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Beryllium 7 radionucleide as a tracer of vertical air mass transport in the troposphere
Advances in Space Research 36 (2005) 828–832 www.elsevier.com/locate/asr
Beryllium 7 radionucleide as a tracer of vertical air mass transport in the troposphere M. Yoshimori Rikkyo University, 3-34-1 Nishi-Ikebukuro, Toshima-ku, Tokyo 171–8501, Japan Received 29 September 2004; received in revised form 29 April 2005; accepted 29 April 2005
Abstract 7
Be is a cosmogenic radionucleide (half life 53.3 days) produced by galactic cosmic rays and solar energetic particles in the upper atmosphere and is a potentially useful tracer of the short-term atmospheric air mass motion. We have measured the surface 7Be concentration in Tokyo (35N, 139E) since 2002 with a high-volume air sampler. The present data of surface Be-7 concentrations exhibit the enhancements in spring and autumn and the seasonal enhancements are not associated with scavenging by precipitation. In order to explain the measured enhancements, we propose the possibility of the downward and upward atmospheric flows in the troposphere caused by a pair of traveling anticyclone and extratropical cyclone that passes over Japan in spring and autumn with a period of a few days. 2005 COSPAR. Published by Elsevier Ltd. All rights reserved. Keywords: Cosmogenic Beryllium 7; Temporal variations; Surface concentrations; Tracer of air mass transport
1. Introduction The 7Be production in the upper atmosphere has been studied by Lal and Peters (1967); Masarik and Beer (1999); Nagai et al. (2000) and Land and Feichter (2003). Their calculations indicated that about 70% of 7 Be is produced in the stratosphere and about 30% in the troposphere. A number of 7Be measurements were previously reported by Reiter and Munzert (1983); Dutkiewicz and Husain (1985); Dibb (1989); Feely et al. (1989); Todd et al. (1989); Baskaran et al. (1993); Dibb et al. (1994, 2003a,b); Megumi et al. (2000); Zanis et al. (2003); Sato et al. (2003). These measurements showed complex temporal variations in the surface 7Be concentration, depending on season, location and local meteorological conditions. Several simulations were carried out for explanation of the measured temporal variations based on the transport models (Viezee and Singh, 1980; E-mail address: [email protected].
Brost et al., 1991; Rehfeld and Heimann, 1995; Liu et al., 2001). A few processes were proposed: (1) The air mass mixing between the stratosphere and troposphere, (2) the vertical atmospheric motion in the troposphere, (3) the air mass transport from middle latitudes into high latitudes and (4) scavenging by precipitation. However, the temporal variations are not yet fully understood. The mean removal time of 7Be in the stratosphere was derived to be 1.4 years from the surface 137Cs experiment (Staley, 1982) and 1.93 years from three-dimensional atmospheric transport simulation (Rehfeld and Heimann, 1995), while the mean removal time in the troposphere is estimated to be 10–35 days (Shapiro and Forbes-Resha, 1976; Bleichrodt, 1978; Graustein and Turekian, 1986; Koch et al., 1996). The estimated removal time in the stratosphere is much longer than the half life of 7Be (53.3 days), suggesting that most stratospheric 7Be decay within the stratosphere and a large part of surface 7Be are of tropospheric origin.
0273-1177/$30 2005 COSPAR. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.asr.2005.04.088
M. Yoshimori / Advances in Space Research 36 (2005) 828–832
We started to measure the surface 7Be concentration in Tokyo (35N, 139E) in 2002 January to study a shorttime atmospheric air mass motion. Tokyo is an important location for study of the air mass motions in the upper atmosphere in the Asian Pacific region. The present data exhibit the enhancements of surface 7Be concentration in spring and autumn that are not associated to precipitation scavenging. We discuss the possibility of the enhancements in the context of meteorological conditions over Japan.
829
185.99 keV and 214Bi lines at 609.31, 1120.29 and 1764.49 keV; Thorium series: 212Pb line at 238.62 keV, 208 Tl lines at 510.72, 583.14 and 860.37 keV, and 212Bi line at 727.17 keV, and 40K line at 1460.8 keV). In addition, the positron annihilation line at 511.0 keV is apparent. Taking account the photo efficiency at 477.59 keV, the decay rate of 7Be and sampled air volume per 1 week, we determine the surface 7Be concentration in unit of mBq/m3 from a total count of 7Be line.
