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Atmospheric deposition of 7Be and 10Be
Geockimica el Cosmochimica Ado Vol. 53, pp. 135-142 Copyright Q 1989 Pergamon Press pk. Printed in USA
0016-7037/89/$3.00 + .CUl
Atmospheric
deposition of ‘Be and “Be
LOUIS BROWN’, GARY J. STENSLAND’,JEFFREYKLEIN~and ROY MIDDLET~N~ ’ Department of Terrestrial Magnetism, Carnegie Institution of Washington, Washington, D.C. 20015, U.S.A. ’ Illinois State Water Survey, Champaign, IL 6 1820, U.S.A. ’ Tandem Accelerator Laboratory, University of Pennsylvania, Philadelphia, PA 19104, U.S.A. (Received April 1I, 1988; accepted in revisedform October 14, 1988)
Abstract-Measurements of “Be in precipitation taken in Hawaii, Illinois and New Jersey over a period of five years arc reported. The problem of contamination by the isotope being resuspended on wind blown soil that is also collected is addressed. Rain collected at Mauna Loa, Hawaii has such low values of dust contamination that it has been taken as clean, and the data from Illinois and New Jersey are evaluated on that assumption. The conclusion is that the deposition in a given amount of rain for the non-resuspended component is the same for ah three stations, and we propose that the annual rate for mid-latitude locations having moderate rainfall is proportional to the local rainfall. ‘Be, which is probably negligibly contributed to the measurements by soil contamination, was measured for individual rains in Illinois and found to have a deposition of 1.4 X 10’ atom/cm3. We have found that concentration variations between precipitation events greater than a factor of 20 exist for both isotopes and that relatively rare, high concentration events dominate deposition, thereby requiring long periods of observation to avoid significant error. Based on our own and other data we conclude that the best value for “‘Be deposition is 1.5 X IO4atom/cm”, uncertain by 201, and for ‘Be is 1.2 x lo4 atom/cm3, uncertain by 25%. A global average deposition rate cannot be inferred directly for either isotope from these kinds of data; however, the theoretical global deposition rate for ‘% is shown to be consistent with the deposition reported here, if the concentration in equatorial rain is about 3300 atom/g.
INTRODUCTION
MAYAJULU,1977; FINKELet al., 1977; RAISBECKet al., 1981; SOUTHON et al., 1987) lacustrine sediments (WAHLEN et al., 1983) and polar ice cores (MCCORKELLet al., 1967; BEER et al., 1984).
MANY OF THE APPLICATIONS of “Be as a geochemical tracer require a knowledge of its atmospheric deposition rate, and indeed one of the early measurements of the isotope by RAISBECK ef al. (1979) addressed this matter. This was the first measurement of “Be concentration in rainwater, made possible by the use of a nuclear particle accelerator as a mass spectrometer. As other observers made use of the new technique additional reports of deposition rate measurements followed. At first glance this would seem to be the simplest kind of “Be investigation, as the sampling appears straightforward and the extraction chemistry simple. STENSLANDet al. (1983), however, demonstrated that for a rural Illinois location the contamination of rainwater with windblown soil and other atmospheric dust introduces significant amounts of “Be, a problem that persists even when wet-only samplers are used (devices that are open only at the onset of the rain), and that can be extreme if no provision is taken to block dust input. We note here a second complicating effect not yet commented on to our knowledge: the variation in the isotope’s concentration from one collection to another is sufficiently great to make that an important contribution to the uncertainty of the result. Additional reports of observations of ‘%e in rain appeared after R4ts~EcK et al. (1979) introduced the accelerator method with RAISBECKet al. ( 198 1b) extending their original work at Orsay, France, and SOMAYAJULU et al. (1984) reporting data from eight stations in India. The first full report that addressed dust contamination came from MONAGHANet al. ( 1985). All of these observations were made with collectors that were open all the time. Measurements of ‘Be deposition are reported by TUREKIAN et al. (1983) together with five references cited therein and by BLEICHRODT(1978) and RAISBECKet al. (198 lb). Indirect measurements of the “Be deposition have been made using pelagic sediments (TANAKA and INOUE, 1979, 1980; So-
Our studies of “Be in rain are in two parts. The first is a two year period of measurements of daily precipitation sampled at the Bondville site of the Illinois State Water Survey. We obtained concentrations of “Be for 62 and 7Be for 63 rains as well as the concentrations of various cations and anions. The second part of the study is an additional three years in which 36 monthly samples were taken at the New Jersey site of the Environmental Measurement Laboratory and 29 weekly or longer period samples (as was often required to obtain usable quantities of water) from the Mauna Loa site of the Hawaiian Global Monitoring for Climate Change Observatory. For comparison 40 monthly samples were analyzed from the Illinois site as well. No measurements of ‘Be were made for any of these samples in the second part because of the uncertain time correction for decay, but all had the same ion determinations as those of the first part. In all cases we sampled with wet-only collectors. Precipitation was measured separately at each site with a weighing bucket gauge as one of the standard observations. The Mauna Loa site was selected because negligible contamination by dust is encountered there. A straightforward interpretation would allow us to use these data as direct measurements of “Be concentration in rain water and allow us to calculate the deposition without complication. The island location and the high altitude of the site make us uneasy with such an interpretation, so we elected to take similar samples at the New Jersey site, which is continental but with lower dust contamination than the Illinois site. Earlier studies of “Sr showed that total deposition was proportional to the rainfall with negligible amounts in dry deposition (HARDY et al., 1968). We made direct measurements in Illinois and the District of Columbia for the dry 135
L. Brown et al.
136
of ‘Be-measurements that are impossible for “Be because of the amount of the isotope on resuspended dustdeposition
and demonstrated that this mechanism with our observational uncertainties.
is negligible compared
DETERMINATION OF ‘Be AND “Be CONCENTRATIONS ‘% is determined with the mass spectrometer technique of isotope dilution, ‘Be with low level counting, so 9Be is added to the samples to serve as a spike for the former and as a carrier for the latter. After spiking with 9Be and acidification with HCl following collection, the Illinois and Hawaii samples were shipped to the Department of Terrestrial Magnetism for the determination of the ‘Be and the preparation of the Be for the determination of the ‘%e with the University of Pennsylvania tandem Van de Graaff accelerator. The New Jersey samples were sent in 20-liter plastic buckets neither spiked nor acidified. In order to ascertain whether ‘Be is lost through sticking to the walls of the collector receptacle or the shipping container or is lost in the chemical extraction, three 9Be-spiked and acidified samples were spiked with ‘Be. Within an experimental error of 5% no loss was observed. Similar experiments with the New Jersey samples showed no Be sticking to the container walls. The samples were evaporated in three stages using a 2000-ml pyrex beaker to reduce the volume to about 200 ml after which it was transfered to a 6OO-ml and then to a 30-ml teflon beaker where it was taken to dryness with a few drops of HClO, to destroy organics. It was then converted to a Be(OH)3 gel that was concentrated in the bottom of a 15-ml centrifuge tube. The ‘Be content of the gel was measured by counting the 0.478MeV y ray with a 7.5-cm NaI well detector located in a lead cylinder of 27 cm diameter, 46 cm height and 10 cm covering plug. This apparatus is located in a basement room constructed for low level counting. The counter efficiency was determined by measuring 0.2 &i of ‘Be with the 7.5 cm counter and then with a calibrated 20cm NaI 4a detector at the National Bureau of Standards. Counts were accumulated in a pulse height analyzer during periods of 80,000 set, beginning at about the same time each day. Measurements were usually made on groups of five or six samples with about the same number of background measurements both preceding and following each group. The average number of background counts for a measurement was about 5.1 X 104, but the uncertainty that typically resulted from a group of background measurements was 330, about double the square root of the number of counts in a single measurement. The ‘Be in the samples gave from 2200 to 12,000 additional counts, yielding uncertainties ranging from 18% to 14%, respectively. The “Be content was determined using accelerator mass spectrometry, an experimental technique that employs a nuclear particle accelerator as a mass spectrometer. For details read LITHERLAND (1980); for a survey of applications read BROWN(1984, 1987) and ELMOREand PHILI.IPS (1987); for details of the Penn tandem read KLEINet al. (1982) and MIDDLETONet al. (1983). The accelerator yields a measure of the ratio “BepBe from which one can determine the number of “Be atoms in the sample from the amount of 9Be added in the spike. The extracted Be in the form of Be(OH)* can be converted to Be0 in a form satisfactory for the ion source by baking it at 550°C for an hour. The details of the mass spectrometry are identical to those described TERAet al. (1986). The uncertainties of the ‘a data were taken to be 15%, typical of standard reproducibility for count rates typical of the rains. STANDARDS
FOR 9Be AND “Be
Isotope standards of “Be/‘Be have been prepared by various investigators by diluting a quantity of “Be with 9Be, the 4 See NAPS Document No. 04642 for five pages of supplementary material. Order from NAPS, % Microfiche Publications, P.O. Box 35 13, Grand Central Station, New York, NY 10163-35 13. Remit in advance, in U.S. Funds only, $7.75 for photocopies or $4.00 for microfiche. Outside the U.S. and Canada, add postage of $4.50 for the first 20 pages and $1.00 for each of the ten pages of material thereafter, or %1.50 for microfiche postage.
