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Atmospheric depositional fluxes of cosmogenic 35S and 7Be: Implications for the turnover rate of sulfur through the biosphere
Atmospheric Environment 45 (2011) 4230e4234
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Atmospheric Environment journal homepage: www.elsevier.com/locate/atmosenv
Atmospheric depositional fluxes of cosmogenic 35S and 7Be: Implications for the turnover rate of sulfur through the biosphere Hyung-Mi Cho, Young-Lim Hong, Guebuem Kim* School of Earth and Environmental Sciences/RIO, Seoul National University, Seoul 151-747, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history: Received 28 October 2010 Received in revised form 15 March 2011 Accepted 3 May 2011
The wet depositional fluxes of cosmogenic 35S and 7Be, together with 210Pb, were measured in Seoul, Korea, from April 2004 to April 2005. Approximately half of the annual fluxes occurred in summer, during which about 60% of the precipitations occurred. Our simple box model shows that the theoretical scavenging ratio of 35S/7Be to the ground is approximately 0.013, which accounts for the decay for the duration of oxidation and settling. However, we observed w50% higher 35S/7Be activity ratios than the theoretical removal ratio over the entire sampling period, with particularly higher (w133%) ratios during November 2004eApril 2005. These higher ratios in the winter cannot be explained by sudden incursions of the stratospheric air or longer aerosol residence times, on the basis of 7Be/210Pb ratios. We hypothesize that the ratios could occur owing to biomass burning or as plants go dormant and dry during the autumn and winter. Based on this hypothesis, we developed a 35S/7Be mass balance model which yields the turnover rate of sulfur in the atmosphere through the biosphere to be 0.015 0.007 d1. Such a rapid sulfur turnover rate should be applied to the prediction model of sulfur inventory changes. Ó 2011 Elsevier Ltd. All rights reserved.
Keywords: Sulfur cycle 35 S 7 Be 210 Pb Precipitation Atmosphere
1. Introduction 35
S (t1/2 ¼ 87 d), the radioactive isotope of sulfur, is produced by cosmic-ray spallation of argon in the stratosphere and upper troposphere (Tanaka and Turekian, 1991; Turekian and Tanaka, 1992). Soon after 35S is produced, it is oxidized to sulfate (35SO4) and attached to the ambient aerosols. 7Be (t1/2 ¼ 53 d) is similar in its origin to 35S, produced by cosmic-ray spallation of oxygen and nitrogen, and adsorbed onto particles immediately (Lal et al., 1958; Tanaka and Turekian, 1995). The stratospheric 35S and 7Be transferred to the troposphere and those produced in the troposphere then settle to the ground by either wet or dry deposition (Turekian and Tanaka, 1992; Osaki et al., 1999). Thus, these two isotopes, 35S and 7Be, form a pair which provides a powerful tool for determining the oxidation rates of SO2 in the atmosphere and the removal rates of aerosols (Turekian and Tanaka, 1992; Tanaka and Turekian, 1995; Osaki et al., 1999). Once settled on the earth’s surface, the pair is a useful tracer for determining groundwater age (Sueker et al., 1999; Plummer et al., 2001), new water input to lakes or runoff (Michel et al., 2000; Shanley et al., 2005), and the hydrology of watersheds (Cooper et al., 1991; Michel et al., 2002) over a time
* Corresponding author. Tel.: þ82 2 880 7508; fax: þ82 2 876 6508. E-mail address: [email protected] (G. Kim). 1352-2310/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.atmosenv.2011.05.002
scale of less than one year. In these aquatic applications of 35S, the fallout flux of 35S should be known. Thus, in this study, our aim was to determine (1) the annual flux of 35S and (2) the cycling of sulfur through the biosphere using the observed 35S/7Be ratios. We hypothesized that the atmospheric depositional fluxes of 35S can be determined using 7Be if a general activity ratio of 35S/7Be is obtained at mid-latitudes in the Northern Hemisphere. We used the pair of 35S and 7Be to determine the turnover rate of 35S, since 7Be is permanently removed from the atmosphere (except for soil resuspension) while 35S can re-enter the troposphere by forming gaseous species. 2. Materials and methods Precipitation samples were collected in a plastic bucket for the analyses of 35S, 7Be, and 210Pb for every precipitation event during one year (April 2004eApril 2005) in Seoul, Korea. The sampling bucket (diameter 26 cm) was cleaned with 5% neutral detergent solution (Merck ExtranÒ MA 03), then rinsed with HNO3 and deionized water. The samples were moved to the laboratory immediately after collection, then acidified to pH 3e4 using 6 M HCl. The analytical procedures of 35S were documented separately by Hong and Kim (2005). Briefly, 0.1 g of a stable sulfur carrier was added to the sample. Then, the sample solution was passed through an anion exchange resin (Amberlite IRA 400) column (15e50 mesh)
H.-M. Cho et al. / Atmospheric Environment 45 (2011) 4230e4234