3. Result 2. Experiment We use a high-volume air sampler (Shibata Scientific Technology Inc. HV-1000F) which is of all-weather type with air sampling rate of 1 m3 per minute. 7Be-borne aerosol particles were collected for one week on a glass fiber filter (203 · 254 mm in area) which has a nominal 0.3 lm cutoff for the sample aerodynamic diameter. A gamma-ray spectrum of the air sample is measured with a germanium gamma-ray spectrometer. The spectrometer consists of a high-purity coaxial germanium detector surrounded with a 10-cm-thick lead shield which enables to reduce the background radiation level. The effective volume of the detector is 90 cm3 and the photopeak efficiency is 5% at 477.59 keV. The energy resolution (FWHM) is 0.86 at 477.59 keV. After air sampling for one week, the gamma-ray spectral data of the collected radionucleides are taken with a computer-based multichannel analyzer (8192 channel MCA, Canberra Inspector 2000). A typical gamma-ray spectrum is shown in Fig. 1. We see a strong cosmogenic 7 Be line at 477.59 keV and numerous gamma-ray lines of terrestrial radionucleides (Uranium series: 210Pb line at 46.50 keV, 214Pb line at 351.92 keV, 226Ra line at
We plot the yearly 7Be concentrations in surface air in 2002 (January–December), 2003 (January–December) and 2004 (January–August) in Fig. 2. The yearly variations are similar in 2002–2004. The surface concentrations are high in spring and autumn (6–8 mBq/cm3). Since the enhancements were not associated with the solar activity (solar flare and coronal mass ejection) and galactic cosmic ray flux, the variations are caused by
Fig. 2. Temporal variations in the surface 7Be concentration.
Fig. 1. Typical gamma-ray spectrum of the sample.
830
M. Yoshimori / Advances in Space Research 36 (2005) 828–832
seasonal atmospheric transport. The present Be-7 concentration values are roughly consistent with those reported by Dibb (1989) (3.3–5.0 mBq/m3 in Maryland, USA), Dibb et al. (1994) (1.5–4.5 mBq/m3 in Northwest Territories, Canada) and Megumi et al. (2000) (3– 9 mBq/m3 in Osaka, Japan). Olsen et al. (1985) indicated that the surface 7Be concentrations vary with season, location and local meteorological conditions and the time scale of variations ranges from within a single precipitation event to seasonal change. Seasonal variations in the surface 7Be concentration were reported by Feely et al. (1989). In particular, the spring maximum was reported by Dibb (1989); Baskaran et al. (1993) and Megumi et al. (2000). However, the enhancements in autumn were not reported.
4. Discussion 7
Be on the surface consist of the stratospheric and tropospheric components. The 7Be concentration depends on the production rate, mean removal times in the stratosphere and troposphere and decay rate. The 7 Be production rate in the stratosphere is 2.4 times as large as that in the troposphere (Lal and Peters, 1967), but the removal time of 7Be in the stratosphere is 1–2 years that is much longer than the half life of 7Be (53.3 days), indicating that most of the stratospheric 7 Be do not reach the EarthÕs surface. If the surface 7Be is of stratospheric origin, a frequent air mass flow to the troposphere is needed. On the other hand, the majority of 7Be produced in the troposphere fall on the surface without the decay because the mean removal time is short (10–35 days). In order to explain the seasonal enhancements of surface 7Be concentration, Turekian et al. (1983), Feely et al. (1989) and Koch et al. (1996) proposed the vertically downward transport within the troposphere, though Viezee and Singh (1980) studied the air mass mixing or exchange between the stratosphere and troposphere. Further, measurements of the 10Be/7Be ratio have been carried out for study of the stratosphere-to-troposphere exchange (Jordan et al., 2003; Land and Feichter, 2003). First, we study the influence of scavenging by precipitation because it is a main process bringing 7Be to the EarthÕs surface (Matsunami et al., 1979; Olsen et al., 1985; Dibb, 1989). Temporal variations in the monthly average 7Be concentration and the monthly precipitation in January 2002–August 2004 are shown in Fig. 3. We do not find a clear relation between these two. Next, we look for another possibility of vertical air mass transport in the troposphere in spring and autumn. It has been well known that a pair of a traveling anticyclone and extratropical cyclone usually passes over Japan in spring and autumn. A period of the passing is a few days. The traveling anticyclone is formed between
Fig. 3. Temporal variations in the monthly average 7Be concentration (upper figure) and monthly precipitation (lower figure).
