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Helium concentration in the Earth's lower atmosphere
0016.7037/84/$3.00
Geochlmxa n Cosmoehimica Acto VolA8. pp. 1759-1767 0 Pergsmon Press Ltd. 1984. Printed in U.S.A.
+ .oO
Helium concentration in the Earth’s lower atmosphere B. M. OLIVER, JAMES G. BRADLEY*and HARRY FARRAR IV Rockwell International Corporation, Canoga Park, California 9 1304, USA (Received August 25, 1981; accepted in revised&m
May 29, 1984)
Abstract-Measurements in 1981 of the helium content of the Earth’s lower atmosphere have given a value of 5.222 + 0.017 ppm by volume. This value, obtained by isotope dilution mass spectrometry, is 0.3% lower, but in essential agreement with the currently accepted value of 5.239 + 0.004 ppm determined by Glue&auf in the 1930%.A consideration of processes that could have altered the helium concentration since the 1930’s indicates that the concentration could have increased measurably due to release of helium by natural gas production. Possible net helium loss from the atmosphere is, however, not readily quantifiable. 1.
INTRODUCTION
from the upper atmosphere, however, are complex, and the measurement of actual eflluxes is difficult, FOR THE PAST four decades, the accepted value for so that the possibility of extraordinary losses in recent the helium concentration in the lower atmosphere times is not easily quantified. has been 5.239 + 0.004 ppm by volume in dry air, In order to assess the possibility of an altered and has been assumed to have been constant. This helium concentration in the atmosphere, measureconcentration was determined after many years of ments of the helium in the near-surface atmosphere work by Glueckauf (GLUECRAUF, 1946; GLUECRAUF have recently been made at Rockwell International. and PANETH, 1946) from the helium/(nitrogen The measurements were made using a gas mass + argon) pressure ratio in 28 air samples collected spectrometer in conjunction with several calibrated from around the world in the 1930’s. ERMOLIN helium isotope dilution “spiking” systems. (1957) made an independent determination of the helium concentration in dry air, but the result, 4.74 2. MASS SPECTROMETER SYSTEM + 0.08 ppm, has not been generally held as credible. There has been no other independent confirmation The mass spectrometer system used at Rockwell for since then. helium concentration measurements has been described GLUECKAUF’S(1946) value is commonly used as briefly elsewhere (FARRARef ol., 1975). The system was specificahy designed to measure small amounts of helium the basis for absolute calibration in studies involving the measurement of helium (CLARKE et al., 1976) and is the basis for calculating equilibrium solubilities to which helium solubilities measured at much higher partial pressures am compared (WEISS, 197 1). The mean residence time of helium in the Earth’s atmosphere, estimated from natural helium influx, is -2 X lo6 y (KCKRARTS, 1973). Thus any change in the helium flow of sufficient magnitude and duration, into or from the atmosphere, in the approximately 40 years since the original determination, should be detectable with sufficiently precise measurements. By far the most significant additional flow of helium to the atmosphere during this time period has come from the extraction, burning, or other release of natural gas, which contains anywhere from <0.01’S to >6W helium. Data on the production of natural gas since 1939 (UNITED NATIONS, 1952, 1976; MCCORMICK, 1977a,b; ENERGY INFORMATIONAn MINISTRATION,1980) indicate that the total helium content of the atmosphere could have risen 0.1 to 0.6% since then. The mechanisms of helium loss
l Present address: Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, California 9 I 109, USA.
in solid samples, but was easily adapted to air measurements with the addition of a few vacuum components. The mass spectrometer itself is a 2-inch (5.08-cm) radius, 60”, permanent magnet instrument (modified Veeco GA-4) with a single electron-multiplier collector. The resolution is AM/M - l/85 (full width at 5% of maximum). It is normally operated while isolated from its vacuum pumps (static mode) for high sensitivity during analysis. The mass spectrometer is connected to the sample gas handling system through a series of getters that purify the helium gas sample and insure that reactive gases, and gases condensible on 77°K activated charcoal, are not admitted. A simplified schematic diagram of the system is shown in Fig. I. The analysis of the ‘He content of an air sample was accomplished by isotope dilution mass spectrometry using a spike of ‘He. The sixes of the various ‘He spikes, in combination with a flexible aliquoting system, allowed ab solute helium measurements for samples in the range of IO’Oto IO” atoms of helium. The analysis procedure consisted of admission of the spike to the sealed-off vacuum system, with admission of the air sample to the vacuum system -10 s later. The background pressure of Or, Nlr Ar, etc. was then removed over a 1 to 2 minute period using liquid nitrogen-cooled charcoal getters. Finally, the measurements were made of the ‘He/3He isotope ratio in 2 or 3 aliquots of the mixture. In the measurements reported here, an aliquot was - 1%or -10% of the total gas volume, depending on the spike system used. The initial gettering of the major atmospheric gases insured that they did not unduly delay the mixing of the ‘He and ‘He. Each aliquot of gas was further purified during
1759
1760
B. M. Oliver. J. G. Bradley and H. Farrar IV CALlsRATED SAMPLE VOLUMES (-4Cdl
LINES
FIG. I. Schematic diagram of mass spectrometer
transfer to the mass spectrometer by passing through another liquid nitrogen-cooled charcoal bed and then over an SAES GT-50 Zr/Al alloy getter operated at room temperature. A separate SAES getter attached to the mass spectrometer, also operated at room temperature, maintained pumping for reactive gases while the spectrometer was isolated from its ion pump during sample analysis. The ‘He’/‘He’ ratios were measured by alternately tuning the ‘He+ and ‘He+ ion current peaks to the electron multiplier collector. The peak intensities were recorded, after amplification by a Cary 401MR vibrating reed electrometer, on a IO-in. (25.4-cm) strip chart recorder without scale expansions. The electrometer has an accuracy specification of -CO.]% and the linearity was confirmed by calibration to be +0.05% of full scale for the scales used. The strip chart recorder has a linearity specification of *O. I% of full scale, and calibration showed a maximum nonlinearity of 0.07% of full scale. Calibration measurements and ‘He measurements reported here were made with ion current ratios in the range of 0.5 to 2.0. All ion and electron currents measured in the present series were sufficiently low that there was no evidence of intensity dependent changes in the multiplier gain. Typical multiplier output currents were 10 nA. The measurement of helium background involved currents as low as 1 PA. The gain of the multiplier has not been measured, but is estimated to be in the range of 2-4 X 10’. The sequence of measure.ments of the ‘He+/.‘He+ ratios of the 2 or 3 aliquots of each sample or calibration analysis permitted correction for small time changes in gas composition during the analysis. These changes occurred from two sources: (1) helium memory in the spectrometer volume. and (2) atmospheric helium diffusion through glass sections into the main gas handling volume. From the first source, the ion current ratios for each individual aliquot typically showed changes of - l-2% during the I-2 minutes of ‘He+ and ‘He+ measurements in the spectrometer. These changes were extrapolated graphically to the time the gas entered the spectrometer. The uncertainty in the extrapolation correction was CO.1%. The second source, atmospheric helium diffusion, resulted in small increases in the extrapolated isotopic ratios measured for successive aliquots. The first aliiuot was typically isolated from the main v handling system -2 minutes after admission of the He spike. The second and third aliquots were analyxed -5 and - 10 minutes after the first ahquot. The small changes in the isotopic composition of helium in the main gas handling system were corrected for by time extrapolation of the ratios measured in the 2 or 3 aliquots. These extrapolations were small, amounting to less than 0.15%. with an uncertainty of ~0.05%. 3. SPIKE SYSTEM Because the accuracy of the final helium abundance data could be no better than the accuracy to which the helium
system
spikes were known, considerable effort was grven to the design and accurate calibration of the spiking system. Figure 2 shows the mercury manometer and a portion of the network of calibrated volumes which were used to fill several of the spike storage volumes, and to dispense known quantities of 3He and “He. These quantities of ‘He and ‘He were used both for calibration and for isotope dilution purposes. Also shown, connected to the spike filling line, are the air collection vial and calibrated volumes, described in Section 4, which were used in the present helium concentration measurements. Glass stopcocks were used throughout the spike system rather than stainless steel valves, because the stopcocks provided a positive and reliable barrier, and because of the ease with which the volumes between stopcocks could be measured. Most of the spike system, including all the stopcocks, was made of borosilicate glass (mostly Coming Type 7740 Pyrex). The volumes which were used for storage of helium for up to one year, however. were constructed primarily from ahrminosilicate glass (Coming Types 1720 and 1723). and stainless steel, with only small amounts (- 10 cm2) of borosilicate glass at the stopcocks, and other glasses in the graded seals. Aluminosilicate glasses are typically - 10’ times less permeable to helium than borosilicate glasses (DUSHMAN, 1962). The amount of ‘He that had entered the ‘He storage volume was measured at least weekly and appropriate corrections were made to the ‘He abundance and mass discrimination measunments. The ‘He/‘He ratio changed from - 1 X 10e5 at the time of ‘He spike filling, to -1 x lo-’ after I year. The amount of ‘He diffusing into the ‘He spike and ‘He-‘He premixturn volumes was assumed to be the same as measured for the ‘He spike, and corrections were made accordingly. The volumes of the small (3 to 100 cm’) glass sections were determined by repeatedly filling them with mercury followed by weighing of the mercury aRer emptying. Temperature corrected mercury densities were used for these calculations. The huger glass and stainless steel (> 100 cm’) volumes were similarly determined by weighing before and after filling with vacuum&gassed distilled water. All measurements were done under closely monitored temperature conditions, and corrections were made for the temperature coefficients of expansion of the liquids and containers and air buoyancy. As a result of this, the volumes of the glass and metal sections were determined with uncertainties less than 0.05%. Both the glass and steel volumes were sufficiently thick walled to prevent any significant volume changes due to distortion from the weight of the liquid inside. Tests were conducted on the largest of the gJass volumes (-5000 cm3 borosilicate) to determine the volume perturbation from the weight of the water. For these tests, a 5 mm I.D. glass tube was attached to the volume and the volume filled with water part way up the tube. The change in the water level with the volume resting on a hard surface in air rcriu\ fully
1761
Helium in Earth’s atmosphere
!?!?.?EJ-d@
PUMPr
, ,
C\
LSAMPLE
t.,.:.
