The Exchange of Carbon Dioxide between the Atmospheric Boundary Layer and the Free Atmosphere: Observational and LES Study

Boundary layer dynamics and surface processes control the daily evolution of carbon dioxide (CO₂) in the fair weather boundary layer. An important process in CO₂ diurnal variability is the CO₂ exchange between the atmospheric boundary layer and the free atmosphere. This CO₂ exchange process depends largely on the initial boundary layer and the boundary layer development.

Two convective boundary layers are studied: Cabauw (the Netherlands, RECAB exp) and Beaumont (USA, CASES-97 exp). Both sites are characterized by grass vegetation. Data from these sites is analyzed. In addition, mixed-layer theory and a Large-Eddy simulation model is used to study the evolution of the mean and CO₂ flux vertical profiles. Potential temperature, specific humidity and their exchanges are also studied as these variables are important for the development of the boundary layer.

The model results show that the CO₂ exchange can be important in the evolution and distribution of CO₂ in the atmospheric boundary layer. The exchange fluxes ranged between 0% and 100% of the surfaceflux values. Also, it is revealed that the initial CO₂ concentration discontinuity has an important role in determining the exchange of CO₂. Especially in the morning this exchange can have a large impact on the CO₂ diurnal variability.