- Email: [email protected]
Accelerator mass spectrometry of particle-bound 10Be
Nuclear Instruments and Methods in Physics Research B 223–224 (2004) 601–607 www.elsevier.com/locate/nimb
Accelerator mass spectrometry of particle-bound
10
Be
Alfred Priller a,*, Michael Berger a,b, Heinz W. G€ aggeler c,d, Evangelos Gerasopoulos e, Peter W. Kubik f, Christoph Schnabel c,g,h, Leonhard Tobler d, Eva-Maria Wild a, Prodromos Zanis i, Christos Zerefos
j
a
c
Vienna Environmental Research Accelerator (VERA), Institut f€ur Isotopenforschung und Kernphysik, Universit€at Wien, W€ahringer Str. 17, 1090 Vienna, Austria b Institut f€ur Analytische Chemie, Universit€ at Wien, W€ahringer Straße 38, 1090 Vienna, Austria Labor f€ ur Radio- und Umweltchemie, Department f€ur Chemie und Biochemie, Universit€at Bern, Freiestraße 3, 3012 Bern, Switzerland d Laboratory for Radiochemistry and Environmental Chemistry, Paul Scherrer Institut, Villingen PSI, 5232 Villingen, Switzerland e Nuclear Physics Department, University of Thessaloniki, 54006 Thessaloniki, Greece f Paul Scherrer Institut, c/o Institute of Particle Physics, ETH Hoenggerberg, 8093 Z€urich, Switzerland g Institute of Particle Physics, ETH Hoenggerberg, 8093 Z€urich, Switzerland h Scottish Universities Environmental Research Centre, Scottish Enterprise Technology Park, Rankine Avenue, East Kilbride, Glasgow G75 OQF, UK i Laboratory of Atmospheric Physics, Aristotle University of Thessaloniki, 54006 Thessaloniki, Campus Box 149, Greece j Laboratory of Climatology and Atmospheric Physics, Geology Department, National and Kapodistrian University of Athens (LAC-UOA), Univ. Campus, 15701 Ilisia, Greece
Abstract In the framework EU Project STACCATO (Influence of Stratosphere–Troposphere Exchange in A Changing Climate on Atmospheric Transport and Oxidation Capacity), the first long-term, simultaneous monitoring of the two cosmogenic radionuclides 7 Be and 10 Be was performed. Emphasis was paid to a high-resolution record of the data, too. A comprehensive data set was created in order to validate model calculations and to provide an independent estimate of the strength of atmospheric transport processes, such as stratosphere–troposphere exchange. For that reason, particlebound beryllium isotopes were collected at three high-alpine meteorological stations: at Sonnblick (Austria), Zugspitze (Germany) and Jungfraujoch (Switzerland), respectively. A total of 400 daily or bi-daily 10 Be measurements are now available. While 7 Be sampling and measurement processes are well standardized, the determination of 10 Be concentrations using AMS is more complicated. For that reason an extensive description of the 10 Be measurement is given. Moreover, the basic characteristics of the 10 Be/7 Be ratios are presented, leading to a mean annual value of 2.08 and 1.82 for Jungfraujoch and Zugspitze, respectively. Analysis in combination with meteorological parameters shows the usefulness of the ratio as an index of stratosphere-to-troposphere transport (STT), especially when wet scavenging becomes important. Regression analysis of the 10 Be/7 Be ratio with 7 Be, 10 Be and relative humidity revealed that the ratio is virtually independent from the effect of wet scavenging while inspection of the weather patterns related to the highest ratios indicated the presence of typical patterns for stratospheric intrusions. Nevertheless, although the 10 Be/7 Be ratio
*
Corresponding author. Tel.: +43-1-4277-51703; fax: +43-1-4277-9517. E-mail address: [email protected] (A. Priller).
0168-583X/$ - see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2004.04.111
602
A. Priller et al. / Nucl. Instr. and Meth. in Phys. Res. B 223–224 (2004) 601–607
can be successful in identifying certain STT cases it is a difficult parameter for an automated stratospheric intrusion detection algorithm. 2004 Elsevier B.V. All rights reserved. PACS: 92.70.Cp; 29.30.)h Keywords: Beryllium10; AMS; Atmosphere
1. Introduction A detailed investigation of the effect of stratosphere-to-troposphere transport (STT) on the tropospheric budget of various constituents has been undertaken during STACCATO. One of the main tasks of the project was the creation of a comprehensive two-year set of tracer isotopes and meteorological data for the validation of model calculations carried out within the project. Under this scope combined measurements of both, 10 Be (t1=2 ¼ 1:51 Ma) and 7 Be (t1=2 ¼ 53:3 d) were carried out regularly throughout the course of a full year at two high alpine peaks, Jungfraujoch, Switzerland and Zugspitze, Germany. These measurements and their ratio were used for an independent estimation of the strength of STT [1]. The use of the 10 Be/7 Be ratio for that purpose was first suggested by Raisbeck et al. in 1981 [2]. The idea behind was that since both cosmogenic radionuclides are attached to the same aerosol species and therefore washed out in the same way, the use of their ratio would significantly cancel the influence of wet deposition. About 67% of the atmospheric beryllium isotopes is produced in the stratosphere and the rest mainly in the upper troposphere [3,4]. This fact, in combination with the residence time of the individual beryllium isotopes in the stratosphere (about one year) and the troposphere (about three weeks), suggests that deep STT events should be reflected in the 10 Be/7 Be ratio measured in aerosol samples collected at surface stations.
