Precise measurement of atmospheric CO₂ concentrations can be used to estimate the net surface exchange flux of carbon, but cannot distinguish CO₂ contributions from net biological exchange and combustion of fossil fuels. High quality estimates of the fossil fuel contribution are therefore needed not only to directly constrain the fossil fuel component, but also to improve estimates of the poorly quantified biological carbon flux.  The fossil fuel contribution to the atmospheric CO₂ signature can be constrained using economic inventories, but difficulties in interpolation limit their accuracy on sub-annual and sub-continental scales.  Independent estimates of fossil fuel CO₂ emissions with quantifiable uncertainties are therefore needed.  We examine the theoretically ideal tracer ¹⁴CO₂, as well as two indirect, correlate tracers (CO and SF₆).

¹⁴C has long been proposed as an ideal tracer for fossil fuel CO₂, but precision and sample size constraints have limited its application.  At the University of Colorado Laboratory for AMS Radiocarbon Preparation and Research (CU), we take advantage of the existing large greenhouse-gas sampling network at the National Oceanic and Atmospheric Administration Climate Monitoring and Diagnostics Laboratory (NOAA/CMDL) to make ¹⁴CO₂ measurements on samples as small as two liters of whole air from sites around the globe.  1-sigma measurement repeatability is 1.8 permil for Δ¹⁴C of CO₂, equating to a fossil fuel CO₂ detection capability of ~0.8 ppm.  The long-term measurement repeatability, as determined by replicate measurements from a single tank of air, is comparable to the single sample precision. 

We use the ¹⁴CO₂ method to quantify the fossil fuel CO₂ contribution to boundary layer air at two locations in North America (Niwot Ridge, Colorado and New England).  Using the indirect tracers CO and SF₆, we obtain two additional independent estimates of the fossil fuel CO₂ component, and compare these results with the ¹⁴CO₂-based estimates. Biases for both CO and SF₆ are seasonally coherent and can be large with respect to the magnitude of the fossil fuel CO₂ signal.