the ridge and trough in the Rossby waves (large-scale meanders of the jet stream) at 300 mb, while the extratropical cyclone is formed between the trough and ridge. The relationship between surface pressure, airflow, front systems and the locations of troughs and ridges in the Rossby waves are schematically shown in Fig. 4. The cold air flows downward from convergence in the Rossby waves to divergence in the surface anticyclone and the warm air flows upward from convergence in the surface cyclone to divergence in the Rossby waves, as shown in Fig. 4. Such vertical air flow in the troposphere is thought to cause the enhancements of the surface 7Be concentration in spring and autumn (Handbook on meteorology, 1995).
Fig. 4. Schematic representation of traveling anticyclone and extratropical cyclone in spring and autumn. H, anticyclone; L, cyclone; C, convergence; D, divergence.
M. Yoshimori / Advances in Space Research 36 (2005) 828–832
Finally, we compare the temporal variations in the surface concentrations of 210Pb with those of 7Be to discuss the seasonal air motion in the troposphere. 210Pb (half life 22.3 years) is the decay daughter of 222Rn (half life 3.8 days) emanated from the EarthÕs crust into the atmosphere and emits a gamma-ray line at 46.50 keV. The temporal variations in the count rate in 2002– 2004 are shown in Fig. 5. The temporal variations in surface concentration of 210Pb is similar to those of 7Be. In general, terrestrial radionucleides are thought to distribute in lower altitudes and their surface concentrations do not vary with season. As an example, our data of terrestrial 226Ra gamma-ray line at 185.99 keV are shown in Fig. 6. The count rate is almost constant in June 2002–August 2004. Further, another terrestrial radionucleide 40K (1461 keV) also indicates nearly constant gamma-ray count rate. Why does the 210Pb exhibit the different temporal variations from the other terrestrial radionucleides? Here, we consider one possibility to explain the seasonal varying 210Pb concentration in the surface atmosphere. There seems to be a difference in the altitude distributions between the 210Pb and other terrestrial radionucleides. Since 226Ra and 40K are metallic, they are quickly attached to atmospheric aerosols, suggest-
Fig. 5. Temporal variations in count rate of 46.5 keV in 2002–2004.
210
Fig. 6. Time profile of count rate of (186.99 keV) in June 2002–August 2004.
226
Pb gamma-ray line at
Ra gamma-ray line
831
ing that these radionucleides distribute at lower altitudes. On the other hand, 222Rn is a noble gas and not attached to an aerosol particle. Further, its average surface concentration is significantly high in Japan (5.9 Bq/m3) moves to higher altitudes and decays to 210 Pb there. Kownacka (2002) exhibited that the 210 Pb concentration was nearly constant (about onethird of the surface concentration) above 1 km and not decreased with altitude. The 210Pb at higher altitudes are efficiently transported to the surface by the vertical atmospheric motion that is strong in spring and autumn, resulting in the enhancements of surface concentration of 210Pb.
Acknowledgements This work has been supported by a Grant-in-Aid for Scientific Research (No. 16654043) of the Ministry of Education, Culture, Sports, Science and Technology, Japan.
