Reference radiocarbon values for 100% biogenic carbon (14Cbio) based on atmospheric 14CO2

University of Groningen (RUG) | Energy and Sustainability Research Institute Groningen (ESRIG) | Centre for Isotope Research (CIO).

Contact person: Safoora Kamjan, s.kamjan rug.nl | Version: 5 July 2021 (update of previous version; 7 November 2019).

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 references of Palstra and Meijer (2014 and 2018; see reference list) in the open (scientific) literature.

Table 1. Annual ¹⁴C_bio values based on averaged monthly mean ¹⁴CO₂ values (January-December) measured at CIO monitoring station Lutjewad, The Netherlands for the period 2003-2020.

For the time period before 2003 we refer to Meijer et al. (1995) and Palstra and Meijer (2018) for data from monitoring station Smilde, The Netherlands (time period 1975 – 2003) and Levin and Kromer (2004) for data from monitoring stations Vermunt (Austria) and Jungfraujoch (Switzerland) (time period 1959 – 2003).

The monthly mean values mentioned in these papers can be used to calculate annual ¹⁴C_bio values for the years before 2003. To obtain insight in atmospheric ¹⁴CO₂ differences on a global scale, see Hua et al. (2013).

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Reference ¹⁴C value for 100% biogenic carbon

In the ¹⁴C-based method to measure the biogenic carbon fraction in a sample material, the principle is used that biogenic carbon contains ¹⁴C in an amount comparable to current atmospheric ¹⁴CO₂ levels, and fossil carbon contains no ¹⁴C anymore. When 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 this value is compared with the ¹⁴C value of the biogenic carbon fraction, ¹⁴C_bio. In international standards this value is also called the reference ¹⁴C value for 100% biogenic carbon: “REF”.

Biogenic carbon fraction, fC_bio = ¹⁴C_sample / ¹⁴C_bio

See Palstra (2016) and the references in that publication for background information on the method and its applications.

Due to temporal and spatial variations in atmospheric ¹⁴CO₂ values (e.g. Levin and Kromer, 2003 and Meijer et al., 1995), the ¹⁴C_bio values of biogenic material show these variations as well. In those cases where the sample material is already a mixture of fossil and biogenic carbon and the biogenic carbon is not separately available for ¹⁴C measurement, the ¹⁴C_bio value should therefore be approximated. In Palstra and Meijer (2014), an approximation method is described in more detail for biogases. The main principle to approximate the ¹⁴C_bio value of a particular sample material, however, can be applied to other bio-based materials as well. In this approximation method biogenic materials can be divided into groups with different average carbon age. That is: the overall time period in which the carbon in the material was taken up from the atmosphere. This can be for instance annual data for products from annual plants from a specific year, or three-year averages for annual-plant-based materials of unknown previous years. Or five-year averages for products that are animal (waste) based. Or an approximated ¹⁴C_bio value based on specific year distributions for wood-based and waste-based materials (Mohn et al., 2008 and Fellner and Rechberger, 2009). The ¹⁴C_bio values for the different groups can be calculated using annual atmospheric ¹⁴CO₂ values as measured at specific representative monitoring stations for the specific and relevant time periods.

Table 1 shows the annual ¹⁴C_bio values for the time period between 2003 and 2020, based on averaged monthly mean atmospheric ¹⁴CO₂ values measured at monitoring station Lutjewad, The Netherlands (University of Groningen). These data can be used as reference values for annual ¹⁴C_bio values in this specific time period.

Due to seasonal and spatial variations in atmospheric ¹⁴CO₂ values, the averaged ¹⁴CO₂ values of Lutjewad of Table 1 might not be fully representative for all investigated biogenic materials. For annual plants that grow in the summer period only, the ¹⁴CO₂ value can be slightly higher compared to the given annual values of Lutjewad, which also includes the winter time period (with increased fossil CO₂ emissions and no plant growth). For example the average April-October monthly mean ¹⁴CO₂ values measured at Lutjewad are in average +0.35% higher. Biomass from areas with many fossil fuel emissions has lower ¹⁴CO₂ values compared to biomass grown in remote areas. We have therefore decided to calculate the average ¹⁴CO₂ value based on all months in a year, including fossil contributions (as measured in the winter period) and remote contributions (as measured in the summer period).

Hence, the ¹⁴CO₂ values measured at monitoring station Lutjewad, located at the coast in the Northern part of the Netherlands (Europe), represent ¹⁴C_bio 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 global averages).

