The role of advection on CO2 flux measurements at the Cabauw tall tower, The Netherlands

Carbon cycle research has gained prominence in recent years due to its established links with global climate change. One important component of carbon cycle research is estimating the annual transfer of CO₂ between the atmosphere and the biosphere (e.g., plants, soils). A global network of over 140 towers has emerged over the last 5-10 years with the common goal of measuring this transfer of CO₂ over a variety of ecosystems worldwide. Each site uses similar techniques (e.g., eddy covariance and storage methods) to measure the turbulent and stored fluxes of CO₂. There exists a third component of the total CO₂ transfer, the advective flux, which is often assumed negligible. Advective fluxes can be caused by heterogeneity of the source area to the flux, or by large-scale changes in the atmospheric concentration of CO₂ that are advected by the tower over periods of seconds to days. This study focuses on measuring the advective component of carbon dioxide transfer at a relatively ideal site, the Cabauw tall tower in the Netherlands, during two months (September, 2003 and April, 2004). The tower is located in flat and homogenous grassland terrain. Measurements are made at four levels on the tall tower (213 m), allowing the advective flux to be estimated by measuring the difference in NEE between levels. Persistent advective fluxes were observed associated with the morning transition period during both months. Strategies are being emplaced to separate the two most likely components of advective flux. NEE is being compared between levels (1) that exhibit homogeneous and heterogeneous source areas based on footprint analysis during periods of no frontal passage and relatively stable CO₂ concentrations, and (2) during periods when different levels are sourced from comparable / homogeneous upwind regions, but the tower experiences a change in CO₂ concentration over periods of hours to days. Results from this analysis will be discussed.