using a peristaltic pump at a flow rate of 70 mL min1. The sulfur in the column was eluted with 300 mL of 3 M NaCl. The entire sulfur in the eluted solution from the resin column was precipitated as BaSO4 by adjusting the pH to 3e4 following the addition of excess barium solution. The precipitates were collected onto a preweighed GF/B filter. After transferring the filter with BaSO4 into a 20-mL plastic vial, 10 mL of distilled water and 10 mL of Ultima gold LLT cocktail were added. The material in this vial was homogenized in an ultrasonic bath. Then, the amount of 35S in the precipitates was directly counted using a super-low-background LSC (Packard Tricarb 3170 TR/SL). The purity of 35S after the source preparation was checked by counting the samples several times (Hong and Kim,
4231
2005). The counting efficiency (cpm/dpm ratio) of the LSC was determined using NIST traceable 35S standard for the same BaSO4 amount, filter, and cocktail. The samples for the analyses of 7Be and 210Pb were dried and transferred to 100 mL bottles, and then adjusted to a volume of 80 mL. The activities of 7Be and 210Pb were determined using a coaxial type (Canberra) detector. The counting efficiency (cpm/dpm ratios) for 210Pb (at 46.5 keV) was determined using the same volume of NIST traceable 210Pb standard. Since we did not have a 7Be standard, corrections of 7Be (at 477 keV) for geometry, self absorption factors, and counting efficiency were made by interpolating 214Pb (352 keV) and 214Bi (609 keV) efficiencies using the 226Ra standard. 3. Results and discussion The precipitation was highest during the summer (Fig. 1). The activities of the radioisotopes in the precipitation were in the range
Fig. 1. The activities of 2004 to April 2005.
35
S, 7Be, and
210
Pb in precipitation in Seoul, Korea, from April
Fig. 2. Precipitation amounts versus the activity of in Seoul, Korea, from April 2004 to April 2005.
35
S, 7Be, and
210
Pb in precipitation
4232
H.-M. Cho et al. / Atmospheric Environment 45 (2011) 4230e4234
of 4e400 Bq m3 for 35S, 210e19,000 Bq m3 for 7Be, and 25e540 Bq m3 for 210Pb. Approximately half of the annual fluxes occurred in the summer due to the high precipitation. The specific activities of 35S, 7Be and 210Pb decreased exponentially as the precipitation amount increased (Fig. 2), owing to the typical diluting effect of the precipitation. In general, the specific activities of 35S, 7Be and 210Pb are higher for smaller precipitations since raindrops with smaller sizes have longer residence times and larger surface areas (Su and Huh, 2006). The activity ratio of 35S/7Be was similar for different regions in the mid-latitudes of the Northern Hemisphere within 17% (Table 1), except for New Haven, Connecticut (Tanaka and Turekian, 1995) where the detection efficiency was determined using 14C. Thus, an estimate of the fallout flux of 35S can be derived within this uncertainty for various applications in aqueous and groundwater studies in the Northern Hemisphere mid-latitudes using this ratio and the regional 7Be flux. In order to obtain the theoretical depositional flux of 35S and 7 Be, we constructed a simple atmospheric mass balance model for two (the stratospheric and tropospheric) boxes (Fig. 3). The production ratio of 35S/7Be in the stratosphere is w0.0103 (Lal and Peters, 1967). Since the residence time of stratospheric aerosols is about 1.0 0.5 yr on the basis of data from bomb-produced 90Sr (Kuroda et al., 1962; Taylor, 1968), the expected flux ratio of 35S and 7 Be activities from the stratosphere to the troposphere is 0.063 0.044, resulting from radioactive decay of 35S and 7Be during the aerosol residence time and oxidation time of 35SO2 to 35 SO4 (w7 days). The production ratio of 35S and 7Be activities in the upper troposphere is known to be w0.0111 (Lal and Peters, 1967). Thus, if we assume that the aerosol residence time in the troposphere for these isotopes transferred from the stratosphere and produced in the troposphere is about 30 5 days on the basis of 7Be results (Shapiro and Forbes-Resha, 1976; Raisbeck et al., 1981; Su and Huh, 2006), the fallout ratio of 35S/7Be activities to the ground would be 0.013 0.003. In this calculation, the same oxidation time of tropospheric 35SO2 is applied. However, the observed 35S/7Be activity ratios range from 0.012 to 0.037 (0.0202 0.0004, mean value) which are generally higher than those of the theoretical fallout ratio (Fig. 4). The ratios for the three exceptionally high periods from November to April were approximately 56% higher than those during the other periods. Although sudden incursions of the stratospheric air or resuspension of soils could result in such high ratios, such events seem unlikely to be the cause since the 7Be/210Pb activity ratios were not
Table 1 Comparison of atmospheric depositional ratios of Location and time of collection Bombay (18 N) 1970 New Haven, Connecticut (41 N) Mar. 1977eFeb. 1978 Dec. 1991eNov. 1992 Stillpond, Maryland (39 N) Oct. 1995eSep. 1996 College Station, Texas (30 N) 1990 1991 Galveston, Texas (30 N) 1990 1991 Fukuoka, Japan (33.4 N) Mar. 1992eFeb. 1993 Seoul (37 N) Apr. 2004eApr. 2005 St. Petersburg (27 N) Jul. 2003eJun. 2004
35
S, 7Be, and
Fig. 3. Box model for cosmogenic 35S and 7Be in the atmosphere. Ps, Pt, Inc, and Tc denote the activity ratios of 35S/7Be for the stratospheric production, the tropospheric production, the stratospheric fallout, and the theoretical scavenging ratios of 35S/7Be from the atmosphere. The values for the input flux and production of 35S in the troposphere corrected for the decay during the oxidation times of 35SO2.
