Role of boundary layer processes on the mixed layer CO2-budget

The diurnal and vertical variability of temperature, humidity and specially CO2 in the atmospheric boundary layer is studied by combining detailed observations taken at Cabauw (The Netherlands), Large-Eddy simulations (LES) and mixed layer theory. The research focus on the role played by the entrainment and other boundary layer driven processes on the distribution and diurnal evolution of CO2 in the boundary layer. The relative importance of this entrained air to ventilate CO2 will be analyzed. During the morning the exchange of CO2 between the residual free tropospheric air masses with the growing boundary layer is a more important contribution than the CO2 uptake by the vegetation, whereas during the afternoon the assimilation by grass at the ground could become the dominant process. This work is completed by quantifying the terms of the budget conservation equation of CO2 using observations and the LES numerical experiments. It will be shown that under non-advective conditions, the flux divergence measurements can correctly reproduce the diurnal variability of temperature, moisture and CO2. The role of boundary layer dynamics on the CO2-budget has direct implications in inferring the CO2-uptake flux from CO2 observations. Our findings show how dynamic factors, as the morning temperature inversion or the lapse rate, are relevant in the retrieval of the CO2 flux. By analyzing the sensitivity of the inferred CO2 flux to the dynamics of the boundary layer and to the CO2 variables, we are able to estimate the uncertainties of the inferred CO2 flux to mean CO2 concentrations