Basis of Tracing Fossil Fuel CO2 Using 14C: Comparison
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Carbon dioxide (CO2), the most important greenhouse gas, is a significant driver of global warming. Radiocarbon (14C), a widely used dating method in archaeology, geosciences, etc., is a direct tracer and a promising method to differentiate the emissions of fossil fuel and non-fossil fuel from atmospheric carbon.

  • radiocarbon
  • fossil fuel carbon dioxide
  • emission estimation

1. Introduction

Carbon dioxide (CO2), the most important greenhouse gas, is a significant driver of global warming. In 2019, the annual average concentration of CO2 reached 410 ppm, which was higher than any time in at least 2 million years [1]. The observed increase in CO2 concentrations since the beginning of the industrial era is unequivocally caused by human activities, among which the combustion of fossil fuels is responsible for most of the total anthropogenic CO2 emissions. Global warming has caused increases in the global temperature of the surface and upper ocean, increases in precipitation and sea level, weather, and climate extremes, and decreases in glaciers and sea ice [2,3,4,5][2][3][4][5]. Besides CO2, fossil fuel combustion is also a primary contributor to air pollutants [6]. Thus, slowing down the increase in fossil fuel CO2 (FFCO2) concentration is of vital importance. According to the Paris Agreement and the sixth assessment report of IPCC (Intergovernmental Panel on Climate Change), CO2 emissions need to be net negative to hold the global surface temperature lower than 1.5 °C or 2 °C at the end of this century (very low and low greenhouse gas emission scenarios, according to IPCC, 2021). This means that the anthropogenic removal of CO2 exceed anthropogenic emissions. Under these circumstances, identifying the contribution of FFCO2 to total atmospheric CO2, as well as its atmospheric process interpretation and emission estimation, is a fundamental work for studies on its climatic and environmental impacts and on the evaluation of mitigation actions.
Multiple tracers that co-emitted with CO2 have been used to quantify FFCO2, including carbon monoxide (CO), sulfur hexafluoride (SF6), tetrachloroethylene (C2Cl4) and even air pollutants, based on the ratio of each tracer to CO2 [7,8,9,10,11,12,13,14][7][8][9][10][11][12][13][14]. However, there are large uncertainties due to the non-fossil emissions of the tracers [15]. Radiocarbon (14C), a widely used dating method in archaeology, geosciences, etc. [16], is a direct tracer and a promising method to differentiate the emissions of fossil fuel and non-fossil fuel from atmospheric carbon. The abundances of three naturally occurring carbon isotopes 12C, 13C and 14C are 98.89%, 1.11%, and ~10−10%, respectively [17]. The radiocarbon content of CO2 is expressed as Δ14C or Δ14CO2 [18,19][18][19]:
Δ C 14 = [ ( C 14 / C 12 ) SN ( C 14 / C 12 ) ABS − 1 ] × 1000 ‰ .
(14C/12C)SN is the 14C to 12C ratio of the sample, and (14C/12C)ABS is related to the commonly used primary measurement standard Oxalic Acid I. Radiocarbon is cosmogenic, and has a radioactive half-life of 5730 ± 40 years [20]. Thus, there are no 14C in fossil fuels because they are all depleted during long-term radioactive decay. Since fossil fuel CO2 contains no 14C whereas CO2 from other sources has similar 14C concentrations with the ambient air, the release of fossil fuel CO2 will cause a decrease in the 14C/12C ratio in the atmosphere. This was first discovered by Hans Suess [21], and is called the “Suess effect”. With industrial development, atmospheric Δ14CO2 decreased by 25‰ between 1890 and 1950 [22]. Then comes the nuclear testing period between the 1950s and the early 1960s, during which large-scale detonations of nuclear bombs produced 14C atoms in the Northern Hemisphere. Atmospheric Δ14CO2 in the Northern Hemisphere increased swiftly and reached a peak value of nearly 1000‰ in 1963, and then decreased after the Limited Nuclear Test Ban Treaty [23,24][23][24]. 

2. The Basis of Tracing Fossil Fuel CO2 Using 14C

2.1. The Theory of Quantifying Fossil Fuel CO

2

Using

14

C

Observed CO2 mole fraction (or concentration) is thought to be a mixing of many components, mainly including atmospheric background CO2, fossil fuel CO2, biospheric CO2 and oceanic CO2. The most commonly used method to constrain recently added FFCO2 in the atmosphere with 14C is called the pseudo-Lagrangian method [25[25][26][27],26,27], in which a parcel of air with an initial CO2 mixing ratio (CO2bg) and Δ14CO2 value (Δbg) moves across a polluted region, and then CO2 mixing ratio and Δ14CO2 value are modified to CO2obs and Δobs by the addition of FFCO2 and other sources or sinks of CO2. If combining other sources (and sinks) together, the mixing ratio and the Δ14CO2 value could be written as CO2other and Δother. Two balance equations for CO2 mixing ratio and Δ14C can be formulated as below. CO 2 obs = CO 2 bg + CO 2 ff + CO 2 other Δ obs CO 2 obs = Δ bg CO 2 bg + Δ ff CO 2 ff + Δ other CO 2 other By combining Equations (2) and (3), CO2ff can be calculated as: CO 2 ff = CO 2 obs ( Δ obs − Δ bg ) Δ ff − Δ bg − CO 2 other ( Δ other − Δ bg ) Δ ff − Δ bg . CO2obs, Δobs are measured in collected samples at interested sites. Δbg is measured in samples from background sites in general, while free tropospheric measurements can also act as Δbg, too [13]. Δff is known to be −1000‰ since CO2ff is 14C-free. The second term of Equation (4) is bias due to the effect of the others: β = CO 2 other ( Δ other − Δ bg ) Δ ff − Δ bg . some researchers assume β to be zero, which means that all other sources have the same Δ14C compared to those of the background atmosphere, Δother = Δbg [26]. The main contributor to uncertainties in β would be heterotrophic respiration, which has large 14C disequilibrium. The ignorance of β would cause a systematic underestimation of CO2ff, up to 0.5 ppm in summer and 0.2 ppm in winter [13,27][13][27]. There are two other factors that influence atmospheric Δ14CO2, air-sea exchange in the oceans, and stratosphere-troposphere transport [28]. However, these exchanges are assumed to affect the background and observed samples equally; thus, normally, they will not be counted in the calculation of FFCO2.