1 1 ,ZmmPYREX w VACUUM
1%2-b
L
,
-...I..-“” I =‘!??== - -IKE
DISPENSING VOLUME t-4 cm31
DILUTION VOLUME PRESSURE
.. .
MERCUR’V MANOM IETER
TOMASSSPECTROMETER SYSTEM
- -..
FIG. 2. Partial diagram of mass spectrometer helium spiking system.
immersed in water at the same temperature indicated a total volume change of ~0.0004%. A critical part of assuring the accuracy of the volume measurements for the spike systems was verifying that the calibrated volumes were not altered by the grease used to seal the stopcocks. A special technique was used which minimized the grease, limiting its effect on the volume to ~0.1%. A correction for this effect was applied in the calculations for the spike filling. To insure that the amount of ‘He and ‘He was accurately known from pressure measurements made during filling of the spike and standard premixture volumes, only high purity helium was used. The commercial supplier specified the ‘He to be 99.998% He and 99.9995% ‘He in total helium. The ‘He was specified as having
by an aliquot. Data from the volume calibrations and spike fillings give an uncertainty (I u) for the various helium spikes of <0.2%, with roughly equal contributions from each source of uncertainty. In addition to the four separate ‘He and ‘He spike systems, there was a third system that dispensed a standard premixture of the two isotopes. This premixture was used for calibration of the relative sensitivity of the mass spectrometer for ‘He and ‘He, i.e.. the mass discrimination factor. The standard premixture was prepared by delivering separately “He and ‘He to a common storage volume using the same filling system and techniques used for the smaller spikes. The effect of the back expansion from Volume D to C (Fig. 2) at the time of admitting the second gas was accounted for in the final pressure ratio calculation. The premixture provided an especially accurate standard for discrimination measurements because. the ‘He/“He pressure ratio delivered to the mass spectrometer system did not depend on the exact size of volumes A, B, and E or on the exact temperatures of Volumes D and E at aliquot dispensing time. The separate ‘He and ‘He spikes were also regularly combined, intercompared, and analyzed each day to provide alternative measurements of the discrimination factor, and to verify that each of the spikes was delivering the expected number of atoms of helium. The mass discrimination factor of the spectrometer was typically 1.65 for the ‘He+/‘He+ ratio. For the helium concentration measurements reported here, the discrimination factor was determined at least twice each day, using different gas mixtures, once before and once after the concentration measurements. The discrimination factor was a constant each day with no time trends detectable. However, because the spectrometer filament was turned off each night, the discrimination varied slightly [-0.7% (I a)] from day to day. Thus, the ‘He+13He+ ratios for a given day were corrected accordingly for that day’s discrimination.
Internal consistency tests of the measurements of the discrimination factor over several years indicate a random uncertainty of about 0.3% (I a) for a single analysis, due primarily to a combination of precision in mass spectrometer tuning and chart recorder readout. Total random uncertainty in any measurement
1762
B. M. Oliver, J. G. Bradley and H. Farrar IV
of helium content is estimated to be -0.5%. This estimate comes from numerous duplicate analyses conducted over the years on samples with homogeneous helium content. As would be expected, this uncertainty is higher than the 0.3% estimated for the discrimination factor alone because a determination of the helium content in a sample involves an additional -0.3% uncertainty (added in quadrature) in the measured helium isotopic ratio. It should be noted that the uncertainty in the absolute 4He/“He pressure ratio used to determine the discrimination factor should be less than that in the absolute amount of gas in a spike because of common volumes and apparatus used in filling the storage volumes. Nevertheless, to be conservative, it is estimated that the systematic bias uncertainty in a day’s discrimination factor is 0.2% ( 1a), i.e., the same as the uncertainty in the absolute size of a spike aliquot, even though at least two different spike systems were used for calibration each day. 4. EXPERIMENTAL
METHOD
The air samples used in the helium concentration measurements were collected using two different techniques. Additionally, the samples collected by the different techniques were analyzed relative to different spike systems. In this manner, certain systematic biases in either technique could be detected as an inconsistency in the separately calculated mean concentrations of helium in air. These two air sampling techniques are discussed below. a. Instrumented air samples
The optimum helium sample size for analysis in the mass spectrometer (‘He*/‘He+ equals 0.5 to 2) using the larger helium spiking system was -6 X lOI atoms. Given the helium concentration in air of -5 ppm, this required analysis of an air sample of - Id0 atoms or -4 cm3 (STP). To obtain this sample size for analysis, a portable homsilicate sampling assembly was constructed which was comprised of four individual air sample volumes (labeled Nos. 1, 2, 3, and 5) of -4 cm’ each, attached to a common 35 mm ID ground glass joint (see Pii. 1 and 2). The individual sampling volumes were isolated by ground glass vacuum stopcocks lubtiatted with Apiezon Type N grease. The common ground glass joint was used to attach the assembly to the appropriate sample line of the mass spectrometer. Each of the air sample volumes, which inch&d the stopcock borrs, was measured in a manner similar to the spiking system volumes, by weighing successive mercury tillings. Ad&nmahy, to fmtber minimize puhubation e&t.5 on the calibrated volumes from excess stopcock grease, the calibrated volumes and assembled stopcock bores were degmamd by thtshing with chloroform prior to use. Measurements were made of the residual grease mass remaining in the bores aI& cleaning and exercising the stopcocks. This was done by cohecting the residual grease using a chloroform wetted length of pipe cleaner. The increase in mass of the pipe chner, after evaporation of the chloroform, allowed a measurement of the grease mass. The measurements indicated a total residual grease volume of <0.0003 cm), or <0.07% of the total space in the calibrated volumes. Prior to sample collection, the volumes were evacuated to <1 mPa and sealed off. At the sampling location, 5 to 15 minutes was allowed for the assembly to reach thermal equilibrium, and for the air in the large ground glass joint
volume to reach compositional equilibrium with the sup rounding air, before opening the stopcocks. After opening the stopcocks, another 5 to I5 minutes was allowed to insure compositional equilibrium with the room air before closing. The length of the equilibration time was selected based on the estimated temperature and humidity differences between the air in the laboratory where the evacuation of the sampling assembly took place and the sampling location. Careful measurements were made of the air temperature. barometric pressure, and relative humidity in the sampling room just prior to the stopcock closure.. Within two hours of lilling, the volumes were attached to the mass spectrometer system, as shown in Fig. 1, and analyzed for total helium, as described in Section 3. Diffusion of 4He either into or out of the calibrated volumes, caused by small temperature, and consequently, pressure differences between the sample tilling location and the Mass Spectrometer Laboratory, was determined from separate measurements of the helium that diffused into one of the calibrated volumes which had been evacuated and stored for several days. This dit%ion, when recalculated for the pressure dit?brences and time intervals involved, was found to be < 1O9atoms of *He, and therefore was negligible. The analyses were corrected for the system helium background. This background was determined for each day’s analyses by analyzing one of the air sample volumes after evacuation (- 1 mPa). In all cases, the background was 4 x 1Oroatoms, contributing negligible uncertainty (401%) to the results. A summary of these and other sources of uncertainty in the instrumented air sampling procedure and in the mass spectrometer analysis is given in Table la. b. Remote air samples
The instrumented air filling operation discussed above precluded the possibility of obtaining air samples at remote locations because of the need to take appropriate temperature, pressure, and humidity readings. Remote sampling was considered important because, even though dfotts were undertaken to insure otherwise, there was no absolute assurance that the helium concentration in the laboratory air was not being artificially changed by some operation being conducted in this or another section of the facility. A second air collection technique was devised, therefore, to allow filling the four calibrated air sample volumes (used in the first technique) with air which had been collected at remote locations in separate uncalibrated -30 cm3 vials. Because storage times up to several hours were sometimes involved, the individual vials used for the remote air collection TABLE
1.
AIR SAMPLING
Aw
MLIU(
ANAlISIS
UNCERTAINTIES
EttiMtRd AQso1ute lo gstr
Measuring
Technique
UllC(LMf RtJ
.___
rla14tivca Uncert4l"t~
,e -
it, -.--
Helium in Earth’s atmosphere were constructed of Corning Type 1720 glass. These were attached to borosilicate stopcocks lubricated with Apiezon Type N grease. Coming Type 1720 glass has a very low oerrneabilitv to helium. and thus the diffusion of ‘He either mto or out of the air sample vials, caused by differences in sample air pressure as compared to laboratory pressure. was made extremely small (~0.01%). and thus no correction for it was necessary. Prior to sampling, each remote collection vial was evacuated to
1763
A summary of the various experimental uncertainties in this second filling procedure is given in Table lb. The accuracies of the mercury barometer used in the instrumented air sampling method, and the mercury U-tube manometer and cathetometers used both in the remote air sampling method and in the spike system filling, were checked by intercomparison with a commercially made precision mercury manometer/barometer (Charles Meriam CO., Model 34EH50-TM). All three agreed to better than 0.05% at several representative pressures in the ranges encountered in these measurements. There
are several
potential
sources
of systematic
error relating specifically to the helium in air measurements that have not yet been discussed. The first is oxygen consumption from the air samples due to its reaction with organic materials (e.g., vacuum grease and vacuum pump oil) in the sampling containers and analysis lines. Oxygen consumption would affect the results by altering the pressure readings used in the remote sampling technique. Within the precision of the analytical techniques discussed here, however, no such oxygen consumption has been detected. In any event, it is difficult to imagine that the instrumented air sampling technique could show a significant oxygen consumption bias. This is because the pressure of the air and the helium in the sampling volume was determined immediately after equilibration with the atmosphere, and subsequent steps partitioned it volumetrically. For the remote air samples, an examination of earlier analyses (not reported here), where the samples were stored for periods of up to twenty days, revealed no trend in apparent helium concentration as a function of storage time in the original sample vials. Additionally, observation of the pressure of an air sample stored in the gas transfer lines for several days showed a pressure decrease of less than 0.2% that could not be attributed to temperature changes. Thus, the possible loss of oxygen in normal I to 2 hour transfer times would be entirely negligible. It is possible to imagine that some rapid time scale (minutes) reaction processes could create very small oxygen consumption biases in either sampling technique: however, to the extent that the results from the two techniques agree, such processes are assumed to be insignificant. Direct loss of helium from the samples, due to absorption by the Apiezon stopcock grease, has also been considered. Although no data are available on the helium solubility in Apiezon Type N grease, it is known that the Hz and Nz saturation solubilities at 100 kPa are -0.2 cm3 (STP) per cm3 of grease (BIDDLE, pers. commun., 1983). It is assumed here that the absorption of helium is no more than this. For the special greasing technique used in the present work, typically ~0.02 cm3 of grease was used for each stopcock with only -0.002 cm3 exposed to the calibrated volumes. Given the smallest calibrated volume on the system of -4 cm3, this would translate into a maximum loss of only -0.02% for two stopcocks. Another possible source of systematic error is
B. M. Oliver, J. G. Bradley and H. Farrar IV
1764
caused by the neon content of the air sample being admitted to the mass spectrometer. As discussed earlier, the major non-helium components in the atmosphere are effectively gettered by the liquid nitrogen-cooled charcoal traps. The exception to this is neon. Since neon is about four times as abundant in the atmosphere as helium, the gas sample admitted to the mass spectrometer will contain approximately four times as much neon as helium. The possibility that this quantity of neon could be affecting, in some way, the measured 4He’/3He+ ratio was investigated by temporarily filling a new spike container on the mass spectrometer system with a known amount of neon. The amount of neon was chosen so that one aliquot, when combined with an aliquot of the known 3He-4He premixture, would be within - 10% of the Ne/4He ratio in the atmosphere. Several analyses were then conducted measuring the 4He+/3He+ ratio from aliquots of the premixture both with and without neon present, To within the -0.1% precision of these special ratio measurements, no measurable change was observed. ~ck~und signal levels near masses 3 and 4 with only the neon present revealed no features that would alter the ‘He+13Hec ratio. The possibility that trace quantities of the major non-helium gas species in air (HrO, 4, Nr, Ar, and CO*) could alter the discrimination factor has also been considered. The effect of these gases was not directly tested with a synthetic gas mixture; however, observation of the mass 2, 15, 16, 18, 28, and 44 peaks in the spectrometer with and without processing an air sample showed no significant differences. Processing an air sample introduced mass 4, 10, 20, 22, and 40 peaks, The relative peak intensities in the mass spectrometer with and without processing an air sample are given in Table 2. The only significant difference between the blank and mass spectrometer background was the absence of masses 15 and 16 peaks in the background, The only very slight increase in signals at unwanted masses was the result of the sequence of traps and getters used in processing an air sample. When an air sample was first admitted to the mass spectrometer gas handling and inlet system, the sorbable gases were trapped by >l cm3 of liquid nitrogen-cooled activated charcoal until the pressure was <5 Pa on a
TABLE
2.