References Baskaran, M., Coleman, C.H., Santschi, P.H. Atmospheric deposition fluxes of Be-7 and Pb-210 at Galveston and College station, Texas. J. Geophys. Res. 98, 20555–20571, 1993. Bleichrodt, J.F. Mean tropospheric residence time of cosmic-rayproduced Be-7 at north temperate latitudes. J. Geophys. Res. 83, 3058–3062, 1978. Brost, R.A., Feichter, J., Heimann, M. Three-dimensional simulation of Be-7 in a global climate model. J. Geophys. Res. 96, 22423– 22445, 1991. Dibb, J.E. Atmospheric deposition of Be-7 in the Chesapeake Bay region. J. Geophys. Res. 94, 2261–2265, 1989. Dibb, J.E. et al. Estimation of stratospheric input to the Arctic troposphere: 7Be and 10Be in aerosols at Alert, Canada. J. Geophys. Res. 99, 12855–12864, 1994. Dibb, J.E. et al. Stratospheric influence on the northern North American free troposphere during TOPSE: Be-7 as a stratospheric tracer. J. Geophys. Res. 108, 8363–8370, 2003a. Dibb, J.E. et al. Stratospheric influence on the northern North American free troposphere during TOPSE: 7Be as a stratospheric tracer. J. Geophys. Res. 108, 8763–8770, 2003b. Dutkiewicz, V.A., Husain, L. Stratospheric and tropospheric 7Be in surface air. J. Geophys. Res. 90, 5783–5788, 1985. Feely, H.W., Larsen, R.J., Sanderson, C.G. Factors that cause seasonal variations in Be-7 concentrations in surface air. J. Environ. Radioactiv. 9, 223–249, 1989. Graustein, W.C., Turekian, K.K. 210Pb and 137Cs in air and soils measure the rate and vertical profile of aesosol scavenging. J. Geophys. Res. 91, 14355–14366, 1986. Asakura, T. et al. (Eds.). Handbook on Meteorology. Asakura Pub. Co., Tokyo, Japan, 1995 (in Japanese). Jordan, C.E., Dibb, J.E., Finkel, R.C. 10Be/7Be tracer of atmospheric transport and stratosphere–troposphere exchange. J. Geophys. Res. 108, 4234–4247, 2003. Koch, D.M., Jacob, D.J., Graustein, W.C. Vertical transport of tropospheric aerosols as indicated by 7Be and 210Pb in a chemical tracer model. J. Geophys. Res. 101, 18,651–18,666, 1996.
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Kownacka, L. Vertical distributions of beryllium-7 and lead-210 in the tropospheric and lower stratospheric air. Nukleonika 47, 79–82, 2002. Lal, D., Peters, B. Cosmic ray produced radioactivity on the Earth. Handb. Phys. 46, 551–612, 1967. Land, C., Feichter, J. Stratosphere–troposphere exchange in a changing climate simulated with the general circulation model MAECHAM4. J. Geophys. Res. 108, 8523–8532, 2003. Liu, H. et al. J. Geophys. Res. 106, 12109–12128, 2001. Masarik, J., Beer, J. Simulation of particle fluxes and cosmogenic nucleid production in EarthÕs atmosphere. J. Geophys. Res. 104, 12099–12111, 1999. Matsunami, T., Mizohara, A., Mamuro, T. Observations of depositions and atmospheric concentrations of 210Pb, 7Be and 137Cs in Osaka, Annual Report of Radiation Center. Osaka Prefecture 20, 1–4, 1979. Megumi, K. et al. Factors, especially sunspot number, causing variations in surface air concentrations and depositions of 7Be in Osaka, Japan. Geophys. Res. Lett. 27, 361–364, 2000. Nagai, H., Tada, W., Kobayashi, T. Production rates of 7Be and 10Be in the atmosphere. Nucl. Instr. Meth. B172, 796–801, 2000. Olsen, C.R. et al. Atmospheric fluxes and marsh-soil inventories of 7 Be and 210Pb. J. Geophys. Res. 90, 10487–10495, 1985. Rehfeld, S., Heimann, M. Three dimensional atmospheric transport simulation of the radioactive tracers, 210Pb, 7Be, 10Be and 90Sr. J. Geophys. Res. 100, 26141–26161, 1995.
