Reactive Trace Gases within and above a Grassland Canopy: Challenges for Flux and Transport Process Determination
Nitrogen oxide (NO), nitrogen dioxide (NO2) and ozone (O3) constitute a reactive trace gas triad, which is highly important for the oxidizing capacity of the atmosphere, the functioning of ecosystems and climate change. Terrestrial ecosystems, such as grasslands, represent considerable sources and sinks for the NO-NO2-O3 triad and, thus, are crucial for atmospheric budgets of these species. The experimental quantification of surface-atmosphere exchange of the NO-NO2-O3 triad is difficult, and permanent flux monitoring networks are still at their infancy. Due to their fast reactivity, vertical gradients and fluxes of these species differ significantly from the respective theoretical descriptions for non-reactive trace gases. This effect is referred to as “chemical flux divergence”. Compared to the layer above, plant canopies exhibit an increased aerodynamic resistance, resulting in prolonged transport times of air within the canopy. This may provide sufficient time for in-canopy flux divergence and for interactions of trace gases such as the NO-NO2-O3 triad with plants. A prominent example is the fate of soilemitted NO that undergoes chemical reaction with O3 within the canopy and is subsequently recaptured by the surrounding vegetation in the form of NO2. This process is referred to as “NOx canopy reduction” (NOx = NO+NO2). NOx canopy reduction is commonly applied in global atmospheric chemistry and transport models, but was never investigated for grassland canopies, which cover vast terrestrial areas. This thesis focusses on the following topics investigated for a natural grassland canopy: (i) the quantification of in-canopy transport characteristics such as aerodynamic resistances and transport times, (ii) validation of the dynamic chamber technique for routine O3 flux measurements at low canopy ecosystems and (iii) the analysis of chemical flux divergence and NOx canopy reduction. The quantification of in-canopy transport characteristics is of a major importance for the investigation of sources, sinks and net fluxes of reactive trace gases within plant canopies. A novel automated measurement system for selective vertical Thoron (Tn) profiles near the earth’s surface has been presented and evaluated, and its suitability for the direct and reliable determination of transport times within a natural grassland canopy has been demonstrated. For the first time, a rigorous determination of systematic and random error of Tn concentrations was performed under field conditions for this type of measurement system. In-canopy transport times were calculated and their uncertainty from the individual errors of the Tn concentration measurements was propagated. The directly measured in-canopy transport times were compared with two empirical parameterizations that are frequently used in modeling studies. The disability of the parameterizations to reproduce the entire diurnal course of the in-canopy transport has been proven. An agreement with the measured transport times was either found during daytime or during nighttime, depending on the parameterization. The incanopy transport characteristics constituted as base for the investigation of chemical flux divergence and NOx canopy reduction. Nowadays, eddy covariance (EC) is the state of the art method to quantify turbulent exchange fluxes. EC requires high-frequency trace gas instruments that are not always available. In the absence of such instruments, fluxes can also be determined using e.g., chamber techniques. However, up to date fluxes of depositing compounds have been rarely determined using chamber techniques, mainly due to a modification of the aerodynamic conditions for the trace gas transport within the chamber. O3 deposition fluxes measured at a natural grassland site by the dynamic chamber technique are presented and, for the first time, validated against the EC method. The raw O3 fluxes of the dynamic chamber method were corrected for gas-phase chemistry and for the modification of the aerodynamic resistances. Simultaneously measured carbon dioxide and water vapor fluxes by both methods were comparable during daytime, documenting an equal vegetation activity inside and outside the chambers. The final corrected O3 deposition fluxes of both methods deviated on average by only 11 % during daytime. This demonstrates the capability of the dynamic chamber method to capture representative O3 deposition fluxes for low canopy ecosystems. The canopy resistance to O3, an important parameter in modeling studies, was assessed by both methods and showed a characteristic diurnal cycle with minimum hourly median values of 180 s m-1 (chambers) and 150 s m-1 (EC) before noon. By using the O3 fluxes resulting from the EC method, it could be shown that the nonstomatal pathway dominated the total O3 deposition to the natural grassland canopy. For the first time, transport times, aerodynamic resistances, vertical profiles of NO-NO2-O3 mixing ratios and micrometeorological quantities were simultaneously measured within and above a natural grassland canopy, and delivered insights on potential NOx canopy reduction and flux divergence. A canopy decoupling was observed during day and nighttime from vertical temperature profiles resulting in inverse stability conditions in the lower and upper grassland canopy. For the lower canopy this interestingly implied a daytime stable stratification and a nighttime unstable stratification. The diurnal courses of in-canopy transport characteristics reflected the stratification. The grassland showed parallels with Amazonian rainforest canopies from the literature. Unfortunately, NOx canopy reduction could not be quantified due to insignificant NO soil emissions at the site. Nevertheless, the obtained results clearly allowed the conclusion that NOx canopy reduction in grassland canopies of similar structure is generally very efficient during daytime at sites where NO is emitted. In addition, a chemical flux divergence for O3 was determined between the EC measurement height and the canopy top. In contrast to previous studies, the chemical flux divergence resulted in a net chemical O3 production during daytime, leading to 10 % underestimation of the O3 flux by the EC method.
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