(21) Koirtyohann, S. R., Pickett. E. E.. A n a / . Chem., 37, 601 (1965). ( 2 2 ) Koirtyohann, S.R., Pickett, E. E., ibid., 38, 585, 1087 (1966). ( 2 3 ) Dick, D. L., Urtamo, S. J., Lichte, F.E., Skogerboe, R. K., Appl. Spectrosc., 27, 467 11973). (24) Arvik, J. H., doctoral dissertation, Colorado State University, Ft. Collins, Colo., 1973. (25) Ostle, B., "Statistics in Research", Iowa State Univ. Press, Ames, Iowa, 1963. (26) Jenne, E. A , , in: "Trace Inorganics in Water", Adc. Chem., 73, 337 (1968).
( 2 7 ) Parks, G. A., Chem. Rei).,65,177 (1965). (28) Perdue, E. M., Beck, K. C., Reuter, J . H., Nature, 260, 418 (1976). (29) Howard, J. H., 111, in "Trace Substances in Environmental Health-V", D. D. Hemphill, Ed., pp 485-95,1971. (30) Schnitzer, M., Skinner, S.I.M., Soil Sci., 102, 361 (1966). Receiced for reuieu, August 12, 1974. Resubmitted August 3, 1976. Accepted J u l y 29,1977. Research supported by lVSFGrant Nos. GI-4 and GI-34813 X .
Vertical Profiles of 222Rnand Its Long-Lived Daughters over the Eastern Pacific Howard E. Moore1*, Stewart E. Poet, and Edward A. Martell National Center for Atmospheric Research, P.O. Box 3000, Boulder. Colo. 80307
Vertical profiles of *Z2Rn,zlOPb, plOBi,and zlOPoobtained offshore near San Francisco, Calif., are compared with similar profiles over the West Central United States and on the slope of Mauna Loa, Hawaii. Based on the profile data and meteorological trajectories, aerosol transit times across the Pacific Ocean are estimated to be about two weeks. Mean residence times for tropospheric aerosols over ocean areas are estimated to be 4-6 days. Isotope activity ratios are significantly altered during transit across ocean areas. 22pRn,a naturally occurring chemically inert radioactive gas, is produced by the decay of nzsRain soils and vegetation. The '22Rn flux per unit area over continents is approximately 100 times that over oceans ( I ) . The only significant mechanism for removing 222Rnfrom the atmosphere is by radioactive decay via its 3.8 day halflife. These facts make 222Rnan ideal tracer for large-scale transport of air masses across oceans (2, 3).z2*Rnmeasurements have been used to delineate monsoon air masses and to trace air masses from Africa to Barbados ( 4 , 5 ) . Recently, we used data for 222Rn and its long-lived daughters, 210Pb (Tl/r = 22 y), 21OBi (T1/2 = 5 d ) and 210Po (Tl/a = 138 d ) , as a basis for estimating transit times from distant continental areas to Hawaii (6). Reliable estimates of transit time require a knowledge of the variation in concentration of these isotopes with altitude in the troposphere to properly account for vertical eddy diffusion during transport. Activity ratios of the above isotopes also have been used to determine aerosol residence times in the atmosphere (6-10). Aerosol residence times can be estimated from the relationship:
dN,/dt = NIX1 - N z
(Xp
+ XR)
=0
(1)
where N1 and N2 refer to the number of atoms of parent and daughter isotopes, t is time, XI and X p are the corresponding radioactive decay constants, and XR is the removal rate for aerosols from the atmosphere. The mean aerosol residence time, T R , is the reciprocal of XR. Equation 1assumes a constant 222Rnflux. Since this flux is much lower over the oceans, the activity ratios are dependent on the time of transit over the ocean as well as on the mean residence time. Thus, residence times estimated from such ratios may be in error, especially near or over the oceans. The residence time of airborne particles also is dependent on particle size. As we have pointed out elsewhere (IO),210Pband 210Biactivity in the troposphere Present address, Department of Physical Sciences, Florida International University, Miami, Fla. 33199.
is largely associated with particles near 0.1 fim diameter, a size interval for which particle deposition mechanisms are least effective. Thus, 210Bi/z10Pbactivity ratios indicate upper limit values for aerosol residence times. T o better understand the changes in concentrations and activity ratios in tropospheric air during transit across the Pacific Ocean, we have determined the vertical distribution of these isotopes in samples collected by aircraft up to 13.1km altitude approximately 160 km west of San Francisco, Calif.
