Journal of Environmental Radioactivity 127 (2014) 111e118

Contents lists available at ScienceDirect

Journal of Environmental Radioactivity journal homepage: www.elsevier.com/locate/jenvrad

Variability of atmospheric krypton-85 activity concentrations observed close to the ITCZ in the southern hemisphere A. Bollhöfer a, *, C. Schlosser b, J.O. Ross c, H. Sartorius b, S. Schmid b a

Environmental Research Institute of the Supervising Scientist (eriss), Department of the Environment, PO Box 461, Darwin, NT 0801, Australia Bundesamt für Strahlenschutz, Rosastr. 9, 79098 Freiburg, Germany c Bundesanstalt für Geowissenschaften und Rohstoffe, Stilleweg 2, 30655 Hannover, Germany b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 10 June 2013 Received in revised form 4 October 2013 Accepted 5 October 2013 Available online 30 October 2013

Krypton-85 activity concentrations in surface air have been measured at Darwin, which is located in northern Australia and is influenced by seasonal monsoonal activity. Measurements between August 2007 and May 2010 covered three wet seasons. The mean activity concentration of krypton-85 measured during this period was 1.31
0.02 Bq m3. A linear model fitted to the average monthly data, using month and monsoon as predictors, shows that krypton-85 activity concentration measured during the sampling period has declined by 0.01 Bq m3 per year. Although there is no statistically significant difference in mean activity concentration of krypton-85 between wet and dry season, the model implies that activity concentration is higher by about 0.015 Bq m3 during months influenced by the monsoon when a north westerly flow prevails. Backward dispersion runs using the Lagrangian particle dispersion model Hysplit4 highlight possible source regions during an active monsoon located deep in the northern hemisphere, and include reprocessing facilities in Japan and India. However, the contribution of these facilities to krypton-85 activity concentrations in Darwin would be less than 0.003 Bq m3. Crown Copyright Ó 2013 Published by Elsevier Ltd. All rights reserved.

Keywords: Krypton-85 Monsoon Southern hemisphere Atmospheric transport

1. Introduction Krypton is a noble gas that is present in the earth’s atmosphere at concentrations of about 1 mL m3 (ANL, 2005). It has eleven major isotopes, six of which are stable. The krypton-85 isotope is unstable and decays with a half-life of 10.76 years via a b decay to rubidium-85 (ICRP, 2008). Less than 106 of the present atmospheric inventory of krypton-85 is produced naturally: in the atmosphere, due to neutron capture reactions of cosmic ray neutrons with krypton-84, and in the lithosphere, due to neutron induced fission of uranium or other actinides (Schröder and Roether, 1975; Styra and Butkus, 1991). The vast majority of krypton-85 in the atmosphere is of anthropogenic origin. Large quantities of krypton-85 are being produced as a by-product of nuclear fission in nuclear power plants, with a cumulative fission yield for krypton-85 of approximately 0.3% for thermal neutrons (Kirk, 1972; IAEA, 2008a). This krypton85 is trapped in the nuclear fuel rods, and decladding of the rods

* Corresponding author. Tel.: þ61 8 89201142. E-mail addresses: [email protected] (A. Bollhöfer), [email protected] (C. Schlosser), [email protected] (J.O. Ross), [email protected] (H. Sartorius), [email protected] (S. Schmid).

and subsequent dissolution during reprocessing releases almost 100% of the trapped krypton-85 (Kirk, 1972; Boeck, 1976; Mian and Nayyar, 2002). Apart from nuclear reprocessing plants, other anthropogenic sources of krypton-85 are nuclear power plants, medical isotope production facilities, naval reactors, nuclear test explosions and nuclear reactor accidents. However, the magnitude of these sources globally is small compared to releases from the reprocessing of nuclear fuel (Ahlswede et al., 2013). As nuclear reprocessing plants are exclusively located in the northern hemisphere (IAEA, 2005, 2008b) almost all of the krypton-85 is released into the atmosphere in the northern hemisphere and then distributed globally (Winger et al., 2005). As krypton-85 is a chemically inert noble gas its only significant sink in the atmosphere is radioactive decay. This and its relatively long half-life make it an ideal tracer to (a) investigate atmospheric transport processes and interhemispheric exchange and (b) validate atmospheric transport models (Weiss et al., 1983, 1992; Levin and Hessheimer, 1996; Ross, 2010). The atmospheric krypton-85 activity concentration has also been suggested as an indicator for clandestine plutonium production as it is released when plutonium is separated from fuel rods during reprocessing (Sittkus and Stockburger, 1976; Mian and Nayyar, 2002; Kalinowski et al., 2004; Ross, 2010). However, there is some debate about the

0265-931X/$ e see front matter Crown Copyright Ó 2013 Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jenvrad.2013.10.003