2. Data and methodology of measurements Aerosol sampling was performed within STACCATO at three high-alpine meteorological stations: at Jungfraujoch (JFJ, Switzerland,
3580 m asl, 46320 N, 7590 E), at Hoher Sonnblick (SB, Austria, 3106 m asl, 47030 N, 12580 E) and at Zugspitze (ZS, Germany, 2962 m asl, 47250 N, 10420 E). All stations used automatic high-volume aerosol samplers, which filtered about 1000–1500 standard cubic meters air per day on either glass fiber (JFJ and SB) or cellulose nitrate filters (ZS). At SB and ZS daily samples were collected, whereas those of JFJ were sampled with a time resolution of 48 h by the NABEL network [5]. The 10 Be concentrations of samples from SB were measured for selected case studies only. The respective filters were measured for their 7 Be activity using its characteristic gamma-radiation. Much more complicated is the process followed for the measurement of 10 Be, which in combination with the increased cost, makes those measurements rather rare, especially on a continuous basis. Two AMS facilities performed the 10 Be measurements, the AMS facility of the Paul Scherrer Institute at the Institute of Particle Physics of the ETH Zurich, (PSI/ETHZ), Switzerland [6], and the Vienna Environmental Research Accelerator (VERA) of the University of Vienna, Austria [7]. 2.1. Sample preparation The separation of 10 Be from the filter material is a very important stage for a correct determination of its activity. The two different methods used by PSI/ETHZ and VERA are extensively presented here. After having determined the 7 Be concentrations by gamma spectrometry (atoms per standard cubic meter) from one half of each filter, a known amount of stable 9 Be was added as a carrier. For the JFJ samples, this amount was approximately 0.65 mg, whereas only 0.45 mg 9 Be per filter was added when at least two filters were combined.
A. Priller et al. / Nucl. Instr. and Meth. in Phys. Res. B 223–224 (2004) 601–607
Each filter was leached separately in nitric acid (wt. 17%) in an ultrasonic bath followed by filtration. A second 7 Be measurement of the resulting solution gave about 99% recovery for 7 Be. Consequently, 99% of the Be isotopes 10 Be and 7 Be was leached from the filter and equilibrated with the stable 9 Be carrier. The results were corrected for these losses and the standard deviation of the yield determination was included in the uncertainty calculation. In order to obtain good decontamination from boron, iron and aluminum, the samples were subjected to a cation exchange process (17 ml DOWEX 50W X 8 resin). The reduced sample volume of about 1 ml was put onto the columns that had been equilibrated with 1 M HCl. After discarding the first three column volumes of 1 M HCl, which removed nearly all of the boron present, five column volumes of 1 M HCl were collected as beryllium containing fraction. Aluminum and iron were eluted from the cation exchange column with two column volumes of 4.5 M HCl. In order to recycle the cation exchange resin material for the next sample, six column volumes of 9 M HCl were washed through the columns followed by equilibration with six column volumes of 1 M HCl. Analysis of chemical blanks including the cation exchange process led to results indistinguishable from analysis of the solution of stable beryllium that was used as carrier. VERA: Before sample preparation, the amount of 7 Be was measured at the Federal Office of Agrobiology, Department of Radiation Protection, Linz, Austria (SB) and the Fraunhofer-Institut f€ ur Atmosph€ arische Umweltforschung, GarmischPartenkirchen, Germany (ZS). Then one half (SB) or the whole filter (ZS) was sent to VERA. The cellulose nitrate filters from ZS were spiked with 1 mg 9 Be-carrier and 10 Be was leached from the filters with 6 M hydrochloric acid at room temperature for 24 h . After filtration and washing of the filter material, the volume of the resulting solution (leachate plus washing solution) was reduced by evaporation. Be(OH)2 was precipitated from this solution with diluted ammonia and Be(OH)2 was transformed to BeO by heating the precipitate to 850 C for 2 h. The BeO powder was mixed with Cu powder (BeO:Cu 1:3) to improve the thermal
603
and the electrical conductivity of the target material. The efficiency for leaching has been tested using an approach different from that one at PSI/ETHZ. Since there was no way to measure the 7 Be activity of the leached filters once more, some filters have been leached a second time (same preparation procedure) and then measured by AMS. Similar to the cellulose nitrate filters the borosilicate glass fiber filters from SB were spiked with 1 mg 9 Be-carrier and leached with 2.5 M HCl for 24 h at room temperature. Again the leaching efficiencies of the procedure were determined by a second leaching (see above). During the leaching step considerable amounts of the filter material were also dissolved. Simply precipitating the hydroxides from the leaching solution results in a mixture of beryllium hydroxide and co-precipitated aluminum and iron hydroxides with inclusions of silica and boron. Using this precipitate (converted to the oxides) for the AMS 10 Be determinations would lead to: (a) a dilution of the BeO concentration in the target material resulting in low BeO -currents, and (b) large amounts of 10 B present in the target material which would seriously interfere with the 10 Be determination with the AMS method. Therefore the clean up of the leachate with an effective as well as less time consuming chemical method was necessary. We achieved the reduction of the Al content by the complexation of Al with EDTA (ethylene diamine tetraacetic acid) prior to the hydroxide precipitation. Silica and boron were removed by HF treatment of the precipitate. The final product of the chemical procedure was Be(OH)2 , which was further processed as described above for the cellulose nitrate filters. The correction for losses during leaching were less than 5% for both filter materials. 2.2.