The given values of table 1 should be considered as indicative annual ¹⁴C_bio values. Spatial differences of ± 0.5 pmC with theses ¹⁴C_bio values are likely to exist. For biomass materials with biomass from different growth years the anomaly can be even larger due to the uncertainty in the approximation of the overall ¹⁴C_bio value for mixed bio-based or (mixed) wood-materials (unknown distribution of carbon uptake during an unknown time period).

Although there might be slight differences on a local and regional scale and also seasonally in atmospheric ¹⁴CO₂ values, the long-term annual (decreasing) trend at Lutjewad follows the global trend. Over the last 15 years the average annual decrease was 0.35 pmC per year.

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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) at a constant flow. The CO₂ from the air is dissolved in this solution. The solution is taken to the RUG-CIO laboratory in Groningen for further preparation and measurement. The CO₂ is released in a special system after the addition of acid (H₃PO₄) to the alkaline solution. Up to 2011 we measured the total CO₂ amount (a few litre) from the sample directly after the extraction using a proportional gas counter. Since 2011 we extract a subsample of the total alkaline solution in a vacuum pumped system and divide the obtained amount of CO₂ over three flasks. Each of the flasks contains approximately 4 ml of CO₂. Each CO₂ sample is graphitized and the graphite targets are measured on the abundance of carbon isotopes ¹²C, ¹³C and ¹⁴C with an Accelerator Mass Spectrometer (AMS). Between 2011 and May 2017 the ¹⁴CO₂ Lutjewad samples were measured with a 2.5 MV Tandetron built by HVEE (van der Plicht et al., 2000). Since May 2017 we measure the samples with a 200 kV AMS, MICADAS (Ionplus AG). 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.

Atmospheric ¹⁴CO₂ values are often also corrected for ¹⁴C decay and expressed in ‰, as “delta” values. Expressed as a percentage the calculated ¹⁴C values can then be symbolized as ¹⁴a_N ^S value (Mook and van der Plicht, 1999). For bio-fossil carbon fraction measurements, however, this particular decay correction is not necessary if the ¹⁴C_bio value, used to calculate the biogenic carbon fraction is also not decay-corrected and is representative in time origin for the biogenic carbon in the sample.

The ¹⁴C_bio values for the years 2003-2020 as given in table 1 are not corrected for decay and are therefore symbolized as ¹⁴a_N values. These ¹⁴C_bio 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, ¹⁴C_sample and ¹⁴C_bio values are expressed with the unit ‘pmC’ (percent modern Carbon).

References

Fellner, J., Rechberger, H., 2009. Abundance of ¹⁴C in biomass fractions of wastes and solid recovered fuels. Waste Management, 29(5), 1495-1503.

Hua, Q., Barbetti, M., Rakowski, A.Z., 2013. Atmospheric radiocarbon for the period 1950- 2010. Radiocarbon, 55(4), 2059-2072.Hua, Q., Barbetti, M., Rakowski, A.Z., 2013. Atmospheric radiocarbon for the period 1950- 2010. Radiocarbon, 55(4), 2059-2072.

Levin, I., Kromer, B., 2004. The tropospheric ¹⁴CO₂ level in mid-latitudes of the Northern Hemisphere (1959-2003), Radiocarbon, 46(3), 1261-1272.

Meijer, H.A.J., van der Plicht, J., Gislefoss, J.S., Nydal, R., 1995. Comparing long-term atmospheric 14C and 3H records near Groningen, The Netherlands with Fruholmen, Norway and Izaña, Canary Islands ¹⁴C stations, Radiocarbon, 37(1), 39-50.

Mohn, J., Szidat, S., Fellner J., Rechberger, H., Quartier, R., Buchmann, B., Emmenegger, L., 2008. Determination of biogenic and fossil CO₂ emitted by waste incineration based on ¹⁴CO₂ and mass balances. Bioresource Technology, 99(14), 6471-6479.

Mook, W., van der Plicht, J., 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.

Palstra, S.W.L., 2016. On ¹⁴C-based methods for measuring the biogenic carbon fraction in fuels and flue gases. PhD thesis, University of Groningen, 2016. 160 p. link to PhD thesis.

Palstra, S.W.L., Meijer, H.A.J., 2018. Poster presentation: “Atmospheric ¹⁴CO₂ data sets from Dutch monitoring stations Smilde (1995-2003) and Lutjewad (2002-present)”, ¹⁴C conference, June 2018, Trondheim, Norway. A paper with an overview of the data is in preparation.

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.