Bridging the Scale Gap from Leaf to Canopy in Biosphere-Atmosphere Gas and Particle Exchanges

Terrestrial ecosystems, occupying more than 25% of the Earth's surface, can serve as 'biological valves' in regulating the anthropogenic emissions of atmospheric aerosol particles and greenhouse gases (GHGs) as responses to their surrounding environments. While the significance of quantifying the exchange rates of GHGs and atmospheric aerosol particles between the terrestrial biosphere and the atmosphere is hardly questioned in many scientific fields, the progress in improving model predictability, data interpretation or the combination of the two remains impeded by the lack of precise framework elucidating their dynamic transport processes over a wide range of spatiotemporal scales. The difficulty in developing prognostic modeling tools to quantify the source or sink strength of these atmospheric substances can be further magnified by the fact that the climate system is also sensitive to the feedback from terrestrial ecosystems forming the so-called 'feedback cycle'. Hence, the emergent need is to reduce uncertainties when assessing this complex and dynamic feedback cycle that is necessary to support the decisions of mitigation and adaptation policies associated with human activities (e.g., anthropogenic emission controls and land use managements) under current and future climate regimes. With the goal to improve the predictions for the biosphere-atmosphere exchange of biologically active gases and atmospheric aerosol particles, the main focus of this dissertation is on revising and up-scaling the biotic and abiotic transport processes from leaf to canopy scales. The validity of previous modeling studies in determining iv the exchange rate of gases and particles is evaluated with detailed descriptions of their limitations. Mechanistic-based modeling approaches along with empirical studies across different scales are employed to renew the mathematical descriptions of surface conductance responsible for gas and particle exchanges as commonly adopted by all operational models. Specifically, how variation in horizontal leaf area density within the vegetated medium, leaf size and leaf microroughness impact the aerodynamic attributes and thereby the ultrane particle collection efficiency at the leaf/branch scale is explored using wind tunnel experiments with interpretations by a porous media model and a scaling analysis. A multi-layered and size-resolved second-order closure model combined with particle fluxes and concentration measurements within and above a forest is used to explore the particle transport processes within the canopy sub-layer and the partitioning of particle deposition onto canopy medium and forest floor. For gases, a modeling framework accounting for the leaf-level boundary layer effects on the stomatal pathway for gas exchange is proposed and combined with sap flux measurements in a wind tunnel to assess how leaf-level transpiration varies with increasing wind speed. How exogenous environmental conditions and endogenous soil-root-stem-leaf hydraulic and eco-physiological properties impact the above- and below-ground water dynamics in the soil-plant system and shape plant responses to droughts is assessed by a porous media model that accommodates the transient water flow within the plant vascular system and is coupled with the aforementioned leaf-level gas exchange model and soil-root interaction model. It should be noted that tackling all aspects of potential issues causing uncertainties in forecasting the feedback cycle between terrestrial ecosystem and the climate is unrealistic in a single dissertation but further research questions and opportunities based on the foundation derived from this dissertation are also briefly discussed.