“Be being determined by p-ray counting. This has not yet led to a generally accepted standard because of uncertainty about the value of the isotope’s half life. Experience with the developing experimental technique over eight years causes us to accept the machine in its present form as yielding absolute ratios more accurate than 5%. Throughout the years of rain observations we normalized our data to a house standard, so all the results reported here are consistent. The 9Be spike solutions were prepared by weighing BeFz after drying overnight at 80°C. Solution acidity was 0.1 M HCl. We measured the concentrations subsequently with an inductively coupled plasma spectrometer and found agreement at the percent level. The 9Be of the spike proved to be contaminated with “Be at the lo-l4 level (MIDDLETON et al., 1984). Data were corrected in the few cases where this could not be neglected. THE DATA-CURSORY
EXAMINATION
The data are grouped as (1) the weekly (or longer) collections at Mauna Loa, (2) the precipitation events at the Illinois
site, (3) the monthly collections at the Illinois site, and (4) the monthly collections at the New Jersey site. The data include ion concentrations for Ca, Mg, Na, K, NH4, Cl, NO3 and SO.,. Let us first describe the various groups after which we shall inquire about correlations. Mauna Loa samples
We discuss the Mauna Loa samples first because their interpretation is simplest. Not all rainfall for the 1100 day period is accounted for because we failed in some of the determinations, owing to the small amount of water collected. Weekly samples were desired but composites of more than one week were often used. The concentrations of Ca and Mg are the best indicators of soil contamination, which implies for continental locations resuspended “Be. The values for Mauna Loa are sufficiently small compared with the other data sets to make this a negligible source of error. In one case of large Ca and Mg concentration, the effects must also be small because this material was volcanic dust, which has “Be at concentrations two orders of magnitude less than most soils (TERA et al., 1986). This allows us to discuss these data assuming with some assurance that only soil-free deposition takes place. The “Be concentrations take on extreme values, from 1.9 X lo3 to 89.4 X lo3 atom/g, a variation of nearly a factor of 50. We do not know what properties of the atmosphere determine the concentration in a given rain and hence do not know what theoretical distribution of concentration is to be expected. It is evident that a few high-concentration rains dominate and cause large uncertainties. The average concentration weighted by precipitation is 16.9 X lo3 atom/g. Interpretation in terms of a normal distribution yields a standard deviation of 113% for a single measurement, implying for the average after three years (29 determinations) an uncertainty of 2 1%. Illinois event samples
The sample group with the next level of complication is made up of 68 precipitation event samples taken at the
Atmospheric ‘Be and ‘“Be
Bondville, Illinois site and for which both Be isotopes were measured with a few exceptions. The rains of these events had to be strong enough to cause the collectors to open and to furnish enough water to measure-generahy 0.5 liter or more. The mid-point time of each event is known and the 7Be measurements were corrected to give the concentration at the time of the rain. The variation in concentration for “Be is large-from 5.2 X lo3 to 69.7 X 10’ atom/g-though not so great as for the Mauna Loa group. The precipitation weighted average concentration is 2 1.2 X lo3 atom/g with a single measurement uncertainty of 75%. The data of 6 May 1983 are excluded and will be discussed later. The variation of the ‘Be concentration ranges almost as far as does the “Be of Mauna Loa, 3.1 X lo3 to 65.1 X 10’ atom/g more than a factor of 20. The weighted average is 14.1 X 10” atom/g with an uncertainty of 59%. The similarity in the distributions of ‘Be and “Be for the Illinois event and Hawaii is illustrated in Fig. 1. Here the complication, found in most land based stations, of “Be resuspended from soil is encountered. Numerous studies of ‘(Be in soils (PAVICHetal., 1986, and references cited therein) show typical ranges of concentrations near the surface of 2 X 10’ to 10’ atom/g with fine particles generally having higher than average values. Dust typical of Illinois is reported by BOERNGEN and SHACKLETTE (1981) to have 0.60% Ca and 0.42% Mg. Typical concentrations of these ions in Bondville rain are 0.2 m&l Ca and 0.03 mg/l Mg. If one assumes a “Be dust concentration of 5 X 10’ atom/gdust, ~n~minations of 16 X lo3 atom/g-in result using Ca and 4 X lo3 atom/g-rain using Mg. One obviously cannot correct the rain data in this way because of the large uncer-
137
taint& in the isotope and elemental concentration of the dust, but the measured ion concentrations indicate that the effect certainly cannot be prudently neglected. Dust particles in the air will also scavenge the non-resuspended Be isotopes attached to submicron particles and bring them in the rain, but this component need not trouble us because it is immaterial how the aerosols am entrapped in the rain. Although ‘Be can also be found in soils the haIf life of 53 days makes its concentration there negligible relative to ‘a, so its resuspension from soil is probably negligible. We shah return to this shortly. The ‘Be data are sufficiently numerous to allow one to examine the question of temporal variation. The average concentrations for the springs of 1982 and 1983 are 18.1 and 2 1.9 X lo3 atom/g compared with 13.9 X lo3 atom/g for other seasons with standard deviations for the mean of about 15%, enough to support the existence of spring maxima. Illinois and New Jersey monthly samples These samples, though numerous, are lower in information content than the two preceding groups because they have neither the negligible soil contamination of Hawaii nor the discrete ‘Be concentration of the Illinois event group. Comparisons of these data are helpful in ascertaining the magnitude of typical dust contamination. The data of both Illinois and New Jersey show very similar extremes in concentration to those of the Illinois event samples: 5.8 X 10’ to 67.9 X lo3 atom/g for the Illinois monthly, and 8.2 X lo3 to 75.0 X lo3 atom/g for the New Jersey monthly. Note that the ratio of the extremes is considerably less than for the Hawaii “Be samples or the Illinois ‘Be, an effect easily attributable to soil increasing the lower concentrations. The average of the 39 Illinois monthly samples is 22.7 X lo3 atom/g with a single measurement uncertainty of 66%, of the 36 New Jersey monthly 16.7 X lo3 atom/g with an uncertainty of 73%. THE DATA-CO~E~TIONS
AND LACK THEREOF
Negative or unimportant correlations
n
:
-I
FIG. 1. Histograms of %e and ‘Be concentrations of different samples. The top panel presents ‘@Bedata from the Hawaii station, the bottom ‘Be data from the IUinois station. The Hawaii samples are generally for cobction periods of a week, the Illinois are for individual precipitation events.
In plotting various data, one against the other, certain patterns can be observed and others seen not to exist. Some of the latter can thus be disposed of in a general way because ah groups show similar behavior. Perhaps the most important lack of correlation is with rainfall. The concentration of both ‘Be and ‘?3e for all sample groups show no dependence on rainfaIl. Indeed none of our data correlate with rainfall There is also no correlation between “Be and Na or Cl. There is no correlation between Na and Cl except for the Hawaii samples (with CI about 60% greater than the Na), hardly a surprise given the insular nature of that station. There are no other correlations found for Hawaii, not even those for Ca and Mg, which show up in the continental stations. There is a hint of NaCl correlation in the Illinois event group. No significant correlations are found with K and NH,, and the anions SO, and NO3. ‘Be against “Be The data of the two Illinois groups show three correlations, two of which will help us to estimate the extent of the dust
L. Brown et al.
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contamination. Of fundamental importance is the relationship of’Be to ‘%e, shown in Fig. 2. The straight line through the data gives a ratio of ‘Be/“Be of 0.68. These data have had no correction for dust con~mination. We shall return to that point later in determining a better value of ‘Be/‘?3e. We shall use a corrected ratio as another approach to determining the deposition of “Be. Correlations with soil indicators The Illinois data, both by event and by month show enough correlation between the ‘%e ~on~en~tion and the dust indicator ions Ca and Mg to allow some estimate of this source. These data are plotted in Figs. 3a and 3b, each showing Ca and Mg against “Be. The New Jersey data show less correlation between “Be and Ca and Mg than the Illinois data, as can be seen by comparing Fig. 3c with the corresponding predecessors, The higher average “Be concentrations in the Illinois samples point qualitatively towards soil con~ination. We have some measure of the amount of soil in the samples (all of which entered with the rain) through the Ca, Mg, Na and K ions but only guesses as to the “Be concentration in the dust. We choose to use the Illinois and New Jersey monthly data as checks on conclusions drawn from the Hawaii ‘s by assuming Hawaii to have the correct value and calculating the “Be concentration of the soils in the Illinois and New Jersey samples from the amount of the isotope present that is in excess of the Hawaii concen~tion. We note first that the New Jersey value of 16.7 X lo3 atom/g is essentially the Hawaii value, indicating that the uncontaminated value to be even lower. The Illinois values of 2 1.3 X IO3 atom/g for the event group and 22.7 X lo3 atom/g for the monthly samples are 5200 and 5800 atom/g in excess of the Hawaii value, implying 25% results from dust. One can determine the “Be concentration of the dust that produces this by using the composition of Illinois soil by BOERNGEN and SHACKLETTE (1981) of 0.60% Ca and 0.42% Mg. From the data of Fig. 3 one obtains a dust of 4 X 10’ atom/g from Ca and 3 X 10’
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FIG. 2. Correlation plot of the concentration of ‘Be against ‘*Be for the Illinois event data. The straight line was fitted by the least squares technique, has a slope of 0.678 and a gave a correlation coefficient of 0.70 1.
