Marten, G. C., Hammond, P. B., Agron. J. 58, 553 and 554 (1966). McCallum, G. J., Woodward, R. N., Nature 209, 69 and 70 (1965). Mitchell, R. L., Reith, J. W., J. Sci. Food Agri. 17, 437-45 (1966). National Air Sampling Network, Public Health Service, U S . Dept. Health, Education, and Welfare, Air Pollution Measurements Rpt. 637 (1958), 978 (1962); and Air Quality Annual Reports (from 1962 onwards). Parizek, J., J . Endocrinol. 15, 56 (1957). Patterson, C. C., Arch. Eizuiroii. Health 11, 344-58 (1965). Pulido, P., Keiichiro, F., Vallee, B. L., Anal. Biochem. 14, 393-404 (1966). Samuelson, O., “Ion Exchange Separations in Analytical Chemistry,” Wiley, New York, 1963, p. 311. Schrenk, H. H., Heimann,H., Clayton, G . D., Gafafer, W. M., Wexler, H., Public Health Service, U S . Dept. Health, Education, and Welfare, Rept. 306 (1949). Schroeder, H. A., J. Amer. Med. Assoc. 195,81-5 (1966). Schroeder, H. A., J. Chron. Dis. 18,647-56 (1965). Schroeder, H. A., Balassa, J.,J. Chron. Dis.14,236-58 (1961).
Sinclair Refining Co., Product Info. Bull. P-74 (1964). Singer, M. J., Hanson, L., Soil Sci. Soc. Amer. Proc. 33, 152 and 153 (1969). Specht, A. W., Myers, A. T., Oda, U., “Methods of Soil Analysis,” Black, C. A., Ed., Amer. Soc. Agron., Agrori. Series 9, 822-48 (1965). Terhaar. G . L.. Holtzmann, R. B.. Lucas. H. F., Nature 216, 353-5‘(1967): Tipton, I. H., “Metal-Binding in Medicine,” Seven, M. J., Johnson, L. A., Eds.. Limincott, Phila, Pa.. 1960. tm.27-42. Toth, S. J:, Prince, A. L.; Wallace, A:, Mikkelsen, D. S., Soil Sci. 66,459-66 (1948). Warren, H. V., Delavault. R. E., J . Sci. Food Anric. 13, 96-8 (1962).
Receiced for reciew July 9, 1969. Accepted March 30, 1970. This report is a coritributiori from the U S .Soils Laboratory, Soil arid Water Consercatiori Research Dicisioii, Agricultural Research Sercice, US.Department of Agriculture, Beltscille, Md.
Determination of “‘Pb Mean Residence Time in the Atmosphere Chester W. Francis’ and Gordon Chesters Department of Soil Science, University of Wisconsin, Madison 53706
Larry A. Haskin Department of Chemistry, University of Wisconsin, Madison 53706
w Rainwater was filtered through a 0.22-p Millipore filter before radioassay of the resulting dust and filtrate fractions for 21OPo. Lead-210 contents of the fractions were obtained by following the growth of 210Poactivity in stored samples as a function of time. At the time of rainfall, the 210Pocontents of the dust and filtrate fractions were approximately equal, but the dust fractions contained less than 3 z of the total 210Pb,Polonium-210 and zloPb in the dust fractions were already in radioactive equilibrium at the time of the rainfall, suggesting that their presence in the dust was not a result of recent scavenging of zlaPb from the troposphere. Ratios of zloPb to 21aPo used for calculations of mean atmospheric residence time for zloPb should, therefore, be those of the filtrate. From those ratios, a mean atmospheric residence time for 210Pbof 9.6 days. * 2 0 z , is obtained.