unusually higher (tracing stratospheric source inputs) or lower (tracing surface soil inputs) (Fig. 4). Alternatively, this unusually high 35S/7Be ratio could be due to the turnover of 35S independently from 7Be through the biosphere in its gaseous form. Although this hypothesis has yet to be confirmed, it is supported by the much higher 35S/7Be ratios (Tanaka and Turekian, 1995) in the groundlevel aerosol samples compared with those in wet deposition samples from all altitude levels ranging from the cloud level to the ground. By assuming that there is a turnover process for 35S between the atmosphere and the land-surface, we can calculate the turnover rate of 35S, as a proxy for anthropogenic sulfur in the atmosphere, using the following mass balance models. At steady state,
210
Pb in mid-latitudes of the Northern Hemisphere.
Rainfall (cm)
35
S/7Be
7
Be/210Pb
Source
263
0.0284 (0.004)
5.06
Lal et al. (1979)
148 e
e 0.0134
19.8 e
Turekian et al. (1983) Tanaka and Turekian (1995)
127
e
18.4
Kim et al. (2000)
98 146
e e
16.3 11.3
Baskaran et al. (1993)
97 150
e e
17.8 13.6
Baskaran et al. (1993)
e
0.0238
e
Osaki et al. (1999)
128
0.0202 (0.0003)
16.5
This study
115
e
15.5
Baskaran and Swarzenski (2007)
H.-M. Cho et al. / Atmospheric Environment 45 (2011) 4230e4234
4233
reciprocal of the mean residence time of 7Be in the troposphere. Thus, the turnover rate of sulfur is approximately 0.015 0.007 d1, with exceptionally higher rates (0.025 0.006 d1) in winter. This implies that atmospheric 35S removed by precipitation is turned over and re-enters to the troposphere within 66 28 days, with more rapid turnover in winter (41 10 days). Similarly, higher atmospheric deposition of 35S was observed in winter precipitation relative to the summer precipitation in Tatsunokuchi, Japan (Yamamoto et al., 2006), which supports our hypothesis. Garten (1988) showed that there are more losses of artificial 35S from the trees, due to the shedding of leaves during OctobereNovember, than in summer. In contrast, soil studies (Novák et al., 2004; Novák and Precová, 1995) showed that sulfur loss from the soil to the atmosphere was insignificant in winter but significant in summer in association with bacterial sulfate reduction. These contradictory results suggest that the turnover flux of high 35S activity, fresh air-borne sulfur through soils is insignificant, and such fresh sulfur re-enters the troposphere predominantly during late autumn and winter owing to biomass burning or as plants go dormant and dry over a global scale. Our result implies that sulfur in the atmosphere undergoes a rapid turnover, perhaps through the biosphere, which should be applied to a sulfur prediction model in the atmosphere. 4. Conclusions
Fig. 4. The variation in the activity ratios of 35S/7Be, 7Be/210Pb and precipitation in Seoul, Korea, from April 2004 to April 2005.