atom/g from Mg. The lower value from Ca may result from the extensive use of crushed limestone on roads near the Illinois site (BARNARD et al., 1986; STENSLANDet al., 1986). otherwise the values are reasonable (PAVICH et al., 1986). Accepting the 25% excess of the Illinois over the Hawaii concentration as correct allows us to go one step further. We can now evaluate a corrected value for the ‘Be/“Be ratio determined from the Illinois event group. The uncorrected ratio of 0.68 is multiplied by I .25 to yield the c0rrecttx3 ratio of 0.85. An anoma~o~ Illinois rain One measurement for the Illinois event samples taken on 6 May 1983 had a “Be concentration of 549 X lo3 atom/g. This resulted from a rain that followed a dust storm and has very high values of 4.49 mgjl Ca and 0.795 mgjl Mg. This was a wet-only sample and demonstrates the potential for soil contamination even if this technique is used. Taking the Ca and Mg values and assuming that 532 X 1O3atom/g was excess resulting from dust leads one to ‘(Be ~n~ntm~ons of the dust of 7 X lOa atom/g from Ca and 3 X 10’ atom/g from Mg, high but within the range of observed soil values. DE~S~ON
RATE
If one accepts that the Be isotopes are deposited in rain and snow and notes that the average “Be concentrations that we observe at sites with greatly differing rainfall are about the same, then one is forced to express the deposition rate in terms of a standard amount of rainfall rather than in terms of atoms per unit area and time. Given the kind of data reported here, a resulting deposition rate can be given a global interpretation only if one knows the average global precipitation and the latitude effect, which are observationally not well known to say the least. LAL and PETERS (1967) and O’BRIEN (1979) estimated global production rates, showing a minimum at the equator. How the meteorological effects of the months the isotopes spend in the stratosphere affect the latitude dependence of deposition is not known. Further complicating matters is the limited knowledge about the concentration of Be isotopes in equatorial rains, which contribute a significant fraction of the Earth’s precipitation, as there is only a single ‘Be measurement (LAL et al., 1979). Inasmuch as there is a limited amount of the isotopes in the atmosphere, one would suspect that such rains have significantly less than those of temperate climates reported here. Rainfall near the equator is about twice that of the temperate latitudes (SELLERS,1965). Such wash out may be the explanation for the deficiency of 7Be found in tropospheric air near the equator by VIEZEEand SINGH(1980). The unknown extent of dry deposition in desert regions adds an additional element to the puzzle. We see three approaches to the question, given the restrictions of our data. ( 1) The Hawaii measurements obviously present the best possibility of avoiding the effects of soil contamination and will be interpreted in a straightforward manner. (2) The Be data of the Illinois events are another set that does not suffer from soil contamination and that can be used indirectly to determine the “Be deposition rate. (3) The monthly samples from Illinois and New Jersey are flawed as
139
Atmospheric ‘Be and ‘?3e
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*’
l
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1
1
r
b
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/
1
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*
0
60
20
7
RG, 3. Correlation plots of the concentration of Ca and Mg against ‘%e for (a) the Illinois event data, (b) the BPnois monthly data and (c) the New Jersey monthly data. The straight lines in the Illinois
data were fitted by the least squares technique. Some of the data lie outside the panel boundaries.
%C
(IO6 atom/g)
a result of the contamination but serve nevertheless as checks on conclusions drawn from the Hawaii data. En the data one finds the isotope ~n~t~tion and the precipitation, from which one can calculate deposition rates per cm of precipitation, which leads to deposition in units of atom/cm3. This unit, trivialally different from atom/g for these data, distinguishes herein deposition from concentration. Depaaitian is, of course, only the wei&t& average of the concentration with rainfall a the weighting quantity. The deposition for “Be in Hawaii is found to be 1.69 X 104atom/ cm’. The deposition of ‘Be for the Illinois event group is I .44 X IO4 atom/cm3. One can convert the ‘Be deposition to a %e deposition with the %e/%e ratio, which we have determined to be 0.85 and which gives a value of 1.69 X 10’ atom/cm’, for obvious msons very nearly the same as for Hawaii. The seven temperate Iatitude depositions of ‘Be found in TUREKWN etal.(1983) average 9.7 X 10’ atom/ cm3. bWBECK etal.f198 f b> report a 7Be &eposition of t 3.3 X lo3 atom/cm3. BLEICHRODT (1978) has rainfall data that can be changed to deposition, giving 9.01 X IO3 atom/cm3 with a remarkably small singk measurement uncertainty of 0.3% for five stations scattered worldwide. If one averages all ofthe reported ‘Be depositions, the result is 10.5 X IQ3atom] cm3 with a single measurement standard deviation of M,
L. Brown et al.
140
implying 13% for the average. From our 7Be/‘aBe of 0.85 we get a “Be deposition of 12.4 X lo3 atom/cm3. We now have three quasi-independent measurements for “Be deposition in temperate latitudes. Expressed in units of 10’ atom/cm3 they are the straight Hawaiian value of 1.69, the New Jersey value of 1.67, which is almost certainly high because dust is known to be present, and the value derived from ‘Be of 1.24. By now the reader will be well aware of the uncertainties inherent in all this and will understand the difficulty of selecting the best value. We shall simply average the three, which gives 1.53 X lo4 atom/cm3 with an uncertainty of 20%. There are three estimates of the global deposition rate made from what knowledge there is of nuclear reactions in the atmosphere. LAL and PETERS (1967) obtained 1.3 X 10” cmd2 yr-‘, O'BRIEN (1979) 0.8 X lo6 cm-’ yr-’ and REYSS et af. (1981) 0.7 X lo6 cm-* yr-‘. Let us now inquire as to whether our datum of 1.S X lo4 atom/cm3 is consistent with the average of the two recent estimates. To do this we calculate the total annual precipitation in an equatorial, two temperate and two polar zones from the data given by SELLERS(1965) on the latitude dependence of precipitation. The results are given in Table 1. If one assumes that the contributions of the polar zones are negligible, that the two temperate zones deposit equal numbers of atoms and that the total number for temperate zones is twice the product of the north temperate precipitation with our value of 1.5 X 1O4atom/cm3, then we have 9.8 X 1O23atom/yr left for the equatorial zone. This yields an average deposition of 3300 atom/cm3. Confi~ation awaits measurements of dust free equatorial rain. The single measurement of ‘Be deposition at Bombay by LAL et al. (1979) is suggestively low, 3.6 X lo3 atom/cm3, which translates into 4.2 X 10’ atom/cm3 for “Be. We note that SOUTHON et a!.(1987) have estimated the average global production rate from the “Be concentration and the sedimentation rate for a pelagic core located at 4 1“N, 22’W to be 6.6 X lo5 cmV2 yr-‘, uncertain by 25%, a result in agreement with both of the two recent theoretical values. Given the long residence time of the isotope in the ocean, a thorough homogenization of equatorial and temperate zone deposition is reasonable. One must be careful in using pelagic sedimentation for determining ‘Qe deposition rate, as there is evidence that the flux is proportional to the sedimentation rate, the sediment having to a first approximation the same concentration of the isotope (TERA etal., 1986, and references therein). Deposition rates so derived will necessarily be strongly dependent on local conditions. As examples RAISBECK et al. (198 1) report deposition rates derived from cores
TABLE
1. Summary
of Global Precipitation
taken in the Indian Ocean with an average of 2.1 X lo6 cm-’ yr-‘, whereas MANGINI et al. (1984) obtained 6.0 X 10’ cm-’ yr-’ for a core at 30”N, 157’W, in good agreement with SOUTHONet al., but they report values an order of magnitude higher in regions of high biological activity. DRY DEPOSITION The extent of primary “Be dry deposition can not be estimated for land stations because of its resuspension in dust, but this source must hold to a much lower degree for ‘Be, probably negligible for the accuracy of the data presented here. To ascertain this rate we made two observations of dry deposition. Plastic buckets offering a total opening of 7550 cm2 were filled to about 1 cm with demineralized water acidified with HCl and spiked with about 1 mg of 9Be. They were exposed to the air in November 1984 at Washington during times when no precipitation occurred. At night the buckets were sealed and brought indoors. These measurements must necessarily be expressed in atoms per unit area and time. In order to compare them with rain we use the product of the previously stated rain deposition and the annual rainfall for the District of Columbia of 103.6 cm (RUDLOFF, 1981). We also made a measurement of ‘*Be collected dry at the Illinois site, where dust is more prevalent than in the District of Columbia. The collection area was 680 cm2 and the observation period was 200 hr; assurances were made beyond the normal use of the dry side of a standard collector that no precipitation took place and that the collection period was not particularly dusty. No 7Be measurement was made on this sample. All results may be compared in Table 2. We note that the dry deposition of 7Be is less than 10% of the amount deposited by rain, rendering it small compared with the general uncertainties of rain deposition. We also note that the dry component of “Be observed in the District of Columbia was of the order of 40% that observed from rainfall. The dry component observed at the Illinois site was 147% that observed from rainfall. Given the degree of soil contamination also found in wet only collectors, one must concede the extreme difficulty of sampling the “Be flux with bulk collectors, i.e. those open at all times. If the ratio ‘Be/“Be of 0.85 that we determined from the Illinois event data applies to the primary dry component, we can calculate the dry flux of primary “Be to be 1.5 X 10’ atom cmm2 yr-’ with an uncertainty of 45%. DISCUSSION