112

A. Bollhöfer et al. / Journal of Environmental Radioactivity 127 (2014) 111e118

lower limit of detection of krypton-85 for safeguard purposes given the background that exists from global nuclear fuel reprocessing (Kalinowski et al., 2004, 2006; Kemp and Schlosser, 2008; Ross, 2010). During the past four decades the atmospheric krypton-85 baseline has increased by about 0.03 Bq m3 per year to a maximum activity concentration of about 1.50 Bq m3 at stations located at mid northern latitudes at around 2003e2005 (Hirota et al., 2004; Schlosser et al., 2010). This increase makes krypton85 a valuable tool for dating young groundwaters (Salvamoser, 1981; Slide, 2006). The specific krypton-85 activity per volume of krypton in the atmosphere (85Kr[dpm]/Kr[ml]) is maintained in precipitation, the watershed and groundwater. Hence, determination of the specific krypton-85 activity in a groundwater sample allows to determine the age or flow time of groundwater. However, the application for groundwater dating has mainly been restricted to the northern hemisphere, because a reliable time series of the specific atmospheric activity of krypton-85 is required, but at present not as well documented in the southern hemisphere. It has been suggested that the increase has come to a halt, indicating equilibrium between emissions of krypton-85 and radioactive decay (Schlosser et al., 2010; BMU, 2012). If indeed the monotony of the atmospheric krypton-85 increase is no longer a given, its applicability for dating groundwater may be complicated, albeit not impossible. The interhemispheric exchange time is in the order of about 1 year (e.g. Weiss et al., 1992; Levin and Hessheimer, 1996; Kjellström et al., 1999; Ross, 2010). Consequently, the increase of the krypton85 activity concentration in the southern hemisphere lags behind the northern hemisphere and activity concentrations are generally lower. In addition, peaks of atmospheric krypton-85 activity concentrations that can be detected in the northern hemisphere due to reprocessing activities at major facilities (such as La Hague that emits approximately 200 PBq per year at present) (Hirota et al., 2004; Kalinowski et al., 2004; Winger et al., 2005) are smoothed out in the southern hemisphere resulting in a flatter time series. There are only a few stations in the southern hemisphere known to the authors, where atmospheric krypton-85 activity concentrations have been measured in the past 30 years. The German Office for Radiation Protection (Bundesamt für Strahlenschutz, BfS) together with collaborating institutions operated sampling stations at the Georg von Neumayer Station, Antarctica (1983e2008); Cape Point and Pelindaba, South Africa (1985e1998); Cape Grim (1987e 1996) (see Weiss et al., 1992; Winger et al., 2005) and the Environmental Research Institute of the Supervising Scientist (eriss) Darwin, Australia (August 2007 to May 2010), as reported in this paper. Continuation of measurements in the southern hemisphere is important to establish a time series of the atmospheric krypton85 specific activity, which can be used for groundwater dating and safeguard applications. Moreover, measurements in the southern hemisphere may confirm whether the global baseline atmospheric krypton-85 activity concentration is continuing to increase, remains constant or is decreasing, as measurements in the southern hemisphere are less likely influenced by spikes originating from krypton-85 emitted by reprocessing plants located in the mid latitudes of the northern hemisphere. In this study, the atmospheric krypton-85 activity concentration in surface air was measured in Darwin located in northern Australia, 12.43 S and 130.9 E at an altitude of 10 m above sea level (asl). Darwin is influenced by seasonal monsoonal activity (Wheeler and McBride, 2005) with an average rainfall of about 1730 mm between November and April and an almost rainless dry season from May to October (BOM, 2007). During the austral winter (dry season) the intertropical convergence zone (ITCZ) moves away from the equator to the North due to the large Asian continental

landmass and is on average located at 10 N, but can move as far as 25 N (Waliser and Gautier, 1993). The portion of the ITCZ which extends into or through the monsoon circulation during the austral summer (wet season) is called a monsoon trough. It can be located well south of Darwin, resulting in a strong north westerly flow and influence of northern hemispheric air masses at Darwin during the monsoon (Manins et al., 2001). The influence of different air masses could be seen for instance in krypton-85 data measured at Tsukuba, Japan (Igarashi et al., 2000; Hirota et al., 2004). It was thus hypothesised that the distinct change in seasons may also result in changes in krypton-85 activity concentrations measured in Darwin, with higher activity concentrations during periods of active monsoon. 2. Methods 2.1. Monsoon periods The mean lower (850 hPa) tropospheric winds shift from being easterly in the austral winter to westerly in summer, with strong cumulonimbus convection and rainfall. The onset of the monsoon is defined as the first westerly burst in each wet season, occurring anywhere between early December to mid January (Wheeler and McBride, 2005). The monsoon is characterised by active and break periods, with 3e6 active spells in Darwin with an average length of 4 days (Shaik and Cleland, 2010). There are many approaches to monitor the AustralianeIndonesian monsoon and a summary of published research is given in Wheeler and McBride (2005). Shaik and Cleland (2010) compare various techniques of monsoon monitoring for Darwin that have been developed in the Darwin Regional Specialised Meteorological Centre. One of the techniques determines the date of the onset or an active spell of monsoon using the 850 hPa level (about 1500 m asl) and 200 hPa level (about 12,500 m asl) winds, similar to a method published by Drosdowsky (1996). In this method, the date of the onset or active spell of the monsoon is defined as the first occurrence of >5 knots westerly winds at the 850 hPa level, >5 knots easterlies at the 200 hPa level, and a Darwin 3-day running mean rainfall above 0.2 mm. Upper air data for Darwin Airport for the period from 2007 to 2010 were downloaded from the University of Wyoming, College of Engineering, Department of Atmospheric Science’s website (University of Wyoming, 2012). A 3-day running mean was applied to the 12 hourly wind data and the date of the onset of the monsoon and the duration of active spells was determined as described in Shaik and Cleland (2010). 2.2. Sampling of krypton-85 Air samples (n ¼ 130) were collected from 15 August 2007 to 31 May 2010 at eriss in Darwin, close to Darwin Airport. Outside air was pumped typically for one week (longer than the typical duration of active monsoon spells) at a flow rate of about 60 L h1 through a cooling trap (about 18  C) and a silica-gel column (about 2.5 L) to remove water vapour, followed by a same sized column filled with a 5  A molecular sieve (bead size: 1e2 mm) to remove the remaining water vapour and CO2. The dry air was then pumped through an activated charcoal adsorbant at 196  C and at a pressure below 500 hPa to avoid condensation of O2 and N2 in the adsorbant. At these temperatures, krypton (and xenon) adsorb onto the charcoal. For more detail of the sampling system see Sartorius et al. (2002) or Hirota et al. (2004). After sampling, the adsorbant was allowed to warm up to room temperature. After one day the remaining water and volatile O2 and N2 was released from the adsorbant by opening the needle valve at

A. Bollhöfer et al. / Journal of Environmental Radioactivity 127 (2014) 111e118

60

a

W

-1

wind speed [m s ]

the air inlet. The adsorbant was then heated to 300  C for one hour to transfer the noble gases released from the charcoal to a 1 L aluminium bottle (Linde Co. Ltd., Minican). N2 gas was then used to rinse the remaining noble gases from the adsorbant into the sample bottle and the bottle was pressurised to 500 kPa. Bottles were sealed and sent to the Noble Gas Laboratory of the German Office for Radiation Protection in Freiburg, Germany.