MASS
PEAKS
AND
INTENSXTIES
Relative Ha55
Possible SOUrCe5
2 3 1:
l
Intensity In
In
Blank
Air
Samplea
+ 0.04 0.04 ;i+ "He+
TABLE 3.
oate of At~lyres 3/31/51
HELM
VOlure No.
1
:
5
4/23/8X
517181
ANALYSES - 1NSTWURkTtO MWLIN6 ~__I____ 4ne (10'4 atcau, Cdlculated 4.004 5.014 4.p87 6.1%
nedsuneii 3.993 5.007 4.975 6.lZM
t4EiWi_
Ratio kasured,'Ealculare~ oats . 0.9973 0.9986 0.9982 0.9966
I
3.988
3.9%
0.9995
2 3
4.994 4.946 6.05OC
i:E
5
4.994 4.967 6.125
1 2 3 5
3.949 4.944 4.917 6.064
3.546" 4.941 4.913 6.063
3.982
: :
4.9% 4.959 6.115
3.911 4.970 4.925 6.075*
Memb
..__.-.
%2.'1975 'O.WiG
0.3964 lo.ua?3
0.9994 3.9991. 0.9992 ">.wl3 0.9998 0.9972 0.9968 4.3352 0.9931 'iJ.LJli?: u.9935
-.-
.
dCalcu14ted d%.uming d value of 5.239 pp~ (lie. o&an valuer for each day and 'lo uncertainty in the dlstr~butli)n. ‘SaWtIle v01Wre's Stqmck had vlslb!e leak. dUnsplked sawle analyzed for both 'lk!and 4tie.
thermocouple gauge. Only then was the remaining gas passed through the large -50 cm3 liquid nitrogencooled main charcoal trap and past the SAES getter into the mass spectrometer. At no time was the gas exposed to a hot getter, this insured that there was no generation of Hz. 5. RESULTS A total of 31 air sample analyses were conducted using eight separate air sample volume fillings. Sixteen of these analyses, shown in Table 3, were of air samples collected in the Mass Standards Laborator)i at Rockwell International in Canaga Park, California, using the instrumented sampling technique discussed in Section 4a. The calculated and measured numbers of 4He atoms in each of the air calibration volumes are shown in Columns 3 and 4 of Table 3, respectively. The corresponding ratios of m~ured-to-calculate values are shown in Columns 5 and 6, and are presented in this manner for ease of comparison. The calculated values were obtained from van der Waal’s equation of state using the measured amount of air in each volume and assuming GLUECKAUF’S (1946) original helium concentration value in air of 5.239 ppm. The mean value of the measured-tocalculated daily ratios in Column 6, with all measurements equally weighted, is 0.9977 f 0.00 18 t+ t u standard deviation). The remaining 15 helium analyses were of air samples collected at various outside locations in Canoga Park (suburban Los Angeles) using the remote collection technique described in Section 4b. The results of these analyses are shown in Table 4. Columns 5 and 6 list again, the ratio of measuredto-calculated vahzs, and the mean value of this ratio for each remotely collected gas sample, respectively. Averaging the daily ratios listed in Column 6 gives a mean value of 0.9959 + 0.0013. The analyses of the samples in Table 4 occurred over a period during which the smaller spike svstem
1765
Helium in Earth’s atmosphere TABLE 4.
Date of Analyses 4/Z/81
4/4/a
4/27/8I
5/15/a
HELIUM ANALYSES - REMTE
VOlUne No.
SAMPLING EETHOO
4He (1013 atoms) Qlculateda
Ratio Measured/Calculated
Measured
Data
1 2 3 5
3.256 4.079 4.057 5.003
3.251 4.070 4.048 4.991
0.9979 0.9978 0.9978 0.9976
1 2 3 5
3.357
0.9937
4.181 5.156
3.336 0.002c 4.161 5.141
: 3 5
3.253 4.074 4.051 4.996
3.238 4.054 4.030 4.9a4d
0.9354 0.9951 0.9948 0.9976
0.9957 to.0013
1 2 :
3.254 4.075 4.998 4.053
3.227 4.054 4.985 4.035
0.9917 0.9948 0.9956 0.9974
0.9949 *0.0024
0.9952 0.9971
hea"b 0.9978 *0.0001
0.9953 to.0017
"Calculated asrmlng a value of 5.239 ppm 4He. b&an values for each day and *lo uncertainty in the distribution. 1i"a backgmund. ‘Empty sanple volune analyzed for dunspiked sample analyzed for both$ tk and 4tk.
was refilled. The mean ratio, 0.9953, determined for the last eight analyses using the refilled spike system, is in excellent agreement with the mean, 0.9967, of the previous seven measurements. It should be noted that the small spike system used for the first seven analyses was filled almost a year earlier and -400 3He spike aliquots had already been dispensed, rep resenting an -37% reduction in the stored gas pressure (0.1150% per aliquot). The consistency of the results before and after refilling helps confirm the basic reproducibility of the mass spectrometer system analyses. The measured standard deviation in the distribution of measurements is <0.2%, which is consistent with the estimated uncertainties discussed in Section 4. Although not included here, earlier data, which were shown to have small biases due to excess grease in the smaller calibrated volumes, had a standard deviation of 0.4%. 6. DISCUSSION
OF RESULTS
The average mean measured-to-calculated value for the two sample collection methods are 0.9977 f 0.0018 and 0.9959 + 0.0013, respectively. The slight difference in the results From the two techniques is well within the estimated uncertainties in the individual spiking systems given in Table 1. It was expected that the data in Tables 3 and 4 would show a slightly smaller variability between analyses made on the same day as compared to different days. This difference would be expected in view of the -0.3% (la) added random uncertainty from the determinations of the mass spectrometer mass discrimination factor measured each day. Any small error in this discrimination factor will introduce an equal systematic error in each of the particular day’s analyses. In addition, since each day’s analyses were done using a single air sampling event, the possibility of some day-today variability due to sampling parameters could not be ruled out. However,
the single analysis variability was large enough so that the effect of the discrimination factor and sampling cannot readily be seen. Calculating the unweighted mean of all the daily means in Tables 3 and 4 (Column 6) yields a final value for the measured-to-calculated ratio of 0.9968 f 0.0017. This value gives a helium concentration in the lower atmosphere of 5.222 -C 0.009 ppm. In determining an uncertainty to be assigned to the value of 5.222 ppm helium in air obtained here, the following four factors are considered: (1) uncertainty in the determination of the mean of the distribution of the measurements, (2) uncertainty in the determination of the true amount of dry air in a sample, (3) uncertainty in the absolute number of helium atoms in the spiking system, and (4) uncertainty due to systematic errors in the determination of the discrimination factor. The standard deviation in the distribution of the 8 daily means is 0.17% so that the uncertainty in the mean is approximately 0.06%. The mean uncertainty in the determination of the actual amount of dry air in a sample for the two sampling techniques (see Table 1) is -0.14%. The uncertainty in the absolute size of the helium spikes is -0.20%. Finally, the potential systematic bias in the discrimination factor is -0.20%. Combining these uncertainties in quadrature gives a final uncertainty (standard deviation) of 0.32%. The helium concentration in air in mid-1981 is therefore 5.222 f 0.017 ppm. It should be noted that the 0.08% probable error (equivalent to 0.12% standard deviation) reported by GLUECKAUF (1946) in the measured helium concentration is divided equally between the probable error in the mean of the distribution and in the uncertainty in the calibration of his apparatus. Although a description of Glueckauf s calibration apparatus has been provided by others (CHACKETT et al., 1950) the details of the volumetric calibration of the apparatus have not been published. Thus, the validity of the calibration uncertainty is difficult to assess. As mentioned earlier, Glueckauf measured the ratio He/(Ne + Ar) in the atmosphere and thus this helium concentration in total air is no more accurate than the Ne + Ar abundance was known. Similarly, the accuracy of the present results could be influenced by abundance changes in other atmospheric components. Hence the results are comparable only to the extent that the “total” atmospheric composition was and is accurately known. 7. POSSIBLE
CHANGES
IN THE ATMOSPHERIC HELIUM CONTENT
It is well established that atmospheric helium (99.9999% 4He) has resulted from a degassing of the Earth’s crust and mantle, where 4He is produced by radioactive decay of uranium and thorium (KOCKARTS, 1973). The flux of 4He out of the Earth’s surface would double the present helium concentration in the atmosphere in -2 X IO6 y (KOCKARTS,