10
Be measurements and quality control
AMS with 10 Be was performed by PSI/ETHZ and the upgraded VERA [8,9]. The 9 Be 16 O currents from the source were 1–3 lA, and the accelerating voltages were 3.0 MV (VERA) and 5.3 MV (PSI/ETHZ), respectively. After stripping, charge state 3+ was selected by both labs.
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However, with respect to suppressing 10 B correlated background, their analyzing beam lines differ appreciably (Fig. 1). In both labs, 10 B, the principally interfering isobar of 10 Be, is stopped completely in an absorber cell placed directly in front of the 10 Be detector due to its higher stopping power. However, 10 B can also interfere in an indirect way with 10 Be because of the reaction 1 H(10 B,a)7 Be, taking place in the entrance foil of the rare isotope detector. Since the energy of the ions at VERA (10.2 MeV) is lower than at PSI/ ETHZ (19 MeV), the production of 7 Be is suppressed by a factor 50 to 60 at VERA. At PSI/ ETHZ, the mass 10 beam is sent to another magnet [6]. At its entrance, a carbon foil is placed to poststrip the 10 Be and the 10 B ions. At energy of 19 MeV the highest stripping yield for 10 B ions is the 5þ charge state, which cannot be populated by Be ions. Consequently, the background from 10 B can
Fig. 1. Analyzing beam lines of the AMS facilities participating in STACCATO. For more details see text. EEA ¼ electrostatic energy analyzer, MMA ¼ magnetic mass analyzer, MQP ¼ magnetic quadrupole doublet, QP ¼ electrostatic quadrupole doublet, RD ¼ rare isotope detector, S ¼ steerer, SF ¼ foil for poststripping, SFC ¼ stable ion Faraday cup.
be reduced by at least a factor of 5 when using Be4þ (90% stripping efficiency). Both AMS laboratories used the same standard material for normalization (S555 supplied by PSI/ETHZ – 10 Be/9 Be ¼ 95.5 · 1012 with an uncertainty of 3%). As a quality control the two laboratories performed an intercomparison of both, sample preparation and measurement methods. Samples and standard material were converted to BeO at VERA and a part of them sent to PSI/ETHZ for AMS and vice versa. The samples were new glass fiber filters soaked with the same standard and spike solutions, as well as real aerosol samples. It was shown that the different sample preparation and the AMS measurement at VERA and PSI/ ETHZ produced data of the same quality. The data sets including blank samples agree within their uncertainties (1r). These samples had isotopic ratios of 10 Be/9 Be between 1 · 1013 to 1 · 1012 . The 10 Be concentrations measured at PSI/ETHZ normalized to those measured at VERA ranged from 1.00 ± 0.09 to 1.09 ± 0.05. Over the time span of STACCATO, the AMS setup for 10 Be has changed significantly at Vienna [8–10]. Thus an additional intercomparison on the measurement procedure was performed by measuring a number of samples collected at JFJ and 10
Fig. 2. 10 Be/9 Be ratios data of samples collected at the Jungfraujoch. Results of an intercomparison exercise between the two AMS laboratories participating in STACCATO. The samples were prepared at PSI/ETHZ.
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prepared by PSI/ETHZ. Once more, the agreement of the two individual measurements was very good (Fig. 2).
3. Results and discussion During STACCATO, measurements of various atmospheric tracers and meteorological parameters such as O3 , relative humidity (RH) and 7 Be, were continued mainly at the three stations in the Alps. Gerasopoulos et al. [11] presented a climatology of the 7 Be data from these stations. Nevertheless, for the first time to our knowledge in Europe, 10 Be was monitored on a continuous basis at two stations (JFJ and ZS) for more than a year, whereas additional samples from SB were also analyzed for the study of specific case studies. Thus, a total of about 400 daily or bi-daily 10 Be measurements were finally available, constituting one of the most compact and long-term data set of 10 Be that is worldwide available. 7 Be and 10 Be were measured from March 2000 to February 2001 at JFJ and from January 2000 to December 2000 at ZS. The 7 Be and 10 Be concentrations as well as their isotopic ratio for both, ZS and JFJ in 2000 are shown in Fig. 3. At JFJ the 10 Be/7 Be ratios range from 1.04 to 3.69 (average of 2.08), while at ZS the ratios range from 0.14 to 6.11 (average of 1.82). The higher value at JFJ than at ZS is likely related to the different altitudes of the two stations, which possibly indicates a stronger influence of STT at JFJ than at ZS. Individual isotopes show increased variability related to their episodic nature while the ratio 10 Be/7 Be shows a seasonal cycle with a clear maximum in May and June at JFJ, which is less pronounced at ZS. Multiple-linear regression analysis revealed that 64% ðn ¼ 163Þ for ZS and 76% ðn ¼ 110Þ for JFJ of the 10 Be/7 Be ratio variability is explained by variability in both 7 Be and 10 Be. In contrast, the variability of the ratio that can be explained by variations of 7 Be only is 7.0% for ZS and 0.6% for JFJ, and that explained by variations of 10 Be only is 1.9% for ZS and 12.7% for JFJ. Thus, the ratio is almost independent from processes that have a clear effect on both radionuclides, such as wet scavenging.