Experimental Methods The procedures used for collection and radiochemical analysis have been described in detail elsewhere (8, 9). Atmospheric aerosol samples are collected by National Center for Atmospheric Research (NCAR) aircraft a t various levels up to 13.1 km altitude. Aerosol samples are collected by passing ambient air through 40-cm-diameter filters a t flow rates of 10-20 m3/min STP maintained by the ram pressure a t NCAR aircraft flight speeds. Prior to aerosol collection the sampling system is purged with air a t altitude. Samples are collected on efficient (>99%) polystyrene filters (Delbag 98/99). 222Rnis collected by passing air through 250 g of 6-10 mesh coconut charcoal, cooled to -80 "C with a dry ice slurry. Prior to sample collection the charcoal is heated to 400 "C and outgassed to less than 15 1 pressure. All intake lines are flushed with air a t altitude through a bypass before sampling. Flow through the system is maintained within the range 25-100 L/min STP by aircraft ram pressure. Before radiochemical analysis each polystrene filter is compacted in a laboratory press and placed in a distillation flask. Stable Pb, Bi, and, when desired, Sr carriers and a calibrated 208Poor 209P0spike are added. The filter material is removed by distillation, followed by digestion with sulfuric and nitric acids. Bismuth is subsequently separated from the mixture as BiOCl and redissolved. A t this point, Po is separated from the Bi by autoplating on silver disks. After plating, the Bi is converted to BiP04 for 210Bicounting. The centrifugate remaining after the BiOCl separation contains the P b carrier and is reserved for P b analysis. After conversion of PbS04, to determine the P b yield, the PbS04 is stored to allow complete in-growth of 210Bi.A second Bi carrier is added, and the aloPb content of the sample is determined by 210Bi daughter assay as before. The Z2ZRnsamples are outgassed a t -80 "C to 750 fi pressure and then allowed to come to room temperature. Each sample is transferred by heating the charcoal to 400 "C and passing the gases successively through a trap cooled to -80 "C to remove water, through ascarite to remove COz, and through a second small charcoal trap cooled to -80 "C to reVolume 11, Number 13, December 1977
1207
Table 1. 222Rn,210Pb,210Bi,and 210Povs. Altitude Offshore near San Francisco Collection dates
TroooDause Ht (km)
2-24-72
10.7
9-28-72
15.2
4-24-73
9-13-73
11-1-73
.
11.9
13.1
11.9
(km)
222Rn
'l0Pb
210B1
4.6 7.6 9.5 11.3 13.1 3.0 4.6 7.6 9.4 11.3 13.1 1.5 3.0 4.6 7.6 9.4 11.3 13.1 1.5 3.0 6.1 7.6 9.4 11.3 13.1 1.5
... ... ... ...
1.63 0.24 0.16 0.24 0.25
0.91 0.14 0.08 0.19 0.17
3.0 4.6 7.6 9.4 11.3 13.1 a
Alr concn (dpm/100 m3, STP)a
Altitude
... 157 528 153 246 538 161 994 912 1440 638 1785 2125 28 7290 234 708 1426 685 1435 1524 1291 457 71 1 525 435 455 87
2'0p0
0.053 0.0 18 0.020 0.046 0.102
...
...
...
1.24 0.42 0.36 0.45 0.37
0.81 0.30 0.36 0.35 0.25
0.029
...
...
... ...
... ...
0.77 0.13 0.22 0.34 0.18
0.53 0.06 0.12 0.21 0.17
0.035 0.010 0.020 0.018 0.070
...
...
... ...
... ...
1.68 0.76 0.26 0.41 0.36
0.76 0.41 0.23 0.22 0.17
0.040 0.023 0.016 0.019 0.013
... ...
... ...
... ...