35
S/210Pb in
Since the fallout activity ratio of 35S/7Be is almost constant within 17% in the mid-latitudes of the Northern Hemisphere, this ratio can be used to obtain the 35S depositional flux if 7Be is known over a regional scale. Such fallout flux can be utilized to enable the use of 35S as a tracer for stream, river, lake, and ground waters. Our monitoring results showed that 35S/7Be ratios in the troposphere are significantly higher than the theoretical fallout ratios. We showed that this is likely due to the turnover of sulfur through the biosphere, perhaps owing to biomass burning or as plants go dormant and dry during the autumn and winter. Our simple mass balance model for 35S/7Be activities revealed that the mean turnover rate of sulfur through the biosphere is about 0.015 0.007 d1, with particularly higher rates in winter (0.025 0.006 d1). Since sulfur is an important element regulating acid precipitation and cloud condensation, we should investigate the turnover rate of sulfur more extensively over the global atmosphere. Acknowledgments
S 0 ¼ Fpro
S lSd Iatm
S lSC Iatm
Be
þ
Be
Be Be Be ld Iatm lC Iatm 0 ¼ Fpro
S lSr Iatm
(1) (2)
where Fpro is total tropospheric production flux, which combines the input fluxes from the stratosphere and the in-situ theoretical production in the troposphere, Iatm is the inventory in the troposphere, ld is the decay constant, lc is the scavenging rate constant, and lr is the turnover rate of 35S. If Eq. (1) is divided by Eq. (2), then Eq. (3) can be obtained as S FPro Be FPro
S lSd þ lSC lSr Iatm ¼ Be Be lBe Iatm d þ lC
(3)
In this equation, the turnover rate (lr) can be calculated using S =I Be Þ of 35S and 7Be if we assume the measured activity ratio ðIatm atm S =F Be Þ of 35S/7Be is that the total tropospheric input ratio ðFpro pro 0.0113 0.0006 (Fig. 3), and that the scavenging rate constant (lc) of 35S and 7Be is 0.033 0.006 d1. Here, lc for 35S and 7Be is the
This work was funded by the Korea Meteorological Administration Research and Development Program under Grant RACS_2010-3008. We would like to thank all EMBL members who helped in the sampling and sample analysis. HMC was partially supported by the BK21 scholarship through the School of Earth and Environmental Sciences, Seoul National University, Korea. References Baskaran, M., Swarzenski, P.W., 2007. Seasonal variations on the residence times and partitioning of short-lived radionuclides (234Th, 7Be and 210Pb) and depositional fluxes of 7Be and 210Pb in Tampa Bay, Florida. Marine Chemistry 104, 27e42. Baskaran, M., Coleman, C.H., Santschi, P.H., 1993. Atmospheric depositional fluxes of 7 Be and 210Pb at Galveston and College Station, Texas. Journal of Geophysical Research 98, 20,555e20,571. Cooper, L.W., Olsen, C.R., Solomon, D.K., Larsen, I.L., Cook, R.B., Grebmeier, J.M., 1991. Stable isotopes of oxygen and natural and fallout radionuclides used for tracing runoff during snowmelt in an Arctic watershed. Water Resources Research 27, 2171e2179. Garten, C.T., 1988. Fate and distribution of sulfur-35 in yellow poplar and red maple trees. Oecologia 76, 43e50. Hong, Y.-L., Kim, G., 2005. Measurement of cosmogenic 35S activity in rainwater and lake water. Analytical Chemistry 77, 3390e3393.
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Kim, G., Hussain, N., Scudlark, J.R., Church, T.M., 2000. Factors influencing the atmospheric depositional fluxes of stable Pb, 210Pb, and 7Be into Chesapeake Bay. Journal of Atmospheric Chemistry 36, 65e79. Kuroda, P.K., Hodges, H.L., Fry, L.M., Moore, H.E., 1962. Stratospheric residence time of strontium-90. Science 137, 15e17. Lal, D., Peters, B., 1967. Cosmic ray produced radioactivity on the earth. Handbuch der Physik 46, 551e612. Lal, D., Malhotra, P.K., Peters, B., 1958. On the production of radioisotopes in the atmosphere by cosmic radiation and their application to meteorology. Journal of Atmospheric and Terrestrial Physics 12, 306e328. Lal, D., Nijampurkar, V.N., Rajagopalan, G., Somayajulu, B.L.K., 1979. Annual fallout of 32 Si, 210Pb, 22Na, 35S and 7Be in rains in India. Proceeding of the Indian Academy of Science 88, 29e40. Michel, R.L., Campbell, D., Clow, D., Turk, J.T., 2000. Timescales for migration of atmospherically derived sulphate through an alpine/subalpine watershed, Loch Vale, Colorado. Water Resources Research 36, 27e36. Michel, R.L., Turk, J.T., Campbell, D.H., Mast, M.A., 2002. Use of natural 35S to trace sulphate cycling in small lakes, Flattops Wilderness Area, Colorado, USA. Water, Air, and Soil Pollution Focus 2, 5e18. e pánová, M., 2004. The missing flux in a 35S Novák, M., Michel, R.L., Prechová, E., St budget for the soils of a small polluted catchment. Water, Air, and Soil Pollution Focus 4, 517e529. Novák, M., Precová, E., 1995. Movement and transformation of 35S-labelled sulphate in the soil of a heavily polluted site in the Northern Czech Republic. Environmental Geochemistry and Health 17, 83e94. Osaki, S., Tagawa, Y., Chijiiwa, T., Sugihara, S., Maeda, Y., 1999. Atmospheric deposition of 35S. Journal of Radioanalytical and Nuclear Chemistry 239, 543e547. Plummer, L.N., Busenberg, E., Böhlke, J.K., Nelms, D.L., Michel, R.L., Schlosser, P., 2001. Groundwater residence times in Shenandoah National Park, Blue Ridge Mountains, Virginia, USA: a multi-tracer approach. Chemical Geology 179, 93e111.