OF PREVIOUS
OBSERVATIONS
Strictly speaking we cannot compare our data with previous observations because we have chosen to report deposition rate for a standard amount of rainfall rather than as a globalannual deposition rate and shall restrict discussion here to precipitation measurements. The problems with the two papers (RAISBECK et al., 1981b; SOMAYAJULUef al., 1984) wherein soil contamination is not considered have been noted and require no further comment. The work of MONACHAN et al. (1985) approaches the problem of global deposition ingeniously but suffers from two defects. (1) They used bulk collectors open for a period of about a year, their object being to determine the total annual flux into the containers. This
141
Atmospheric ‘Be and “Be TABLE
Samnlz
2. Summary
DC-1
necessarily enhances the recycled component, which they estimate by chemical analysis aimed at determining the amount of soil deposited and correcting for this component by assuming the ‘OBe concentration in the soil to have been 5 X 1O* atom/g, regardless of the location of the station. Corrections for their seven continental stations ranged from 8% to 35%, amounts similar to what we have observed for wet only collection and substanti~ly less than what we have observed for dry deposition. (2) Rain concentrations were not measured because the open collectors were intended to sample the total loBe flux. Unfortunately their “collectors were placed on top of buildings or on the ground.” Sampling locations not selected according to standard meteorological method, especially the tops of buildings, have long been known to give erratic values of rainfall, to which the flux is directly ~ropo~io~al (ABBE, f899; KURTYIGQ, 1953). It is better to determine rainfall by standard rain gauges as we have done. CONCLUSIONS The determination of the deposition rates of the cosmogenie Be isotopes would seem to be a problem that suffers rather than gains from increasing study. Since the initial reports, complications have been found to lie in contaminations that are difficult to avoid or correct and in extreme variations of concentration from rainfall to rainfall necessitating long periods of observation. Even if these troubles are surmounted the problem of converting rainfall deposition into a global rate remains and the matter oflatitude dependence has been approached only superficially. In this work we have demonstrated clearly the effects of soil contamination on the Illinois and New Jersey samples and used knowledge of it to strenghthen our assumption that the Hawaiian rains have the same amount of “Be as uncontaminated samples from a continental station would have. Our data for 7Be, which do not suffer from soil contamination, have a distribution in concentration that is similar to the Hawaiian “Be. This is important support for the Hawaiian data, as they are less numerous, which would make their extreme values suspect but for the similar pattern in 7Be. And note that for both Hawaiian “Be and Illinois ‘Be the deposition is dominated by high concentration events. The determination of the distribution in concentration, both observationally and theoretically, lies in the future. Despite the morose temper of the preceding paragraph, we find our study has clarified certain elements of the problem. ( 1) There is moderate evidence that rains at middle lat-
of
Dry Deposition Results
DC-2
IL-1
itudes and in temperate climates have the same average ‘OBe concentrations, (2) This has as a consequence that local primary, i.e. not resuspended, deposition is overwhelmingly p~po~ion~ to rainfall, which we evaluate from our own and other data to be 1.5 X 104atom/cm3, uncertain by 20%. It is worth mentioning at this point that this deposition is in agreement with measurements of Arctic ice, whose measured concent~tions correspond to the de~nition of deposition. For Greenland ice laid down between 1860 and 1890 the value is 1.6 X lo4 atom/cm’ (BEER et al., 1984). It is not in agreement with Antarctic ice. RAISBECKet al. (198 la) found the concentration for roughly the same period in the Dome C core to be 5 X t09 atom/cm3. (3) Observations of rBe, especially in conjunction with “Be, are useful in circumventing the problem of soil contamination, which is sufficiently serious to preclude using loBe taken Born continental stations not shown to be essentially dust free, (4) These data are consistent with theoretical global estimates, if equatorial rain has an average concentration of the order of 3300 atom/ g, (5) Our best estimate for mid-latitude 7Be deposition is 1.O X lo4 atom/cm3, uncertain by 40%. Acknowledgements-The samples from Hawaii were provided by Alan Yoshinaga of the Mauna Loa Observatory, the New Jersey samples, by Donald C. Bogen of the Dept. of Energy Environmental Measurements Laboratory, New York City. Without their help this work would have much lower value. The support of the National Science Foundation, through the Division of Nuclear Physics and the Dept. of Energy through grant no. DE-FGO2-88ER-60635 are gratefully acknowledged. A reviewer pointed out the existence of some ‘Be data that had escaped our attention and for which we are grateful.
REFERENCES AWE C. f 1899) Effect of wind on
catch of rainfall. ~~~~~~~We&~r Rev. 27,308-310. BARNARD W. R., STENSLAND G. J. and GATZ D. F. (1986) Alkaline
materials flux from unpaved roads: Source streqth, chemistry and potential for acid rain neutralization. Water, Air Soil P&t. 30, 285-293. BEERJ., OESCHGERH., ANDRE M., BONANIG., SUTERM., WOEFLI W. and LANGWAYC. C., JR. (1984)Temporal variations in the loBe concentration levels found in the Dye 3 ice core, Greenland. Ann. Glacioc. 5, 16-127. BLEICHRODT J. F. ( 1978) Mean tropospheric residence time ofcosmio ray-produced beryllium 7 at north temperate latitudes. f. Geophys. Res. 83,3058-3062.
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BOERNGENJ. C. and SHACKLETTEH. T. (198 1) Chemical analyses
of soils and other surficial materials in the coterminous United States. U.S. Geol. Surv. Rept. 81, 197. BROWNL. (1984) Applications of accelerator mass spectrometry. Ann. Rev. Earth Planet. Sci. 12, 39-59. BROWNL. (1987) “‘Be: recent applications in Earth Sciences. Phil. Trans. Roy. Sot. London A323,75-86. ELMORED. and PHILLIPSF. M. (1987)Accelerator mass spectrometry for measurement of long-lived radioisotopes. Science 236, 543550. ~NKEL R., KRISHNASWAMI S. and CLARKD. L. (1977) “Be in arctic ocean sediments. Earth Planet. Sci. Lett. 35, 199-204. HARDY E. P. (1968) Strontium-90 on the Earth’s surface. Nature 219,584-587. KLEIN J., MIDDLETONR. and TANG H. (1982) Modifications of an FN Tandem for quantitative ‘OBemeasurement. Nucl. Inst. Meth. 193,601-616. KURTYKA J. C. (1953) Precipitation measurements study. Report of Investigation No. 20, State Water Survey Division: Urbana, Illinois. LAL D. and PETERSB. (1967) Cosmic ray produced radioactivity on the Earth. Handbuch der Physik 46/2,551-612. LAL D., NIJAMPURKARN., RAJAGOPALANG. and SOMAYAJULU B. L. K. (1979) Annual fallout of 32Si,“‘Pb, 22Na, “S and ‘Be in rains in India. Proc. Indian Acad. Sci. 88A, 29-40. LITHERLANDA. E. (1980) Ultrasensitive mass spectrometry with accelerators. Ann. Rev. Nucl. Sci. 30, 437-473. MCCORKELLR., FIREMANE. L. and LANGWAYC. JR. (1967) Aluminum-26 and beryllium- 10 in Greenland ice. Science 158, 16901692. MANGINI A., SEGL M., BONANIG., HOFMANNH. J., MORENZONI E., NESSIM., SUTERM., WOELFLIW. and TUREKIANK. K. (1984) Mass-spectrometric “‘Be dating of deepsea sediments applying the Zuerich tandem accelerator. Nucl. Inst. Meth. B5, 353-358. MIDDLETONR., KLEIN J., RAISBECKG. M. and YIOU F. (1983) Accelerator mass spectrometry with 26Al.Nucl. Inst. Meth. 218, 430-438. MIDDLETONR., KLEIN J., BROWNL. and TERA F. (1984) “‘Be in commercial beryllium. Nucl. Inst. Meth. B5, 5 I l-5 13. MONAGHANM. C., KRISHNASWAMI S. and TUREKIANK. K. (1985) The global-average production rate of “‘Be. Earth Planet Sci. Lett. 76, 279-287. O’BRIENK. ( 1979) Secular variations in the production of cosmogenic isotopes in the Earth’s atmosphere. J. Geophys. Res. 84,423-43 1. PAVICHM. J., BROWNL., HARDENJ., KLEIN J. and MIDDLETON R. (1986) “Be distribution in soils from Merced River terraces, California. Geochim. Cosmochim. Acta 50, 1727-1735. RAISBECKG. M., YIOU F., FRUNEAUM., LQISEAUXJ. M., LIEUVIN M. and RAVELJ. C. (1979) Deposition rate and seasonal variations in precipitation of cosmogenic “Be. Nature 282, 279-280. RAISBECKG. M., YIOU F., FRUNEAUM., LOISEAUXJ. M., LIEUVIN M., RAVELJ. C. and I.,CIRIUS C. (1981a) Cosmogenic “‘Be con-