2.2. Air Sampling and Measurement

Atmospheric Δ14CO2 can be measured with direct air sampling. Whole air samples are normally collected using flasks or bags. Short-period and integrated samples can be collected by pump and acid solution, respectively. CO2 samples can be collected by static absorption using CO2-free sodium hydroxide (NaOH) or barium hydroxide (BaOH) solutions in flasks [25,29,30][25][29][30]. The primary collection method is the static absorption of CO2 using CO2-free sodium hydroxide (NaOH) or barium hydroxide (BaOH) solutions in discrete glass flasks [25,29][25][29]. The flasks are exposed to air for collection of integrated samples. Besides ground sites, tall towers, aircrafts, balloons, and even kites are all effective platforms to collect CO2 samples [13,31,32,33,34][13][31][32][33][34]. Air samples reflect near real-time atmospheric Δ14CO2, can be used to characterize the FFCO2 temporal variations with high resolution effectively. However, the representativeness of the air samples is limited to those of the sampling region and period, while little information (spatial and temporal distribution) is known beyond that. In addition, the sample collecting process and/or the site maintenance is labor and cost intensive. Direct sampling of air is not the only way to analyze atmospheric Δ14CO2. Plants fix CO2 from the atmosphere via photosynthesis, offering a unique complementary analysis method. For plants, their carbon isotopic composition can be used to reflect the mean atmosphere Δ14CO2 isotopic composition of their growing period. By collecting plant samples in different regions and analyzing 14C, FFCO2 spatial distribution on a large scale can be mapped out. Compared to air samples, collecting plant materials is more convenient and relatively cheap. Tree rings and annual leaves (grasses) are two main types used to reveal the spatiotemporal distribution of FFCO2 [30,35,36,37,38,39,40][30][35][36][37][38][39][40]. Each plant species may have its own advantages in addition to those illustrated above. Maize is grown in many countries, so it is convenient to map out the large-scale spatial distribution of fossil fuel influences using corn leaves. Gingko is a perennial and deciduous tree that is widely planted in East Asian countries, urban areas, and rural areas. Thus, it is feasible to separate samples of clean sites from samples of polluted sites. Wine ethanol is a unique plant material that can represent previous sampling years, since the harvest year and region are all written on the label of the wine bottle. Tree rings, a unique plant material, help in the reconstruction of annual atmospheric Δ14CO2 for decades or hundreds of years. In practice, however, the sampling of tree rings may be more difficult than that of annual plants since it is difficult to separate one annual ring from the others. Comparisons of Δ14CO2 and/or FFCO2 between plant materials and air samples show nearly consistent results [30[30][41][42][43],41,42,43], which verifies the usage of plant materials. Xiong et al. [44] found a significant difference in Δ14CO2 between respired CO2 and bulk organic matter from 21 plant species, suggesting that bias associated with dark respiration should be considered when use 14C in plants to quantify atmospheric FFCO2. It should be noted that biomass accumulated by plants only represents daytime Δ14CO2 (when photosynthesis occurs), and the sampling should be well planned for different plant species considering their growing period and local climate. Before the analysis of 14C, the preparations of air samples included extracting the CO2 (purification), and the reduction of CO2 to graphite. The extraction of CO2 is to remove water cryogenically, freeze CO2 completely together with N2O (non-interfering), and without freeze O2 or CH4 [13,45][13][45]. Graphite is produced by adding hydrogen gas to CO2 over an iron catalyst [46,47][46][47]. The atom counting of each graphite sample is then performed by an accelerator mass spectrometer (AMS). The preparation of annual leaves is a little different from air samples: plant samples need to (1) be cleaned by pure water and then dried, (2) be combusted to CO2 and then reduced to graphite [35]. Direct atom-counting of 14C using AMS is a great progress of 14C analysis methods. Before that, the conventional methods were decay counting, solid carbon using a Geiger–Muller counter, and liquid scintillation counting [48]. The sensitivity was improved around 106 times by AMS over the decay counting methods [49]. With the attempts to reduce sample size and to increase precision, the detection limits have been reduced to ~5 µg of carbon [50[50][51],51], and the reported precisions have reached 1‰ [17].

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