I766
B. M. Ohver, J. G.
Bradley and H. Farrar IV
1973) if helium were not continually being lost to space from the upper atmosphere. The helium concentration is a constant fraction in the atmosphere because of vertical mixing up to - 100 km, at which point its proportion increases with altitude because of diffusive separation (KOCKARTS, 1973). However, the long mean residence time of helium does not preclude the possibility of short-term changes that could alter the ~on~n~tion in the lower atmosphere. The rate at which helium has been transported from the lower atmosphere and lost in the ionosphere during the last 40 y is not readily quantifiable. Simple models of the lower atmosphere suggest that there may be a significant barrier to vertical transport of hehum due to eddy diffusion time constants as long as -5 y at heights of -20 km (WOFSY et al., 1973). Such a barrier would tend to average the effect of fluctuations in effluxes from the ionosphere. Detailed examination of data on atmospheric transpott, however, indicates that such a diffusion barrier is subject to significant durations and seasonal fluctuations (REITER, 1978) so that the averaging time may be much shorter than -5 y. The loss of helium to space from the ionosphere is a complex function involving photoionization, neutralization, and the Barth’s magnetic and electric fields, and is subject to approximateIy a factor of 20 change between summer and winter hemispheres. as well as changes due to solar activity (RArr”r et al., 1978). The complexity of the processes and a lack of historical flux measurements make impractical a quantitative estimate of helium concentration change in the last 40 y. However, a 40-y increase in average efflux would have to be - 150 times the 2 X lo6 y average in order to decrease the helium content of the lower atmosphere by -0.3%. There are human activities that could have produced a significant increase in the 4He in the lower atmosphere; in particular, the release of natural gas from 1939 through 1980. It is estimated that -9.5% of all natural gas production has occurred since 1939 with -50% between 1971 and 1981 (UNITED NATIONS, 1952, 1976; MCCORMICK, 1977a,b: ENERGY INFORMATION ADMINISTRATION,1980)for a total of 3.0 X lOI m3 (STP). The helium content of natural gas ranges from -0% to -6% in the U.S. with a mean concentration of -0.25% (ZARTMAN el al., 1961; TONGISH, 1980) in a log-normal distribution. The mean helium concentration in gas from outside the U.S. is less well known, but a small sample indicates that it may be -0.1% (MOORE, 1976). With -60% of the world total gas production occurring in the U.S., and allowing a factor of two uncertainty in mean helium concentration, it is estimated that -3 to 12 X 10” m3 (STP) of 4He has been released into the atmosphere fmm natural gas between 1939 and 1981. This is a potential increase of from -0.1 to 0.6% in the 1981 inventory of -2.1 X 1O’j m3 of helium in the atmosphere. The 198 1 inventory was calculated assuming 5.2 ppm He in a lOO-kPa atmosphere uniform over the Earth.
Much smaller amounts of helium have been rtleased by mining and refining of uranium and coal. Assuming that the -7 X 10’ kg U,O, mined worldwide since 1939. which is three times the U.S. total (U.S. Bureau of Census, 1975; JOLLY. 19791. had accumulated helium for -4.6 X 10’ y (approximate age of the Earth), then refining would have released only -7 X 10’ m3 (STP) helium. If 3 X lo’-‘ k&y of coal has been burned worldwide since 1939 {UNITED NATIONS, 1952. 1976), the typical 1 ppm of uranium m coal would have released only - 1 X lOa m’ (STP) even if the uranium had retained all the helium for 4.6 X 10’ y. Both of these sources are small compared to natural gas. Concerning crustal degassing, a complete cessation in the helium entering the Earth’s atmosphere over the last 40 y would have had a negligible effect on the helium content of the atmosphere. An order of magnitude increase in the average crustal 4He degassing rate in the last 40 y would have increased. on the other hand, the helium content of the atmosphere by only 0.02%. Such an increase in degassing. if it were accompanied by a comparable increase in tectonic activity. would not have escaped notice. Thus, increased degassing seems unlikely to have occurred. The question of how much the con~ent~t~on of helium in the atmosphere has actually changed couId be resolved directly by the analysis of air samples from 10 to 40 years ago. However, such samples. regardless of storage method, would almost certainly be subjected to depletion of their oxygen content by reaction with the storage container. Thus, measurement and comparison of He/Ne or He/Ar ratios in old and new samples might be the most appropriate technique for detecting historic trends. It would also be desirable to improve the accuracy of the helium concentration measurement techniques and to observe any changes in the Earth’s atmospheric hehum content over the next few years. 8. CONCLUSIONS The measurements presented here indicate a I981 helium concentration in the Earth’s lower atmosphere of 5.222 10.017 ppm. This value is 0.3 1+_ 0.3% lower than the currently accepted value of 5.239 I 0.004 ppm measured by GLUECKAUF (1946). The results do not establish whether the helium concentration in the atmosphere has changed in the past 40 years, but they set reasonable limits. Pending further m~uremen~, and a possible systematic check of old samples, it is suggested that the appropriate value for helium content of dry air in 198 I is 5.222 + 0.017 ppm by volume. On a statistical basis, however, it cannot be said that this value is preferred over the earlier one. .~‘.~nowiedgemenrs-The authors wish to acknowledge contributions to this work by J. F. Johnson, D. W. Kneff. M. M. Nakata, W. E. Parkins, H. Pearlman, R. P. Skowronskr. and H. C. Wieseneck. all of Rockwell lnternatlonal
Helium in Earth’s atmosphere Valuable suggestions were obtained from W. B. Clarke (McMaster University, Hamilton, Canada), E. Farrar (Queen’s University, Kingston, Canada), E. Glueckauf (Chilton, Oxon, England), and J. C. Huneke (Charles Evans and Associates, San Mateo, California), ail of whom reviewed drafts of this paper before it underwent significant revision. We also wish to express our thanks to M. M. Cohen, R. S. Caron, P. B. Hemmig, J. W. Lewellen, T. C. Reuther, Jr., and K. M. Zwilsky of the U.S. Department of Energy (DDE) for their continued interest and support. This work was in part supported by DOE Contract DE-AT0381SF11561. REFERENCES CHACKHT K. F., PANETH F. A. and WILSON E. J. (1950)
Chemical analysis of stratospheric samples from 50 and 70 km height. J. Atmos. Terrest. Phys. 1, 49-55. CLARKE W. B., JENKINSW. J. and TOP Z. (1976) Determination of tritium by mass spectrometric measurements of ‘He. Int. J. Appl. Radial. Isot. 21, 515-522. DUSHMANS. (1962) Sorption of gases by “active” charcoal, silicates (including glasses) and cellulose (revised by G. L. Gaines, Jr.). In Scientific Foundations of Vacuum Technique (ed. J. M. LAFFERTY),2nd Ed, Chap 7, pp. 435515. Wiley. ENERGY INFORMATIONADMINISTRATION( 1980) World natural gas: 1978. Energy Data Report, U.S. Department of Energy, DOE/EIA-O133 (78). ERMOLING. M. (1957) Toward a method of quantitative separation of helium-neon mixtures. Trudy Radievogo Inst. im. V. G. Khlopina 6, Trans. in Works of V. G. Khlopin Radium Institute 6. 124-145. AEC-tr-4208. FARRA~ IV H., MCELROY W. N. and LIPPINCOTTE. P. (1975) Helium production cross section of boron for fast reactor neutron spectra. Nucl. Technol. 25, 305-329. GLUECKAUFE. (1946) A microanalysis of the helium and neon contents of air. Proc. Roy. Sot. Ser A 185.98-I 19. GLUECKAUF E. and PANETH F. A. (1946) The helium content of atmospheric air. Proc. Roy. Sot. Ser. A 185, 89-97.
1767
JOLLY J. H. (1979) In Commodity Year Rook I979 (ed. W. L. EMERY).Commodity Research Bureau, Inc. KOCKARTSG. (I 973) Helium in the terrestrial atmosphere. Space Sci. Rev. 14, 723-757. MCCORMICKW. T. (1977a) The future for world gas supply. Gas Supply Rev. 5, 1-3, Amer. Gas Assoc. MCCORMICK W. T. (1977b) Summary of United States natural gas statistics for the period 1945 to 1976. Gas Supply Rev. 5, Supply and Production Supplement. I-4, Amer. Gas Assoc. MCIOREB. J. (1976) Analyses of natural gases, 1917-1974. U.S. Bureau of Mines, National Technical Information Service, PB-25 1202. RAIN W. J., SCHUNK R. W. and BANKS P. M. ( 1978) Helium ion outflow from the terrestrial ionosphere. Planet. Space Sci. 26, 255-268.
REITER E. R. (1978) Atmospheric Transport Processes, Part 4: Radioactive Tracers. U.S. Department of Energy, (TID27114). TONGISHC. A. (1980) Helium-its relationship to geologic systems and its occurrence with the natural gases, nitrogen, carbon dioxide, and argon. U.S. Bureau of Mines, Report of Investigation RI8444. UNITED NATIONS(1952), Statistical Office, Department of Economic Affairs, World Energy Supplies in Selected Years 1929-1950. (ST/STAT/SER.J/ 1). UNITED NATIONS (1976), Department of Economic and Social Affairs, World Energy Supplies 1950-1974 (ST/ ESA/STAT/SER.J/ 19). U.S. BUREAUOF CENSUS(1975) Historical Statistics of the United States, Colonial Times to 1970. Bicentennial Edition. WEISS R. F. (197 1) Solubility of helium and neon in water and seawater. J. Chem. Eng. Data 16, 235-241. WOFSY S. C. and MCELROY M. B. (1973) On vertical mixing in the upper stratosphere and lower mesosphere. J. Geophys. Res. 78, 2619-2624.
ZARTMANR. E., WASSERBURGG. J. and REYNOLDSJ. H. (196 1) Helium, argon, and carbon in some natural gases. J. Geophys. Res. 66, 277-306.
