The use of disjunct eddy sampling methods for the determination of ecosystem level fluxes of trace gases

The concept of disjunct eddy sampling (DES) for use in measuring ecosystem-level micrometeorological fluxes is re-examined. The governing equations are discussed as well as other practical considerations and guidelines concerning this sampling method as it is applied to either the disjunct eddy covariance (DEC) or disjunct eddy accumulation (DEA) techniques. A disjunct eddy sampling system was constructed that could either be combined with relatively slow sensors (response time of 2 to 40 s) to measure fluxes using DEC, or could also be used to accumulate samples in stable reservoirs for later laboratory analysis (DEA technique). Both the DEC and DEA modes of this sampler were tested against conventional eddy covariance (EC) for fluxes of either CO2 (DEC) or isoprene (DEA). Good agreement in both modes was observed relative to the EC systems. However, the uncertainty in a single DEA flux measurement was considerable (40%) due to both the reduced statistical sampling and the analytical precision of the concentration difference measurements. We have also re-investigated the effects of nonzero mean vertical wind velocity on accumulation techniques as it relates to our DEA measurements. Despite the higher uncertainty, disjunct eddy sampling can provide an alternative technique to eddy covariance for determining ecosystem-level fluxes for species where fast sensors do not currently exist

Scaling isoprene fluxes from leaves to canopies: test cases over a boreal aspen and a mixed species temperate forest

The rate at which isoprene is emitted by a forest depends on an array of environmental variables, the forest’s biomass, and its species composition. At present it is unclear whether errors in canopy-scale and process-level isoprene emission models are due to inadequacies in leaf-to-canopy integration theory or the imperfect assessment of the isoprene-emitting biomass in the flux footprint. To address this issue, an isoprene emission model (CAN- VEG) was tested over a uniform aspen stand and a mixed-species, broad-leaved forest. The isoprene emission model consists of coupled micrometeorological and physiological modules. The mi- crometeorological module computes leaf and soil energy exchange, turbulent diffusion, scalar concentration profiles, and radiative transfer through the canopy. Environmental variables that are computed by the micro- meteorological module, in turn, drive physiological modules that calculate leaf photosynthesis, stomatal con- ductance, transpiration and leaf, bole and soil/root respiration, and rates of isoprene emission. The isoprene emission model accurately predicted the diurnal variation of isoprene emission rates over the boreal aspen stand, as compared with micrometeorological flux measurements. The model’s ability to simulate isoprene emission rates over the mixed temperate forest, on the other hand, depended strongly upon the amount of isoprene-emitting biomass, which, in a mixed-species forest, is a function of the wind direction and the horizontal dimensions of the flux footprint. When information on the spatial distribution of biomass and the flux footprint probability distribution function were included, the CANVEG model produced values of isoprene emission that compared well with micrometeorological measurements. The authors conclude that a mass and energy exchange model, which couples flows of carbon, water, and nutrients, can be a reliable tool for integrating leaf-scale, isoprene emission algorithms to the canopy dimension over dissimilar vegetation types as long as the vegetation is characterized appropriately