I
n conjunction with studies of 210Po uptake by plants (Francis and Chesters, 1967), several samples of rainwater were collected for measurement of their 21oPo concentrations. From the data obtained, mean atmospheric residence times for some decay products of 222Rncan be estimated. Such residence times depend sensitively on ratios of 210Pbto 210Po,which were found to differ greatly between dust and filtrate fractions of rainwater. Estimates of residence time ’Present address: Health Physics Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37830 586 Environmental Science & Technology
calculated from these data are compared with those of other investigators whose procedures apparently did not include filtration. Polinium-210 is an alpha emitter of half-life 138 days and is the fifteenth member of the 238Udecay chain. It is the seventh decay product after 3.82-day 22ZRn,the species which diffuses from the earth’s surface into the atmosphere. Polonium-210 is immediately preceded by 5.01-day 210Bi.which is, in turn, preceded by 22-year *I0Pb. The nuclides between 222Rnand ?I0Pball have half-lives shorter than 30 min. and can be considered in radioactive secular equilibrium with 222Rnfor the purposes of this paper. Previous measurements, as well as those reported here, show that 210Pb,zlOBi,and 210Poare not in radioactive equilibrium with each other or with 22zRnin atmosphere, principally because they are washed out by rain. When a rain falls, the region of the atmosphere in which it forms is presumably cleansed of ?loPb, 210Bi,and 210Po,but not of the noble gas 222Rn.Thus, in principle, values for the relative amounts of zloPb and 210Bi,or of zloPb and *laPo in freshly fallen rainwater, if substituted into the equations that describe successive radioactive decays, allow calculation of the time elapsed since that region of the atmosphere was cleansed of the decay products of 222Rnby the preceding rain. In fact, intermixing among source regions complicates such easy measurement, so that most residence times are based on a model for an average condition of the atmosphere. The concentration of 222Rnand, consequently, the production rate ( R ) of 210Pbare assumed to remain constant with time. The rate of loss of ?I0Pbfrom the atmosphere is presumed to be caused partially by radioactive decay (rate constant Apt,)
and partially by washout by rain, which is also written as a first-order kinetic process (washout constant k ) . The net rate of change of concentration of 210Pbin the atmosphere, according to the model, is given by Equation 1, in which the symbol Npbrefers to the number of atoms of *l0Pbin the system. (dNrbjdt) = R
-
(kpb
f
ki'b)N~>b
(1)
I t is next assumed that *lOPband 2 2 2 R nare in steady-state concentration equilibrium in the atmosphere, and Equation 1 is set equal to zero. Similar equations and assumptions are made for zlOBiand *loPo.If the washout constant ( k ) is presumed to have the same value for 210Pb,*loBi, and zloPo, then Equation 2 results, in terms of washout and decay constants and the ratio of the number of atoms of 210Pb to 210Po :
h2
+ ( A B > + ki.,)k
-L
X B , X I J ~- X B ~ X I ~ ~ ( N I ~ =J N 0 P(2) ~)
The positive root ( k ) of Equation 2 is the reciprocal of the mean residence time for *lOPb(and 210Biand 210Po)in the atmosphere. Such residence times cannot, of course, be more than reasonable estimates of average residence times for the atoms measured. The value of k is not known to be the same for *lnPb,210Bi,and 210Po,or even to be constant for any one of them. Atmospheric concentrations of ***Rnvary, averaging about 200 d.p.m./kg. of air over land and about 4 d.p.ni./kg. over oceans (Israel, 1951). Lead-210 concentrations in the atmosphere, as determined by direct collection of' that nuclide on air filters in the vicinity of Harwell, England, range from about 7 X d.p.m./kg. of air at ground level to about 70 X d.p.m./kg. of air in the lower stratosphere (around 14 km.) (Burton and Stewart, 1960). The gradient of concentration with respect to altitude was found to be similar to that for fission products. Entry of zloPb(or its 222Rnancestor) into the stratosphere d o n g with rising warm air, and then descent at more northerly latitudes, were suggested