The entrainment process of carbon dioxide in the atmospheric boundary layer

Aircraft and surface measurements of turbulent thermodynamic variables and carbon dioxide (CO2) were taken above a grass land in a convective atmospheric boundary layer. The observations were analyzed to assess the importance of the entrainment process for the distribution and evolution of carbon dioxide in the boundary layer. From the observations, we were able to estimate the vertical profiles of the fluxes, the correlation coefficients and the skewness. These profiles indicate that important entrainment events occurred during the observed period. The data were also used to estimate the budgets for heat, moisture and carbon dioxide.

By studying this observational data, we find that the entrainment of air parcels containing lower concentrations of water vapor and carbon dioxide significantly dries and dilutes the concentration of these two constituents in the boundary layer. This process is particularly important in the morning hours that are characterized by a rapidly growing boundary layer. The observations show that the CO2 concentration in the boundary layer is reduced much more effectively by the ventilation with entrained air than by CO2 uptake by the vegetation. We quantify this effect by calculating the ratio of tthe entrainment flux of CO2 to the surface flux of CO2 Bc=-(wc)e/(wc)o. A value of Bc equal to 2.7 is estimated from the vertical profile of the carbon dioxide flux.

We corroborate this observational evidence by reproducing the observed situation using a mixed layer model. The mixed layer model also yields the variation in time of Bc. During the morning, the ventilation process is more important than the CO2-uptake by the vegetation (Bc>1) whereas in the afternoon the assimilation by grass at the surface becomes the dominant processes (Bc<1). This research points out the relevance of the entrainment process on the budget of carbon dioxide in the lower troposphere and the relevance of boundary layer dynamics in controlling the diurnal variation of carbon dioxide.