Relation of Radon Concentration in the Atmosphere to Total Moisture

25 Relation of Radon Concentration in the Atmosphere to Total Moisture Detention in Soil and Atmospheric Thermal Stability

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WILLIAM M. COX, RICHARD L. BLANCHARD, and BERND KAHN Department of Health, Education, and Welfare, Division of Environmental Radiation, Radiological Engineering Laboratory, 5555 Ridge Ave., Cincinnati, Ohio 45213

Average monthly morning radon concentrations in ground-level air at Cincinnati, Ohio, based on daily measurements of radon-daughter concentrations, are reported for the period 1959-1966. Monthly averages ranged from 120 to 1520 pCi/cu. meters; maxima occurred from August to October and minima from February to April. Two alternative linear equations for predicting average monthly radon concentrations from atmospheric stability and total moisture detention in soil were derived. Radon concentrations varied directly with atmospheric stability and inversely with total moisture detention. Both correlation coefficients were 0.94, and 50 mean monthly values were predicted within 20% on the average.

aseous radon-222 and its particulate short lived daughters are usually the major radioactive constituents in air. Presented here are monthly average radon-222 concentrations based on the measurement of short lived radon-222 daughters i n ground-level air at the same location daily in the morning for eight successive years. To aid i n predicting radon-222 concentrations, results are analyzed for seasonal trends and the effects of atmospheric thermal stability and soil moisture content. I n an earlier paper ( 1 ), morning values during the first four years of the study were compared with afternoon radon-222 concentrations, morning radon-220 concentrations, w i n d speeds, and amount of precipitation, and typical diurnal cycles of radon-222 concentrations were shown. 436

Freiling; Radionuclides in the Environment Advances in Chemistry; American Chemical Society: Washington, DC, 1970.


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Total Moisture Detention

437

The concentrations of radon-222 and its daughters i n ground-level air depend on the concentration and distribution of their radium-226 precursor i n soil and rock, the emanating power of radon from its point of formation, the diffusion of radon to the surface of the ground, the resistance of ground cover (snow or vegetation) to the passage of radon, the vertical and horizontal transport of radon i n the atmosphere, particle formation and attachment to larger particles by radon daughters, and washout from the atmosphere by precipitation ( 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14). A t a specific location, atmospheric thermal stability and soil moisture content appear to be especially significant i n predicting radon concentrations, as indicated by accumulation of radon during inversions and by higher radon concentrations during drier seasons (2, 4, 8, 9,12,14). To evaluate among numerous other factors the influence of atmos pheric thermal stability and soil moisture content, their monthly average values were compared with monthly average radon-222 concentrations by least squares analysis i n terms of linear equations. Radon-222 con centrations were computed from radon-daughter measurements at C i n cinnati from 1959 to 1966. The Cincinnati station of the U . S. Weather Bureau provided temperature and rainfall records from 1962 to 1966 for computing atmospheric thermal stability and soil moisture content. The latter was derived i n terms of total moisture detention ( TMD ) as defined by Thornthwaite (15, 16). This presentation of soil moisture data has been applied to irrigation problems for the past 20 years and has been tested and improved in this context. Its use appears advantageous be cause only Weather Bureau records and determination of soil waterholding capacity are required, and direct measurements of soil moisture content are unnecessary. In correlated studies the influence of precipi tation on radon-daughter concentrations was observed for specific sta bility ranges, and radon concentrations computed from radon-daughter measurements were compared with directly measured radon concentra tions i n 114 morning observations. Procedure

and

Calculations

Airborne particles were collected outside the laboratory by drawing air at the rate of 30 cu. meters/day through a 4.7 cm. diameter, 0.8-μ pore size, membrane filter (1). The air intake was 5 meters above ground and was shielded from precipitation by a glass tube. A i r was drawn through the filter for 24-hour periods. The filter was collected at 8:30 a.m. (either E S T or E D T , as applicable), Tuesdays through Fridays, and the α-activity was measured with a scintillation detector within 2 minutes of collection. The concentration of radon i n air was computed from the α-count by assuming that 3.8-day R n and its short-lived daughters—2.0 min. 2 2 2


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P o , 27-min. P b , 20-min. B i , and 164-/xsec. Po—were i n secular equilibrium. The measured α-radiation, emitted by P o , was corrected for the decay of its parents P b and B i between collection and count ing, for slight periodic changes i n counter efficiency as determined w i t h a long lived standard, and for the presence of short lived daughters of R n . The corrected count rates were converted to radon concentrations in air by taking into account the counter efficiency and the amount of air that passed through the filter. For the eight-year record, all daily morning values were averaged on a monthly basis; for comparisons with stability and TMD values, radon values were averaged only on days for which stability data were available, and monthly averages were used if stability data were available for more than one-half of the days. As a result, only 50 monthly averages were considered for the period 1962-

218

2 1 4

2 1 4

214

214

2 1 4

2 1 4


2 2 0

1966.