Raisbeck, G.M., Yiou, F., Fruneau, M., Loiseaux, J.M., Lieuvin, M., Ravel, J.C., 1981. Cosmogenic 10Be/7Be as a probe of atmospheric transport processes. Geophysical Research Letters 8, 1015e1018. Shanley, J.B., Mayer, B., Mitchell, M.J., Michel, R.L., Bailey, S.W., Kendall, C., 2005. Tracing sources of streamwater sulfate during snowmelt using S and O isotope ratios of sulfate and 35S activity. Biogeochemistry 76, 161e185. Shapiro, M.H., Forbes-Resha, J.L., 1976. Mean residence time of 7Be-bearing aerosols in the troposphere. Journal of Geophysical Research 81, 2647e2649. Su, C.-C., Huh, C.-A., 2006. Measurements of 7Be and 210Pb in cloudwaters: toward a better understanding of aerosol transport and scavenging. Geophysical Research Letters 33 (L04807). Sueker, J.K., Turk, J.T., Michel, R.L., 1999. Use of cosmogenic 35S for comparing ages of water from three alpineesubalpine basins in the Colorado Front Range. Geomorphology 27, 61e74. Tanaka, N., Turekian, K.K., 1991. Use of cosmogenic 35S to determine the rates of removal of atmospheric SO2. Nature 352, 226e228. Tanaka, N., Turekian, K.K., 1995. Determine of the dry deposition flux of SO2 using cosmogenic 35S and 7Be measurements. Journal of Geophysical Research 100, 2841e2848. Taylor, C.B., 1968. A comparison of tritium and strontium-90 fallout in the Southern Hemisphere. Tellus 20, 559e576. Turekian, K.K., Tanaka, N., 1992. The use of atmospheric cosmogenic 35S and 7Be in determining depositional fluxes of SO2. Geophysical Research Letters 19, 1767e1770. Turekian, K.K., Benninger, L.K., Dion, E.P., 1983. 7Be and 210Pb total deposition fluxes at New Haven, Connecticut and at Bermuda. Journal of Geophysical Research 88, 5411e5415. Yamamoto, M., Sakaguchi, A., Funatsu, M., Kofuji, H., Tokuyama, H., 2006. Determination of the deposition of cosmogenic radionuclide 35S and sulfate in a heavy-snowfall area facing the Sea of Japan. Journal of Radioanalytical and Nuclear Chemistry 268, 569e577.
Contents lists available at ScienceDirect
Atmospheric Environment journal homepage: www.elsevier.com/locate/atmosenv
Atmospheric depositional fluxes of cosmogenic 35S and 7Be: Implications for the turnover rate of sulfur through the biosphere Hyung-Mi Cho, Young-Lim Hong, Guebuem Kim* School of Earth and Environmental Sciences/RIO, Seoul National University, Seoul 151-747, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history: Received 28 October 2010 Received in revised form 15 March 2011 Accepted 3 May 2011
The wet depositional fluxes of cosmogenic 35S and 7Be, together with 210Pb, were measured in Seoul, Korea, from April 2004 to April 2005. Approximately half of the annual fluxes occurred in summer, during which about 60% of the precipitations occurred. Our simple box model shows that the theoretical scavenging ratio of 35S/7Be to the ground is approximately 0.013, which accounts for the decay for the duration of oxidation and settling. However, we observed w50% higher 35S/7Be activity ratios than the theoretical removal ratio over the entire sampling period, with particularly higher (w133%) ratios during November 2004eApril 2005. These higher ratios in the winter cannot be explained by sudden incursions of the stratospheric air or longer aerosol residence times, on the basis of 7Be/210Pb ratios. We hypothesize that the ratios could occur owing to biomass burning or as plants go dormant and dry during the autumn and winter. Based on this hypothesis, we developed a 35S/7Be mass balance model which yields the turnover rate of sulfur in the atmosphere through the biosphere to be 0.015 0.007 d1. Such a rapid sulfur turnover rate should be applied to the prediction model of sulfur inventory changes. Ó 2011 Elsevier Ltd. All rights reserved.
