The influence of nocturnal CO2 advection on CO2 flux measurements

Eddy covariance (EC) measurements of CO2 are widely used to assess the CO2 budget of ecosystems. However, important uncertainties exist about how to obtain flux values that are representative for the regional scale from local-scale EC measurements. This paper addresses the issues of nocturnal CO2 advection in non-flat topography, and the representativeness of the local clover-rich grass surface for the larger region, at the end of the growing season. Local EC measurements were compared with the fluxes obtained from a boundary-layer budget method (BLB) which employed tethered balloon measured vertical profiles of CO2 concentration in the lowest few hundred meters of the atmosphere. It was found that the diurnal change in CO2 concentration was restricted to the lowest approximate to 450 m of the atmosphere. The apparent discrepancies between the CO2 fluxes obtained via the EC and the BLB method disappeared when an appropriate scaling of the EC flux measurements was carried out. Best agreement between the two methods was found when EC fluxes were scaled with a factor of 1.85 and CO2 advection during the first half of the night was treated specially by scaling the EC flux component pointing away from the surface with a factor of 12.5 instead. The factor 1.85 was interpreted as the scaling factor going from the local surface probed by the EC method to the average of the regional surface. The special factor of 12.5 was interpreted as the product of the scaling factor 1.85 and of a factor of 6.8 relating the surface area that contributes to CO2 advection to the unit area of the turbulent EC flux measurements. This is a first attempt to find ways of scaling up local-scale EC measurements to the regional-scale where net biome productivity (NBP) is sought. In this approach, NBP is expressed as a simple combination of the turbulent flux measurements (net ecosystem exchange NEE) with a net non-turbulent exchange component (NNTE) such as nocturnal advection of CO2.