centrations in antarctic ice during the past 30,000 years. Nature 292,825-826. RAISBECKG. M., Ylou F., LIEUVINM., RAVELJ. C., FRUNEAUM. and LOISEAUXJ. M. t 198lb) ‘%e in the environment: Some recent results and their applications. In Proc. Symp. Accel. Mass Spectrom. (eds. W. HENNING,W. KUTSCHERA,R. K. SMITHERand J. L. YNTEMA),pp. 228-243. Argonne Natl. Lab., Argonne, Illinois. RAISBECKG. M., YIOU F., FRUNEAUM., LOISEAUXJ. M., LIEUVIN M. and RAVELJ. C. (1981) Cosmogenic “Be/‘Be as a probe of atmospheric transport processes. Geophys. Res. Left. 8, 1015-1018. REYSSJ.-L., YOKOYAMAY. and GUICHARDF. (198 1) Production cross sections of 26AI,22Na, ‘Be from argon and of ‘%, ‘Be from nitrogen: Implications for production rates of 26Aland “Be in the atmosphere. Earth Planet. Sci. Lett. 53, 203-2 10. RUDLOFFW. (1981) World-Climates with Tables of Climatic Data and Practical Suggestions. Wissenschaftliche Verlagsgesellschaft, Stuttgart. SELLERSW. D. (1965) Physical Climatology. University of Chicago Press, Chicago. SOMAYAJULUB. L. K. (1977) Analysis of causes for the beryllium10 variations in deep sea sediments. Geochim. Cosmochim. Acta 41,909-913. SOMAYAJULU B. L. K., SHARMAP., BEERJ., BONANIG., HOFMANN H.-J., MORENZONIE., NESSI M., SUTER M. and WOELFLIW. (1984) “Be annual fallout in rains in India. Nucl. Inst. Meth. B5, 398-403. SOUTHONJ. R., Ku T. L., NELSOND. E., REYSSJ. L., DUPLESSY J. C. and VOGELJ. S. (1987) “Be in a deepsea core: Implications regarding “Be production changes over the past 420 ka. Earth Planet. Sci. Lett. 85, 356-364. STENSLAND G. J., BROWNL., KLEINJ. and MIDDLETONR. (1983) Beryllium-10 in rain. Eos 64, 283. STENSLAND G. J., WHELPDALED. M. and OEHLERTG. (1986) Precipitation chemistry. In Acid Deposition: Long-term Trends. (ed. JAMESH. GIBSON).Natl. Acad. Press, Washington, D.C. TANAKAS. and INOUET. (1979) ‘%e dating of North Pacific sediment cores up to 2.5 million year B.P. Earth Planet. Sci. Left. 45, 18 I187. TANAKAS. and INOUET. (1980) ‘%e evidence for geochemical events in the North Pacific during the Pliocene. Earth Planet. Sci. Lett. 49, 34-38. TERA F., BROWNL., MORRISJ., SACKSI. S., KLEIN J. and MIDDLETONR. (1986) Sediment incorporation in island-arc magmas: Inferences from “Be. Geochim. Cosmochim. Acta 50, 535-550. TUREKIANK. K., BENNINGERL. K. and DION E. P. (1983) ‘Be and 2’oPb total deposition fluxes at New Haven, Connecticut and at Bermuda. J. Geophys. Res. 88, 541 l-5415. VIEZEEW. and SINGHH. B. (1980)The distribution of beryllium-7 in the troposphere: Implications on stratospheric air exchange. Geophys. Res. Lett. I, 805-808. WAHLENM., KOTHARIB., ELMORED., BANERJEES. K. and LESKEE W. (I 983) “Be in lake systems. Eos 64, 282.
0016-7037/89/$3.00 + .CUl
Atmospheric
deposition of ‘Be and “Be
LOUIS BROWN’, GARY J. STENSLAND’,JEFFREYKLEIN~and ROY MIDDLET~N~ ’ Department of Terrestrial Magnetism, Carnegie Institution of Washington, Washington, D.C. 20015, U.S.A. ’ Illinois State Water Survey, Champaign, IL 6 1820, U.S.A. ’ Tandem Accelerator Laboratory, University of Pennsylvania, Philadelphia, PA 19104, U.S.A. (Received April 1I, 1988; accepted in revisedform October 14, 1988)
Abstract-Measurements of “Be in precipitation taken in Hawaii, Illinois and New Jersey over a period of five years arc reported. The problem of contamination by the isotope being resuspended on wind blown soil that is also collected is addressed. Rain collected at Mauna Loa, Hawaii has such low values of dust contamination that it has been taken as clean, and the data from Illinois and New Jersey are evaluated on that assumption. The conclusion is that the deposition in a given amount of rain for the non-resuspended component is the same for ah three stations, and we propose that the annual rate for mid-latitude locations having moderate rainfall is proportional to the local rainfall. ‘Be, which is probably negligibly contributed to the measurements by soil contamination, was measured for individual rains in Illinois and found to have a deposition of 1.4 X 10’ atom/cm3. We have found that concentration variations between precipitation events greater than a factor of 20 exist for both isotopes and that relatively rare, high concentration events dominate deposition, thereby requiring long periods of observation to avoid significant error. Based on our own and other data we conclude that the best value for “‘Be deposition is 1.5 X IO4atom/cm”, uncertain by 201, and for ‘Be is 1.2 x lo4 atom/cm3, uncertain by 25%. A global average deposition rate cannot be inferred directly for either isotope from these kinds of data; however, the theoretical global deposition rate for ‘% is shown to be consistent with the deposition reported here, if the concentration in equatorial rain is about 3300 atom/g.
INTRODUCTION
MAYAJULU,1977; FINKELet al., 1977; RAISBECKet al., 1981; SOUTHON et al., 1987) lacustrine sediments (WAHLEN et al., 1983) and polar ice cores (MCCORKELLet al., 1967; BEER et al., 1984).
MANY OF THE APPLICATIONS of “Be as a geochemical tracer require a knowledge of its atmospheric deposition rate, and indeed one of the early measurements of the isotope by RAISBECK ef al. (1979) addressed this matter. This was the first measurement of “Be concentration in rainwater, made possible by the use of a nuclear particle accelerator as a mass spectrometer. As other observers made use of the new technique additional reports of deposition rate measurements followed. At first glance this would seem to be the simplest kind of “Be investigation, as the sampling appears straightforward and the extraction chemistry simple. STENSLANDet al. (1983), however, demonstrated that for a rural Illinois location the contamination of rainwater with windblown soil and other atmospheric dust introduces significant amounts of “Be, a problem that persists even when wet-only samplers are used (devices that are open only at the onset of the rain), and that can be extreme if no provision is taken to block dust input. We note here a second complicating effect not yet commented on to our knowledge: the variation in the isotope’s concentration from one collection to another is sufficiently great to make that an important contribution to the uncertainty of the result. Additional reports of observations of ‘%e in rain appeared after R4ts~EcK et al. (1979) introduced the accelerator method with RAISBECKet al. ( 198 1b) extending their original work at Orsay, France, and SOMAYAJULU et al. (1984) reporting data from eight stations in India. The first full report that addressed dust contamination came from MONAGHANet al. ( 1985). All of these observations were made with collectors that were open all the time. Measurements of ‘Be deposition are reported by TUREKIAN et al. (1983) together with five references cited therein and by BLEICHRODT(1978) and RAISBECKet al. (198 lb). Indirect measurements of the “Be deposition have been made using pelagic sediments (TANAKA and INOUE, 1979, 1980; So-
Our studies of “Be in rain are in two parts. The first is a two year period of measurements of daily precipitation sampled at the Bondville site of the Illinois State Water Survey. We obtained concentrations of “Be for 62 and 7Be for 63 rains as well as the concentrations of various cations and anions. The second part of the study is an additional three years in which 36 monthly samples were taken at the New Jersey site of the Environmental Measurement Laboratory and 29 weekly or longer period samples (as was often required to obtain usable quantities of water) from the Mauna Loa site of the Hawaiian Global Monitoring for Climate Change Observatory. For comparison 40 monthly samples were analyzed from the Illinois site as well. No measurements of ‘Be were made for any of these samples in the second part because of the uncertain time correction for decay, but all had the same ion determinations as those of the first part. In all cases we sampled with wet-only collectors. Precipitation was measured separately at each site with a weighing bucket gauge as one of the standard observations. The Mauna Loa site was selected because negligible contamination by dust is encountered there. A straightforward interpretation would allow us to use these data as direct measurements of “Be concentration in rain water and allow us to calculate the deposition without complication. The island location and the high altitude of the site make us uneasy with such an interpretation, so we elected to take similar samples at the New Jersey site, which is continental but with lower dust contamination than the Illinois site. Earlier studies of “Sr showed that total deposition was proportional to the rainfall with negligible amounts in dry deposition (HARDY et al., 1968). We made direct measurements in Illinois and the District of Columbia for the dry 135
L. Brown et al.
136
of ‘Be-measurements that are impossible for “Be because of the amount of the isotope on resuspended dustdeposition
and demonstrated that this mechanism with our observational uncertainties.
