Abstract
Abstract. The 14C-free fossil carbon added to atmospheric CO2 by combustion dilutes the atmospheric 14C/C ratio (Δ14C), potentially providing a means to verify fossil CO2 emissions calculated using economic inventories. However, sources of 14C from nuclear power generation and spent fuel reprocessing can counteract this dilution and may bias 14C/C-based estimates of fossil fuel-derived CO2 if these nuclear influences are not correctly accounted for. Previous studies have examined nuclear influences on local scales, but the potential for continental-scale influences on Δ14C has not yet been explored. We estimate annual 14C emissions from each nuclear site in the world and conduct an Eulerian transport modeling study to investigate the continental-scale, steady-state gradients of Δ14C caused by nuclear activities and fossil fuel combustion. Over large regions of Europe, North America and East Asia, nuclear enrichment may offset at least 20% of the fossil fuel dilution in Δ14C, corresponding to potential biases of more than −0.25 ppm in the CO2 attributed to fossil fuel emissions, larger than the bias from plant and soil respiration in some areas. Model grid cells including high 14C-release reactors or fuel reprocessing sites showed much larger nuclear enrichment, despite the coarse model resolution of 1.8°×1.8°. The recent growth of nuclear 14C emissions increased the potential nuclear bias over 1985–2005, suggesting that changing nuclear activities may complicate the use of Δ14C observations to identify trends in fossil fuel emissions. The magnitude of the potential nuclear bias is largely independent of the choice of reference station in the context of continental-scale Eulerian transport and inversion studies, but could potentially be reduced by an appropriate choice of reference station in the context of local-scale assessments.
Highlights
Since radiocarbon (14C) is absent in highly aged fossil fuels, fossil fuel combustion strongly dilutes the ratio of 14C/C in atmospheric CO2, reported as 14C including corrections for age and fractionation
One way that 14C observations may be employed to estimate CO2 emissions from fossil fuel combustion is through the joint inversion of atmospheric CO2 and its 14C/C ratio on continental scales (Peters et al, 2007; Pacala et al, 2010)
Accounting for the spatial distribution of nuclear sites reveals several regions with a high density of 14C sources that are important to consider in determining continental-scale influences on 14C
Summary
Since radiocarbon (14C) is absent in highly aged fossil fuels, fossil fuel combustion strongly dilutes the ratio of 14C/C in atmospheric CO2, reported as 14C including corrections for age and fractionation. Atmospheric observations can be used to quantify the dilution of 14C and thereby provide an estimate of the amount of CO2 added by fossil fuel combustion, relative to a clean air reference site (e.g., Levin et al, 2003). One way that 14C observations may be employed to estimate CO2 emissions from fossil fuel combustion is through the joint inversion of atmospheric CO2 and its 14C/C ratio on continental scales (Peters et al, 2007; Pacala et al, 2010). Implementing observation sites along such 14C gradients may allow continental-scale fossil fuel emissions to be estimated with inversion schemes such as CarbonTracker (Peters et al, 2007).
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