During 1965, morning concentrations of radon and radon daughters were compared by passing 1.4 cu. meters of air i n 2 hours through a membrane filter and then through 60 grams of activated charcoal at a temperature of —78°C. (13). The radon daughters on the membrane filter were measured i n an α-counter immediately after collection; the radon adsorbed on the charcoal was measured three hours after collection by counting with a spectrometer the y-rays from the P b and Bi daughters. 2 1 4

2 1 4

Stability values—differences between temperatures at elevations of 30 and 3 meters—were obtained from records of the U. S. Weather Bureau station at Gest St., 6 miles from the laboratory and i n a topo graphically similar location. Stability records were begun i n 1962. Hourly values were averaged for 3, 6, 9,12, and 15 hours before collecting the radon-daughter filter and then averaged for each month. Average daily temperatures at the same location were also obtained from this source and converted to monthly averages. Hourly precipitation data at Abbe Observatory, 1 mile from the laboratory, were also obtained from the U. S. Weather Bureau. Monthly averages were derived for T M D calculations, and monthly averages for the 12 hours prior to filter col lection were computed to evaluate the combined effect of stability and rainfall. Total moisture detention was computed by a monthly bookkeeping procedure developed by Thornthwaite for precipitation, évapotranspiration, storage of water i n the ground and as snow, and run-off (15). Briefly, T M D values are derived from the mean monthly rainfall at Cincinnati, an average water-holding capacity of soil at Cincinnati of 300 mm. (16), and potential évapotranspiration values that were obtained from a nomogram based on mean monthly temperatures and the number of daylight hours. Every month, the retained rainfall is added to, and évapotranspiration subtracted from, the water stored during the previous months. W h e n the water-holding capacity of the soil is reached, the TMD remains constant, subject to the retention of additional water as snow and ice, and the presence of some water i n the process of running off. Thornthwaite provides examples of this lengthy but not complex procedure (15).


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Table I.


A L .

439

Average Monthly Morning Radon-222 p C i / c u . meter

Concentrations,

Month

1959

1960

J96I

1962

1963

1964

1965

1966

Average

Jan. Feb. March April May June July Aug. Sept. Oct. Nov. Dec.

230 240 270 — — 390 540 840 700 620 310 280

170 160 150 320 290 280 610 580 790 660 320 210

310 280 120 170 230 450 440 640 570 820 360 240

180 130 180 360 420 470 490 640 560 500 460 470

350 200 120 260 320 580 590 720 1050 1520 620 410

390 360 — 170 350 420 640 640 870 1130 660 390

260 240 120 200 400 300 610 700 530 440 440 410

270 210 280 200 400 720 590 970 690 910 370 280

270 230 180 240 340 450 560 710 720 830 440 310

Figure 1.

Mean and extreme average monthly morning radon concentrations, 1959-1966

Results and

Discussion

Average monthly morning radon concentrations i n air computed from radon-daughter measurements (Table I and Figure 1) consistently reached annual maxima during August-October and minima during F e b r u a r y - A p r i l . Individual monthly averages ranged from 120 to 1520 p C i . / c u . meters, and October averages i n the eight-year period were more



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than four times as high as March averages. The same pattern has been observed at other inland locations (5, 7, 9 ) , but it is not universally applicable (7). Monthly averages based on daily average radon concentrations would differ from the values in Table I and Figure 1 i n that (a) morning values usually are daily maxima, and (b) radon concentrations appear to be underestimated by measuring radon daughters. A t Livermore, Calif., for example, afternoon radon concentrations (usually the daily minima) were on the average 0.5 of morning values ( 5 ) , and the average concentration measured in the morning by radon daughters relative to direct measurement was 0.77 ± 0.09 (6). A t Cincinnati, the average ratio of 8:30 a.m. to 3:00 p.m. concentrations was 0.41 ± .18 (1 σ of individual monthly values) from 1959 to 1962 (1). The average ratio of morning radon concentration by radon-daughter measurement to direct measurement was 0.87 db 0.14 (1 σ of individual values), as measured i n this study. Neither ratio varied greatly from month to month. Thus, the average computed from the mean of maximum and minimum daily values would be approximately (1 + 0.41 ) / ( 2 X 0.87) — 0.8 of the values shown in Table I and Figure 1. To indicate numerically the correlation of trends for average monthly radon concentrations i n air, nighttime atmospheric stability, and soil moisture content, coefficients for alternative simple empirical equations were obtained by least-square analysis: C ^ ^ S + L (TMD^

and

(1)

