Tall Tower and Aircraft Measurements of Carbon Dioxide: Prospects for the North American Carbon Program (NACP)
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For over 10 years, we have monitored carbon dioxide mixing ratios over North America from tall (up to 600 m) transmitter towers (e.g., Figure 1) and aircraft (up to 8 km). The data, though sparse, give a measure of the temporal and 3-D spatial variability of CO2 over the continent. Analysis of these data suggests ways in which measurements from a more extensive network of sites, such as is envisioned for the NACP, could be used to estimate net CO2 surface fluxes on a regional scale (i.e., roughly one million square kilometers). We also evaluate the use of mixing ratio data from short (30-100 m) towers for regional flux estimates. Short towers are widely used for local-scale (a few hectares) measurements of atmosphere-surface exchange of CO2 (FLUXNET). The results indicate that measurements of CO2 mixing ratios at continental tower sites can be used to constrain estimates of regional CO2 fluxes. However, at present, CO2 mixing ratios are measured with sufficient accuracy relative to global reference gas standards at only a few continental sites.