We propose a simple equilibrium boundary layer model [an extension of Betts, J. Hydrometeorol., 2000] to link the mixed layer equilibrium of potential temperature, water vapor, CO2 (and other tracers such as radon) with the surface energy and water budgets and net ecosystem exchange (and the surface radon flux). We show equilibrium solutions as a function of a set of external parameters: soil water content, which directly impacts respiration and photosynthesis, and hence transpiration; the surface net shortwave, directly linked to net radiation, which is also coupled to photosynthesis and transpiration; and the radiative cooling of the ML, which in the equilibrium model directly affects the surface sensible heat flux. Our other variable model parameters are mid-tropospheric values of vapor mixing ratio, CO2 and radon, the properties of air entrained into the CBL, and the lapse-rate above cloud-base. We considered two idealized ecosystems, forest (based on observations in Wisconsin) and grassland (using parameter estimates from the literature) to show how the vegetation model affect the ML equilibrium. We show how the mass transport out of the sub-cloud layer and the mass exchange with the free troposphere couples the mixed layer equilibrium of water vapor, CO2, and radon with the corresponding surface fluxes.