Partitioning of Terrestrial Carbon Sources Using ¹⁴CO₂: Observations and Modeling

S. Lehman¹, J.B. Miller², P.P. Tans², C. Sweeney², A.E. Andrews², J. Southon³, B.W. LaFranchi⁴, T.P. Guilderson⁴ and J.C. Turnbull⁵

¹University of Colorado, Boulder, CO 80309; 303 492 8980, E-mail: Scott.Lehman@colorado.edu
²NOAA Earth System Research Laboratory, Boulder, CO 80305
³University of California at Irvine, Irvine, CA 92697
⁴Lawrence Livermore National Laboratory, Livermore, CA 94550
⁵Rafter Radiocarbon Laboratory, Lower Hutt, New Zealand

The small radiocarbon fraction of total CO₂ (~1:10^{12 14}C:C) has proven to be an ideal tracer for its fossil fuel derived component.  Unlike all other significant contributions to the atmospheric CO₂ budget, the fossil fuel component is devoid of radiocarbon, so that temporal and spatial gradients in recently added fossil fuel CO₂ can be readily identified as radiocarbon gradients provided there is adequate precision in the measurements. Over large industrialized land areas such as Eurasia and North America, the use of ¹⁴C to isolate the recently added fossil fuel contribution also quantifies (by difference) the change in atmospheric CO₂ due to uptake and release by the terrestrial biosphere. Simple mass balance considerations suggest that in order to apportion fossil fuel and biological components in the continental CO₂ observations to ±1 ppm, a ¹⁴CO₂ measurement repeatability of ~2 per mil (1-sigma ppt deviation from standard) is needed. Here we will report on i) our efforts to maintain the necessary measurement precision in a growing number of air craft and tall tower sampling sites around the U.S., and on ii) the ability of the TM5 transport model (as currently implemented for CO₂ and ¹⁴CO₂) to represent the  Δ¹⁴CO₂ observations. The latter is an important step towards using ¹⁴CO₂ as an additional constraint on regional fossil fuel emissions and Net Ecosystem Exchange flux retreivals in CarbonTracker

Figure 1. Model representations of a) Δ¹⁴C (left panel) and b) the fossil fuel component of total CO₂ (C_ff; right panel) in the atmosphere near the surface over North America for a week in January of 2006. The color scales correspond to the expected mass balance relationship between Δ¹⁴C and C_ff of -2.7 ‰/ppm. The sites labeled in white are existing ¹⁴C sampling sites, as are Mount Wilson Observatory (MWO) and Niwot Ridge (NWR) (which underlays Boulder Atmospheric Observatory (BAO)).