Reiter, R., Munzert, K. Cosmogenic radionucleides at a mountain station under fallout background conditions with consideration of the stratospheric residence time. Arch. Met. Biocl. B33, 187–197, 1983. Sato, S. et al. Atmospheric concentration of 210Pb and 7Be at Sasafune, Hokkaido, Japan. J. Radioanal. Nucl. Chem. 255, 351– 353, 2003. Shapiro, M.H., Forbes-Resha, J.L. Mean residence time of 7Bebearing aerosols in the troposphere. J. Geophys Res. 81, 2647– 2649, 1976. Staley, D.O. Strontium-90 in surface air and the stratosphere: some interpretations of the 1963–75 data. J. Atmos. Sci. 39, 1571–1790, 1982. Todd, J.F. et al. Atmospheric depositional characteristics of beryllium 7 and lead 210 along the southeastern Virginia coast. J. Geophys. Res. 95, 11106–11116, 1989. Turekian, K.K., Benninger, L.K., Dion, E.P. 7Be and 210Pb total deposition fluxes at New Heaven, Connecticut and at Bermuda. J. Geophys. Res. 88, 5411–5415, 1983. Viezee, W., Singh, H.B. The distribution of Be-7 in the troposphere: implications on stratospheric/tropospheric air exchange. Geophys. Res. Lett. 7, 805–808, 1980. Zanis, P. et al. An estimate of the impact of stratosphere-totroposphere transport on the lower free tropospheric ozone over the Alps using Be-10 and Be-7 measurements. J. Geophys. Res. 108, 8520–8528, 2003.
Beryllium 7 radionucleide as a tracer of vertical air mass transport in the troposphere M. Yoshimori Rikkyo University, 3-34-1 Nishi-Ikebukuro, Toshima-ku, Tokyo 171–8501, Japan Received 29 September 2004; received in revised form 29 April 2005; accepted 29 April 2005
Abstract 7
Be is a cosmogenic radionucleide (half life 53.3 days) produced by galactic cosmic rays and solar energetic particles in the upper atmosphere and is a potentially useful tracer of the short-term atmospheric air mass motion. We have measured the surface 7Be concentration in Tokyo (35N, 139E) since 2002 with a high-volume air sampler. The present data of surface Be-7 concentrations exhibit the enhancements in spring and autumn and the seasonal enhancements are not associated with scavenging by precipitation. In order to explain the measured enhancements, we propose the possibility of the downward and upward atmospheric flows in the troposphere caused by a pair of traveling anticyclone and extratropical cyclone that passes over Japan in spring and autumn with a period of a few days. 2005 COSPAR. Published by Elsevier Ltd. All rights reserved. Keywords: Cosmogenic Beryllium 7; Temporal variations; Surface concentrations; Tracer of air mass transport
1. Introduction The 7Be production in the upper atmosphere has been studied by Lal and Peters (1967); Masarik and Beer (1999); Nagai et al. (2000) and Land and Feichter (2003). Their calculations indicated that about 70% of 7 Be is produced in the stratosphere and about 30% in the troposphere. A number of 7Be measurements were previously reported by Reiter and Munzert (1983); Dutkiewicz and Husain (1985); Dibb (1989); Feely et al. (1989); Todd et al. (1989); Baskaran et al. (1993); Dibb et al. (1994, 2003a,b); Megumi et al. (2000); Zanis et al. (2003); Sato et al. (2003). These measurements showed complex temporal variations in the surface 7Be concentration, depending on season, location and local meteorological conditions. Several simulations were carried out for explanation of the measured temporal variations based on the transport models (Viezee and Singh, 1980; E-mail address: [email protected].
Brost et al., 1991; Rehfeld and Heimann, 1995; Liu et al., 2001). A few processes were proposed: (1) The air mass mixing between the stratosphere and troposphere, (2) the vertical atmospheric motion in the troposphere, (3) the air mass transport from middle latitudes into high latitudes and (4) scavenging by precipitation. However, the temporal variations are not yet fully understood. The mean removal time of 7Be in the stratosphere was derived to be 1.4 years from the surface 137Cs experiment (Staley, 1982) and 1.93 years from three-dimensional atmospheric transport simulation (Rehfeld and Heimann, 1995), while the mean removal time in the troposphere is estimated to be 10–35 days (Shapiro and Forbes-Resha, 1976; Bleichrodt, 1978; Graustein and Turekian, 1986; Koch et al., 1996). The estimated removal time in the stratosphere is much longer than the half life of 7Be (53.3 days), suggesting that most stratospheric 7Be decay within the stratosphere and a large part of surface 7Be are of tropospheric origin.