Fig. 3. Surface concentrations of 7 Be, 10 Be, the 10 Be/7 Be ratio and their corresponding moving averages (weekly), as measured at Jungfraujoch (JFJ) and Zugspitze (ZS), respectively. 7 Be/m3 and 10 Be/m3 are normalized to standard air conditions.
At ZS, where the time resolution of 7 Be and Be measurements is higher (daily samples every second day), the above conclusion is strengthened further by the fact that the correlation coefficients of 7 Be and 10 Be with RH daily averages are )0.62 and )0.48, respectively, whereas the correlation between their ratio and RH is +0.32 indicating that the ratio is substantially relieved from the effect of wet scavenging. As RH can serve as a proxy for wet scavenging processes [11], at least 10
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such taking place close to the measurement site, this indicates that the ratio is virtually independent from wet scavenging. On the other hand, RH has been used as a tracer to isolate events of stratosphere-to-troposphere exchange (STE), since stratospheric and tropospheric air masses are distinguished by very different water vapor concentrations [11]. Thus the anti-correlation between 7 Be or 10 Be and RH can be alternatively explained as events of downward transport of dry upper tropospheric or stratospheric air. In this case one would expect that specific humidity (SH), which is a better tracer than RH, to be also a good predictor of 7 Be and 10 Be concentrations [11]. However, this is not the case as the correlation coefficients of 7 Be and 10 Be with SH daily averages at ZS are )0.14 and )0.07, respectively. This result also supports indirectly the previous result that the ratio is virtually independent from wet scavenging. In order to investigate to what extent 7 Be, 10 Be and 10 Be/7 Be trace intrusions of stratospheric air, geopotential height charts at 500 hPa were inspected for all the days with 10 Be/7 Be within the upper 10% quantile (10 Be/7 Be > 2.7) at ZS where daily values are available. The vast majority of the synoptic patterns revealed a deep upper trough extending southwards, with strong northerly advection affecting the Alpine stations. This synoptic situation is known to be often related to stratospheric intrusion events [12,13]. There were also a few occasions with cut-off low systems affecting central Europe, which are also synoptic patterns that are often linked to stratospheric intrusions [14,15]. Back trajectory analysis for all these selected days indicated downward transport from the upper troposphere or lower stratosphere, thus providing evidence for the use of the ratio as a stratospheric tracer and a confirmation that the ratio is mostly successful in identifying STT cases [1]. Nevertheless, when correlating the 10 Be/7 Be ratio with SH or ozone, which both are also expected to serve as an index for stratospheric intrusions, the interpretation of the correlation becomes a very complex task [1]. Hence, although the 10 Be/7 Be ratio can be successful in identifying certain STT cases it is a difficult parameter for an automated stratospheric intrusion detection algo-
rithm, because it is influenced also by other factors than STT such as production of 10 Be, 7 Be in the upper troposphere, accompanied by vertical mixing within the troposphere [1].
Acknowledgements The AMS measurements of aerosol filters from the Jungfraujoch, Sonnblick and Zugspitze were carried out within the STACCATO project (Contract No. EVK2-CT1999-00050) funded by the European Community under the Fifth Framework. This study has also been funded by the Bundesamt f€ ur Bildung und Wissenschaft (BBW) of Switzerland. The sampling and the delivery of the filters by the EMPA (Eidgen€ ossische Materialpr€ ufungs- und Forschungsanstalt, D€ ubendorf, CH, operator of the NABEL network) are highly appreciated. We thank MR E. Henrich from the Ministry of Agriculture and Forestry, Environment and Water Management who gave us the possibility to use aerosol filters from the Sonnblick observatory for AMS measurements. We appreciate the supporting work done by M. Mandl from the Zentralanstalt f€ ur Meteorologie und Geodynamik, Wetterdienststelle f€ ur Salzburg und Ober€ osterreich, and W. Ringer from the Federal Office of Agrobiology, Dep. of Radiation Protection, Linz.
References [1] P. Zanis, E. Gerasopoulos, A. Priller, C. Schnabel, A. Stohl, C.S. Zerefos, H.W. Gaeggeler, L. Tobler, P.W. Kubik, H.J. Kanter, H.E. Scheel, J. Luterbacher, M. Berger, J. Geophys. Res. 108/D12 (2003) 8520. [2] G. Raisbeck, F. Yiou, M. Fruneau, J.M. Loiseaux, M. Lieuvin, J.C. Ravel, Geophys. Res. Lett. 8–9 (1981) 1015. [3] D. Lal, Annu. Rev. Earth Planet. Sci. 16 (1988) 355. [4] J. Masarik, J. Beer, J. Geophys. Res. 104/D10 (1999) 12099. [5] Nationales Beobachtungsnetz f€ ur Luftfremdstoffe (NABEL) Luftbelastung 1999, Bundesamt f€ ur Umwelt, Wald und Landschaft, Switzerland (BUWAL), SRU-316, 2000. [6] H.-A. Synal, G. Bonani, M. D€ obeli, R.M. Ender, P. Gartenmann, P.W. Kubik, Ch. Schnabel, M. Suter, Nucl. Instr. and Meth. B 123 (1997) 62.
