Measurements of carbon dioxide (CO2) mixing ratios have been carried out since 1992 on a 610-m tall communications tower in North Carolina and since 1994 on a 447-m tall tower in Wisconsin. The data provide insights into the influence of pollution (fossil fuel combustion), biological exchange, boundary layer dynamics, and advective transport on CO2 mixing ratios over the continents. In this paper, we provide an overview of the data, describe access to the data, and suggest ways in which these results could be used to improve simulations of the global carbon cycle. In particular, the data will be very useful to constrain model estimates of covariance between terrestrial ecosystem fluxes of CO2 and diurnal and seasonal variations of planetary boundary layer mixing.