is negligible compared
DETERMINATION OF ‘Be AND “Be CONCENTRATIONS ‘% is determined with the mass spectrometer technique of isotope dilution, ‘Be with low level counting, so 9Be is added to the samples to serve as a spike for the former and as a carrier for the latter. After spiking with 9Be and acidification with HCl following collection, the Illinois and Hawaii samples were shipped to the Department of Terrestrial Magnetism for the determination of the ‘Be and the preparation of the Be for the determination of the ‘%e with the University of Pennsylvania tandem Van de Graaff accelerator. The New Jersey samples were sent in 20-liter plastic buckets neither spiked nor acidified. In order to ascertain whether ‘Be is lost through sticking to the walls of the collector receptacle or the shipping container or is lost in the chemical extraction, three 9Be-spiked and acidified samples were spiked with ‘Be. Within an experimental error of 5% no loss was observed. Similar experiments with the New Jersey samples showed no Be sticking to the container walls. The samples were evaporated in three stages using a 2000-ml pyrex beaker to reduce the volume to about 200 ml after which it was transfered to a 6OO-ml and then to a 30-ml teflon beaker where it was taken to dryness with a few drops of HClO, to destroy organics. It was then converted to a Be(OH)3 gel that was concentrated in the bottom of a 15-ml centrifuge tube. The ‘Be content of the gel was measured by counting the 0.478MeV y ray with a 7.5-cm NaI well detector located in a lead cylinder of 27 cm diameter, 46 cm height and 10 cm covering plug. This apparatus is located in a basement room constructed for low level counting. The counter efficiency was determined by measuring 0.2 &i of ‘Be with the 7.5 cm counter and then with a calibrated 20cm NaI 4a detector at the National Bureau of Standards. Counts were accumulated in a pulse height analyzer during periods of 80,000 set, beginning at about the same time each day. Measurements were usually made on groups of five or six samples with about the same number of background measurements both preceding and following each group. The average number of background counts for a measurement was about 5.1 X 104, but the uncertainty that typically resulted from a group of background measurements was 330, about double the square root of the number of counts in a single measurement. The ‘Be in the samples gave from 2200 to 12,000 additional counts, yielding uncertainties ranging from 18% to 14%, respectively. The “Be content was determined using accelerator mass spectrometry, an experimental technique that employs a nuclear particle accelerator as a mass spectrometer. For details read LITHERLAND (1980); for a survey of applications read BROWN(1984, 1987) and ELMOREand PHILI.IPS (1987); for details of the Penn tandem read KLEINet al. (1982) and MIDDLETONet al. (1983). The accelerator yields a measure of the ratio “BepBe from which one can determine the number of “Be atoms in the sample from the amount of 9Be added in the spike. The extracted Be in the form of Be(OH)* can be converted to Be0 in a form satisfactory for the ion source by baking it at 550°C for an hour. The details of the mass spectrometry are identical to those described TERAet al. (1986). The uncertainties of the ‘a data were taken to be 15%, typical of standard reproducibility for count rates typical of the rains. STANDARDS
FOR 9Be AND “Be
Isotope standards of “Be/‘Be have been prepared by various investigators by diluting a quantity of “Be with 9Be, the 4 See NAPS Document No. 04642 for five pages of supplementary material. Order from NAPS, % Microfiche Publications, P.O. Box 35 13, Grand Central Station, New York, NY 10163-35 13. Remit in advance, in U.S. Funds only, $7.75 for photocopies or $4.00 for microfiche. Outside the U.S. and Canada, add postage of $4.50 for the first 20 pages and $1.00 for each of the ten pages of material thereafter, or %1.50 for microfiche postage.
“Be being determined by p-ray counting. This has not yet led to a generally accepted standard because of uncertainty about the value of the isotope’s half life. Experience with the developing experimental technique over eight years causes us to accept the machine in its present form as yielding absolute ratios more accurate than 5%. Throughout the years of rain observations we normalized our data to a house standard, so all the results reported here are consistent. The 9Be spike solutions were prepared by weighing BeFz after drying overnight at 80°C. Solution acidity was 0.1 M HCl. We measured the concentrations subsequently with an inductively coupled plasma spectrometer and found agreement at the percent level. The 9Be of the spike proved to be contaminated with “Be at the lo-l4 level (MIDDLETON et al., 1984). Data were corrected in the few cases where this could not be neglected. THE DATA-CURSORY
EXAMINATION
The data are grouped as (1) the weekly (or longer) collections at Mauna Loa, (2) the precipitation events at the Illinois
site, (3) the monthly collections at the Illinois site, and (4) the monthly collections at the New Jersey site. The data include ion concentrations for Ca, Mg, Na, K, NH4, Cl, NO3 and SO.,. Let us first describe the various groups after which we shall inquire about correlations. Mauna Loa samples
We discuss the Mauna Loa samples first because their interpretation is simplest. Not all rainfall for the 1100 day period is accounted for because we failed in some of the determinations, owing to the small amount of water collected. Weekly samples were desired but composites of more than one week were often used. The concentrations of Ca and Mg are the best indicators of soil contamination, which implies for continental locations resuspended “Be. The values for Mauna Loa are sufficiently small compared with the other data sets to make this a negligible source of error. In one case of large Ca and Mg concentration, the effects must also be small because this material was volcanic dust, which has “Be at concentrations two orders of magnitude less than most soils (TERA et al., 1986). This allows us to discuss these data assuming with some assurance that only soil-free deposition takes place. The “Be concentrations take on extreme values, from 1.9 X lo3 to 89.4 X lo3 atom/g, a variation of nearly a factor of 50. We do not know what properties of the atmosphere determine the concentration in a given rain and hence do not know what theoretical distribution of concentration is to be expected. It is evident that a few high-concentration rains dominate and cause large uncertainties. The average concentration weighted by precipitation is 16.9 X lo3 atom/g. Interpretation in terms of a normal distribution yields a standard deviation of 113% for a single measurement, implying for the average after three years (29 determinations) an uncertainty of 2 1%. Illinois event samples
The sample group with the next level of complication is made up of 68 precipitation event samples taken at the
Atmospheric ‘Be and ‘“Be
Bondville, Illinois site and for which both Be isotopes were measured with a few exceptions. The rains of these events had to be strong enough to cause the collectors to open and to furnish enough water to measure-generahy 0.5 liter or more. The mid-point time of each event is known and the 7Be measurements were corrected to give the concentration at the time of the rain. The variation in concentration for “Be is large-from 5.2 X lo3 to 69.7 X 10’ atom/g-though not so great as for the Mauna Loa group. The precipitation weighted average concentration is 2 1.2 X lo3 atom/g with a single measurement uncertainty of 75%. The data of 6 May 1983 are excluded and will be discussed later. The variation of the ‘Be concentration ranges almost as far as does the “Be of Mauna Loa, 3.1 X lo3 to 65.1 X 10’ atom/g more than a factor of 20. The weighted average is 14.1 X 10” atom/g with an uncertainty of 59%. The similarity in the distributions of ‘Be and “Be for the Illinois event and Hawaii is illustrated in Fig. 1. Here the complication, found in most land based stations, of “Be resuspended from soil is encountered. Numerous studies of ‘(Be in soils (PAVICHetal., 1986, and references cited therein) show typical ranges of concentrations near the surface of 2 X 10’ to 10’ atom/g with fine particles generally having higher than average values. Dust typical of Illinois is reported by BOERNGEN and SHACKLETTE (1981) to have 0.60% Ca and 0.42% Mg. Typical concentrations of these ions in Bondville rain are 0.2 m&l Ca and 0.03 mg/l Mg. If one assumes a “Be dust concentration of 5 X 10’ atom/gdust, ~n~minations of 16 X lo3 atom/g-in result using Ca and 4 X lo3 atom/g-rain using Mg. One obviously cannot correct the rain data in this way because of the large uncer-
137
taint& in the isotope and elemental concentration of the dust, but the measured ion concentrations indicate that the effect certainly cannot be prudently neglected. Dust particles in the air will also scavenge the non-resuspended Be isotopes attached to submicron particles and bring them in the rain, but this component need not trouble us because it is immaterial how the aerosols am entrapped in the rain. Although ‘Be can also be found in soils the haIf life of 53 days makes its concentration there negligible relative to ‘a, so its resuspension from soil is probably negligible. We shah return to this shortly. The ‘Be data are sufficiently numerous to allow one to examine the question of temporal variation. The average concentrations for the springs of 1982 and 1983 are 18.1 and 2 1.9 X lo3 atom/g compared with 13.9 X lo3 atom/g for other seasons with standard deviations for the mean of about 15%, enough to support the existence of spring maxima. Illinois and New Jersey monthly samples These samples, though numerous, are lower in information content than the two preceding groups because they have neither the negligible soil contamination of Hawaii nor the discrete ‘Be concentration of the Illinois event group. Comparisons of these data are helpful in ascertaining the magnitude of typical dust contamination. The data of both Illinois and New Jersey show very similar extremes in concentration to those of the Illinois event samples: 5.8 X 10’ to 67.9 X lo3 atom/g for the Illinois monthly, and 8.2 X lo3 to 75.0 X lo3 atom/g for the New Jersey monthly. Note that the ratio of the extremes is considerably less than for the Hawaii “Be samples or the Illinois ‘Be, an effect easily attributable to soil increasing the lower concentrations. The average of the 39 Illinois monthly samples is 22.7 X lo3 atom/g with a single measurement uncertainty of 66%, of the 36 New Jersey monthly 16.7 X lo3 atom/g with an uncertainty of 73%. THE DATA-CO~E~TIONS
AND LACK THEREOF
Negative or unimportant correlations
n
:
-I
FIG. 1. Histograms of %e and ‘Be concentrations of different samples. The top panel presents ‘@Bedata from the Hawaii station, the bottom ‘Be data from the IUinois station. The Hawaii samples are generally for cobction periods of a week, the Illinois are for individual precipitation events.
