DISCUSSION: PRESENTATION OF ATMOSPHERIC 14CO2 DATA

Observations of radiocarbon (C-14) in Earth's atmosphere and other carbon reservoirs are important to quantify exchanges of CO2 between reservoirs. The amount of C-14 is commonly reported in the so-called Delta notation, i.e., Delta C-14, the decay- and fractionation-corrected departure of the ratio of C-14 to total C from that ratio in an absolute international standard; this Delta notation permits direct comparison of C-14/C ratios in the several reservoirs. However, as Delta C-14 of atmospheric CO2, Delta(CO2)-C-14 is based on the ratio of (CO2)-C-14 to total atmospheric CO2, its value can and does change not just because of change in the amount of atmospheric (CO2)-C-14 but also because of change in the amount of total atmospheric CO2, complicating ascription of change in Delta(14)CO2 to change in one or the other quantity. Here we suggest that presentation of atmospheric (CO2)-C-14 amount as mole fraction relative to dry air (moles of (CO2)-C-14 per moles of dry air in Earth's atmosphere), or as moles or molecules of (CO2)-C-14 in Earth's atmosphere, all readily calculated from Delta(CO2)-C-14 and the amount of atmospheric CO2 (with slight dependence on delta(CO2)-C-13), complements presentation only as Delta(CO2)-C-14, and can provide valuable insight into the evolving budget and distribution of atmospheric (CO2)-C-14.