Continuous eddy-covariance measurements are now widely used to study surface-atmosphere exchange of CO2, H2O, sensible heat and momentum. Long-term integrals of CO2, in particular, are sensitive to small systematic errors since the net flux is a relatively small difference of large gross photosynthetic and respiratory fluxes. These measurements of net ecosystem-atmosphere exchange (NEE) are actually the solution of a one-dimensional surface layer budget where mean advection and turbulent horizontal flux divergence are assumed to be negligible. In truth advective phenomena such as drainage flows in the stable boundary layer have been suggested as a source of significant, systematic contributions to net annual CO2 budgets at many flux towers. Direct observations of these advective flows, however, have proven elusive. Investigation of the role of advection in the boundary layer budget of CO2 at a unique pair of towers in northern Wisconsin has revealed several persistent phenomena. Past results, including nighttime drainage and advection during the morning turbulence transition will be reviewed. Several new observations of advection will be added. One tower appears to capture the turbulent venting of large amounts of CO2 pooled via drainage flows. This venting, which occurs under very specific meteorological conditions, amounts to a net flux 270 gC m-2 integrated over an annual cycle. Two towers with high-precision, high-accuracy CO2 mixing ratio measurements have been used to estimate persistent horizontal advection generated by spatial gradients in CO2 mixing ratio between the towers. Average daytime advection is shown to be negligible. Nighttime advection, while modest, is measurable. Vertical flux divergence measurements obtained from multiple flux measurements on the 447m WLEF-TV transmitter tower provide an independent means of detecting persistent advection. Finally extreme episodes of horizontal advection, including one increase of 22 ppm of CO2 in less than 2 minutes, are described. These moderately rare events appear to represent coherent masses of air transported from Lake Superior, located approximately 70 km to the north of the towers. During the growing season these air masses (the blobs) are found to be cool, dry, and rich in CO2 relative to the forest air. They are found during stable conditions (at night). Multiple-level, multiple-tower observations provide data concerning the structure and extent of these drifting, mesoscale features.