In plotting various data, one against the other, certain patterns can be observed and others seen not to exist. Some of the latter can thus be disposed of in a general way because ah groups show similar behavior. Perhaps the most important lack of correlation is with rainfall. The concentration of both ‘Be and ‘?3e for all sample groups show no dependence on rainfaIl. Indeed none of our data correlate with rainfall There is also no correlation between “Be and Na or Cl. There is no correlation between Na and Cl except for the Hawaii samples (with CI about 60% greater than the Na), hardly a surprise given the insular nature of that station. There are no other correlations found for Hawaii, not even those for Ca and Mg, which show up in the continental stations. There is a hint of NaCl correlation in the Illinois event group. No significant correlations are found with K and NH,, and the anions SO, and NO3. ‘Be against “Be The data of the two Illinois groups show three correlations, two of which will help us to estimate the extent of the dust
L. Brown et al.
138
contamination. Of fundamental importance is the relationship of’Be to ‘%e, shown in Fig. 2. The straight line through the data gives a ratio of ‘Be/“Be of 0.68. These data have had no correction for dust con~mination. We shall return to that point later in determining a better value of ‘Be/‘?3e. We shall use a corrected ratio as another approach to determining the deposition of “Be. Correlations with soil indicators The Illinois data, both by event and by month show enough correlation between the ‘%e ~on~en~tion and the dust indicator ions Ca and Mg to allow some estimate of this source. These data are plotted in Figs. 3a and 3b, each showing Ca and Mg against “Be. The New Jersey data show less correlation between “Be and Ca and Mg than the Illinois data, as can be seen by comparing Fig. 3c with the corresponding predecessors, The higher average “Be concentrations in the Illinois samples point qualitatively towards soil con~ination. We have some measure of the amount of soil in the samples (all of which entered with the rain) through the Ca, Mg, Na and K ions but only guesses as to the “Be concentration in the dust. We choose to use the Illinois and New Jersey monthly data as checks on conclusions drawn from the Hawaii ‘s by assuming Hawaii to have the correct value and calculating the “Be concentration of the soils in the Illinois and New Jersey samples from the amount of the isotope present that is in excess of the Hawaii concen~tion. We note first that the New Jersey value of 16.7 X lo3 atom/g is essentially the Hawaii value, indicating that the uncontaminated value to be even lower. The Illinois values of 2 1.3 X IO3 atom/g for the event group and 22.7 X lo3 atom/g for the monthly samples are 5200 and 5800 atom/g in excess of the Hawaii value, implying 25% results from dust. One can determine the “Be concentration of the dust that produces this by using the composition of Illinois soil by BOERNGEN and SHACKLETTE (1981) of 0.60% Ca and 0.42% Mg. From the data of Fig. 3 one obtains a dust of 4 X 10’ atom/g from Ca and 3 X 10’
I
I
I
I
I
I
I
FIG. 2. Correlation plot of the concentration of ‘Be against ‘*Be for the Illinois event data. The straight line was fitted by the least squares technique, has a slope of 0.678 and a gave a correlation coefficient of 0.70 1.
atom/g from Mg. The lower value from Ca may result from the extensive use of crushed limestone on roads near the Illinois site (BARNARD et al., 1986; STENSLANDet al., 1986). otherwise the values are reasonable (PAVICH et al., 1986). Accepting the 25% excess of the Illinois over the Hawaii concentration as correct allows us to go one step further. We can now evaluate a corrected value for the ‘Be/“Be ratio determined from the Illinois event group. The uncorrected ratio of 0.68 is multiplied by I .25 to yield the c0rrecttx3 ratio of 0.85. An anoma~o~ Illinois rain One measurement for the Illinois event samples taken on 6 May 1983 had a “Be concentration of 549 X lo3 atom/g. This resulted from a rain that followed a dust storm and has very high values of 4.49 mgjl Ca and 0.795 mgjl Mg. This was a wet-only sample and demonstrates the potential for soil contamination even if this technique is used. Taking the Ca and Mg values and assuming that 532 X 1O3atom/g was excess resulting from dust leads one to ‘(Be ~n~ntm~ons of the dust of 7 X lOa atom/g from Ca and 3 X 10’ atom/g from Mg, high but within the range of observed soil values. DE~S~ON
RATE
If one accepts that the Be isotopes are deposited in rain and snow and notes that the average “Be concentrations that we observe at sites with greatly differing rainfall are about the same, then one is forced to express the deposition rate in terms of a standard amount of rainfall rather than in terms of atoms per unit area and time. Given the kind of data reported here, a resulting deposition rate can be given a global interpretation only if one knows the average global precipitation and the latitude effect, which are observationally not well known to say the least. LAL and PETERS (1967) and O’BRIEN (1979) estimated global production rates, showing a minimum at the equator. How the meteorological effects of the months the isotopes spend in the stratosphere affect the latitude dependence of deposition is not known. Further complicating matters is the limited knowledge about the concentration of Be isotopes in equatorial rains, which contribute a significant fraction of the Earth’s precipitation, as there is only a single ‘Be measurement (LAL et al., 1979). Inasmuch as there is a limited amount of the isotopes in the atmosphere, one would suspect that such rains have significantly less than those of temperate climates reported here. Rainfall near the equator is about twice that of the temperate latitudes (SELLERS,1965). Such wash out may be the explanation for the deficiency of 7Be found in tropospheric air near the equator by VIEZEEand SINGH(1980). The unknown extent of dry deposition in desert regions adds an additional element to the puzzle. We see three approaches to the question, given the restrictions of our data. ( 1) The Hawaii measurements obviously present the best possibility of avoiding the effects of soil contamination and will be interpreted in a straightforward manner. (2) The Be data of the Illinois events are another set that does not suffer from soil contamination and that can be used indirectly to determine the “Be deposition rate. (3) The monthly samples from Illinois and New Jersey are flawed as
139
Atmospheric ‘Be and ‘?3e
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20
7
RG, 3. Correlation plots of the concentration of Ca and Mg against ‘%e for (a) the Illinois event data, (b) the BPnois monthly data and (c) the New Jersey monthly data. The straight lines in the Illinois
data were fitted by the least squares technique. Some of the data lie outside the panel boundaries.
%C
(IO6 atom/g)
a result of the contamination but serve nevertheless as checks on conclusions drawn from the Hawaii data. En the data one finds the isotope ~n~t~tion and the precipitation, from which one can calculate deposition rates per cm of precipitation, which leads to deposition in units of atom/cm3. This unit, trivialally different from atom/g for these data, distinguishes herein deposition from concentration. Depaaitian is, of course, only the wei&t& average of the concentration with rainfall a the weighting quantity. The deposition for “Be in Hawaii is found to be 1.69 X 104atom/ cm’. The deposition of ‘Be for the Illinois event group is I .44 X IO4 atom/cm3. One can convert the ‘Be deposition to a %e deposition with the %e/%e ratio, which we have determined to be 0.85 and which gives a value of 1.69 X 10’ atom/cm’, for obvious msons very nearly the same as for Hawaii. The seven temperate Iatitude depositions of ‘Be found in TUREKWN etal.(1983) average 9.7 X 10’ atom/ cm3. bWBECK etal.f198 f b> report a 7Be &eposition of t 3.3 X lo3 atom/cm3. BLEICHRODT (1978) has rainfall data that can be changed to deposition, giving 9.01 X IO3 atom/cm3 with a remarkably small singk measurement uncertainty of 0.3% for five stations scattered worldwide. If one averages all ofthe reported ‘Be depositions, the result is 10.5 X IQ3atom] cm3 with a single measurement standard deviation of M,