113

40 20 0 -20 E

2.3. Measurement of krypton-85

a

b

W N

-1

wind speed [m s ]

20

2.4. Atmospheric modelling To investigate potential source regions of air arriving in Darwin, the Lagrangian particle dispersion model Hysplit4 (Hybrid SingleParticle Lagrangian Integrated Trajectory) was applied (Draxler and Rolph, 2003). Meteorological data from the Global Data Assimilation System hosted by the US National Oceanic and Atmospheric Administration (NOAA) Air Resources Laboratory for the years 2007e2010 was used. The horizontal resolution of the meteorological data was 1 1, the temporal resolution used in the atmospheric transport modelling was 6 hourly. Backward dispersion runs for air samples collected in Darwin were calculated with 1 million particles included per model run for a duration of about 20 days. Longer simulation periods demand larger numbers of Lagrangian model particles to keep sensitivities per grid cell statistically meaningful. Besides the computational effort, longer backtracking is not expected to deliver additional significant source regions of krypton-85, due to the high background concentrations and smooth gradients.

10 0 -10 -20

E S

c rainfall [mm]

200 150 100

· 50 0

d

1.45 -3

Kr-85 [Bq m ]

Samples were subjected to an additional pre concentration step to remove remaining traces of CO2 and dry the gas. The krypton fraction in the remaining about 100 mL gas sample was separated quantitatively using a 2-stage gas chromatograph with methane as the carrier gas, and then injected into a proportional counter to measure the beta decays of krypton-85. More detailed information on the chromatographic system can be found in Stockburger et al. (1977). The proportional counters to measure the beta activity have a volume of approximately 230 cm3, and are housed within an anti-coincidence system and a 10 cm lead shield. The background count rates are typically between 0.03 and 0.05 counts per second. The volume of the stable krypton in the proportional counter was determined chromatographically by comparison with a calibration gas of similar composition (krypton/methane). The average volume of krypton sampled in Darwin per sample was approximately 4 cm3 (all the volumes given are at standard temperature and pressure (STP) condition: 273.15 K and 1013.25 hPa). As the volume concentration of krypton in air is known (1.14 cm3 m3) the activity concentration of krypton-85 in air could be determined.

1.40 1.35 1.30 1.25 1.20 Jul-07

Nov-07 Mar-08

Jul-08

Nov-08 Mar-09

Jul-09

Nov-09 Mar-10

Date Fig. 1. (a) Easterly and westerly wind speeds for the 200 hPa (about 12,500 m asl) and 850 hPa (about 1500 m asl) levels, (b) northerly and southerly wind speeds for the 850 hPa level, (c) daily rainfall and (d) krypton-85 activity concentration measured at Darwin airport from August 2007 to May 2010. The shaded areas indicate periods of active monsoon.

onset of the monsoon was in late December and it lasted, with short break periods, to mid February with a less pronounced northerly flow at wind speeds < 10 m s1. In 2009/10 the wet season was above average, with an annual rainfall of about 2100 mm and an active monsoon in January and at the end of February to early March 2010. The active spells of monsoon lasted from a couple of days to a couple of weeks.

3. Results

3.2. Krypton-85

3.1. Meteorological data

Table 1 shows the krypton-85 activity concentrations measured at Darwin Airport, the sampling periods and the detection limits calculated for each sample. The statistical uncertainty (1 sd) of the krypton-85 activity concentration measurements was less than 1%, the systematic uncertainty was 3%. Typical reproducibility for the measurement system was 1% (Sartorius et al., 2002) the detection limit was about 5  103 Bq m3, which is about 400 times lower than the activity concentrations measured in the samples. Fig. 1d shows the results of the krypton-85 activity concentration measurements plotted against sampling date. The minimum krypton-85 activity concentration was 1.28 Bq m3 (9e16 April 2010), the maximum was 1.47 Bq m3 (16e22 January 2008). The mean (arithmetic and geometric) over the sampling period was

Fig. 1a and b shows the wind data and Fig. 1c shows the daily rainfall measured at Darwin airport between 1 July 2007 and 1 June 2010. The grey shaded areas highlight periods of an active monsoon at Darwin determined using the 850 hPa and 200 hPa East to West wind data. The 2007/08 wet season was one of the largest wet seasons on record in Darwin, with an annual rainfall of about 2700 mm and active monsoon periods in January and February, followed by a short spell of monsoon in March 2008. During January and February northerly flow was pronounced. The 2008/09 wet season was about average with an annual rainfall of about 1600 mm. The

114

A. Bollhöfer et al. / Journal of Environmental Radioactivity 127 (2014) 111e118

Table 1 Krypton-85 activity concentrations and uncertainties measured at Darwin airport, and detection limits [Bq m3] associated with each sample. The total uncertainty is given (k ¼ 1). Sampling On

Off

Bq m3 air

DL

15/08/2007 21/08/2007 28/08/2007 04/09/2007 11/09/2007 18/09/2007 25/09/2007 04/10/2007 09/10/2007 16/10/2007 23/10/2007 30/10/2007 06/11/2007 21/11/2007 29/11/2007 04/12/2007 11/12/2007 18/12/2007 24/12/2007 02/01/2008 10/01/2008 16/01/2008 22/01/2008 31/01/2008 05/02/2008 19/02/2008 26/02/2008 04/03/2008 11/03/2008 18/03/2008 28/03/2008 07/04/2008 14/04/2008 22/04/2008 29/04/2008 06/05/2008 14/05/2008 21/05/2008 28/05/2008 02/06/2008 10/06/2008 17/06/2008 25/06/2008 04/07/2008 16/07/2008 22/07/2008 29/07/2008 07/08/2008 12/08/2008 19/08/2008 02/09/2008 11/09/2008 17/09/2008 25/09/2008 21/11/2008 01/12/2008 10/12/2008 19/12/2008 30/12/2008 07/01/2009 15/01/2009 21/01/2009 28/01/2009 04/02/2009 25/02/2009 03/03/2009 12/03/2009 19/03/2009 25/03/2009 01/04/2009 07/04/2009 15/04/2009