0273-1177/$30 2005 COSPAR. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.asr.2005.04.088
M. Yoshimori / Advances in Space Research 36 (2005) 828–832
We started to measure the surface 7Be concentration in Tokyo (35N, 139E) in 2002 January to study a shorttime atmospheric air mass motion. Tokyo is an important location for study of the air mass motions in the upper atmosphere in the Asian Pacific region. The present data exhibit the enhancements of surface 7Be concentration in spring and autumn that are not associated to precipitation scavenging. We discuss the possibility of the enhancements in the context of meteorological conditions over Japan.
829
185.99 keV and 214Bi lines at 609.31, 1120.29 and 1764.49 keV; Thorium series: 212Pb line at 238.62 keV, 208 Tl lines at 510.72, 583.14 and 860.37 keV, and 212Bi line at 727.17 keV, and 40K line at 1460.8 keV). In addition, the positron annihilation line at 511.0 keV is apparent. Taking account the photo efficiency at 477.59 keV, the decay rate of 7Be and sampled air volume per 1 week, we determine the surface 7Be concentration in unit of mBq/m3 from a total count of 7Be line.
3. Result 2. Experiment We use a high-volume air sampler (Shibata Scientific Technology Inc. HV-1000F) which is of all-weather type with air sampling rate of 1 m3 per minute. 7Be-borne aerosol particles were collected for one week on a glass fiber filter (203 · 254 mm in area) which has a nominal 0.3 lm cutoff for the sample aerodynamic diameter. A gamma-ray spectrum of the air sample is measured with a germanium gamma-ray spectrometer. The spectrometer consists of a high-purity coaxial germanium detector surrounded with a 10-cm-thick lead shield which enables to reduce the background radiation level. The effective volume of the detector is 90 cm3 and the photopeak efficiency is 5% at 477.59 keV. The energy resolution (FWHM) is 0.86 at 477.59 keV. After air sampling for one week, the gamma-ray spectral data of the collected radionucleides are taken with a computer-based multichannel analyzer (8192 channel MCA, Canberra Inspector 2000). A typical gamma-ray spectrum is shown in Fig. 1. We see a strong cosmogenic 7 Be line at 477.59 keV and numerous gamma-ray lines of terrestrial radionucleides (Uranium series: 210Pb line at 46.50 keV, 214Pb line at 351.92 keV, 226Ra line at
We plot the yearly 7Be concentrations in surface air in 2002 (January–December), 2003 (January–December) and 2004 (January–August) in Fig. 2. The yearly variations are similar in 2002–2004. The surface concentrations are high in spring and autumn (6–8 mBq/cm3). Since the enhancements were not associated with the solar activity (solar flare and coronal mass ejection) and galactic cosmic ray flux, the variations are caused by
Fig. 2. Temporal variations in the surface 7Be concentration.
Fig. 1. Typical gamma-ray spectrum of the sample.
830
M. Yoshimori / Advances in Space Research 36 (2005) 828–832
seasonal atmospheric transport. The present Be-7 concentration values are roughly consistent with those reported by Dibb (1989) (3.3–5.0 mBq/m3 in Maryland, USA), Dibb et al. (1994) (1.5–4.5 mBq/m3 in Northwest Territories, Canada) and Megumi et al. (2000) (3– 9 mBq/m3 in Osaka, Japan). Olsen et al. (1985) indicated that the surface 7Be concentrations vary with season, location and local meteorological conditions and the time scale of variations ranges from within a single precipitation event to seasonal change. Seasonal variations in the surface 7Be concentration were reported by Feely et al. (1989). In particular, the spring maximum was reported by Dibb (1989); Baskaran et al. (1993) and Megumi et al. (2000). However, the enhancements in autumn were not reported.