Keywords: Sulfur cycle 35 S 7 Be 210 Pb Precipitation Atmosphere
1. Introduction 35
S (t1/2 ¼ 87 d), the radioactive isotope of sulfur, is produced by cosmic-ray spallation of argon in the stratosphere and upper troposphere (Tanaka and Turekian, 1991; Turekian and Tanaka, 1992). Soon after 35S is produced, it is oxidized to sulfate (35SO4) and attached to the ambient aerosols. 7Be (t1/2 ¼ 53 d) is similar in its origin to 35S, produced by cosmic-ray spallation of oxygen and nitrogen, and adsorbed onto particles immediately (Lal et al., 1958; Tanaka and Turekian, 1995). The stratospheric 35S and 7Be transferred to the troposphere and those produced in the troposphere then settle to the ground by either wet or dry deposition (Turekian and Tanaka, 1992; Osaki et al., 1999). Thus, these two isotopes, 35S and 7Be, form a pair which provides a powerful tool for determining the oxidation rates of SO2 in the atmosphere and the removal rates of aerosols (Turekian and Tanaka, 1992; Tanaka and Turekian, 1995; Osaki et al., 1999). Once settled on the earth’s surface, the pair is a useful tracer for determining groundwater age (Sueker et al., 1999; Plummer et al., 2001), new water input to lakes or runoff (Michel et al., 2000; Shanley et al., 2005), and the hydrology of watersheds (Cooper et al., 1991; Michel et al., 2002) over a time
* Corresponding author. Tel.: þ82 2 880 7508; fax: þ82 2 876 6508. E-mail address: [email protected] (G. Kim). 1352-2310/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.atmosenv.2011.05.002
scale of less than one year. In these aquatic applications of 35S, the fallout flux of 35S should be known. Thus, in this study, our aim was to determine (1) the annual flux of 35S and (2) the cycling of sulfur through the biosphere using the observed 35S/7Be ratios. We hypothesized that the atmospheric depositional fluxes of 35S can be determined using 7Be if a general activity ratio of 35S/7Be is obtained at mid-latitudes in the Northern Hemisphere. We used the pair of 35S and 7Be to determine the turnover rate of 35S, since 7Be is permanently removed from the atmosphere (except for soil resuspension) while 35S can re-enter the troposphere by forming gaseous species. 2. Materials and methods Precipitation samples were collected in a plastic bucket for the analyses of 35S, 7Be, and 210Pb for every precipitation event during one year (April 2004eApril 2005) in Seoul, Korea. The sampling bucket (diameter 26 cm) was cleaned with 5% neutral detergent solution (Merck ExtranÒ MA 03), then rinsed with HNO3 and deionized water. The samples were moved to the laboratory immediately after collection, then acidified to pH 3e4 using 6 M HCl. The analytical procedures of 35S were documented separately by Hong and Kim (2005). Briefly, 0.1 g of a stable sulfur carrier was added to the sample. Then, the sample solution was passed through an anion exchange resin (Amberlite IRA 400) column (15e50 mesh)
H.-M. Cho et al. / Atmospheric Environment 45 (2011) 4230e4234
using a peristaltic pump at a flow rate of 70 mL min1. The sulfur in the column was eluted with 300 mL of 3 M NaCl. The entire sulfur in the eluted solution from the resin column was precipitated as BaSO4 by adjusting the pH to 3e4 following the addition of excess barium solution. The precipitates were collected onto a preweighed GF/B filter. After transferring the filter with BaSO4 into a 20-mL plastic vial, 10 mL of distilled water and 10 mL of Ultima gold LLT cocktail were added. The material in this vial was homogenized in an ultrasonic bath. Then, the amount of 35S in the precipitates was directly counted using a super-low-background LSC (Packard Tricarb 3170 TR/SL). The purity of 35S after the source preparation was checked by counting the samples several times (Hong and Kim,
4231
2005). The counting efficiency (cpm/dpm ratio) of the LSC was determined using NIST traceable 35S standard for the same BaSO4 amount, filter, and cocktail. The samples for the analyses of 7Be and 210Pb were dried and transferred to 100 mL bottles, and then adjusted to a volume of 80 mL. The activities of 7Be and 210Pb were determined using a coaxial type (Canberra) detector. The counting efficiency (cpm/dpm ratios) for 210Pb (at 46.5 keV) was determined using the same volume of NIST traceable 210Pb standard. Since we did not have a 7Be standard, corrections of 7Be (at 477 keV) for geometry, self absorption factors, and counting efficiency were made by interpolating 214Pb (352 keV) and 214Bi (609 keV) efficiencies using the 226Ra standard. 3. Results and discussion The precipitation was highest during the summer (Fig. 1). The activities of the radioisotopes in the precipitation were in the range
Fig. 1. The activities of 2004 to April 2005.
