Krypton-85 datasets of the northern and southern hemisphere collected over the past 60 years

With a half-life of 10.7 years, the noble gas radioisotope 85Kr is perfectly suited as a tracer to date ice and water that formed during the past half century. Furthermore, due to its inhomogeneous input into the atmosphere, it is a useful tool to investigate atmospheric circulation and back-trajectory analysis. The data presented here represent a comprehensive time series of atmospheric 85Kr activity concentrations in ground level air that can be used to model northern and southern hemispheric input functions, which is essential to apply 85Kr as a dating tracer. The collection comprises 11 datasets from 4 monitoring stations in the northern and 7 monitoring stations in the southern hemisphere, respectively. In total, it contains about 8000 measurements performed over the past 60 years, making it the largest published 85Kr record.


b s t r a c t
With a half-life of 10.7 years, the noble gas radioisotope 85 Kr is perfectly suited as a tracer to date ice and water that formed during the past half century. Furthermore, due to its inhomogeneous input into the atmosphere, it is a useful tool to investigate atmospheric circulation and backtrajectory analysis. The data presented here represent a comprehensive time series of atmospheric 85 Kr activity concentrations in ground level air that can be used to model northern and southern hemispheric input functions, which is essential to apply 85 Kr as a dating tracer. The collection comprises 11 datasets from 4 monitoring stations in the northern and 7 monitoring stations in the southern hemisphere, respectively. In total, it contains about 80 0 0 measurements performed over the past 60 years, making it the largest published 85 Kr record. © 2020 Published by Elsevier Inc. This is an open access article under the CC BY-NC-ND license ( http://creativecommons.org/licenses/by-nc-nd/4.0/ ) Table   Subject Atmospheric Science Specific subject area Monitoring of the atmospheric 85 Kr concentrations in ground level air in the northern and southern hemisphere Type of data Excel file in data repository,

Value of the Data
• This comprehensive dataset is important for the application of 85 Kr as a dating tracer in water and ice • Researchers in the field of tracer hydrology can benefit from these data as it allows deriving a 85 Kr input function for dating • The 85 Kr data is useful for investigating atmospheric circulation and it can support back trajectory models due to nuclear reprocessing plants as point like sources of 85 Kr. • The dataset will support the potential future application of 85 Kr as a tool for the verification of nuclear arms control treaties.
All measurements were conducted via β-decay counting in gas proportional counters with a measurement uncertainty of about 3%. However, for the datasets "Tahiti" and "Terre-Adelie" no errors were given in the original publications and a conservative estimate of 10% measurement uncertainty was taken.
As seen in Fig. 1 , the northern hemispheric data represents a coherent 60 years long series of measurements, while the southern hemispheric data set contains gaps of about 5 years between around 1980 and in the early 2010s. The 85 Kr activity concentrations in the Freiburg, Schauinsland and Jungfraujoch dataset reach up to 6 Bq/m ³ air, while the southern hemispheric data do not exceed 1.5 Bq/m ³ air.

Table 1
List of all the datasets with start and end of the sampling as well as the number of total samples and the elevation of the monitoring station.  [ 3 , 4 , 5 , 6 , 7 , 8 , 9 , 10 , 11 , 12 ] 13  [ 5 , 14 ] 18.08.1969 15.11.1976 18 10 Terre-Adélie [ 5 , 14 ] 17.03.1968 05.10.1977 32 10 * At these monitoring stations, sampling is still ongoing. The date just refers to the last sample in the dataset.

Experimental Design, Materials and Methods
The collection of krypton samples for the analysis of 85 Kr follows the same principle for all datasets. In a multistage process, 2 to 5 ml of pure krypton are separated from about 10 m ³ of air and the 85 Kr activity concentration is determined via radioactive β-decay counting in gas proportional counters.
The first separation step is done by pumping air for one week with a constant flow of about 1 L/min through a liquid nitrogen cooled activated charcoal column. The pressure in the column is regulated to about 500 mbar to avoid condensation of oxygen and nitrogen, while most of the krypton is trapped [15] . After one week, the activated charcoal column is replaced with a clean column, to ensure continuous sampling. The charged column is heated to 300 °C and the released gas is flushed into a 1 L aluminium container with helium as carrier gas. For the second purification step, the 1 L aluminium container is shipped to the laboratories of the "Bundesamt für Strahlenschutz" in Freiburg, Germany.
Via cryogenic purification, CO 2 is removed, and the residual gas mixture is flushed with helium through a smaller liquid nitrogen cooled activated charcoal trap to further remove the lighter air components, mainly O 2 , N 2 and Ar.
In a third step, krypton is separated from xenon by gas chromatography with methane serving as a carrier and counting gas. The highly enriched krypton fraction is then flushed into a gas proportional counter to measure its 85 Kr activity.
The overall measurement uncertainty for an atmospheric 85 Kr measurement is about 3% with a 85 Kr detection limit of typically around 4 mBq/m ³ air.