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Reference radiocarbon values for 100% biogenic carbon, based on radiocarbon values of atmospheric CO2

Sanne Palstra

These data may be freely used to obtain 14C reference values for 100% biogenic carbon, provided that reference is made to this document (webpage) in reports and also to the reference Palstra and Meijer (2014; see reference list) in the open (scientific) literature.

Table 1. Annual 14Cbio values based on averaged monthly mean 14CO2 values (January-December) measured at CIO monitoring station Lutjewad, The Netherlands for the period 2003-2014.
Table 1. Annual 14Cbio values based on averaged monthly mean 14CO2 values (January-December) measured at CIO monitoring station Lutjewad, The Netherlands for the period 2003-2014.

Reference 14C value for 100% biogenic carbon

14 C-based methods to measure the biogenic carbon fraction in a sample material make use of the principle that biogenic carbon contains 14C and fossil carbon does not (due to decay of the radioactive carbon atom over millions of years). If biogenic and fossil carbon atoms are mixed, the overall relative 14C amount in the total carbon decreases proportional with the decrease in biogenic carbon fraction.

To calculate the biogenic carbon fraction in a sample material, 14C is measured in the carbon of the sample and the measured 14C value is compared with the 14C value of the biogenic carbon fraction.

The 14C value of the biogenic carbon fraction, 14Cbio, depends on the 14C value of the total atmospheric CO2 that was taken up by plants during their growth (photosynthesis). Atmospheric 14CO2 values, which are measured in the lower troposphere (0-3km above earth surface) are not constant and vary on a daily, seasonal and annual scale and also show spatial differences. Large annual differences over the last 100 years are mainly due to a combination of above ground nuclear bomb tests in the 1950s and 1960s (large increase of 14C in the stratosphere), carbon exchange between atmosphere, oceans and biosphere (differences in 14C between the carbon reservoirs after the bomb tests) and the combustion of fossil fuels (relative decrease of 14C in the atmosphere). On a seasonal time scale the measured atmospheric 14CO2 values also differ due to increased atmospheric mixing of stratospheric 14C-rich air with troposphere air in the summer period and due to the increased use of fossil fuels during the winter period. Daily variations are mainly due to daily variations in atmospheric mixing.

Because 14CO2 values show such large temporal variations, plants and trees, which have a large variation in time period of 14CO2 uptake, have different 14C values (14Cbio) as well. As 14Cbio is not a constant value for all different biomass-based materials it needs to be measured for each sample under investigation. In general, samples under investigation are already mixed biogenic and fossil carbon materials and the biogenic carbon fraction cannot be measured on its 14C value separately (which would give the 14Cbio value). For those cases the 14Cbio value needs to be approximated based on information (if available at all) about the growth period of the used biomass materials in the sample and atmospheric 14CO2 measurement results for this same time period. The approximation of 14Cbio introduces a systematic error in the calculated biogenic carbon fraction, which can be minimized as shown by Palstra and Meijer (2014).

In several international standards (ASTM, CEN, ISO) 14C-based methods are used to measure the biogenic carbon fraction in different sample materials. To avoid the additional needed research to approximate 14Cbio for the sample material under investigation, an approximated constant 14Cbio value is usually given in these standards. This constant value is, despite an annual decrease in 14Cbio values of approximately 0.5 pMC/year, used for a period of at least 5 years. The chosen 14Cbio value in these standards depends here usually only on the type of sample materials for which the standard is written. Waste materials and wood based materials usually have higher 14Cbio values than biomass from annual plants, due to differences in the time period of CO2 uptake.

Table 1 gives annual 14Cbio values based on the average of monthly mean 14CO2 values (atmospheric CO2 sampled during 1 month for 24 hours a day), measured at RUG/CIO-monitoring station Lutjewad in a particular year, for the period 2003-2014.

These values do not fully represent the 14Cbio values of all 100% biomass-based materials, as atmospheric 14CO2 values and the CO2 uptake by plants and trees show temporal and spatial variations. The 14CO2 values measured at this monitoring station, located at the coast in the Northern part of the Netherlands (Europe), give, however, a representative indication of 14Cbio values for sites that are neither remote (with relatively higher 14C values compared to global averages) nor urban (with relatively lower 14C values compared to averages).

The given values of table 1 should be considered as indicative annual 14Cbio values. Spatial and intra-annual differences of ±1% with this chosen 14Cbio are likely to exist. For biomass materials with biomass from different growth years the anomaly can be even larger (usually 14Cbio should be higher for these materials). The given variation of 0.3% in table 1 is the standard deviation in the average year-value only, and does not represent the possible variation in 14Cbio values for different biomass materials.

Additional information about the measured atmospheric 14CO2 data at Lutjewad

The applied method to obtain the monthly mean 14CO2 values is as following. For each month of the year, atmospheric air is pumped during one month, 24 hours a day, through an alkaline solution (1.5 M NaOH). The CO2 from the air is dissolved in this solution. The solution is taken to the RUG-CIO laboratory in Groningen and the CO2 is released in a vacuum pumped system after the addition of acid (H3PO4) to alkaline solution. The CO2 (4 ml is sufficient) is cryogenically trapped in a flask and then graphitized. The graphite is measured on its abundance of carbon isotopes 12C, 13C and 14C with an Accelerator Mass Spectrometer (AMS) (van der Plicht et al., 2000). The measured 14C signal is calculated relative to the measured 12C signal and is corrected for background counts. The isotope ratio is then calculated relative to a reference material with standardized 14C amount and is corrected for isotope fractionation, based on measured 13C/12C ratios:

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Atmospheric 14CO2 values are often also corrected for 14C decay and expressed in per mill, as “delta” values. Expressed as a percentage the calculated 14C values can be symbolized as 14aN S value (Mook and van der Plicht, 1999). For bio-fossil carbon fraction measurements, however, this particular decay correction is not necessary if the 14Cbio value, used to calculated the biogenic carbon fraction (fCbio = 14Csample/14Cbio) is also not decay-corrected (and the chosen 14Cbio value is representative for the biogenic carbon in the sample).

The 14Cbio values for the years 2003-2014 as given in table 1 are not corrected for decay and can be symbolized as 14aN values. These 14Cbio values are calculated in %, but because the biogenic carbon fraction is also a percentage this can result in confusion whether a given 14C value expressed in % already shows a biogenic carbon fraction or not. Therefore, 14Csample and 14Cbio values are often expressed with the unit ‘pMC’ (percent Modern Carbon).

Figure 1 shows the monthly mean atmospheric 14CO2 values as measured at Lutjewad. It demonstrates the temporal (annual and seasonal) variation in atmospheric 14CO2 values. The annual decrease is approximately 0.5 pMC/year.

Figure 1
Figure 1

References

Mook, W., van der Plicht, 1999. Reporting 14C activities and concentrations. Radiocarbon, 41(3), 227-239.

Palstra, S.W.L., Meijer, H.A.J., 2014. Biogenic carbon fraction of biogas and natural gas fuel mixtures determined with 14C, Radiocarbon, 56(1), 7-28.

Van de Plicht, J., Wijma, S., Aerts, A.T., Pertuisot, M.H., Meijer, H.A.J., 2000. Status report: the Groningen AMS facility. Nuclear Instruments in Physics Research B, 172(1-4), 58-65.

Laatst gewijzigd:12 april 2017 15:49