L. Brown et al.
140
implying 13% for the average. From our 7Be/‘aBe of 0.85 we get a “Be deposition of 12.4 X lo3 atom/cm3. We now have three quasi-independent measurements for “Be deposition in temperate latitudes. Expressed in units of 10’ atom/cm3 they are the straight Hawaiian value of 1.69, the New Jersey value of 1.67, which is almost certainly high because dust is known to be present, and the value derived from ‘Be of 1.24. By now the reader will be well aware of the uncertainties inherent in all this and will understand the difficulty of selecting the best value. We shall simply average the three, which gives 1.53 X lo4 atom/cm3 with an uncertainty of 20%. There are three estimates of the global deposition rate made from what knowledge there is of nuclear reactions in the atmosphere. LAL and PETERS (1967) obtained 1.3 X 10” cmd2 yr-‘, O'BRIEN (1979) 0.8 X lo6 cm-’ yr-’ and REYSS et af. (1981) 0.7 X lo6 cm-* yr-‘. Let us now inquire as to whether our datum of 1.S X lo4 atom/cm3 is consistent with the average of the two recent estimates. To do this we calculate the total annual precipitation in an equatorial, two temperate and two polar zones from the data given by SELLERS(1965) on the latitude dependence of precipitation. The results are given in Table 1. If one assumes that the contributions of the polar zones are negligible, that the two temperate zones deposit equal numbers of atoms and that the total number for temperate zones is twice the product of the north temperate precipitation with our value of 1.5 X 1O4atom/cm3, then we have 9.8 X 1O23atom/yr left for the equatorial zone. This yields an average deposition of 3300 atom/cm3. Confi~ation awaits measurements of dust free equatorial rain. The single measurement of ‘Be deposition at Bombay by LAL et al. (1979) is suggestively low, 3.6 X lo3 atom/cm3, which translates into 4.2 X 10’ atom/cm3 for “Be. We note that SOUTHON et a!.(1987) have estimated the average global production rate from the “Be concentration and the sedimentation rate for a pelagic core located at 4 1“N, 22’W to be 6.6 X lo5 cmV2 yr-‘, uncertain by 25%, a result in agreement with both of the two recent theoretical values. Given the long residence time of the isotope in the ocean, a thorough homogenization of equatorial and temperate zone deposition is reasonable. One must be careful in using pelagic sedimentation for determining ‘Qe deposition rate, as there is evidence that the flux is proportional to the sedimentation rate, the sediment having to a first approximation the same concentration of the isotope (TERA etal., 1986, and references therein). Deposition rates so derived will necessarily be strongly dependent on local conditions. As examples RAISBECK et al. (198 1) report deposition rates derived from cores
TABLE
1. Summary
of Global Precipitation
taken in the Indian Ocean with an average of 2.1 X lo6 cm-’ yr-‘, whereas MANGINI et al. (1984) obtained 6.0 X 10’ cm-’ yr-’ for a core at 30”N, 157’W, in good agreement with SOUTHONet al., but they report values an order of magnitude higher in regions of high biological activity. DRY DEPOSITION The extent of primary “Be dry deposition can not be estimated for land stations because of its resuspension in dust, but this source must hold to a much lower degree for ‘Be, probably negligible for the accuracy of the data presented here. To ascertain this rate we made two observations of dry deposition. Plastic buckets offering a total opening of 7550 cm2 were filled to about 1 cm with demineralized water acidified with HCl and spiked with about 1 mg of 9Be. They were exposed to the air in November 1984 at Washington during times when no precipitation occurred. At night the buckets were sealed and brought indoors. These measurements must necessarily be expressed in atoms per unit area and time. In order to compare them with rain we use the product of the previously stated rain deposition and the annual rainfall for the District of Columbia of 103.6 cm (RUDLOFF, 1981). We also made a measurement of ‘*Be collected dry at the Illinois site, where dust is more prevalent than in the District of Columbia. The collection area was 680 cm2 and the observation period was 200 hr; assurances were made beyond the normal use of the dry side of a standard collector that no precipitation took place and that the collection period was not particularly dusty. No 7Be measurement was made on this sample. All results may be compared in Table 2. We note that the dry deposition of 7Be is less than 10% of the amount deposited by rain, rendering it small compared with the general uncertainties of rain deposition. We also note that the dry component of “Be observed in the District of Columbia was of the order of 40% that observed from rainfall. The dry component observed at the Illinois site was 147% that observed from rainfall. Given the degree of soil contamination also found in wet only collectors, one must concede the extreme difficulty of sampling the “Be flux with bulk collectors, i.e. those open at all times. If the ratio ‘Be/“Be of 0.85 that we determined from the Illinois event data applies to the primary dry component, we can calculate the dry flux of primary “Be to be 1.5 X 10’ atom cmm2 yr-’ with an uncertainty of 45%. DISCUSSION
OF PREVIOUS
OBSERVATIONS
Strictly speaking we cannot compare our data with previous observations because we have chosen to report deposition rate for a standard amount of rainfall rather than as a globalannual deposition rate and shall restrict discussion here to precipitation measurements. The problems with the two papers (RAISBECK et al., 1981b; SOMAYAJULUef al., 1984) wherein soil contamination is not considered have been noted and require no further comment. The work of MONACHAN et al. (1985) approaches the problem of global deposition ingeniously but suffers from two defects. (1) They used bulk collectors open for a period of about a year, their object being to determine the total annual flux into the containers. This
141
Atmospheric ‘Be and “Be TABLE
Samnlz
2. Summary
DC-1
necessarily enhances the recycled component, which they estimate by chemical analysis aimed at determining the amount of soil deposited and correcting for this component by assuming the ‘OBe concentration in the soil to have been 5 X 1O* atom/g, regardless of the location of the station. Corrections for their seven continental stations ranged from 8% to 35%, amounts similar to what we have observed for wet only collection and substanti~ly less than what we have observed for dry deposition. (2) Rain concentrations were not measured because the open collectors were intended to sample the total loBe flux. Unfortunately their “collectors were placed on top of buildings or on the ground.” Sampling locations not selected according to standard meteorological method, especially the tops of buildings, have long been known to give erratic values of rainfall, to which the flux is directly ~ropo~io~al (ABBE, f899; KURTYIGQ, 1953). It is better to determine rainfall by standard rain gauges as we have done. CONCLUSIONS The determination of the deposition rates of the cosmogenie Be isotopes would seem to be a problem that suffers rather than gains from increasing study. Since the initial reports, complications have been found to lie in contaminations that are difficult to avoid or correct and in extreme variations of concentration from rainfall to rainfall necessitating long periods of observation. Even if these troubles are surmounted the problem of converting rainfall deposition into a global rate remains and the matter oflatitude dependence has been approached only superficially. In this work we have demonstrated clearly the effects of soil contamination on the Illinois and New Jersey samples and used knowledge of it to strenghthen our assumption that the Hawaiian rains have the same amount of “Be as uncontaminated samples from a continental station would have. Our data for 7Be, which do not suffer from soil contamination, have a distribution in concentration that is similar to the Hawaiian “Be. This is important support for the Hawaiian data, as they are less numerous, which would make their extreme values suspect but for the similar pattern in 7Be. And note that for both Hawaiian “Be and Illinois ‘Be the deposition is dominated by high concentration events. The determination of the distribution in concentration, both observationally and theoretically, lies in the future. Despite the morose temper of the preceding paragraph, we find our study has clarified certain elements of the problem. ( 1) There is moderate evidence that rains at middle lat-
of
Dry Deposition Results
DC-2
IL-1
itudes and in temperate climates have the same average ‘OBe concentrations, (2) This has as a consequence that local primary, i.e. not resuspended, deposition is overwhelmingly p~po~ion~ to rainfall, which we evaluate from our own and other data to be 1.5 X 104atom/cm3, uncertain by 20%. It is worth mentioning at this point that this deposition is in agreement with measurements of Arctic ice, whose measured concent~tions correspond to the de~nition of deposition. For Greenland ice laid down between 1860 and 1890 the value is 1.6 X lo4 atom/cm’ (BEER et al., 1984). It is not in agreement with Antarctic ice. RAISBECKet al. (198 la) found the concentration for roughly the same period in the Dome C core to be 5 X t09 atom/cm3. (3) Observations of rBe, especially in conjunction with “Be, are useful in circumventing the problem of soil contamination, which is sufficiently serious to preclude using loBe taken Born continental stations not shown to be essentially dust free, (4) These data are consistent with theoretical global estimates, if equatorial rain has an average concentration of the order of 3300 atom/ g, (5) Our best estimate for mid-latitude 7Be deposition is 1.O X lo4 atom/cm3, uncertain by 40%. Acknowledgements-The samples from Hawaii were provided by Alan Yoshinaga of the Mauna Loa Observatory, the New Jersey samples, by Donald C. Bogen of the Dept. of Energy Environmental Measurements Laboratory, New York City. Without their help this work would have much lower value. The support of the National Science Foundation, through the Division of Nuclear Physics and the Dept. of Energy through grant no. DE-FGO2-88ER-60635 are gratefully acknowledged. A reviewer pointed out the existence of some ‘Be data that had escaped our attention and for which we are grateful.
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