21/08/2007 28/08/2007 04/09/2007 11/09/2007 18/09/2007 24/09/2007 02/10/2007 09/10/2007 16/10/2007 23/10/2007 30/10/2007 06/11/2007 15/11/2007 29/11/2007 04/12/2007 11/12/2007 18/12/2007 24/12/2007 02/01/2008 10/01/2008 16/01/2008 22/01/2008 29/01/2008 05/02/2008 11/02/2008 26/02/2008 04/03/2008 10/03/2008 18/03/2008 26/03/2008 07/04/2008 14/04/2008 22/04/2008 28/04/2008 06/05/2008 13/05/2008 20/05/2008 27/05/2008 02/06/2008 10/06/2008 17/06/2008 23/06/2008 02/07/2008 11/07/2008 22/07/2008 29/07/2008 05/08/2008 12/08/2008 19/08/2008 26/08/2008 09/09/2008 16/09/2008 23/09/2008 01/10/2008 29/11/2008 10/12/2008 17/12/2008 30/12/2008 07/01/2009 15/01/2009 21/01/2009 28/01/2009 02/02/2009 11/02/2009 03/03/2009 10/03/2009 18/03/2009 25/03/2009 31/03/2009 07/04/2009 14/04/2009 22/04/2009

1.34
0.04 1.32
0.04 1.31
0.04 1.32
0.04 1.31
0.04 1.32
0.04 1.39
0.04 1.34
0.04 1.33
0.04 1.31
0.04 1.32
0.04 1.31
0.04 1.31
0.04 1.32
0.04 1.31
0.04 1.31
0.04 1.34
0.04 1.33
0.04 1.31
0.04 1.31
0.04 1.34
0.04 1.47
0.04 1.30
0.04 1.31
0.05 1.32
0.04 1.31
0.04 1.32
0.05 1.35
0.04 1.34
0.04 1.30
0.04 1.31
0.04 1.30
0.04 1.31
0.04 1.30
0.04 1.31
0.04 1.29
0.04 1.30
0.04 1.32
0.04 1.30
0.04 1.31
0.04 1.30
0.04 1.31
0.04 1.30
0.04 1.31
0.04 1.31
0.04 1.32
0.04 1.32
0.04 1.32
0.04 1.33
0.04 1.33
0.04 1.34
0.04 1.32
0.04 1.32
0.04 1.31
0.04 1.30
0.04 1.29
0.04 1.30
0.04 1.28
0.04 1.30
0.04 1.29
0.04 1.34
0.04 1.35
0.04 1.32
0.04 1.32
0.04 1.36
0.04 1.31
0.04 1.32
0.04 1.30
0.04 1.29
0.04 1.32
0.04 1.31
0.04 1.33
0.04

0.005 0.002 0.001 0.005 0.004 0.001 0.016 0.001 0.001 0.001 0.001 0.001 0.006 0.005 0.002 0.006 0.002 0.005 0.005 0.001 0.003 0.013 0.003 0.006 0.005 0.002 0.005 0.043 0.005 0.007 0.008 0.004 0.001 0.003 0.001 0.004 0.002 0.015 0.006 0.005 0.005 0.001 0.006 0.006 0.004 0.004 0.001 0.006 0.002 0.002 0.005 0.002 0.002 0.001 0.014 0.004 0.001 0.005 0.005 0.002 0.005 0.001 0.006 0.004 0.039 0.005 0.001 0.004 0.004 0.002 0.006 0.004

Table 1 (continued ) Sampling On

Off

Bq m3 air

DL

22/04/2009 29/04/2009 05/05/2009 26/05/2009 03/06/2009 09/06/2009 16/06/2009 25/06/2009 01/07/2009 08/07/2009 15/07/2009 11/08/2009 18/08/2009 25/08/2009 01/09/2009 11/09/2009 21/09/2009 28/09/2009 05/10/2009 12/10/2009 19/10/2009 29/10/2009 04/11/2009 11/11/2009 18/11/2009 25/11/2009 01/12/2009 09/12/2009 17/12/2009 24/12/2009 06/01/2010 12/01/2010 20/01/2010 04/02/2010 11/02/2010 17/02/2010 25/02/2010 04/03/2010 11/03/2010 18/03/2010 25/03/2010 31/03/2010 09/04/2010 20/04/2010 27/04/2010 04/05/2010 11/05/2010 24/05/2010

29/04/2009 05/05/2009 12/05/2009 02/06/2009 09/06/2009 16/06/2009 25/06/2009 01/07/2009 08/07/2009 15/07/2009 22/07/2009 18/08/2009 25/08/2009 01/09/2009 08/09/2009 18/09/2009 28/09/2009 05/10/2009 12/10/2009 19/10/2009 27/10/2009 04/11/2009 11/11/2009 18/11/2009 25/11/2009 01/12/2009 08/12/2009 15/12/2009 22/12/2009 04/01/2010 12/01/2010 19/01/2010 27/01/2010 11/02/2010 17/02/2010 24/02/2010 04/03/2010 10/03/2010 18/03/2010 25/03/2010 31/03/2010 07/04/2010 16/04/2010 27/04/2010 04/05/2010 11/05/2010 18/05/2010 31/05/2010

1.31
0.04 1.30
0.04 1.29
0.04 1.30
0.04 1.31
0.04 1.29
0.04 1.29
0.04 1.31
0.04 1.30
0.04 1.30
0.04 1.30
0.04 1.31
0.04 1.30
0.04 1.31
0.04 1.31
0.04 1.31
0.04 1.30
0.04 1.29
0.04 1.32
0.04 1.31
0.04 1.28
0.04 1.31
0.04 1.30
0.04 1.33
0.04 1.29
0.04 1.30
0.04 1.34
0.04 1.29
0.04 1.30
0.04 1.28
0.04 1.30
0.04 1.31
0.04 1.29
0.04 1.28
0.04 1.29
0.04 1.32
0.04 1.30
0.04 1.30
0.04 1.31
0.04 1.38
0.10 1.33
0.04 1.29
0.04 1.28
0.04 1.29
0.04 1.30
0.04 1.28
0.04 1.30
0.04 1.32
0.04

0.005 0.002 0.005 0.004 0.001 0.005 0.007 0.001 0.006 0.005 0.001 0.001 0.005 0.001 0.001 0.001 0.006 0.001 0.006 0.001 0.007 0.001 0.006 0.003 0.001 0.001 0.009 0.001 0.005 0.012 0.003 0.005 0.005 0.005 0.003 0.002 0.005 0.001 0.003 0.084 0.005 0.005 0.017 0.001 0.005 0.004 0.001 0.005