0.35 0.13 0.14 0.25 0.34
0.16 0.06 0.09 0.17 0.29
0.017 0.006 0.012 0.018 0.044
0.044 0.060 0.030 0.032
The one sigma counting errors for 222Rnare 1.5 f 1.1% of the values listed. Corresponding errors for the other isotopes are: 2i0Pb, 5.3 f 3.6%; 210Bi,7.2
f 4.5%; 2ioPo, 24.6 f 13.0%.
move 222Rn.The zzzRn is finally transferred to a radon counting cell (11) and counted after the 2z2Rnhas come to secular equilibrium with its short-lived daughters. The overall efficiency of the collection and transfer system is 98%. An additional 15% loss in efficiency is due to the counting configuration. The zloBi is counted by placing thin samples in close geometry between two thin-walled flow counters with shielding anticoincidence counters surrounded by 15 cm of steel shielding. This configuration provides nearly 4 K geometry with a background of only 0.25-0.30 cpm. The combined external absorption, scattering, and geometry factor for zloBi in this counter is 1.32. An additional small correction is made for self-absorption, which is dependent on sample thickness. The 210Po(Y activity is counted in a vaccum chamber a t room temperature with a silicon surface barrier detector. The counting geometry is 0.54 IT,and the background is about 1 count per day in the energy range of the Po isotopes. Suitable standards are processed in the same manner as the samples to ensure the integrity of the procedures.
Discussion Data for five vertical profiles are listed in Table I. In Figures 1 and 2, these data are compared to the average profile obtained a t several locations over the West Central United States (9) and a t Mauna Loa Observatory, Hawaii (6). The average radon profiles over central continental areas (9) do not appear to vary greatly. Mean profiles reported by different investigators at various locations over the U S . and the USSR are shown in Figure 3 for comparison. If the continental profile 1208
Environmental Science & Technology
plotted in Figure 1 is assumed to be representative of the average profile over eastern Asia, the transit time from Asia to California can be estimated from the relation:
A1 = A ; exp ( - A l t )
(2)
where A: and A1 are the total activities in a column of air over eastern Asia and off the California coast, respectively. Integration of the two curves in Figure l yields a 13.6 day transit time. Air trajectories for the profile obtained on 24 April 1973 are shown in Figure 4. For lack of adequate meteorological data, it was not possible to trace these trajectories back more than 2.5 days, to the mid-Pacific Ocean area. If we assume a similarly meandering trajectory from the Asian mainland to the mid-Pacific Ocean area, the 13.6 day transit time from Asia to California appears reasonable. The zloPb profile west of San Francisco, Calif. (Figure 2 ) , is quite similar to that for the Central United States. This would appear to be contradictory to the conclusion that the mean aerosol residence time in the troposphere is about 4-6 days (8,9).If the 210Pb profile over Asia is similar to that over the Central United States, such a short residence time would be expected to result in less zloPbactivity offshore near California. A more probable explanation is that the total 210Pb in the vertical column over Asia is perhaps twice that shown in Figure 2. Although vertical profiles of ZloRn and its longlived daughters over eastern Asia are not available for comparison, zlOPb deposition rates tend to support this conclusion. Tsunogai and Fukuda (12) found the highest 210Pbconcentrations in snow from continental air masses associated with the north-west monsoon. They found aerosol residence times of 2-7 days for these air masses. 210Pb fluxes down to the
I3 I2 I1
IO
us
01
1
I
10 IO0 CONCENTRATION, dpm/rn3 STP
'"9n
165"W
Figure 1. Comparison of average vertical profile for radon offshore near San Francisco with that over the West Central United States Error bars: 1 SD from mean
155OW
145OW
135OW
125"W
Figure 4. Mean air mass trajectories at designated elevations vs time in days prior to time of sample collection
Table II. Mean Acitivity Ratios 210Bi/210Pb
13
West Central United States Eastern Pacific Hawaii
210p~/210pb
0.52 f 0.04 0.068 0.61 f 0.15 0.067 0.82 f 0.11 0.079
f 0.008 f 0.037 f 0.039
2 2 2 ~ ~ / 2 1 0 pab
4980 2661 518
f 1480 f 2190 f 117a
The 222Rn/2'0Pbvalues for Hawaii include only those at Mauna Loa Observatory. Tabulated errors are 1 SD from the mean.