A. Priller et al. / Nucl. Instr. and Meth. in Phys. Res. B 223–224 (2004) 601–607 [7] A. Priller, T. Brandl, R. Golser, W. Kutschera, S. Puchegger, W. Rom, P. Steier, C. Vockenhuber, A. Wallner, E. Wild, Nucl. Instr. and Meth. B 172 (2000) 100. [8] Ch. Vockenhuber, I. Ahmad, R. Golser, W. Kutschera, V. Liechtenstein, A. Priller, P. Steier, S. Winkler, Int. J. Mass Spectrom. 223 (2003) 713. [9] P. Steier, R. Golser, W. Kutschera, A. Priller, Ch. Vockenhuber, S. Winkler, Nucl. Instr. and Meth. B, these Proceedings. doi:10.1016/j.nimb.2004.04.017. [10] A. Priller, R. Golser, W. Kutschera, P. Steier, Ch. Vockenhuber, S. Winkler, in: R. Hellborg, M. Faarinen, C.E. Magnusson, P. Persson, G. Skog, K. Stenst€ orm, (Eds.), Symposium of North Eastern Accelerator Personnel
[11]
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SNEAP XXXIV 22–25 October 2001, Lund University, Sweden, 2002, p. 68, ISBN 91-631-2676-1. E. Gerasopoulos, P. Zanis, A. Stohl, C.S. Zerefos, C. Papastefanou, W. Ringer, L. Tobler, S. Huebener, H.J. Kanter, L. Tositti, S. Sandrini, Atmos. Environ. 35/36 (2001) 6347. R. Reiter, R. Sladkovich, K. P€ otzl, W. Carnuth, H.J. Kanter, Arch. Met. Geoph. Biokl. A 20 (1971) 211. P. Zanis, E. Schuepbach, H.W. Gaeggeler, S. Huebener, L. Tobler, Tellus 51 (4) (1999) 789. W. Viezee, H.B. Singh, Geophys. Res. Lett. 7 (1980) 805. G. Vaughan, J.D. Price, in: R.D. Bojkov, P. Fabian (Eds.), Deepak Publ., 1989, p. 415.
Accelerator mass spectrometry of particle-bound
10
Be
Alfred Priller a,*, Michael Berger a,b, Heinz W. G€ aggeler c,d, Evangelos Gerasopoulos e, Peter W. Kubik f, Christoph Schnabel c,g,h, Leonhard Tobler d, Eva-Maria Wild a, Prodromos Zanis i, Christos Zerefos
j
a
c
Vienna Environmental Research Accelerator (VERA), Institut f€ur Isotopenforschung und Kernphysik, Universit€at Wien, W€ahringer Str. 17, 1090 Vienna, Austria b Institut f€ur Analytische Chemie, Universit€ at Wien, W€ahringer Straße 38, 1090 Vienna, Austria Labor f€ ur Radio- und Umweltchemie, Department f€ur Chemie und Biochemie, Universit€at Bern, Freiestraße 3, 3012 Bern, Switzerland d Laboratory for Radiochemistry and Environmental Chemistry, Paul Scherrer Institut, Villingen PSI, 5232 Villingen, Switzerland e Nuclear Physics Department, University of Thessaloniki, 54006 Thessaloniki, Greece f Paul Scherrer Institut, c/o Institute of Particle Physics, ETH Hoenggerberg, 8093 Z€urich, Switzerland g Institute of Particle Physics, ETH Hoenggerberg, 8093 Z€urich, Switzerland h Scottish Universities Environmental Research Centre, Scottish Enterprise Technology Park, Rankine Avenue, East Kilbride, Glasgow G75 OQF, UK i Laboratory of Atmospheric Physics, Aristotle University of Thessaloniki, 54006 Thessaloniki, Campus Box 149, Greece j Laboratory of Climatology and Atmospheric Physics, Geology Department, National and Kapodistrian University of Athens (LAC-UOA), Univ. Campus, 15701 Ilisia, Greece
Abstract In the framework EU Project STACCATO (Influence of Stratosphere–Troposphere Exchange in A Changing Climate on Atmospheric Transport and Oxidation Capacity), the first long-term, simultaneous monitoring of the two cosmogenic radionuclides 7 Be and 10 Be was performed. Emphasis was paid to a high-resolution record of the data, too. A comprehensive data set was created in order to validate model calculations and to provide an independent estimate of the strength of atmospheric transport processes, such as stratosphere–troposphere exchange. For that reason, particlebound beryllium isotopes were collected at three high-alpine meteorological stations: at Sonnblick (Austria), Zugspitze (Germany) and Jungfraujoch (Switzerland), respectively. A total of 400 daily or bi-daily 10 Be measurements are now available. While 7 Be sampling and measurement processes are well standardized, the determination of 10 Be concentrations using AMS is more complicated. For that reason an extensive description of the 10 Be measurement is given. Moreover, the basic characteristics of the 10 Be/7 Be ratios are presented, leading to a mean annual value of 2.08 and 1.82 for Jungfraujoch and Zugspitze, respectively. Analysis in combination with meteorological parameters shows the usefulness of the ratio as an index of stratosphere-to-troposphere transport (STT), especially when wet scavenging becomes important. Regression analysis of the 10 Be/7 Be ratio with 7 Be, 10 Be and relative humidity revealed that the ratio is virtually independent from the effect of wet scavenging while inspection of the weather patterns related to the highest ratios indicated the presence of typical patterns for stratospheric intrusions. Nevertheless, although the 10 Be/7 Be ratio
*
Corresponding author. Tel.: +43-1-4277-51703; fax: +43-1-4277-9517. E-mail address: [email protected] (A. Priller).