1.31 Bq m3. A simple 2-sample t-test indicated that there was no statistically significant difference in the means of the krypton-85 activity concentration for wet season (monsoon on) and dry season (monsoon off) samples (T ¼ 1.44; Tc ¼ 2.42; P ¼ 0.17). A linear regression to the data implied that the krypton-85 activity concentration decreased by about 0.01 Bq m3 per year (P < 0.05; R2 ¼ 0.12). To confirm the significance of the decrease of the krypton-85 activity concentration in Darwin over the measurement period, a linear model was fitted to the monthly krypton-85 activity concentration data and it was tested whether the time series data was correlated, using the DurbineWatson statistic in MiniTab (version 16). Monthly mean krypton-85 activity concentrations were calculated from the activity concentration measured in a sample and weighted by sampling duration. Samples collected over consecutive months were included in the calculation of the monthly means and weighted by the sampling duration in the respective month. Chosen predictors for the multiple regression were month and monsoon (monsoon on ¼ 1; off ¼ 0). The results of this regression indicate a statistically significant decrease (P  0.05; R2 ¼ 0.64) of the atmospheric krypton-85 activity

A. Bollhöfer et al. / Journal of Environmental Radioactivity 127 (2014) 111e118

115

concentration over the sampling period by 0.01 Bq m3 per year. The monsoon describes about 30% of the krypton-85 variability in this model, and the activity concentration during months that were influenced by monsoon spells is higher by about 0.015 Bq m3 than during months without monsoonal influence. The DurbineWatson statistic was 1.85, significantly larger than the upper bound (1.58) of the critical value, meaning that residuals were not auto-correlated. 4. Discussion 4.1. Variability in krypton-85 observed in Darwin Fig. 2 shows boxplots of krypton-85 activity concentrations measured during months with and without monsoonal influence, and the residuals of the regression plotted against month and monsoon (on/off), respectively. It shows that despite no statistically significant difference in the average krypton-85 activity concentrations for wet (monsoon on) and dry season (monsoon off) months, the variability of the wet season data is significantly larger. This is due to the ITCZ moving south across Darwin and influence of northern hemispheric air masses during periods of active monsoon spells but predominantly southern hemispheric influence during breaks in the monsoon. As monthly averages are shown and the duration of active monsoon spells within one wet season month can vary from a couple of days to a couple of weeks, variability of the monthly averages is larger during the wet season. In contrast, in the dry season, air masses are predominantly of southern hemispheric origin and the ITCZ is located way north of Darwin in the northern hemisphere and thus variability is lower. To further understand the variability of krypton-85 activity concentrations measured in Darwin, atmospheric transport modelling was conducted to characterise possible source regions of krypton-85 for typical monsoonal and dry season conditions, respectively. The retro-plumes calculated using Hysplit4 present graphically the regions where the air containing the krypton-85 at Darwin airport passed by 20 days before it reached Darwin. Fig. 3ae c shows the results of the retro-plume calculations for 20 January 2008 (monsoon; 1.47 Bq m3), 28 January 2008 (break in monsoon; 1.30 Bq m3) and 17 June 2008 (dry season; 1.30 Bq m3). In addition, a dilution factor is shown, representing the total activity of krypton-85 that would need to be released in a 1 1 grid cell in 6 h, to lead to a detection of 1 Bq m3 of krypton-85 in Darwin. 4.1.1. Source regions of krypton-85 during an active monsoon (16e 22 January 2008) Fig. 3a shows that during periods of active monsoon some of the air present in Darwin has its origin north of the ITCZ. Due to the location of the ITCZ well south of Darwin during active monsoon periods (Waliser and Gautier, 1993; Manins et al., 2001) krypton-85 activity concentrations are quite similar to concentrations characteristic for the northern hemispheric baseline (Fig. 4). Atmospheric transport modelling shows that source regions of air arriving at Darwin airport include Japan and India. The reprocessing plant at Rokkasho, Japan, was between 2006 and 2008 one of the greatest emitters of krypton-85. In 2007, the average annual total emission for this source was 47.8 PBq (JNFL, 2007). For comparison, La Hague in France and Sellafield in the UK have emitted 235 PBq and 14 PBq, respectively, in 2007. The Kalpakkam reprocessing facility is located at the coast in southeast India approximately 80 km south of the city of Chennai (Mian and Nayyar, 2002). In 2007, approximately 6 PBq of krypton-85 were emitted from this facility (Ahlswede et al., 2009). Both reprocessing facilities may have contributed to the krypton-85 activity measured in the sample taken 16e22 January 2008 in Darwin. With a dilution factor between 1016 and 1017 (Fig. 3) these two sources combined may

Fig. 2. Boxplot of the krypton-85 activity concentrations during monsoon on and off conditions, and residuals of the linear fit to the monthly krypton-85 activity concentration data plotted versus monsoon and month, respectively. Boxes show interquartile ranges, black dots show the average, whiskers extend to the maximum and minimum data point within 1.5 box heights and stars indicate outliers.

have added between 0.3 and 3 mBq m3 to the krypton-85 activity concentration measured in Darwin, but cannot be discriminated against in our measurements. The majority represents typical northern hemispheric air masses with an estimated activity concentration during the sampling period of about 1.45 Bq m3. An additional forward simulation over 25 days starting at the reprocessing facility Rokkasho (Japan) was performed for a hypothetical 4-day release beginning 26 December 2007. The results show a transport path to the Southern Hemisphere. The maximum simulated 6-h averaged concentration at Darwin occurred on 17 January 2008 with a dilution of 2.5  1018 per m3 compared to the 4-day release. Thus, each PBq released would contribute 2.5 mBq m3 to the krypton-85 activity concentration detected at Darwin. According to emission data, 1 PBq per 4 days is a realistic order of magnitude for Rokkasho releases for December 2007. This is consistent to the conclusion from backward modelling and raises confidence that only small parts of the elevation of krypton-85 activity concentration level seen in a weekly sample in Darwin may originate from current emissions of known active facilities in Japan. 4.1.2. Source regions of krypton-85 during breaks in the monsoon (22e29 January 2008) There is some influence from northern hemispheric air masses, but this influence is much less pronounced than during peak monsoonal conditions. Backward dispersion modelling does not include source regions in India or Japan, located deep in the northern hemisphere (Fig. 3b). Indonesia is included in the 20 day backward dispersion plumes for Darwin, but there is only one small medical isotope production facility in Serpong (Matthews et al., 2011) close to the capital Jakarta. Further afield in the southern hemisphere, there is a large medical isotope production facility in South Africa at Pelindaba, which is one of the four largest isotope production facilities worldwide (Saey et al., 2010). There is also a medical isotope production facility at the Australian Nuclear Science and Technology Organisation (ANSTO) in Sydney, Australia, however this facility did not operate from mid 2006 to October 2008. Ahlswede et al. (2013) have shown that annual krypton-85 emissions even from the largest isotope production facility are small compared to reprocessing facilities and more than 3e4 orders of magnitude lower than emissions from Kalpakkam or Rokkasho, respectively. Taking into account the dilution factors indicated in Fig. 3b, emissions from these isotope production facilities are unlikely to influence the krypton-85 activity concentrations in air at Darwin.

