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

<p>Terrestrial ecosystems, occupying more than 25% of the Earth's surface, can serve asp><p>`biological valves' in regulating the anthropogenic emissions of atmospheric aerosolp><p>particles and greenhouse gases (GHGs) as responses to their surrounding environments.p><p>While the signicance of quantifying the exchange rates of GHGs and atmosphericp><p>aerosol particles between the terrestrial biosphere and the atmosphere isp><p>hardly questioned in many scientic elds, the progress in improving model predictability,p><p>data interpretation or the combination of the two remains impeded byp><p>the lack of precise framework elucidating their dynamic transport processes over ap><p>wide range of spatiotemporal scales. The diculty in developing prognostic modelingp><p>tools to quantify the source or sink strength of these atmospheric substancesp><p>can be further magnied by the fact that the climate system is also sensitive to thep><p>feedback from terrestrial ecosystems forming the so-called `feedback cycle'. Hence,p><p>the emergent need is to reduce uncertainties when assessing this complex and dynamicp><p>feedback cycle that is necessary to support the decisions of mitigation andp><p>adaptation policies associated with human activities (e.g., anthropogenic emissionp><p>controls and land use managements) under current and future climate regimes.p><p>With the goal to improve the predictions for the biosphere-atmosphere exchangep><p>of biologically active gases and atmospheric aerosol particles, the main focus of thisp><p>dissertation is on revising and up-scaling the biotic and abiotic transport processesp><p>from leaf to canopy scales. The validity of previous modeling studies in determiningp><p>ivp><p>the exchange rate of gases and particles is evaluated with detailed descriptions of theirp><p>limitations. Mechanistic-based modeling approaches along with empirical studiesp><p>across dierent scales are employed to rene the mathematical descriptions of surfacep><p>conductance responsible for gas and particle exchanges as commonly adopted by allp><p>operational models. Specically, how variation in horizontal leaf area density withinp><p>the vegetated medium, leaf size and leaf microroughness impact the aerodynamic attributesp><p>and thereby the ultrane particle collection eciency at the leaf/branch scalep><p>is explored using wind tunnel experiments with interpretations by a porous mediap><p>model and a scaling analysis. A multi-layered and size-resolved second-order closurep><p>model combined with particle p><p>uxes and concentration measurements within andp><p>above a forest is used to explore the particle transport processes within the canopyp><p>sub-layer and the partitioning of particle deposition onto canopy medium and forestp><p>oor. For gases, a modeling framework accounting for the leaf-level boundary layerp><p>eects on the stomatal pathway for gas exchange is proposed and combined with sapp><p>ux measurements in a wind tunnel to assess how leaf-level transpiration varies withp><p>increasing wind speed. How exogenous environmental conditions and endogenousp><p>soil-root-stem-leaf hydraulic and eco-physiological properties impact the above- andp><p>below-ground water dynamics in the soil-plant system and shape plant responsesp><p>to droughts is assessed by a porous media model that accommodates the transientp><p>water p><p>ow within the plant vascular system and is coupled with the aforementionedp><p>leaf-level gas exchange model and soil-root interaction model. It should be notedp><p>that tackling all aspects of potential issues causing uncertainties in forecasting thep><p>feedback cycle between terrestrial ecosystem and the climate is unrealistic in a singlep><p>dissertation but further research questions and opportunities based on the foundationp><p>derived from this dissertation are also briep><p>y discussed.p>