Estimates of net CO₂ flux by application of equilibrium boundary layer concepts to CO₂ and water vapor measurements from a tall tower

Convective turbulence within the atmospheric boundary layer (ABL) and movement of the ABL over the surface results in a large spatial (10⁴-10⁵ km²) integration of surface fluxes that affects the CO₂ and water vapor mixing ratios. We apply quasi-equilibrium concepts for the terrestrial ABL to measurements of CO₂ and water vapor made within the ABL from a tall tower (396 m) in Wisconsin. We suppose that CO₂ and water vapor mixing ratios in the ABL approach an equilibrium on timescales longer than a day: a balance between the surface fluxes and the exchange with the free troposphere above. By using monthly averaged ABL-to-free-tropospheric water vapor differences and surface water vapor flux, realistic estimates of vertical velocity exchange with the free troposphere can be obtained. We then estimated the net surface flux of CO₂ on a monthly basis for the year of 2000, using ABL-to-free-tropospheric CO₂ differences, and our flux difference estimate of the vertical exchange. These ABL-scale estimates of net CO₂ flux gave close agreement with eddy covariance measurements. Considering the large surface area which affects scalars in the ABL over synoptic timescales, the flux difference approach presented here could potentially provide regional-scale estimates of net CO₂ flux.