116

A. Bollhöfer et al. / Journal of Environmental Radioactivity 127 (2014) 111e118

visible in the Schauinsland time series. These peaks are caused by reprocessing plants in La Hague in France and to a lesser degree Sellafield in the UK (see for example Winger et al., 2005; Schlosser et al., 2010). In addition to the Schauinsland data, the minimum monthly values measured at various krypton-85 stations in Germany, Austria and Poland between 1973 and 2012 (labelled ‘NH baseline data’) and a 3rd order polynomial fit to these data (labelled ‘baseline NH’) are shown. Data measured in the southern hemisphere by the German Office for Radiation Protection from 1983, including the Darwin data, are also shown in Fig. 4. Northern hemispheric baseline krypton-85 activity concentrations have been steadily increasing at a rate of 0.03 Bq m3 per year between 1973 and 2003 (Hirota et al., 2004; Schlosser et al., 2010). It appears that this increase in the northern hemisphere has stalled or even decreased from about 2003 onwards. This behaviour can be explained by the decreasing krypton-85 releases from the main reprocessing plant in La Hague (Schlosser et al., 2010). A linear regression to the northern hemisphere baseline data measured from January 2005 to September 2012 using date and month as predictors indicates a statistically significant decrease of the krypton-85 baseline concentration by about 0.005 Bq m3 per year (Pdate ¼ 0.02; Pmonth ¼ 0.01) despite the low coefficient of determination (R2 ¼ 0.13). The DurbineWatson statistic is 1.37, which is lower than the lower bound (1.64) of the critical value for this data set, meaning that residuals may be auto-correlated. This is not surprising, as krypton-85 baseline activity concentration in the northern hemisphere is influenced by krypton-85 emission pulses from large reprocessing facilities. These pulses are detected as krypton activity concentration peaks at the Schauinsland station, which can last from a few days to more than two weeks (Fig. 4) and also influence the baseline. Hence, residuals may not be independent (higher krypton-85 baseline activity concentrations one month will most likely be followed by higher values in the following month) and thus the decrease is less significant than estimated with our model. The krypton-85 activity concentrations measured at Antarctica have also been increasing at a rate of 0.03 Bq m3 per year up until about 2005. The krypton-85 activity concentrations measured in Darwin during the dry season (1.31 Bq m3; n ¼ 100) appear to continue the southern hemisphere time series and are similar to those measured at Antarctica from 2007 to 2008 (1.30 Bq m3; n ¼ 4). The krypton-85 activity concentrations measured in Darwin decreased between August 2007 and May 2010 by about Fig. 3. Possible source regions (20 day backward plumes) and dilution factor for krypton-85 in air at Darwin airport, modelled using HYSPLIT4 for (a) 20 January 2008 (monsoon); (b) 29 January 2008 (break in monsoon) and (c) 17 June 2008 (dry season).

4.1.3. Source regions of krypton-85 during dry season conditions (10e17 June 2008) Fig. 3c shows that during the dry season in June 2007, source regions of krypton-85 were in the southern hemisphere, with no influence from northern hemispheric air. Almost the entire southern hemisphere is included in the backward modelling for this sampling period and krypton-85 activity concentrations measured in Darwin (1.30
0.04 Bq m3) represent typical southern hemisphere background levels.

4.2. The global context Fig. 4 shows the krypton-85 activity concentration measured in air at the Schauinsland mountain located in southern Germany (47.9 N, 7.9 E; 1284 m asl). As the majority of krypton-85 is emitted between 30 N and 60 N, peaks in krypton-85 activity concentration originating from reprocessing activities in Europe are clearly

Fig. 4. Krypton-85 activity concentrations measured from 1983 to 2011 at the Schauinsland mountain (Germany), lowest monthly values measured in Europe (þ) and results from various stations in the southern hemisphere, including Darwin (data from BfS database, published e.g. in Weiss et al. (1992), Winger et al. (2005) and Ross (2010)). In addition the northern hemispheric baseline is indicated by the dashed line.

A. Bollhöfer et al. / Journal of Environmental Radioactivity 127 (2014) 111e118

117

0.01 Bq m3 per year, and this decrease is indicative of a decrease in the Southern Hemisphere. The trend may not continue, and further measurements in the southern hemisphere are required to confirm that emissions of krypton-85 into the global atmosphere are declining. Interhemispheric exchange times have been estimated using a simple two-box model (northern and southern hemisphere) (Ross, 2010). It was assumed that sources of krypton-85 are located in the northern hemisphere only and that interhemispheric flow of air across the ITCZ is equal in both directions. The rate of change in krypton-85 activity in the northern and southern hemispheres can thus be described with equations (1) and (2):

was 1.47
0.04 Bq m3 typical for the northern hemispheric baseline. 20-day backward dispersion runs using Hysplit4 identified possible source regions deep in the northern hemisphere including Japan and India, albeit the contribution from sources in these countries to the krypton-85 levels measured in Darwin would be less than 0.003 Bq m3. During the dry season, backward dispersion runs show that air masses at Darwin are almost entirely of southern hemispheric origin and represent typical southern hemisphere background krypton-85 levels of 1.30
0.04 Bq m3.