to explain the gradient. Any *loPbsampled in tropospheric air or in rain would presumably be partly of recent stratospheric origin and partly of purely tropospheric origin. The concentration of about 7 X loF3 d.p.m.,/kg. of air a t ground level is consistent with the measurements of King, Lockhart, et a/. (1956) at Washington, D.C. Other investigators, e.g., Peirson, et a/. (1966) made monthly zloPb measurements over England in 1958 and observed similar concentrations and gradients; however, not as steep as those found by Burton and Stewart (1960). Burton and Stewart (1960) found the average 21uPbconcentration in rainwater at Harwell to be 2.3 picocuries per liter (pCi/l.). King, Lockhart, ef a/. (1956) reported a n average of 2.5 pCi/l. for *lOPbin rainwater collected a t Washington, D.C., and Glenview, Illinois, and values as much as 10 times less for rainwater at remote island sites. No significant correlation was observed between *I0Pbcontent and character of the rains, the seasons, or the quantities of dust in the water. Fry and Menon (1962) reported values between 1 and 8 pCi/l. for '"Pb in rainwater collected at Fayetteville, Arkansas. By comparing the quantity of *loPbin a vertical column of air between the ground and the tropopause with the mean time rate of deposition of 210Pbby rain, Burton and Stewart (1960) obtained a mean tropospheric residence time for 210Pb of 17 days. They commented that the value would be slightly greater if the efficiency of the air filters for trapping ?loPb were less than 100%. The value would be a little lower if the ground-level concentration for 210Pbin air had been used instead of a concentration based o n their observed altitude
gradient. As a rough check of their value, Burton and Stewart (1960) calculated mean residence times of 29 days for *laPb of mixed stratospheric origins and 22 days for the purely tropospheric component, based on the 210Pb/210Poratios in their water samples and the model described in detail above. Fry and Menon (1962) measured 210Pb/210Biratios in water samples from 12 rains and, using a n equation analogous to Equation 2, obtained mean residence times ranging from 2.4 to 25.6 days. The averages of residence times based o n data for rains falling in the interval February to April and June t o August were 5.9 to 6.6 days, showing no measurable seasonal effects. These values are much shorter than those given by Burton and Stewart (1960) and those of 14 and 33 days based on *10Pb/210Po ratios reported by Lehmann and Sittkus (1959). I t would appear from these reports that none of the above investigators filtered the rainwater before concentrating the selected nuclides for radioassay. It could reasonably be assumed that the 210Pb, *l0Bi,and 210Powashed out in rainfall would be attached to fine particulate matter present in the atmosphere and that filtration would be unnecessary or unwise. The data presented below seem to suggest otherwise. Experiineiital
Samples of rainwater were collected at Madison, Wisconsin, during the summer of 1966. To reduce contamination from dust at ground level, the polyethylene-lined collecting tube was placed o n top of a 20-ft.-high building, and only at the onset and for the duration of each rain. Once collected, the rainwater was acidified to 0.03 N with H 2 S 0 4and stored in glass containers. Immediately before each zlOPoanalysis, a 4-liter aliquot was filtered (Millipore, 0.22-p) to provide a dust fraction (particles of diameter >0.22 p) and a filtrate fraction (including any particles of diameter 0.22 p ) Polonium-210 content of Size of content dust (>0.22 p ) - ~. rainfall Counting of rain (in.) date (mg./l.) pCi/l. pCi/g. 3.64 June 1 ... 0 . 1 4 f 0.04 ... July 3 ... 0.20 =t 0.04 ... Aug. 1 ... ... ... Sept. 14 ... 0 . 1 7 =t0.04 ... 0.40 June 3 14.85 0.26 f 0.04 17.5 i 2 . 7 0.25 June 8 21.87 0.41 =t 0.05 18.7 f 2 . 3 0.17 July 13 ... 0 . 4 3 i. 0.05 ... 166 + 50 1.66 July 17 0.60 0.10 i 0.03 121 July 19 0.58 0.07 i0.04 57 145 i 72 Aug. 15 0.55 0 . 0 8 i 0.04 99 i 42 Sept. 14 0.70 0.07 i0.03 40 i. 1 1 0.64 July 28 2.72 0.11 i 0.03 61 =k 11 Aug. 28 2.77 0 . 1 7 i 0.03 46 + 8 1.50 Aug. 16 4.96 0 . 2 3 =k 0 . 0 4
*
Filtrate fraction of rainwater containing particulate matter of