Continental U S
k 7
c
East Pacific Ocean
+
2
O
01
05
'l0Pb
IO
50
CONCENTRATION, dprn/IOO rn3 STP
Figure 2. Comparison of average vertical profile for lead-210 offshore near San Francisco with that over the West Central United States Error bars: 1 SD from mean
\
4 c
:t 'd "'Rn
Concentration, dpm/m3 STP
Figure 3. Mean 222Rnprofiles over U.S. and USSR Profiles 1,3,4, and 5 are over U.S. ( 18,20, 9, 21);profiles 2 and 6 are over USSR ( 19, 22)
surface of the earth a t north temperate latitudes over North America vary from 0.84 to 1.6 dpm cm-2 y-l, while a t Hakodate, Japan, a flux of 2.0 dpm cm-* y-' has been measured (13, 1 4 ) . The higher 210Pbflux over Japan suggests that the atmospheric concentration of *IOPbover eastern Asia may be about twice that over the West Central United States. The Eurasian continent is the largest continuous land mass traveled by air parcels and therefore should contain *lOPba t higher levels more nearly in equilibrium with production and removal processes. Conversely, the 210Pbmay be appreciably below equilibrium levels with these processes in areas over the West Central United States. If the latter is true, the residence time estimated for the West Central United States based on 222Rn/210Pb activity ratios should be less than those based on *10Bi/210Pbratios. The residence times estimated previously are 3.4 f 2.3 and 7.8 f 1.2 days, respectively (9).Doubling the 210Pb concentrations would correspondingly double the residence times calculated on the basis of 222Rn/210Pbactivity ratios. Variations of such magnitude in these ratios are common (9).If we assume the concentration of *IOPbover eastern Asia is double the levels shown in Figure 2, a transit time of 13.6 days and a mean aerosol residence time of about 5.5 days would give rise to the *lOPbconcentrations observed offshore at'california. This does not necessitate any change in the vertical profile for ***Rn.However, it would double the residence time estimated from the 222Rn/210Pbactivity ratio in air over eastern Asia. For reasons indicated above, this would appear to be quite reasonable. The changes in activity ratios during transit over the Pacific Ocean are indicated in Table 11. The 210Bi/210Pb and ***Rn/*lOPbratios at Hawaii are indicative of a longer transit time which was estimated to be between 15 and 20 days (6). Errors shown for the results obtained off the California coast are somewhat greater than those for other locations. This was especially true for errors in the 222Rn/210Pb ratios and may be Volume 11, Number 13, December 1977
1209
attributed to the fact that there is an abrupt, large change in the z22Rn flux in this coastal region. Small admixtures of continental air into the oceanic air mass will change the 222Rn/210Pb ratio drastically. The apparent lack of variation in the 210Po/210Pbratios is less easily explained. The mean aerosol residence time based on the 210Po/210Pb ratio over the eastern Pacific is 20.2 f 8.9 days. This value appears to support the work of various investigators (15,16) who feel a one-month or longer aerosol residence time is correct. An alternate possibility is that the relatively constant 210Po/210Pbratio is due to two factors working in opposition. I t has been inferred that airborne 210Poin the troposphere orginates mainly from wind-generated soil dust, with minor contributions from 222Rndecay in the troposphere (16). The 210Po/210Pbactivity ratio and the 210Poconcentration in the atmosphere increase significantly in dust storms (17). Most of the zloPb and *loPooriginating from 222Rndecay in the troposphere is attached to -0.1-pdiameter particles, while dust particles with a higher 210Po/210Pbactivity ratio would have a size range distribution extending to larger diameters. These larger particles, while having a higher 210Po/210Pbratio, would have a shorter residence time than the smaller size particles to which most of the 210Pbis attached. Thus, more rapid removal of