A_ NH ¼ lANH  4ANH þ 4ASH þ S

(1)

A_ SH ¼ lASH  4ASH þ 4ANH

(2)

Results of this research were presented at the 12th South Pacific Environmental Radioactivity Association (SPERA) conference at ANSTO in Sydney, 16e19 October 2012. Andrew Esparon and Gary Fox are acknowledged for their assistance with sample collection. Che Doering is thanked for his comments on an earlier version of the manuscript.

With:

l: decay constant of krypton-85 A: activity of krypton-85 in the northern (southern) hemisphere 4: air mass exchange coefficient S: krypton-85 activity emission rate in the northern hemisphere. The solution of this differential equation allows calculating the characteristic interhemispheric exchange time s as:



s ¼ ðANH  ASH Þ= A_ SH þ lASH



(3)

Assuming that northern and southern hemisphere volumes are equal and activity concentrations homogeneous horizontally and vertically in each hemisphere, krypton-85 activities have been substituted by activity concentrations in equation (3). Using the baseline krypton-85 activity concentration values for the northern hemisphere from Fig. 4, the monthly averaged krypton-85 results of measurements conducted in Darwin between August 2007 and May 2010, and a decrease of the krypton-85 activity concentration in Darwin by 0.01 Bq m3 per year during this time period allows estimation of the interhemispheric exchange time to 1.6
0.2 years. This value is similar to values determined by Levin and Hessheimer (1996) but is at the upper end of interhemispheric exchange times determined previously (e.g. Kjellström et al., 1999; Weiss et al., 1992; Ross, 2010). This is due to the assumption of vertical homogeneity in the atmospheric krypton-85 activity concentration. It has been shown that krypton-85 activity concentration decreases with height in the northern hemisphere (Weiss et al., 1983), whereas it increases with height south of the ITCZ. Not taking into account this vertical variability results in longer estimated interhemispheric exchange times.

5. Conclusions Measurements in Darwin conducted between August 2007 and May 2010 show a statistically significant yearly decrease of atmospheric krypton-85 activity concentrations by about 0.01 Bq m3, indicating that global krypton-85 emissions may have declined during that time and were less than 6.2% (based on the half-life of krypton-85) of the global existing atmospheric inventory (which was 5200 PBq in 2008). There is no statistically difference in the mean krypton-85 activity concentrations in Darwin between samples influenced by the monsoon and dry season samples, but variability of krypton-85 activity concentrations is larger during the monsoon. The highest krypton-85 activity concentration measured during the monsoon

Acknowledgements

References Ahlswede, J., Hebel, S., Kalinowski, M.B., Ross, J.O., 2009. Update of the Global Krypton-85 Emission Inventory. Occasional paper no. 9. Carl Friedrich von Weizsäcker Centre for Science and Peace Research, University of Hamburg, p. 21. Ahlswede, J., Hebel, S., Ross, J.O., Schoetter, R., Kalinowski, M.B., 2013. Update and improvement of the global krypton-85 emission inventory. J. Environ. Radioact. 115, 34e42. ANL, August 2005. Krypton. In: Argonne National Laboratory Human Health Fact Sheet. BMU, 2012. Bundesministerium für Umwelt, Naturschutz und Reaktorsicherheit (BMU), Umweltradioaktivität und Strahlenbelastung. Jahresbericht 2010, p. 319. BOM, 2007. Australian Bureau of Meteorology. http://www.bom.gov.au/climate/ data/. Boeck, W.L., 1976. Meteorological consequences of atmospheric krypton-85. Science 193, 195e198. Draxler, R.R., Rolph, G.D., 2003. HYSPLIT (HYbrid Single-Particle Lagrangian Integrated Trajectory) Model. Technical report. NOAA Air Resources Laboratory, Silver Spring, MD. Drosdowsky, W., 1996. Variability of the Australian summer monsoon at Darwin: 1957e1992. J. Climate 9, 85e96. Hirota, M., Nemota, K., Wada, A., Igarashi, Y., Aoyama, M., Matsueda, H., Hirose, K., Sartorius, H., Schlosser, C., Schmid, S., Weiss, W., Fujii, K., 2004. Spatial and temporal variations of atmospheric 85Kr observed during 1995e2001 in Japan: estimation of atmospheric 85Kr inventory in the northern hemisphere. J. Radiat. Res. 45 (3), 405e413. IAEA, 2005. Status and Trends in Spent Fuel Reprocessing. IAEA-TECDOC-1467. International Atomic Energy Agency, Vienna. IAEA, 2008a. Handbook of Nuclear Data for Safeguards: Database Extensions, August 2008. International Atomic Energy Agency, Vienna. IAEA, 2008b. Spent Fuel Reprocessing Options. IAEA-TECDOC-1587. International Atomic Energy Agency, Vienna. ICRP, 2008. Nuclear Decay Data for Dosimetric Calculations. In: International Commission on Radiological Protection Publication, vol. 107, p. 96. Ann. ICRP 38. Igarashi, Y., Sartorius, H., Miyao, T., Weiss, W., Fushimi, K., Aoyama, M., Hirose, K., Inoue, H.Y., 2000. 85Kr and 133Xe monitoring at MRI, Tsukuba and its importance. J. Environ. Radioact. 48, 191e202. JNFL, 2007. Japan Nuclear Fuel Limited. http://www.jnfl.co.jp/monitoring/discharge/ content-cycle.html. Kalinowski, M.B., Sartorius, H., Uhl, S., Weiss, W., 2004. Conclusions on plutonium separation from atmospheric krypton-85 measured at various distances from the Karlsruhe reprocessing plant. J. Environ. Radioact. 73, 203e222. Kalinowski, M.B., Feichter, J., Nikkinen, M., Schlosser, C., 2006. Environmental sample analysis. In: Avenhaus, R., Kyriakopoulos, N., Richard, Michel, Stein, G. (Eds.), Verifying Treaty Compliance. Springer Verlag, pp. 367e387. Kemp, R.S., Schlosser, C., 2008. A performance estimate for the detection of undeclared nuclear-fuel reprocessing by atmospheric Kr-85. J. Environ. Radioact. 99 (8), 1341e1348. Erratum, J. Environ. Radioact. 100 (2009), 99. Kirk, W.P., 1972. Krypton 85. A Review of the Literature and an Analysis of Radiation Hazards. US Environmental Protection Agency, Office of Research and Monitoring, Washington, DC. Kjellström, E., Feichter, J., Hoffmann, G., 1999. Transport of SF6 and 14CO2 in the atmospheric general circulation model ECHAM4. Tellus 52B, 1e18. Levin, I., Hessheimer, V., 1996. Refining of atmospheric transport model entries by the globally observed passive tracer distributions of 85krypton and sulfur hexafluoride (SF6). J. Geophys. Res. 101, 16745e16755. Manins, P., Allan, R., Beer, T., Fraser, P., Holper, P., Suppiah, R., Walsh, K., 2001. Atmosphere. Australia State of the environment report 2001 (theme report).

