mesophyll. The discussion covers the methodological developments of this study, the relevance of the different controlling factors of ozone flux, the partition amongst its component, and the possible mechanisms of non-stomatal uptake.