Styles, J.M., P.S. Bakwin, K.J. Davis, and B.E. Law, submitted. A simple atmospheric boundary layer budget validated with tall tower CO2 concentration and flux measurements, Journal of Geophysical Research-Atmospheres.

A simple slab model of the atmospheric boundary layer (ABL) was applied to data from the 400-meter-tall WLEF tower in Wisconsin for the full year of 1998. Atmospheric CO2 concentrations within the ABL were simulated within the model, driven by hourly eddy covariance measurements of CO2 flux on the tower. Diurnal patterns of modeled ABL CO2 concentration closely followed vertically integrated profile concentration measurements on the tower. While there were often differences between modeled and measured concentration build-up overnight due to respiration, the model generally captured the daytime draw-down in concentration due to photosynthesis very well. Winter-time boundary layer concentration appeared to be significantly influenced by advection, but model simulations showed that daytime concentration draw-down was largely independent of these events. An integral boundary layer budget was used to predict daytime CO2 flux from draw-down in daytime CO2 concentration, with validated model simplifications. Flux estimates from measured concentrations agreed well with eddy covariance flux measurements when averaged over several days. Excepting May and September, monthly averages agreed to within 10 gC m-2mo-1 all year round, or better than 10% in the summer months and better than ~50% in other months. The ABL budget underestimated daytime CO2 flux in May and September by 30-40% compared to measurements. Flux estimates from daytime concentration draw-down at the 30-meter level on the tower gave similarly good agreement after accounting for the vertical concentration gradient within the stable nighttime boundary layer. This agreement supports the use of the slab model budget in analyzing near-surface concentration data to infer regional CO2 fluxes at continental sites where residual layers may persist for several days.