118

A. Bollhöfer et al. / Journal of Environmental Radioactivity 127 (2014) 111e118

CSIRO Publishing, on behalf of the Department of the Environment and Heritage, Canberra. Matthews, M., Deconninck, B., Papastefanou, C., Achim, P., DeGeer, L.-E., Piefer, G., Auer, M., Druce, M., Quintana, E., Bell, R., Friese, J., Ross, O., Bielgalski, S., Hague, R., Rotty, M., Bowyer, T., Hoffman, I., Sabzian, M., Braekers, D., Hoffmann, E., Saey, P., Bradley, E., Khrustalev, K., Sameh, A., Briyatmoko, B., Lucas, J., Safari, M., Berglund, H., Mattassi, G., Schoppner, M., Camps, J., Mattila, A., Siebert, P., Carranza, E., Miley, H., Unger, K., Carty, F., Nava, E., Vargas, A., DeCaire, R., Nikkinen, M., November 2011. WOSMIP II e Workshop on Signatures of Medical and Industrial Isotope Production. Prepared for the U.S. Department of Energy. Pacific Northwest National Laboratory, Richland, Washington. Mian, Z., Nayyar, A.H., 2002. An initial analysis of 85Kr production and dispersion from reprocessing in India and Pakistan. Sci. Glob. Secur. 10, 151e179. Ross, J.O., 2010. Simulation of Atmospheric Krypton-85 Transport to Assess the Detectability of Clandestine Nuclear Reprocessing. Reports on earth systems science 82/2010. International Max Planck Research School on Earth System Modelling, Hamburg, p. 125. Saey, P.R.J., Bowyer, T.W., Ringbom, A., 2010. Isotopic noble gas signatures released from medical isotope production facilities e simulations and measurements. Appl. Radiat. Isotopes 68, 1846e1854. Salvamoser, J., 1981. Vergleich der Altersbestimmung von Grundwasser mit Hilfe des tritium- und krypton-85-gehalts. Naturwissenschaften 68, 328e329. Sartorius, H., Schlosser, C., Schmid, S., Weiss, W., 2002. Verfahren zur Bestimmung der Aktivitätskonzentration der atmosphärischen Edelgase Krypton-85 und Xenon-133. Loseblattsammlung FS-78-15-AKU. Empfehlungen zur Überwachung der Umweltradioaktivität. Blatt 3.4.9, p. 16. Schlosser, C., Auer, M., Sartorius, H., Kumberg, T., 2010. Noble gas measurements at BfS for IMIS and CTBTO. In: Schriftenreihe Fachgespräch, Überwachung der Umweltradioaktivität. 14. Fachgespräch, Freiburg, 24e26.03.2009. IMIS e Integrierendes System zur Umweltüberwachung und Notfallvorsorge, Bundesministerium für Umwelt, Naturschutz und Reaktorsicherheit (BMU), p. 399.

Schröder, K.J.P., Roether, T., 1975. The Releases of Krypton-85 and Tritium to the Environment and Tritium to Krypton-85 Ratios as Source Indicators. International Atomic Energy Agency, Vienna. IAEA-SM-191/30. Sittkus, A., Stockburger, H., 1976. Krypton-85 als Indikator des Kernbrennstoffverbrauchs. Naturwissenschaften 63, 266e272. Shaik, H.A., Cleland, S.J., 2010. Onset, active and break periods of the Australian monsoon. Earth Environ. Sci. 11, 012008. Slide, W.C., 2006. Apparent 85Kr ages of groundwater within the Royal watershed, Maine, USA. J. Environ. Radioact. 91, 113e127. Stockburger, H., Sartorius, H., Sittkus, A., 1977. Messung der krypton-85- und xenon133-Aktivität der atmosphärischen Luft. Z. Naturforsch. 32a, 1249e1263. Styra, B., Butkus, K., 1991. Geophysical Problems of Krypton-85 in the Atmosphere. Taylor and Francis, Hemisphere Pub. Corp., New York, p. 160. University of Wyoming, 2012. Atmospheric Soundings. College of Engineering, Department of Atmospheric Sciences. http://weather.uwyo.edu/upperair/ sounding.html (website accessed August 2012.). Waliser, D.E., Gautier, C., 1993. A satellite-derived climatology of the ITCZ. J. Climate 6, 2162e2174. Weiss, W., Sittkus, A., Stockburger, H., Sartorius, H., Münnich, K.O., 1983. Large-scale atmospheric mixing derived from meridional profiles of krypton 85. J. Geophys. Res. 88 (C13), 8574e8578. Weiss, W., Sartorius, H., Stockburger, H., 1992. Global Distribution of Atmospheric Krypton-85, a Data Base for the Verification of Transport and Mixing Models. Technical report of the results of a consultants meeting ‘Isotopes of Noble Gases as Tracers in Environmental Studies’. Technical report series no. 332. IAEA, Vienna, pp. 29e62. Winger, K., Feichter, J., Kalinowski, M.B., Sartorius, H., Schlosser, C., 2005. A new compilation of the atmospheric 85krypton inventories from 1945 to 2000 and its evaluation in a global transport model. J. Environ. Radioact. 80, 183e215. Wheeler, M.C., McBride, J.L., 2005. AustralianeIndonesian monsoon. In: Lau, W.K.M., Waliser, D.E. (Eds.), Intraseasonal Variability in the AtmosphereOcean Climate System. Springer Berlin Heidelberg, pp. 125e173.