Geochlmxa n Cosmoehimica Acto VolA8. pp. 1759-1767 0 Pergsmon Press Ltd. 1984. Printed in U.S.A.
+ .oO
Helium concentration in the Earth’s lower atmosphere B. M. OLIVER, JAMES G. BRADLEY*and HARRY FARRAR IV Rockwell International Corporation, Canoga Park, California 9 1304, USA (Received August 25, 1981; accepted in revised&m
May 29, 1984)
Abstract-Measurements in 1981 of the helium content of the Earth’s lower atmosphere have given a value of 5.222 + 0.017 ppm by volume. This value, obtained by isotope dilution mass spectrometry, is 0.3% lower, but in essential agreement with the currently accepted value of 5.239 + 0.004 ppm determined by Glue&auf in the 1930%.A consideration of processes that could have altered the helium concentration since the 1930’s indicates that the concentration could have increased measurably due to release of helium by natural gas production. Possible net helium loss from the atmosphere is, however, not readily quantifiable. 1.
INTRODUCTION
from the upper atmosphere, however, are complex, and the measurement of actual eflluxes is difficult, FOR THE PAST four decades, the accepted value for so that the possibility of extraordinary losses in recent the helium concentration in the lower atmosphere times is not easily quantified. has been 5.239 + 0.004 ppm by volume in dry air, In order to assess the possibility of an altered and has been assumed to have been constant. This helium concentration in the atmosphere, measureconcentration was determined after many years of ments of the helium in the near-surface atmosphere work by Glueckauf (GLUECRAUF, 1946; GLUECRAUF have recently been made at Rockwell International. and PANETH, 1946) from the helium/(nitrogen The measurements were made using a gas mass + argon) pressure ratio in 28 air samples collected spectrometer in conjunction with several calibrated from around the world in the 1930’s. ERMOLIN helium isotope dilution “spiking” systems. (1957) made an independent determination of the helium concentration in dry air, but the result, 4.74 2. MASS SPECTROMETER SYSTEM + 0.08 ppm, has not been generally held as credible. There has been no other independent confirmation The mass spectrometer system used at Rockwell for since then. helium concentration measurements has been described GLUECKAUF’S(1946) value is commonly used as briefly elsewhere (FARRARef ol., 1975). The system was specificahy designed to measure small amounts of helium the basis for absolute calibration in studies involving the measurement of helium (CLARKE et al., 1976) and is the basis for calculating equilibrium solubilities to which helium solubilities measured at much higher partial pressures am compared (WEISS, 197 1). The mean residence time of helium in the Earth’s atmosphere, estimated from natural helium influx, is -2 X lo6 y (KCKRARTS, 1973). Thus any change in the helium flow of sufficient magnitude and duration, into or from the atmosphere, in the approximately 40 years since the original determination, should be detectable with sufficiently precise measurements. By far the most significant additional flow of helium to the atmosphere during this time period has come from the extraction, burning, or other release of natural gas, which contains anywhere from <0.01’S to >6W helium. Data on the production of natural gas since 1939 (UNITED NATIONS, 1952, 1976; MCCORMICK, 1977a,b; ENERGY INFORMATIONAn MINISTRATION,1980) indicate that the total helium content of the atmosphere could have risen 0.1 to 0.6% since then. The mechanisms of helium loss
l Present address: Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, California 9 I 109, USA.
in solid samples, but was easily adapted to air measurements with the addition of a few vacuum components. The mass spectrometer itself is a 2-inch (5.08-cm) radius, 60”, permanent magnet instrument (modified Veeco GA-4) with a single electron-multiplier collector. The resolution is AM/M - l/85 (full width at 5% of maximum). It is normally operated while isolated from its vacuum pumps (static mode) for high sensitivity during analysis. The mass spectrometer is connected to the sample gas handling system through a series of getters that purify the helium gas sample and insure that reactive gases, and gases condensible on 77°K activated charcoal, are not admitted. A simplified schematic diagram of the system is shown in Fig. I. The analysis of the ‘He content of an air sample was accomplished by isotope dilution mass spectrometry using a spike of ‘He. The sixes of the various ‘He spikes, in combination with a flexible aliquoting system, allowed ab solute helium measurements for samples in the range of IO’Oto IO” atoms of helium. The analysis procedure consisted of admission of the spike to the sealed-off vacuum system, with admission of the air sample to the vacuum system -10 s later. The background pressure of Or, Nlr Ar, etc. was then removed over a 1 to 2 minute period using liquid nitrogen-cooled charcoal getters. Finally, the measurements were made of the ‘He/3He isotope ratio in 2 or 3 aliquots of the mixture. In the measurements reported here, an aliquot was - 1%or -10% of the total gas volume, depending on the spike system used. The initial gettering of the major atmospheric gases insured that they did not unduly delay the mixing of the ‘He and ‘He. Each aliquot of gas was further purified during
1759
1760
B. M. Oliver. J. G. Bradley and H. Farrar IV CALlsRATED SAMPLE VOLUMES (-4Cdl
LINES
FIG. I. Schematic diagram of mass spectrometer
transfer to the mass spectrometer by passing through another liquid nitrogen-cooled charcoal bed and then over an SAES GT-50 Zr/Al alloy getter operated at room temperature. A separate SAES getter attached to the mass spectrometer, also operated at room temperature, maintained pumping for reactive gases while the spectrometer was isolated from its ion pump during sample analysis. The ‘He’/‘He’ ratios were measured by alternately tuning the ‘He+ and ‘He+ ion current peaks to the electron multiplier collector. The peak intensities were recorded, after amplification by a Cary 401MR vibrating reed electrometer, on a IO-in. (25.4-cm) strip chart recorder without scale expansions. The electrometer has an accuracy specification of -CO.]% and the linearity was confirmed by calibration to be +0.05% of full scale for the scales used. The strip chart recorder has a linearity specification of *O. I% of full scale, and calibration showed a maximum nonlinearity of 0.07% of full scale. Calibration measurements and ‘He measurements reported here were made with ion current ratios in the range of 0.5 to 2.0. All ion and electron currents measured in the present series were sufficiently low that there was no evidence of intensity dependent changes in the multiplier gain. Typical multiplier output currents were 10 nA. The measurement of helium background involved currents as low as 1 PA. The gain of the multiplier has not been measured, but is estimated to be in the range of 2-4 X 10’. The sequence of measure.ments of the ‘He+/.‘He+ ratios of the 2 or 3 aliquots of each sample or calibration analysis permitted correction for small time changes in gas composition during the analysis. These changes occurred from two sources: (1) helium memory in the spectrometer volume. and (2) atmospheric helium diffusion through glass sections into the main gas handling volume. From the first source, the ion current ratios for each individual aliquot typically showed changes of - l-2% during the I-2 minutes of ‘He+ and ‘He+ measurements in the spectrometer. These changes were extrapolated graphically to the time the gas entered the spectrometer. The uncertainty in the extrapolation correction was CO.1%. The second source, atmospheric helium diffusion, resulted in small increases in the extrapolated isotopic ratios measured for successive aliquots. The first aliiuot was typically isolated from the main v handling system -2 minutes after admission of the He spike. The second and third aliquots were analyxed -5 and - 10 minutes after the first ahquot. The small changes in the isotopic composition of helium in the main gas handling system were corrected for by time extrapolation of the ratios measured in the 2 or 3 aliquots. These extrapolations were small, amounting to less than 0.15%. with an uncertainty of ~0.05%. 3. SPIKE SYSTEM Because the accuracy of the final helium abundance data could be no better than the accuracy to which the helium
system
spikes were known, considerable effort was grven to the design and accurate calibration of the spiking system. Figure 2 shows the mercury manometer and a portion of the network of calibrated volumes which were used to fill several of the spike storage volumes, and to dispense known quantities of 3He and “He. These quantities of ‘He and ‘He were used both for calibration and for isotope dilution purposes. Also shown, connected to the spike filling line, are the air collection vial and calibrated volumes, described in Section 4, which were used in the present helium concentration measurements. Glass stopcocks were used throughout the spike system rather than stainless steel valves, because the stopcocks provided a positive and reliable barrier, and because of the ease with which the volumes between stopcocks could be measured. Most of the spike system, including all the stopcocks, was made of borosilicate glass (mostly Coming Type 7740 Pyrex). The volumes which were used for storage of helium for up to one year, however. were constructed primarily from ahrminosilicate glass (Coming Types 1720 and 1723). and stainless steel, with only small amounts (- 10 cm2) of borosilicate glass at the stopcocks, and other glasses in the graded seals. Aluminosilicate glasses are typically - 10’ times less permeable to helium than borosilicate glasses (DUSHMAN, 1962). The amount of ‘He that had entered the ‘He storage volume was measured at least weekly and appropriate corrections were made to the ‘He abundance and mass discrimination measunments. The ‘He/‘He ratio changed from - 1 X 10e5 at the time of ‘He spike filling, to -1 x lo-’ after I year. The amount of ‘He diffusing into the ‘He spike and ‘He-‘He premixturn volumes was assumed to be the same as measured for the ‘He spike, and corrections were made accordingly. The volumes of the small (3 to 100 cm’) glass sections were determined by repeatedly filling them with mercury followed by weighing of the mercury aRer emptying. Temperature corrected mercury densities were used for these calculations. The huger glass and stainless steel (> 100 cm’) volumes were similarly determined by weighing before and after filling with vacuum&gassed distilled water. All measurements were done under closely monitored temperature conditions, and corrections were made for the temperature coefficients of expansion of the liquids and containers and air buoyancy. As a result of this, the volumes of the glass and metal sections were determined with uncertainties less than 0.05%. Both the glass and steel volumes were sufficiently thick walled to prevent any significant volume changes due to distortion from the weight of the liquid inside. Tests were conducted on the largest of the gJass volumes (-5000 cm3 borosilicate) to determine the volume perturbation from the weight of the water. For these tests, a 5 mm I.D. glass tube was attached to the volume and the volume filled with water part way up the tube. The change in the water level with the volume resting on a hard surface in air rcriu\ fully
1761
Helium in Earth’s atmosphere
!?!?.?EJ-d@
PUMPr
, ,
C\
LSAMPLE
t.,.:.
1 1 ,ZmmPYREX w VACUUM
1%2-b
L
,
-...I..-“” I =‘!??== - -IKE
DISPENSING VOLUME t-4 cm31
DILUTION VOLUME PRESSURE
.. .