0168-583X/$ - see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2004.04.111
602
A. Priller et al. / Nucl. Instr. and Meth. in Phys. Res. B 223–224 (2004) 601–607
can be successful in identifying certain STT cases it is a difficult parameter for an automated stratospheric intrusion detection algorithm. 2004 Elsevier B.V. All rights reserved. PACS: 92.70.Cp; 29.30.)h Keywords: Beryllium10; AMS; Atmosphere
1. Introduction A detailed investigation of the effect of stratosphere-to-troposphere transport (STT) on the tropospheric budget of various constituents has been undertaken during STACCATO. One of the main tasks of the project was the creation of a comprehensive two-year set of tracer isotopes and meteorological data for the validation of model calculations carried out within the project. Under this scope combined measurements of both, 10 Be (t1=2 ¼ 1:51 Ma) and 7 Be (t1=2 ¼ 53:3 d) were carried out regularly throughout the course of a full year at two high alpine peaks, Jungfraujoch, Switzerland and Zugspitze, Germany. These measurements and their ratio were used for an independent estimation of the strength of STT [1]. The use of the 10 Be/7 Be ratio for that purpose was first suggested by Raisbeck et al. in 1981 [2]. The idea behind was that since both cosmogenic radionuclides are attached to the same aerosol species and therefore washed out in the same way, the use of their ratio would significantly cancel the influence of wet deposition. About 67% of the atmospheric beryllium isotopes is produced in the stratosphere and the rest mainly in the upper troposphere [3,4]. This fact, in combination with the residence time of the individual beryllium isotopes in the stratosphere (about one year) and the troposphere (about three weeks), suggests that deep STT events should be reflected in the 10 Be/7 Be ratio measured in aerosol samples collected at surface stations.
2. Data and methodology of measurements Aerosol sampling was performed within STACCATO at three high-alpine meteorological stations: at Jungfraujoch (JFJ, Switzerland,
3580 m asl, 46320 N, 7590 E), at Hoher Sonnblick (SB, Austria, 3106 m asl, 47030 N, 12580 E) and at Zugspitze (ZS, Germany, 2962 m asl, 47250 N, 10420 E). All stations used automatic high-volume aerosol samplers, which filtered about 1000–1500 standard cubic meters air per day on either glass fiber (JFJ and SB) or cellulose nitrate filters (ZS). At SB and ZS daily samples were collected, whereas those of JFJ were sampled with a time resolution of 48 h by the NABEL network [5]. The 10 Be concentrations of samples from SB were measured for selected case studies only. The respective filters were measured for their 7 Be activity using its characteristic gamma-radiation. Much more complicated is the process followed for the measurement of 10 Be, which in combination with the increased cost, makes those measurements rather rare, especially on a continuous basis. Two AMS facilities performed the 10 Be measurements, the AMS facility of the Paul Scherrer Institute at the Institute of Particle Physics of the ETH Zurich, (PSI/ETHZ), Switzerland [6], and the Vienna Environmental Research Accelerator (VERA) of the University of Vienna, Austria [7]. 2.1. Sample preparation The separation of 10 Be from the filter material is a very important stage for a correct determination of its activity. The two different methods used by PSI/ETHZ and VERA are extensively presented here. After having determined the 7 Be concentrations by gamma spectrometry (atoms per standard cubic meter) from one half of each filter, a known amount of stable 9 Be was added as a carrier. For the JFJ samples, this amount was approximately 0.65 mg, whereas only 0.45 mg 9 Be per filter was added when at least two filters were combined.
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Each filter was leached separately in nitric acid (wt. 17%) in an ultrasonic bath followed by filtration. A second 7 Be measurement of the resulting solution gave about 99% recovery for 7 Be. Consequently, 99% of the Be isotopes 10 Be and 7 Be was leached from the filter and equilibrated with the stable 9 Be carrier. The results were corrected for these losses and the standard deviation of the yield determination was included in the uncertainty calculation. In order to obtain good decontamination from boron, iron and aluminum, the samples were subjected to a cation exchange process (17 ml DOWEX 50W X 8 resin). The reduced sample volume of about 1 ml was put onto the columns that had been equilibrated with 1 M HCl. After discarding the first three column volumes of 1 M HCl, which removed nearly all of the boron present, five column volumes of 1 M HCl were collected as beryllium containing fraction. Aluminum and iron were eluted from the cation exchange column with two column volumes of 4.5 M HCl. In order to recycle the cation exchange resin material for the next sample, six column volumes of 9 M HCl were washed through the columns followed by equilibration with six column volumes of 1 M HCl. Analysis of chemical blanks including the cation exchange process led to results indistinguishable from analysis of the solution of stable beryllium that was used as carrier. VERA: Before sample preparation, the amount of 7 Be was measured at the Federal Office of Agrobiology, Department of Radiation Protection, Linz, Austria (SB) and the Fraunhofer-Institut f€ ur Atmosph€ arische Umweltforschung, GarmischPartenkirchen, Germany (ZS). Then one half (SB) or the whole filter (ZS) was sent to VERA. The cellulose nitrate filters from ZS were spiked with 1 mg 9 Be-carrier and 10 Be was leached from the filters with 6 M hydrochloric acid at room temperature for 24 h . After filtration and washing of the filter material, the volume of the resulting solution (leachate plus washing solution) was reduced by evaporation. Be(OH)2 was precipitated from this solution with diluted ammonia and Be(OH)2 was transformed to BeO by heating the precipitate to 850 C for 2 h. The BeO powder was mixed with Cu powder (BeO:Cu 1:3) to improve the thermal