35
S, 7Be, and
210
Pb in precipitation in Seoul, Korea, from April
Fig. 2. Precipitation amounts versus the activity of in Seoul, Korea, from April 2004 to April 2005.
35
S, 7Be, and
210
Pb in precipitation
4232
H.-M. Cho et al. / Atmospheric Environment 45 (2011) 4230e4234
of 4e400 Bq m3 for 35S, 210e19,000 Bq m3 for 7Be, and 25e540 Bq m3 for 210Pb. Approximately half of the annual fluxes occurred in the summer due to the high precipitation. The specific activities of 35S, 7Be and 210Pb decreased exponentially as the precipitation amount increased (Fig. 2), owing to the typical diluting effect of the precipitation. In general, the specific activities of 35S, 7Be and 210Pb are higher for smaller precipitations since raindrops with smaller sizes have longer residence times and larger surface areas (Su and Huh, 2006). The activity ratio of 35S/7Be was similar for different regions in the mid-latitudes of the Northern Hemisphere within 17% (Table 1), except for New Haven, Connecticut (Tanaka and Turekian, 1995) where the detection efficiency was determined using 14C. Thus, an estimate of the fallout flux of 35S can be derived within this uncertainty for various applications in aqueous and groundwater studies in the Northern Hemisphere mid-latitudes using this ratio and the regional 7Be flux. In order to obtain the theoretical depositional flux of 35S and 7 Be, we constructed a simple atmospheric mass balance model for two (the stratospheric and tropospheric) boxes (Fig. 3). The production ratio of 35S/7Be in the stratosphere is w0.0103 (Lal and Peters, 1967). Since the residence time of stratospheric aerosols is about 1.0 0.5 yr on the basis of data from bomb-produced 90Sr (Kuroda et al., 1962; Taylor, 1968), the expected flux ratio of 35S and 7 Be activities from the stratosphere to the troposphere is 0.063 0.044, resulting from radioactive decay of 35S and 7Be during the aerosol residence time and oxidation time of 35SO2 to 35 SO4 (w7 days). The production ratio of 35S and 7Be activities in the upper troposphere is known to be w0.0111 (Lal and Peters, 1967). Thus, if we assume that the aerosol residence time in the troposphere for these isotopes transferred from the stratosphere and produced in the troposphere is about 30 5 days on the basis of 7Be results (Shapiro and Forbes-Resha, 1976; Raisbeck et al., 1981; Su and Huh, 2006), the fallout ratio of 35S/7Be activities to the ground would be 0.013 0.003. In this calculation, the same oxidation time of tropospheric 35SO2 is applied. However, the observed 35S/7Be activity ratios range from 0.012 to 0.037 (0.0202 0.0004, mean value) which are generally higher than those of the theoretical fallout ratio (Fig. 4). The ratios for the three exceptionally high periods from November to April were approximately 56% higher than those during the other periods. Although sudden incursions of the stratospheric air or resuspension of soils could result in such high ratios, such events seem unlikely to be the cause since the 7Be/210Pb activity ratios were not
Table 1 Comparison of atmospheric depositional ratios of Location and time of collection Bombay (18 N) 1970 New Haven, Connecticut (41 N) Mar. 1977eFeb. 1978 Dec. 1991eNov. 1992 Stillpond, Maryland (39 N) Oct. 1995eSep. 1996 College Station, Texas (30 N) 1990 1991 Galveston, Texas (30 N) 1990 1991 Fukuoka, Japan (33.4 N) Mar. 1992eFeb. 1993 Seoul (37 N) Apr. 2004eApr. 2005 St. Petersburg (27 N) Jul. 2003eJun. 2004
35
S, 7Be, and
Fig. 3. Box model for cosmogenic 35S and 7Be in the atmosphere. Ps, Pt, Inc, and Tc denote the activity ratios of 35S/7Be for the stratospheric production, the tropospheric production, the stratospheric fallout, and the theoretical scavenging ratios of 35S/7Be from the atmosphere. The values for the input flux and production of 35S in the troposphere corrected for the decay during the oxidation times of 35SO2.
unusually higher (tracing stratospheric source inputs) or lower (tracing surface soil inputs) (Fig. 4). Alternatively, this unusually high 35S/7Be ratio could be due to the turnover of 35S independently from 7Be through the biosphere in its gaseous form. Although this hypothesis has yet to be confirmed, it is supported by the much higher 35S/7Be ratios (Tanaka and Turekian, 1995) in the groundlevel aerosol samples compared with those in wet deposition samples from all altitude levels ranging from the cloud level to the ground. By assuming that there is a turnover process for 35S between the atmosphere and the land-surface, we can calculate the turnover rate of 35S, as a proxy for anthropogenic sulfur in the atmosphere, using the following mass balance models. At steady state,
210
Pb in mid-latitudes of the Northern Hemisphere.