MERCUR’V MANOM IETER
TOMASSSPECTROMETER SYSTEM
- -..
FIG. 2. Partial diagram of mass spectrometer helium spiking system.
immersed in water at the same temperature indicated a total volume change of ~0.0004%. A critical part of assuring the accuracy of the volume measurements for the spike systems was verifying that the calibrated volumes were not altered by the grease used to seal the stopcocks. A special technique was used which minimized the grease, limiting its effect on the volume to ~0.1%. A correction for this effect was applied in the calculations for the spike filling. To insure that the amount of ‘He and ‘He was accurately known from pressure measurements made during filling of the spike and standard premixture volumes, only high purity helium was used. The commercial supplier specified the ‘He to be 99.998% He and 99.9995% ‘He in total helium. The ‘He was specified as having
by an aliquot. Data from the volume calibrations and spike fillings give an uncertainty (I u) for the various helium spikes of <0.2%, with roughly equal contributions from each source of uncertainty. In addition to the four separate ‘He and ‘He spike systems, there was a third system that dispensed a standard premixture of the two isotopes. This premixture was used for calibration of the relative sensitivity of the mass spectrometer for ‘He and ‘He, i.e.. the mass discrimination factor. The standard premixture was prepared by delivering separately “He and ‘He to a common storage volume using the same filling system and techniques used for the smaller spikes. The effect of the back expansion from Volume D to C (Fig. 2) at the time of admitting the second gas was accounted for in the final pressure ratio calculation. The premixture provided an especially accurate standard for discrimination measurements because. the ‘He/“He pressure ratio delivered to the mass spectrometer system did not depend on the exact size of volumes A, B, and E or on the exact temperatures of Volumes D and E at aliquot dispensing time. The separate ‘He and ‘He spikes were also regularly combined, intercompared, and analyzed each day to provide alternative measurements of the discrimination factor, and to verify that each of the spikes was delivering the expected number of atoms of helium. The mass discrimination factor of the spectrometer was typically 1.65 for the ‘He+/‘He+ ratio. For the helium concentration measurements reported here, the discrimination factor was determined at least twice each day, using different gas mixtures, once before and once after the concentration measurements. The discrimination factor was a constant each day with no time trends detectable. However, because the spectrometer filament was turned off each night, the discrimination varied slightly [-0.7% (I a)] from day to day. Thus, the ‘He+13He+ ratios for a given day were corrected accordingly for that day’s discrimination.
Internal consistency tests of the measurements of the discrimination factor over several years indicate a random uncertainty of about 0.3% (I a) for a single analysis, due primarily to a combination of precision in mass spectrometer tuning and chart recorder readout. Total random uncertainty in any measurement
1762
B. M. Oliver, J. G. Bradley and H. Farrar IV
of helium content is estimated to be -0.5%. This estimate comes from numerous duplicate analyses conducted over the years on samples with homogeneous helium content. As would be expected, this uncertainty is higher than the 0.3% estimated for the discrimination factor alone because a determination of the helium content in a sample involves an additional -0.3% uncertainty (added in quadrature) in the measured helium isotopic ratio. It should be noted that the uncertainty in the absolute 4He/“He pressure ratio used to determine the discrimination factor should be less than that in the absolute amount of gas in a spike because of common volumes and apparatus used in filling the storage volumes. Nevertheless, to be conservative, it is estimated that the systematic bias uncertainty in a day’s discrimination factor is 0.2% ( 1a), i.e., the same as the uncertainty in the absolute size of a spike aliquot, even though at least two different spike systems were used for calibration each day. 4. EXPERIMENTAL
METHOD
The air samples used in the helium concentration measurements were collected using two different techniques. Additionally, the samples collected by the different techniques were analyzed relative to different spike systems. In this manner, certain systematic biases in either technique could be detected as an inconsistency in the separately calculated mean concentrations of helium in air. These two air sampling techniques are discussed below. a. Instrumented air samples
The optimum helium sample size for analysis in the mass spectrometer (‘He*/‘He+ equals 0.5 to 2) using the larger helium spiking system was -6 X lOI atoms. Given the helium concentration in air of -5 ppm, this required analysis of an air sample of - Id0 atoms or -4 cm3 (STP). To obtain this sample size for analysis, a portable homsilicate sampling assembly was constructed which was comprised of four individual air sample volumes (labeled Nos. 1, 2, 3, and 5) of -4 cm’ each, attached to a common 35 mm ID ground glass joint (see Pii. 1 and 2). The individual sampling volumes were isolated by ground glass vacuum stopcocks lubtiatted with Apiezon Type N grease. The common ground glass joint was used to attach the assembly to the appropriate sample line of the mass spectrometer. Each of the air sample volumes, which inch&d the stopcock borrs, was measured in a manner similar to the spiking system volumes, by weighing successive mercury tillings. Ad&nmahy, to fmtber minimize puhubation e&t.5 on the calibrated volumes from excess stopcock grease, the calibrated volumes and assembled stopcock bores were degmamd by thtshing with chloroform prior to use. Measurements were made of the residual grease mass remaining in the bores aI& cleaning and exercising the stopcocks. This was done by cohecting the residual grease using a chloroform wetted length of pipe cleaner. The increase in mass of the pipe chner, after evaporation of the chloroform, allowed a measurement of the grease mass. The measurements indicated a total residual grease volume of <0.0003 cm), or <0.07% of the total space in the calibrated volumes. Prior to sample collection, the volumes were evacuated to <1 mPa and sealed off. At the sampling location, 5 to 15 minutes was allowed for the assembly to reach thermal equilibrium, and for the air in the large ground glass joint
volume to reach compositional equilibrium with the sup rounding air, before opening the stopcocks. After opening the stopcocks, another 5 to I5 minutes was allowed to insure compositional equilibrium with the room air before closing. The length of the equilibration time was selected based on the estimated temperature and humidity differences between the air in the laboratory where the evacuation of the sampling assembly took place and the sampling location. Careful measurements were made of the air temperature. barometric pressure, and relative humidity in the sampling room just prior to the stopcock closure.. Within two hours of lilling, the volumes were attached to the mass spectrometer system, as shown in Fig. 1, and analyzed for total helium, as described in Section 3. Diffusion of 4He either into or out of the calibrated volumes, caused by small temperature, and consequently, pressure differences between the sample tilling location and the Mass Spectrometer Laboratory, was determined from separate measurements of the helium that diffused into one of the calibrated volumes which had been evacuated and stored for several days. This dit%ion, when recalculated for the pressure dit?brences and time intervals involved, was found to be < 1O9atoms of *He, and therefore was negligible. The analyses were corrected for the system helium background. This background was determined for each day’s analyses by analyzing one of the air sample volumes after evacuation (- 1 mPa). In all cases, the background was 4 x 1Oroatoms, contributing negligible uncertainty (401%) to the results. A summary of these and other sources of uncertainty in the instrumented air sampling procedure and in the mass spectrometer analysis is given in Table la. b. Remote air samples
The instrumented air filling operation discussed above precluded the possibility of obtaining air samples at remote locations because of the need to take appropriate temperature, pressure, and humidity readings. Remote sampling was considered important because, even though dfotts were undertaken to insure otherwise, there was no absolute assurance that the helium concentration in the laboratory air was not being artificially changed by some operation being conducted in this or another section of the facility. A second air collection technique was devised, therefore, to allow filling the four calibrated air sample volumes (used in the first technique) with air which had been collected at remote locations in separate uncalibrated -30 cm3 vials. Because storage times up to several hours were sometimes involved, the individual vials used for the remote air collection TABLE
1.
AIR SAMPLING
Aw
MLIU(
ANAlISIS
UNCERTAINTIES
EttiMtRd AQso1ute lo gstr
Measuring
Technique
UllC(LMf RtJ
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rla14tivca Uncert4l"t~
,e -
it, -.--
Helium in Earth’s atmosphere were constructed of Corning Type 1720 glass. These were attached to borosilicate stopcocks lubricated with Apiezon Type N grease. Coming Type 1720 glass has a very low oerrneabilitv to helium. and thus the diffusion of ‘He either mto or out of the air sample vials, caused by differences in sample air pressure as compared to laboratory pressure. was made extremely small (~0.01%). and thus no correction for it was necessary. Prior to sampling, each remote collection vial was evacuated to
1763
A summary of the various experimental uncertainties in this second filling procedure is given in Table lb. The accuracies of the mercury barometer used in the instrumented air sampling method, and the mercury U-tube manometer and cathetometers used both in the remote air sampling method and in the spike system filling, were checked by intercomparison with a commercially made precision mercury manometer/barometer (Charles Meriam CO., Model 34EH50-TM). All three agreed to better than 0.05% at several representative pressures in the ranges encountered in these measurements. There
are several
potential
sources
of systematic
error relating specifically to the helium in air measurements that have not yet been discussed. The first is oxygen consumption from the air samples due to its reaction with organic materials (e.g., vacuum grease and vacuum pump oil) in the sampling containers and analysis lines. Oxygen consumption would affect the results by altering the pressure readings used in the remote sampling technique. Within the precision of the analytical techniques discussed here, however, no such oxygen consumption has been detected. In any event, it is difficult to imagine that the instrumented air sampling technique could show a significant oxygen consumption bias. This is because the pressure of the air and the helium in the sampling volume was determined immediately after equilibration with the atmosphere, and subsequent steps partitioned it volumetrically. For the remote air samples, an examination of earlier analyses (not reported here), where the samples were stored for periods of up to twenty days, revealed no trend in apparent helium concentration as a function of storage time in the original sample vials. Additionally, observation of the pressure of an air sample stored in the gas transfer lines for several days showed a pressure decrease of less than 0.2% that could not be attributed to temperature changes. Thus, the possible loss of oxygen in normal I to 2 hour transfer times would be entirely negligible. It is possible to imagine that some rapid time scale (minutes) reaction processes could create very small oxygen consumption biases in either sampling technique: however, to the extent that the results from the two techniques agree, such processes are assumed to be insignificant. Direct loss of helium from the samples, due to absorption by the Apiezon stopcock grease, has also been considered. Although no data are available on the helium solubility in Apiezon Type N grease, it is known that the Hz and Nz saturation solubilities at 100 kPa are -0.2 cm3 (STP) per cm3 of grease (BIDDLE, pers. commun., 1983). It is assumed here that the absorption of helium is no more than this. For the special greasing technique used in the present work, typically ~0.02 cm3 of grease was used for each stopcock with only -0.002 cm3 exposed to the calibrated volumes. Given the smallest calibrated volume on the system of -4 cm3, this would translate into a maximum loss of only -0.02% for two stopcocks. Another possible source of systematic error is
B. M. Oliver, J. G. Bradley and H. Farrar IV
1764
caused by the neon content of the air sample being admitted to the mass spectrometer. As discussed earlier, the major non-helium components in the atmosphere are effectively gettered by the liquid nitrogen-cooled charcoal traps. The exception to this is neon. Since neon is about four times as abundant in the atmosphere as helium, the gas sample admitted to the mass spectrometer will contain approximately four times as much neon as helium. The possibility that this quantity of neon could be affecting, in some way, the measured 4He’/3He+ ratio was investigated by temporarily filling a new spike container on the mass spectrometer system with a known amount of neon. The amount of neon was chosen so that one aliquot, when combined with an aliquot of the known 3He-4He premixture, would be within - 10% of the Ne/4He ratio in the atmosphere. Several analyses were then conducted measuring the 4He+/3He+ ratio from aliquots of the premixture both with and without neon present, To within the -0.1% precision of these special ratio measurements, no measurable change was observed. ~ck~und signal levels near masses 3 and 4 with only the neon present revealed no features that would alter the ‘He+13Hec ratio. The possibility that trace quantities of the major non-helium gas species in air (HrO, 4, Nr, Ar, and CO*) could alter the discrimination factor has also been considered. The effect of these gases was not directly tested with a synthetic gas mixture; however, observation of the mass 2, 15, 16, 18, 28, and 44 peaks in the spectrometer with and without processing an air sample showed no significant differences. Processing an air sample introduced mass 4, 10, 20, 22, and 40 peaks, The relative peak intensities in the mass spectrometer with and without processing an air sample are given in Table 2. The only significant difference between the blank and mass spectrometer background was the absence of masses 15 and 16 peaks in the background, The only very slight increase in signals at unwanted masses was the result of the sequence of traps and getters used in processing an air sample. When an air sample was first admitted to the mass spectrometer gas handling and inlet system, the sorbable gases were trapped by >l cm3 of liquid nitrogen-cooled activated charcoal until the pressure was <5 Pa on a
TABLE
2.