4. Discussion 7
Be on the surface consist of the stratospheric and tropospheric components. The 7Be concentration depends on the production rate, mean removal times in the stratosphere and troposphere and decay rate. The 7 Be production rate in the stratosphere is 2.4 times as large as that in the troposphere (Lal and Peters, 1967), but the removal time of 7Be in the stratosphere is 1–2 years that is much longer than the half life of 7Be (53.3 days), indicating that most of the stratospheric 7 Be do not reach the EarthÕs surface. If the surface 7Be is of stratospheric origin, a frequent air mass flow to the troposphere is needed. On the other hand, the majority of 7Be produced in the troposphere fall on the surface without the decay because the mean removal time is short (10–35 days). In order to explain the seasonal enhancements of surface 7Be concentration, Turekian et al. (1983), Feely et al. (1989) and Koch et al. (1996) proposed the vertically downward transport within the troposphere, though Viezee and Singh (1980) studied the air mass mixing or exchange between the stratosphere and troposphere. Further, measurements of the 10Be/7Be ratio have been carried out for study of the stratosphere-to-troposphere exchange (Jordan et al., 2003; Land and Feichter, 2003). First, we study the influence of scavenging by precipitation because it is a main process bringing 7Be to the EarthÕs surface (Matsunami et al., 1979; Olsen et al., 1985; Dibb, 1989). Temporal variations in the monthly average 7Be concentration and the monthly precipitation in January 2002–August 2004 are shown in Fig. 3. We do not find a clear relation between these two. Next, we look for another possibility of vertical air mass transport in the troposphere in spring and autumn. It has been well known that a pair of a traveling anticyclone and extratropical cyclone usually passes over Japan in spring and autumn. A period of the passing is a few days. The traveling anticyclone is formed between
Fig. 3. Temporal variations in the monthly average 7Be concentration (upper figure) and monthly precipitation (lower figure).
the ridge and trough in the Rossby waves (large-scale meanders of the jet stream) at 300 mb, while the extratropical cyclone is formed between the trough and ridge. The relationship between surface pressure, airflow, front systems and the locations of troughs and ridges in the Rossby waves are schematically shown in Fig. 4. The cold air flows downward from convergence in the Rossby waves to divergence in the surface anticyclone and the warm air flows upward from convergence in the surface cyclone to divergence in the Rossby waves, as shown in Fig. 4. Such vertical air flow in the troposphere is thought to cause the enhancements of the surface 7Be concentration in spring and autumn (Handbook on meteorology, 1995).
Fig. 4. Schematic representation of traveling anticyclone and extratropical cyclone in spring and autumn. H, anticyclone; L, cyclone; C, convergence; D, divergence.
M. Yoshimori / Advances in Space Research 36 (2005) 828–832
Finally, we compare the temporal variations in the surface concentrations of 210Pb with those of 7Be to discuss the seasonal air motion in the troposphere. 210Pb (half life 22.3 years) is the decay daughter of 222Rn (half life 3.8 days) emanated from the EarthÕs crust into the atmosphere and emits a gamma-ray line at 46.50 keV. The temporal variations in the count rate in 2002– 2004 are shown in Fig. 5. The temporal variations in surface concentration of 210Pb is similar to those of 7Be. In general, terrestrial radionucleides are thought to distribute in lower altitudes and their surface concentrations do not vary with season. As an example, our data of terrestrial 226Ra gamma-ray line at 185.99 keV are shown in Fig. 6. The count rate is almost constant in June 2002–August 2004. Further, another terrestrial radionucleide 40K (1461 keV) also indicates nearly constant gamma-ray count rate. Why does the 210Pb exhibit the different temporal variations from the other terrestrial radionucleides? Here, we consider one possibility to explain the seasonal varying 210Pb concentration in the surface atmosphere. There seems to be a difference in the altitude distributions between the 210Pb and other terrestrial radionucleides. Since 226Ra and 40K are metallic, they are quickly attached to atmospheric aerosols, suggest-
Fig. 5. Temporal variations in count rate of 46.5 keV in 2002–2004.
210
Fig. 6. Time profile of count rate of (186.99 keV) in June 2002–August 2004.
226
Pb gamma-ray line at
Ra gamma-ray line
831
ing that these radionucleides distribute at lower altitudes. On the other hand, 222Rn is a noble gas and not attached to an aerosol particle. Further, its average surface concentration is significantly high in Japan (5.9 Bq/m3) moves to higher altitudes and decays to 210 Pb there. Kownacka (2002) exhibited that the 210 Pb concentration was nearly constant (about onethird of the surface concentration) above 1 km and not decreased with altitude. The 210Pb at higher altitudes are efficiently transported to the surface by the vertical atmospheric motion that is strong in spring and autumn, resulting in the enhancements of surface concentration of 210Pb.
Acknowledgements This work has been supported by a Grant-in-Aid for Scientific Research (No. 16654043) of the Ministry of Education, Culture, Sports, Science and Technology, Japan.
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