Boundary layer dynamics, including surface pro- cesses determine the daily evolution of temperature and moisture in the atmospheric boundary layer (ABL, Lemone et al., 2002). Moreover, turbulent mixing drives the exchange of CO2 between the atmosphere and bio- sphere and, finally, the advection influences the distribu- tion of carbon dioxide near the surface (Yi et al., 2000; Werner et al., 2006; Eugster and Siegrist, 2000), and as a consequence Net Ecosystem-atmosphere Exchange (NEE) of carbon dioxide. NEE is continuously estimated around the world usu- ally from measurements at one height near the ground of the time evolution of the CO2 concentration and turbu- lent flux (Valentini et al., 2000; Baldocchi et al., 2001). In these observations, among other approximations, advec- tion is supposed negligible (Yi et al., 2000), and a well- mixed boundary layer is considered during the whole day. In this study the diurnal variability of the CO2 con- centration is studied by means of tower measurements, mixed-layer theory and Large-Eddy simulations done with the Dutch model DALES (Heuss et al., 2010). The work presented here is an extension of Casso-Torralba et al. (2008), where observations were presented. Particular attention is devoted to analyze the role of boundary layer processes on the CO2 evolution and to study the uncer- tainties in the calculation the Net ecosystem-atmosphere Exchange for CO2.

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