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and the electrical conductivity of the target material. The efficiency for leaching has been tested using an approach different from that one at PSI/ETHZ. Since there was no way to measure the 7 Be activity of the leached filters once more, some filters have been leached a second time (same preparation procedure) and then measured by AMS. Similar to the cellulose nitrate filters the borosilicate glass fiber filters from SB were spiked with 1 mg 9 Be-carrier and leached with 2.5 M HCl for 24 h at room temperature. Again the leaching efficiencies of the procedure were determined by a second leaching (see above). During the leaching step considerable amounts of the filter material were also dissolved. Simply precipitating the hydroxides from the leaching solution results in a mixture of beryllium hydroxide and co-precipitated aluminum and iron hydroxides with inclusions of silica and boron. Using this precipitate (converted to the oxides) for the AMS 10 Be determinations would lead to: (a) a dilution of the BeO concentration in the target material resulting in low BeO -currents, and (b) large amounts of 10 B present in the target material which would seriously interfere with the 10 Be determination with the AMS method. Therefore the clean up of the leachate with an effective as well as less time consuming chemical method was necessary. We achieved the reduction of the Al content by the complexation of Al with EDTA (ethylene diamine tetraacetic acid) prior to the hydroxide precipitation. Silica and boron were removed by HF treatment of the precipitate. The final product of the chemical procedure was Be(OH)2 , which was further processed as described above for the cellulose nitrate filters. The correction for losses during leaching were less than 5% for both filter materials. 2.2.
10
Be measurements and quality control
AMS with 10 Be was performed by PSI/ETHZ and the upgraded VERA [8,9]. The 9 Be 16 O currents from the source were 1–3 lA, and the accelerating voltages were 3.0 MV (VERA) and 5.3 MV (PSI/ETHZ), respectively. After stripping, charge state 3+ was selected by both labs.
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However, with respect to suppressing 10 B correlated background, their analyzing beam lines differ appreciably (Fig. 1). In both labs, 10 B, the principally interfering isobar of 10 Be, is stopped completely in an absorber cell placed directly in front of the 10 Be detector due to its higher stopping power. However, 10 B can also interfere in an indirect way with 10 Be because of the reaction 1 H(10 B,a)7 Be, taking place in the entrance foil of the rare isotope detector. Since the energy of the ions at VERA (10.2 MeV) is lower than at PSI/ ETHZ (19 MeV), the production of 7 Be is suppressed by a factor 50 to 60 at VERA. At PSI/ ETHZ, the mass 10 beam is sent to another magnet [6]. At its entrance, a carbon foil is placed to poststrip the 10 Be and the 10 B ions. At energy of 19 MeV the highest stripping yield for 10 B ions is the 5þ charge state, which cannot be populated by Be ions. Consequently, the background from 10 B can
Fig. 1. Analyzing beam lines of the AMS facilities participating in STACCATO. For more details see text. EEA ¼ electrostatic energy analyzer, MMA ¼ magnetic mass analyzer, MQP ¼ magnetic quadrupole doublet, QP ¼ electrostatic quadrupole doublet, RD ¼ rare isotope detector, S ¼ steerer, SF ¼ foil for poststripping, SFC ¼ stable ion Faraday cup.
be reduced by at least a factor of 5 when using Be4þ (90% stripping efficiency). Both AMS laboratories used the same standard material for normalization (S555 supplied by PSI/ETHZ – 10 Be/9 Be ¼ 95.5 · 1012 with an uncertainty of 3%). As a quality control the two laboratories performed an intercomparison of both, sample preparation and measurement methods. Samples and standard material were converted to BeO at VERA and a part of them sent to PSI/ETHZ for AMS and vice versa. The samples were new glass fiber filters soaked with the same standard and spike solutions, as well as real aerosol samples. It was shown that the different sample preparation and the AMS measurement at VERA and PSI/ ETHZ produced data of the same quality. The data sets including blank samples agree within their uncertainties (1r). These samples had isotopic ratios of 10 Be/9 Be between 1 · 1013 to 1 · 1012 . The 10 Be concentrations measured at PSI/ETHZ normalized to those measured at VERA ranged from 1.00 ± 0.09 to 1.09 ± 0.05. Over the time span of STACCATO, the AMS setup for 10 Be has changed significantly at Vienna [8–10]. Thus an additional intercomparison on the measurement procedure was performed by measuring a number of samples collected at JFJ and 10
Fig. 2. 10 Be/9 Be ratios data of samples collected at the Jungfraujoch. Results of an intercomparison exercise between the two AMS laboratories participating in STACCATO. The samples were prepared at PSI/ETHZ.
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prepared by PSI/ETHZ. Once more, the agreement of the two individual measurements was very good (Fig. 2).
