What is the concentration footprint of a tall tower?

Studies that have attempted to estimate sources and sinks of trace gases such as CO2 with inverse calculations unanimously identify the lack of continental stations as a prime obstacle. Continental stations have traditionally been avoided because of the difficulty of interpretation due to large time-variability of trace substance mixing ratios. Large variability is caused by the proximity to the strongly variable sources in space and time and the complicated airflow within the lowermost 100-200 m of the planetary boundary layer. To address the need for continental stations and to overcome the problems associated with them, the National Oceanic and Atmospheric Administration Climate Monitoring and Diagnostics Laboratory started in 1992 to measure CO2 and other trace gases on tall television transmission towers [Bakwin et al, 1995]. An essential question in connection with these tower measurements is the area around the tower from which fluxes substantially contribute to the observed short-term variability of trace gas mixing ratios. We present here a simple data and back trajectory-based method to estimate the fraction of the observed short-term variability explained by a localized flux around a tall television transmission tower in Wisconsin in dependence of its location relative to the tower (the concentration "footprint"). We find that the timescale over which the imprint of surface fluxes on air parcels before its arrival at the tower axe still discernible in the mixing ratio variations observed at the tower is of the order of 1.5 days. Based on this timescale and the characteristics of air parcel trajectories, we infer a spatial extent of the footprint of the order of 10(6) km(2), or roughly a tenth of the area of the United States. This is encouraging evidence that tall tower measurements may be useful in global inversions and may also have implications for monitoring fluxes of anthropogenic trace substances on regional scales.