the larger particles would tend to lower the 210Po/210Pbactivity ratio, while ingrowth of zlOPo on the smaller particles would tend to increase the ratio, and the ratio could remain constant even with a relatively short mean aerosol residence time. Although the above mechanism remains unproved, we have found in recently completed unpublished work higher 210Po/210Pbactivity ratios in larger size range particles. The two values for 2z2Rn concentrations in the lower stratosphere (Table I) indicate strikingly low levels a t midlatitudes. These results indicate that radon may be very useful as a natural tracer for the experimental investigation of troposphere-stratosphere exchange rates and the meridional exchange pattern. Concluding Remarks
Although the 222Rndata presented yield a oceanic transit time in agreement with our previous data, the 210Pb and 210Po data are consistent with our earlier data only if certain assumptions are made concerning the concentrations of these isotopes over the Asian continent and concerning mechanisms of aerosol origin and removal. Relationships between concentrations of 222Rnand its long-lived daughters and their activity ratios are complex but provide one of the more promising experimental tools for the study of air mass trans-
1210
Environmental Science & Technology
port and mixing processes in the troposphere and of tropospheric aerosol scavenging mechanisms of aerosol origin and removal. Acknowledgments
We thank Dennis Deaven for the air mass trajectory analysis (Figure 4) and Art Fong for assistance with the radiochemical analysis of samples. We gratefully acknowledge the help of the NCAR Research Aviation Facility for aircraft flight support in the collection of vertical profile samples. L i t e r a t u r e Cited (1) Wilkening, M. H., Clements, W. E., J. Geophys. Res., 80,3828-30
(1975). (2) Servant, J., Tellus, 18,663-71 (1966). (3) Larson, R. E., Lamontagne, R. A,, Wilkness, P. E., Wittman, W. I., Nature, 240,345-7 (1972). (4) Rama, J . Geophys. Res., 75,2227-9 (1970). (5) Prospero, J. M., Carlson, T. N., Science, 167,974-7 (1970. (6) Moore. H. E.. Poet. S. E.. Martell. E. A.. Wilkenina, M. H.. J . Geophys. Res.,’79,5019-24’(1974). ’ ( 7 ) Moore. H. E., Poet. S. E., Martell, E. A., “Natural Radiation Environment 11”,J.A.S. Adams, W. M. Lowder, and T. F. Gesell, Eds., U.S. Energy Research and Development Agency, CONF72-0805-P2, pp 775-86, Oak Ridge, Tenn., 1975. (8) Poet, S. E., Moore, H. E., Martell, E. A,, J. Geophys. Res., 77, 6515-27 (1972). (9) Moore, H. E., Poet, S. E., Martell, E. A,, ibid., 78, 7065-75 (1973). (10) Martell, E. A,, Moore, H. E., J . Rech. Atmos., 903-10 (1974). (11) Lucas, H. F., Reu. Sci. Instrum., 28,680-3 (1957). (12) Tsunogai, S., Fukuda, K., Geochem. J., 8,141-52 (1974). (13) Moore, H. E., Poet, S.E., J . Geophys. Res., 81,1056-8 (1976). (14) Nozaki, Y., Tsunogai, S., Earth Planet. Sci. Lett., 20, 88-92 (1973). (15) Nevissi, A., Beck, J. N., Kuroda, P . K.,HealthPhys., 27,181-8 11974). (16) Gavini, M. B., Beck, J. N., Kuroda, P. K., J. Geophys. Res., 79, 4447-52 (1974). (17) Moore, H. E., Martell, E. A,, Poet, S.E., Enuiron. Sci. Technol., 10,586-91 (1976). (18) Bradley, W. E., Pearson, J . E., J . Geophys. Res., 75, 5890-4 _
~
I
L .
(1 ~ 970). - -_,. .
(19) Kirichenko, L. V., “Problems in Nuclear Meteorology”, I. L. Karol and S. G. Malaknov, Eds., State Publ. House for Literature in the Field of Atomic Science and Engineering, p p 92-124, Moscow, USSR, 1962. (20) Wexler, H., Machta, L., Pock, D. H., White, F. D.,Proc. Int. Conf. Peaceful Uses At. Energy, l s t , 13,333-44 (1956). (21) Wilkening, M. H., J . Geophys. Res., 75,1733-40 (1970). ( 2 2 ) Nazarov, L. E., Kuzenkov, A. F., Malakhov, S.G., Volokitina, L. A,, Gaziyev, Ya. I., Vasil’yev, A. S., ibid., pp 3575-88. Received for reuieu March 31,1977. Accepted August I , 1977. Work done at the National Center for Atmospheric Research which is sponsored by the National Science Foundation.