MASS
PEAKS
AND
INTENSXTIES
Relative Ha55
Possible SOUrCe5
2 3 1:
l
Intensity In
In
Blank
Air
Samplea
+ 0.04 0.04 ;i+ "He+
TABLE 3.
oate of At~lyres 3/31/51
HELM
VOlure No.
1
:
5
4/23/8X
517181
ANALYSES - 1NSTWURkTtO MWLIN6 ~__I____ 4ne (10'4 atcau, Cdlculated 4.004 5.014 4.p87 6.1%
nedsuneii 3.993 5.007 4.975 6.lZM
t4EiWi_
Ratio kasured,'Ealculare~ oats . 0.9973 0.9986 0.9982 0.9966
I
3.988
3.9%
0.9995
2 3
4.994 4.946 6.05OC
i:E
5
4.994 4.967 6.125
1 2 3 5
3.949 4.944 4.917 6.064
3.546" 4.941 4.913 6.063
3.982
: :
4.9% 4.959 6.115
3.911 4.970 4.925 6.075*
Memb
..__.-.
%2.'1975 'O.WiG
0.3964 lo.ua?3
0.9994 3.9991. 0.9992 ">.wl3 0.9998 0.9972 0.9968 4.3352 0.9931 'iJ.LJli?: u.9935
-.-
.
dCalcu14ted d%.uming d value of 5.239 pp~ (lie. o&an valuer for each day and 'lo uncertainty in the dlstr~butli)n. ‘SaWtIle v01Wre's Stqmck had vlslb!e leak. dUnsplked sawle analyzed for both 'lk!and 4tie.
thermocouple gauge. Only then was the remaining gas passed through the large -50 cm3 liquid nitrogencooled main charcoal trap and past the SAES getter into the mass spectrometer. At no time was the gas exposed to a hot getter, this insured that there was no generation of Hz. 5. RESULTS A total of 31 air sample analyses were conducted using eight separate air sample volume fillings. Sixteen of these analyses, shown in Table 3, were of air samples collected in the Mass Standards Laborator)i at Rockwell International in Canaga Park, California, using the instrumented sampling technique discussed in Section 4a. The calculated and measured numbers of 4He atoms in each of the air calibration volumes are shown in Columns 3 and 4 of Table 3, respectively. The corresponding ratios of m~ured-to-calculate values are shown in Columns 5 and 6, and are presented in this manner for ease of comparison. The calculated values were obtained from van der Waal’s equation of state using the measured amount of air in each volume and assuming GLUECKAUF’S (1946) original helium concentration value in air of 5.239 ppm. The mean value of the measured-tocalculated daily ratios in Column 6, with all measurements equally weighted, is 0.9977 f 0.00 18 t+ t u standard deviation). The remaining 15 helium analyses were of air samples collected at various outside locations in Canoga Park (suburban Los Angeles) using the remote collection technique described in Section 4b. The results of these analyses are shown in Table 4. Columns 5 and 6 list again, the ratio of measuredto-calculated vahzs, and the mean value of this ratio for each remotely collected gas sample, respectively. Averaging the daily ratios listed in Column 6 gives a mean value of 0.9959 + 0.0013. The analyses of the samples in Table 4 occurred over a period during which the smaller spike svstem
1765
Helium in Earth’s atmosphere TABLE 4.
Date of Analyses 4/Z/81
4/4/a
4/27/8I
5/15/a
HELIUM ANALYSES - REMTE
VOlUne No.
SAMPLING EETHOO
4He (1013 atoms) Qlculateda
Ratio Measured/Calculated
Measured
Data
1 2 3 5
3.256 4.079 4.057 5.003
3.251 4.070 4.048 4.991
0.9979 0.9978 0.9978 0.9976
1 2 3 5
3.357
0.9937
4.181 5.156
3.336 0.002c 4.161 5.141
: 3 5
3.253 4.074 4.051 4.996
3.238 4.054 4.030 4.9a4d
0.9354 0.9951 0.9948 0.9976
0.9957 to.0013
1 2 :
3.254 4.075 4.998 4.053
3.227 4.054 4.985 4.035
0.9917 0.9948 0.9956 0.9974
0.9949 *0.0024
0.9952 0.9971
hea"b 0.9978 *0.0001
0.9953 to.0017
"Calculated asrmlng a value of 5.239 ppm 4He. b&an values for each day and *lo uncertainty in the distribution. 1i"a backgmund. ‘Empty sanple volune analyzed for dunspiked sample analyzed for both$ tk and 4tk.
was refilled. The mean ratio, 0.9953, determined for the last eight analyses using the refilled spike system, is in excellent agreement with the mean, 0.9967, of the previous seven measurements. It should be noted that the small spike system used for the first seven analyses was filled almost a year earlier and -400 3He spike aliquots had already been dispensed, rep resenting an -37% reduction in the stored gas pressure (0.1150% per aliquot). The consistency of the results before and after refilling helps confirm the basic reproducibility of the mass spectrometer system analyses. The measured standard deviation in the distribution of measurements is <0.2%, which is consistent with the estimated uncertainties discussed in Section 4. Although not included here, earlier data, which were shown to have small biases due to excess grease in the smaller calibrated volumes, had a standard deviation of 0.4%. 6. DISCUSSION
OF RESULTS
The average mean measured-to-calculated value for the two sample collection methods are 0.9977 f 0.0018 and 0.9959 + 0.0013, respectively. The slight difference in the results From the two techniques is well within the estimated uncertainties in the individual spiking systems given in Table 1. It was expected that the data in Tables 3 and 4 would show a slightly smaller variability between analyses made on the same day as compared to different days. This difference would be expected in view of the -0.3% (la) added random uncertainty from the determinations of the mass spectrometer mass discrimination factor measured each day. Any small error in this discrimination factor will introduce an equal systematic error in each of the particular day’s analyses. In addition, since each day’s analyses were done using a single air sampling event, the possibility of some day-today variability due to sampling parameters could not be ruled out. However,
the single analysis variability was large enough so that the effect of the discrimination factor and sampling cannot readily be seen. Calculating the unweighted mean of all the daily means in Tables 3 and 4 (Column 6) yields a final value for the measured-to-calculated ratio of 0.9968 f 0.0017. This value gives a helium concentration in the lower atmosphere of 5.222 -C 0.009 ppm. In determining an uncertainty to be assigned to the value of 5.222 ppm helium in air obtained here, the following four factors are considered: (1) uncertainty in the determination of the mean of the distribution of the measurements, (2) uncertainty in the determination of the true amount of dry air in a sample, (3) uncertainty in the absolute number of helium atoms in the spiking system, and (4) uncertainty due to systematic errors in the determination of the discrimination factor. The standard deviation in the distribution of the 8 daily means is 0.17% so that the uncertainty in the mean is approximately 0.06%. The mean uncertainty in the determination of the actual amount of dry air in a sample for the two sampling techniques (see Table 1) is -0.14%. The uncertainty in the absolute size of the helium spikes is -0.20%. Finally, the potential systematic bias in the discrimination factor is -0.20%. Combining these uncertainties in quadrature gives a final uncertainty (standard deviation) of 0.32%. The helium concentration in air in mid-1981 is therefore 5.222 f 0.017 ppm. It should be noted that the 0.08% probable error (equivalent to 0.12% standard deviation) reported by GLUECKAUF (1946) in the measured helium concentration is divided equally between the probable error in the mean of the distribution and in the uncertainty in the calibration of his apparatus. Although a description of Glueckauf s calibration apparatus has been provided by others (CHACKETT et al., 1950) the details of the volumetric calibration of the apparatus have not been published. Thus, the validity of the calibration uncertainty is difficult to assess. As mentioned earlier, Glueckauf measured the ratio He/(Ne + Ar) in the atmosphere and thus this helium concentration in total air is no more accurate than the Ne + Ar abundance was known. Similarly, the accuracy of the present results could be influenced by abundance changes in other atmospheric components. Hence the results are comparable only to the extent that the “total” atmospheric composition was and is accurately known. 7. POSSIBLE
CHANGES
IN THE ATMOSPHERIC HELIUM CONTENT
It is well established that atmospheric helium (99.9999% 4He) has resulted from a degassing of the Earth’s crust and mantle, where 4He is produced by radioactive decay of uranium and thorium (KOCKARTS, 1973). The flux of 4He out of the Earth’s surface would double the present helium concentration in the atmosphere in -2 X IO6 y (KOCKARTS,
