Comparison of sampling and measurement methods for atmospheric 14C activity

Abstract

Radiocarbon (14C) is both the cosmogenic and anthropogenic isotope. Anthropogenic 14C is produced (1) in atmospheric nuclear bomb tests, and (2) in various nuclear facilities. The bomb-produced 14C has been globally distributed across the planet and can be considered as a new-natural level (clean-air sites). Furthermore, combustion of fossil fuels that do not contain 14C causes increase of atmospheric CO2 concentration and consequently depletion of local 14C levels. Naturally produced CO2 and that formed by fossil fuel combustion are characterized by different content of the stable isotope 13C (d13C values). Therefore, the carbon isotopes (13C and 14C) of the atmospheric CO2 can indicate sources of CO2 at each location, if it is far from nuclear facilities. 14C activity in the atmospheric CO2 in Zagreb, Croatia, has been monitored since 1985. Recently we started monitoring at several other locations with the aim of determining influence of fossil fuel combustion on atmospheric 14C activity and d13C values at clean-air sites and urban sites supposing that the urban sites are affected by fossil fuel uses. A static sampling method has been used for monitoring 14C activity – concentrated NaOH is exposed in a tray for 3-4 weeks. The method is simple and requires neither electric power nor any control, so it can be used at remote sites. However, the method introduces large isotope fractionation in 13C due to high alkalinity of NaOH. Thus, it cannot be applied for monitoring 13C. For the new monitoring program we would like to have a method that can simultaneously deliver both δ13C and 14C activity data. A dynamic method of continuous CO2 collection on NaOH (3M) with a constant air-flow requires source of electricity for pumping and heating, and occasional control of sampler performances. Control of air flow velocity and temperature enables determination of the absorbed CO2 amount. Both static and dynamic sampling methods give an average 14C activity over the sampling period. The amount of collected sample allows benzene synthesis for liquid scintillation spectrometry (LSC-B) that requires about 4 g of carbon. We also developed an instant sampling of atmospheric air in 10-L bags. However, the amount of CO2 is enough for measurement by accelerator mass spectrometry (AMS) that requires about 2 mg of carbon. Comparison of LSC-B and AMS measurement methods resulted in good agreement of 14C activities for samples collected by the static method. The 14C activity of the instant sample revealed comparable results with the samples collected by the static method indicating that the two sampling methods could be combined. d13C values of samples collected by the dynamic method showed no fractionation in 13C and comparable 14C activities with the static method. Instant samples can provide simultaneously 14C activity and d13C values. All 14C activity and d13C data will be used to differentiate carbon of global origin and carbon from local fossil fuel combustion.