3. Results and discussion During STACCATO, measurements of various atmospheric tracers and meteorological parameters such as O3 , relative humidity (RH) and 7 Be, were continued mainly at the three stations in the Alps. Gerasopoulos et al. [11] presented a climatology of the 7 Be data from these stations. Nevertheless, for the first time to our knowledge in Europe, 10 Be was monitored on a continuous basis at two stations (JFJ and ZS) for more than a year, whereas additional samples from SB were also analyzed for the study of specific case studies. Thus, a total of about 400 daily or bi-daily 10 Be measurements were finally available, constituting one of the most compact and long-term data set of 10 Be that is worldwide available. 7 Be and 10 Be were measured from March 2000 to February 2001 at JFJ and from January 2000 to December 2000 at ZS. The 7 Be and 10 Be concentrations as well as their isotopic ratio for both, ZS and JFJ in 2000 are shown in Fig. 3. At JFJ the 10 Be/7 Be ratios range from 1.04 to 3.69 (average of 2.08), while at ZS the ratios range from 0.14 to 6.11 (average of 1.82). The higher value at JFJ than at ZS is likely related to the different altitudes of the two stations, which possibly indicates a stronger influence of STT at JFJ than at ZS. Individual isotopes show increased variability related to their episodic nature while the ratio 10 Be/7 Be shows a seasonal cycle with a clear maximum in May and June at JFJ, which is less pronounced at ZS. Multiple-linear regression analysis revealed that 64% ðn ¼ 163Þ for ZS and 76% ðn ¼ 110Þ for JFJ of the 10 Be/7 Be ratio variability is explained by variability in both 7 Be and 10 Be. In contrast, the variability of the ratio that can be explained by variations of 7 Be only is 7.0% for ZS and 0.6% for JFJ, and that explained by variations of 10 Be only is 1.9% for ZS and 12.7% for JFJ. Thus, the ratio is almost independent from processes that have a clear effect on both radionuclides, such as wet scavenging.
Fig. 3. Surface concentrations of 7 Be, 10 Be, the 10 Be/7 Be ratio and their corresponding moving averages (weekly), as measured at Jungfraujoch (JFJ) and Zugspitze (ZS), respectively. 7 Be/m3 and 10 Be/m3 are normalized to standard air conditions.
At ZS, where the time resolution of 7 Be and Be measurements is higher (daily samples every second day), the above conclusion is strengthened further by the fact that the correlation coefficients of 7 Be and 10 Be with RH daily averages are )0.62 and )0.48, respectively, whereas the correlation between their ratio and RH is +0.32 indicating that the ratio is substantially relieved from the effect of wet scavenging. As RH can serve as a proxy for wet scavenging processes [11], at least 10
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such taking place close to the measurement site, this indicates that the ratio is virtually independent from wet scavenging. On the other hand, RH has been used as a tracer to isolate events of stratosphere-to-troposphere exchange (STE), since stratospheric and tropospheric air masses are distinguished by very different water vapor concentrations [11]. Thus the anti-correlation between 7 Be or 10 Be and RH can be alternatively explained as events of downward transport of dry upper tropospheric or stratospheric air. In this case one would expect that specific humidity (SH), which is a better tracer than RH, to be also a good predictor of 7 Be and 10 Be concentrations [11]. However, this is not the case as the correlation coefficients of 7 Be and 10 Be with SH daily averages at ZS are )0.14 and )0.07, respectively. This result also supports indirectly the previous result that the ratio is virtually independent from wet scavenging. In order to investigate to what extent 7 Be, 10 Be and 10 Be/7 Be trace intrusions of stratospheric air, geopotential height charts at 500 hPa were inspected for all the days with 10 Be/7 Be within the upper 10% quantile (10 Be/7 Be > 2.7) at ZS where daily values are available. The vast majority of the synoptic patterns revealed a deep upper trough extending southwards, with strong northerly advection affecting the Alpine stations. This synoptic situation is known to be often related to stratospheric intrusion events [12,13]. There were also a few occasions with cut-off low systems affecting central Europe, which are also synoptic patterns that are often linked to stratospheric intrusions [14,15]. Back trajectory analysis for all these selected days indicated downward transport from the upper troposphere or lower stratosphere, thus providing evidence for the use of the ratio as a stratospheric tracer and a confirmation that the ratio is mostly successful in identifying STT cases [1]. Nevertheless, when correlating the 10 Be/7 Be ratio with SH or ozone, which both are also expected to serve as an index for stratospheric intrusions, the interpretation of the correlation becomes a very complex task [1]. Hence, although the 10 Be/7 Be ratio can be successful in identifying certain STT cases it is a difficult parameter for an automated stratospheric intrusion detection algo-
rithm, because it is influenced also by other factors than STT such as production of 10 Be, 7 Be in the upper troposphere, accompanied by vertical mixing within the troposphere [1].
Acknowledgements The AMS measurements of aerosol filters from the Jungfraujoch, Sonnblick and Zugspitze were carried out within the STACCATO project (Contract No. EVK2-CT1999-00050) funded by the European Community under the Fifth Framework. This study has also been funded by the Bundesamt f€ ur Bildung und Wissenschaft (BBW) of Switzerland. The sampling and the delivery of the filters by the EMPA (Eidgen€ ossische Materialpr€ ufungs- und Forschungsanstalt, D€ ubendorf, CH, operator of the NABEL network) are highly appreciated. We thank MR E. Henrich from the Ministry of Agriculture and Forestry, Environment and Water Management who gave us the possibility to use aerosol filters from the Sonnblick observatory for AMS measurements. We appreciate the supporting work done by M. Mandl from the Zentralanstalt f€ ur Meteorologie und Geodynamik, Wetterdienststelle f€ ur Salzburg und Ober€ osterreich, and W. Ringer from the Federal Office of Agrobiology, Dep. of Radiation Protection, Linz.
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