I766
B. M. Ohver, J. G.
Bradley and H. Farrar IV
1973) if helium were not continually being lost to space from the upper atmosphere. The helium concentration is a constant fraction in the atmosphere because of vertical mixing up to - 100 km, at which point its proportion increases with altitude because of diffusive separation (KOCKARTS, 1973). However, the long mean residence time of helium does not preclude the possibility of short-term changes that could alter the ~on~n~tion in the lower atmosphere. The rate at which helium has been transported from the lower atmosphere and lost in the ionosphere during the last 40 y is not readily quantifiable. Simple models of the lower atmosphere suggest that there may be a significant barrier to vertical transport of hehum due to eddy diffusion time constants as long as -5 y at heights of -20 km (WOFSY et al., 1973). Such a barrier would tend to average the effect of fluctuations in effluxes from the ionosphere. Detailed examination of data on atmospheric transpott, however, indicates that such a diffusion barrier is subject to significant durations and seasonal fluctuations (REITER, 1978) so that the averaging time may be much shorter than -5 y. The loss of helium to space from the ionosphere is a complex function involving photoionization, neutralization, and the Barth’s magnetic and electric fields, and is subject to approximateIy a factor of 20 change between summer and winter hemispheres. as well as changes due to solar activity (RArr”r et al., 1978). The complexity of the processes and a lack of historical flux measurements make impractical a quantitative estimate of helium concentration change in the last 40 y. However, a 40-y increase in average efflux would have to be - 150 times the 2 X lo6 y average in order to decrease the helium content of the lower atmosphere by -0.3%. There are human activities that could have produced a significant increase in the 4He in the lower atmosphere; in particular, the release of natural gas from 1939 through 1980. It is estimated that -9.5% of all natural gas production has occurred since 1939 with -50% between 1971 and 1981 (UNITED NATIONS, 1952, 1976; MCCORMICK, 1977a,b: ENERGY INFORMATION ADMINISTRATION,1980)for a total of 3.0 X lOI m3 (STP). The helium content of natural gas ranges from -0% to -6% in the U.S. with a mean concentration of -0.25% (ZARTMAN el al., 1961; TONGISH, 1980) in a log-normal distribution. The mean helium concentration in gas from outside the U.S. is less well known, but a small sample indicates that it may be -0.1% (MOORE, 1976). With -60% of the world total gas production occurring in the U.S., and allowing a factor of two uncertainty in mean helium concentration, it is estimated that -3 to 12 X 10” m3 (STP) of 4He has been released into the atmosphere fmm natural gas between 1939 and 1981. This is a potential increase of from -0.1 to 0.6% in the 1981 inventory of -2.1 X 1O’j m3 of helium in the atmosphere. The 198 1 inventory was calculated assuming 5.2 ppm He in a lOO-kPa atmosphere uniform over the Earth.
Much smaller amounts of helium have been rtleased by mining and refining of uranium and coal. Assuming that the -7 X 10’ kg U,O, mined worldwide since 1939. which is three times the U.S. total (U.S. Bureau of Census, 1975; JOLLY. 19791. had accumulated helium for -4.6 X 10’ y (approximate age of the Earth), then refining would have released only -7 X 10’ m3 (STP) helium. If 3 X lo’-‘ k&y of coal has been burned worldwide since 1939 {UNITED NATIONS, 1952. 1976), the typical 1 ppm of uranium m coal would have released only - 1 X lOa m’ (STP) even if the uranium had retained all the helium for 4.6 X 10’ y. Both of these sources are small compared to natural gas. Concerning crustal degassing, a complete cessation in the helium entering the Earth’s atmosphere over the last 40 y would have had a negligible effect on the helium content of the atmosphere. An order of magnitude increase in the average crustal 4He degassing rate in the last 40 y would have increased. on the other hand, the helium content of the atmosphere by only 0.02%. Such an increase in degassing. if it were accompanied by a comparable increase in tectonic activity. would not have escaped notice. Thus, increased degassing seems unlikely to have occurred. The question of how much the con~ent~t~on of helium in the atmosphere has actually changed couId be resolved directly by the analysis of air samples from 10 to 40 years ago. However, such samples. regardless of storage method, would almost certainly be subjected to depletion of their oxygen content by reaction with the storage container. Thus, measurement and comparison of He/Ne or He/Ar ratios in old and new samples might be the most appropriate technique for detecting historic trends. It would also be desirable to improve the accuracy of the helium concentration measurement techniques and to observe any changes in the Earth’s atmospheric hehum content over the next few years. 8. CONCLUSIONS The measurements presented here indicate a I981 helium concentration in the Earth’s lower atmosphere of 5.222 10.017 ppm. This value is 0.3 1+_ 0.3% lower than the currently accepted value of 5.239 I 0.004 ppm measured by GLUECKAUF (1946). The results do not establish whether the helium concentration in the atmosphere has changed in the past 40 years, but they set reasonable limits. Pending further m~uremen~, and a possible systematic check of old samples, it is suggested that the appropriate value for helium content of dry air in 198 I is 5.222 + 0.017 ppm by volume. On a statistical basis, however, it cannot be said that this value is preferred over the earlier one. .~‘.~nowiedgemenrs-The authors wish to acknowledge contributions to this work by J. F. Johnson, D. W. Kneff. M. M. Nakata, W. E. Parkins, H. Pearlman, R. P. Skowronskr. and H. C. Wieseneck. all of Rockwell lnternatlonal
Helium in Earth’s atmosphere Valuable suggestions were obtained from W. B. Clarke (McMaster University, Hamilton, Canada), E. Farrar (Queen’s University, Kingston, Canada), E. Glueckauf (Chilton, Oxon, England), and J. C. Huneke (Charles Evans and Associates, San Mateo, California), ail of whom reviewed drafts of this paper before it underwent significant revision. We also wish to express our thanks to M. M. Cohen, R. S. Caron, P. B. Hemmig, J. W. Lewellen, T. C. Reuther, Jr., and K. M. Zwilsky of the U.S. Department of Energy (DDE) for their continued interest and support. This work was in part supported by DOE Contract DE-AT0381SF11561. REFERENCES CHACKHT K. F., PANETH F. A. and WILSON E. J. (1950)
Chemical analysis of stratospheric samples from 50 and 70 km height. J. Atmos. Terrest. Phys. 1, 49-55. CLARKE W. B., JENKINSW. J. and TOP Z. (1976) Determination of tritium by mass spectrometric measurements of ‘He. Int. J. Appl. Radial. Isot. 21, 515-522. DUSHMANS. (1962) Sorption of gases by “active” charcoal, silicates (including glasses) and cellulose (revised by G. L. Gaines, Jr.). In Scientific Foundations of Vacuum Technique (ed. J. M. LAFFERTY),2nd Ed, Chap 7, pp. 435515. Wiley. ENERGY INFORMATIONADMINISTRATION( 1980) World natural gas: 1978. Energy Data Report, U.S. Department of Energy, DOE/EIA-O133 (78). ERMOLING. M. (1957) Toward a method of quantitative separation of helium-neon mixtures. Trudy Radievogo Inst. im. V. G. Khlopina 6, Trans. in Works of V. G. Khlopin Radium Institute 6. 124-145. AEC-tr-4208. FARRA~ IV H., MCELROY W. N. and LIPPINCOTTE. P. (1975) Helium production cross section of boron for fast reactor neutron spectra. Nucl. Technol. 25, 305-329. GLUECKAUFE. (1946) A microanalysis of the helium and neon contents of air. Proc. Roy. Sot. Ser A 185.98-I 19. GLUECKAUF E. and PANETH F. A. (1946) The helium content of atmospheric air. Proc. Roy. Sot. Ser. A 185, 89-97.
1767
JOLLY J. H. (1979) In Commodity Year Rook I979 (ed. W. L. EMERY).Commodity Research Bureau, Inc. KOCKARTSG. (I 973) Helium in the terrestrial atmosphere. Space Sci. Rev. 14, 723-757. MCCORMICKW. T. (1977a) The future for world gas supply. Gas Supply Rev. 5, 1-3, Amer. Gas Assoc. MCCORMICK W. T. (1977b) Summary of United States natural gas statistics for the period 1945 to 1976. Gas Supply Rev. 5, Supply and Production Supplement. I-4, Amer. Gas Assoc. MCIOREB. J. (1976) Analyses of natural gases, 1917-1974. U.S. Bureau of Mines, National Technical Information Service, PB-25 1202. RAIN W. J., SCHUNK R. W. and BANKS P. M. ( 1978) Helium ion outflow from the terrestrial ionosphere. Planet. Space Sci. 26, 255-268.
REITER E. R. (1978) Atmospheric Transport Processes, Part 4: Radioactive Tracers. U.S. Department of Energy, (TID27114). TONGISHC. A. (1980) Helium-its relationship to geologic systems and its occurrence with the natural gases, nitrogen, carbon dioxide, and argon. U.S. Bureau of Mines, Report of Investigation RI8444. UNITED NATIONS(1952), Statistical Office, Department of Economic Affairs, World Energy Supplies in Selected Years 1929-1950. (ST/STAT/SER.J/ 1). UNITED NATIONS (1976), Department of Economic and Social Affairs, World Energy Supplies 1950-1974 (ST/ ESA/STAT/SER.J/ 19). U.S. BUREAUOF CENSUS(1975) Historical Statistics of the United States, Colonial Times to 1970. Bicentennial Edition. WEISS R. F. (197 1) Solubility of helium and neon in water and seawater. J. Chem. Eng. Data 16, 235-241. WOFSY S. C. and MCELROY M. B. (1973) On vertical mixing in the upper stratosphere and lower mesosphere. J. Geophys. Res. 78, 2619-2624.
ZARTMANR. E., WASSERBURGG. J. and REYNOLDSJ. H. (196 1) Helium, argon, and carbon in some natural gases. J. Geophys. Res. 66, 277-306.