Rainfall (cm)
35
S/7Be
7
Be/210Pb
Source
263
0.0284 (0.004)
5.06
Lal et al. (1979)
148 e
e 0.0134
19.8 e
Turekian et al. (1983) Tanaka and Turekian (1995)
127
e
18.4
Kim et al. (2000)
98 146
e e
16.3 11.3
Baskaran et al. (1993)
97 150
e e
17.8 13.6
Baskaran et al. (1993)
e
0.0238
e
Osaki et al. (1999)
128
0.0202 (0.0003)
16.5
This study
115
e
15.5
Baskaran and Swarzenski (2007)
H.-M. Cho et al. / Atmospheric Environment 45 (2011) 4230e4234
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reciprocal of the mean residence time of 7Be in the troposphere. Thus, the turnover rate of sulfur is approximately 0.015 0.007 d1, with exceptionally higher rates (0.025 0.006 d1) in winter. This implies that atmospheric 35S removed by precipitation is turned over and re-enters to the troposphere within 66 28 days, with more rapid turnover in winter (41 10 days). Similarly, higher atmospheric deposition of 35S was observed in winter precipitation relative to the summer precipitation in Tatsunokuchi, Japan (Yamamoto et al., 2006), which supports our hypothesis. Garten (1988) showed that there are more losses of artificial 35S from the trees, due to the shedding of leaves during OctobereNovember, than in summer. In contrast, soil studies (Novák et al., 2004; Novák and Precová, 1995) showed that sulfur loss from the soil to the atmosphere was insignificant in winter but significant in summer in association with bacterial sulfate reduction. These contradictory results suggest that the turnover flux of high 35S activity, fresh air-borne sulfur through soils is insignificant, and such fresh sulfur re-enters the troposphere predominantly during late autumn and winter owing to biomass burning or as plants go dormant and dry over a global scale. Our result implies that sulfur in the atmosphere undergoes a rapid turnover, perhaps through the biosphere, which should be applied to a sulfur prediction model in the atmosphere. 4. Conclusions
Fig. 4. The variation in the activity ratios of 35S/7Be, 7Be/210Pb and precipitation in Seoul, Korea, from April 2004 to April 2005.
35
S/210Pb in
Since the fallout activity ratio of 35S/7Be is almost constant within 17% in the mid-latitudes of the Northern Hemisphere, this ratio can be used to obtain the 35S depositional flux if 7Be is known over a regional scale. Such fallout flux can be utilized to enable the use of 35S as a tracer for stream, river, lake, and ground waters. Our monitoring results showed that 35S/7Be ratios in the troposphere are significantly higher than the theoretical fallout ratios. We showed that this is likely due to the turnover of sulfur through the biosphere, perhaps owing to biomass burning or as plants go dormant and dry during the autumn and winter. Our simple mass balance model for 35S/7Be activities revealed that the mean turnover rate of sulfur through the biosphere is about 0.015 0.007 d1, with particularly higher rates in winter (0.025 0.006 d1). Since sulfur is an important element regulating acid precipitation and cloud condensation, we should investigate the turnover rate of sulfur more extensively over the global atmosphere. Acknowledgments
S 0 ¼ Fpro
S lSd Iatm
S lSC Iatm
Be
þ
Be
Be Be Be ld Iatm lC Iatm 0 ¼ Fpro
S lSr Iatm
(1) (2)
where Fpro is total tropospheric production flux, which combines the input fluxes from the stratosphere and the in-situ theoretical production in the troposphere, Iatm is the inventory in the troposphere, ld is the decay constant, lc is the scavenging rate constant, and lr is the turnover rate of 35S. If Eq. (1) is divided by Eq. (2), then Eq. (3) can be obtained as S FPro Be FPro
S lSd þ lSC lSr Iatm ¼ Be Be lBe Iatm d þ lC
(3)
In this equation, the turnover rate (lr) can be calculated using S =I Be Þ of 35S and 7Be if we assume the measured activity ratio ðIatm atm S =F Be Þ of 35S/7Be is that the total tropospheric input ratio ðFpro pro 0.0113 0.0006 (Fig. 3), and that the scavenging rate constant (lc) of 35S and 7Be is 0.033 0.006 d1. Here, lc for 35S and 7Be is the
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