+ k*

In C = * i ' S - k ' (TMD) + Jfc' 2

(2)

a

C is the average monthly morning radon concentration i n p C i . / c u . meter computed from radon daughter measurements, S is the stability i n °F./27 meters, and TMD is the total moisture detention i n mm. The constants k and k' and the correlation coefficients for the equations were obtained for a l l three variables and also separately for C vs. stability and C vs. TMD. Figure 2 shows the trend in average monthly radon concentrations with average monthly nighttime atmospheric stability. The squared cor relation coefficients, R , of 0.53 and 0.54 ( F ratios of 54 and 56) indicate that the two variables are definitely correlated. The scattering of values about the curves i n Figure 2, however, suggests that the indicated equa tions do not alone explain the variation i n radon concentrations. [ R pro vides the F ratio according to: 2

2

^

F ratio =

(n — p)l

'

1



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Figure 2.

441


Average monthly radon concentration as function of stability

where η is the number of observations, and ρ is the number of parame ters. In this study, η = 50, and ρ = 2 or 3. Relations are significant according to the F ratio at the 99.9% confidence level if F > 13.] The 12-hour averages for stability used i n Figure 2 yield a higher correlation than averages over shorter or longer periods as shown below: Period

R

3 hours 6 9 12 15

0.05 0.32 0.46 0.53 0.51

2

That the R value for the 12-hour period is largest suggests that radondaughter measurements were affected more by the accumulation of radon near ground level during the entire night than by the accumulation of the short lived radon daughters in air immediately before collection. Precipitation during the 12-hour period before filter collection de creased the radon concentration, as shown i n Table II for three stability categories. The effect is especially apparent at high stabilities: radon concentrations were almost two-fold lower during precepitation at sta2


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Table II. Effect of Stability and Rainfall on Average Morning Radon Concentrations, pCi/cu. meter Average Stability (9 p.m-9 a.m.), °F./27 meters

β

Precipitation

0.01 inch

194 ( 91 ) 158 (16)

- 0 . 5 to 0.5

>0.5

337 ( 281 ) 218 (54)

793 ( 290 ) 398 (17)

Number of values in parentheses.

Table III. Downloaded by UNIV OF CINCINNATI on May 30, 2016 | http://pubs.acs.org Publication Date: January 1, 1970 | doi: 10.1021/ba-1970-0093.ch025

α

Total Moisture Detention in Soil at Cincinnati, Ohio, mm.

Month Jan. Feb. March April May June July Aug. Sept. Oct. Nov. Dec.

1962 405 464 450 351 314 226 239 172 156 191 229 269

1964 187 227 398 408 279 277 186 136 119 107 146 256

1963 321 344 452 388 364 244 197 154 116 91 89 112

1965 331 383 378 387 257 177 141 113 201 254 280 296

1966 387 417 382 392 328 223 147 116 144 129 217 300

bility values above 0.5 °F./27 meters. A decrease of radon concentration after rain had been noted previously (1,2), and can be ascribed to wash out of radon and/or its daughters from air and to a reduction of radon diffusion through the ground when the surface soil is moist. The pattern of high TMD values i n late winter and low values i n late summer is shown i n Table III, and the high degree of predictive power of both equations for average radon concentrations i n air from TMD values is apparent i n Figure 3. For the linear relation, R is 0.84, and the F ratio is 252; the logarithmic relation is correlated only slightly less ( R — 0.79, F ratio = 181). Prediction of average monthly morning concentrations i n air is fur ther improved by considering both stability and TMD; by least-squares analysis, Equations 1 and 2 are: 2

2

C = 139 S + 93,000 ( TMD y

1

- 14

[R = 0.89]

( la )

[R = 0.89]

(lb )

2

and In C = 0.332 S - 0.00384 ( TMD ) + 6.877

2

The F ratios equal 380, and the two equations are the same i n their over-all predictive power. Different values are predicted for individual months, however, as shown i n Figures 4 and 5. Both equations predict


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443


1,600)

I 1962

1,400

^

Ι,20θ|

O

1963

•

1964

A

1965

+

1966

V

J 1,000 α Έ 8 o

800| 1510 ·

600

0.0049*


0.79

200h

200

Figure 3. Average monthly radon concentration as function of total moisture detention in soil 1 IJ500

Γ

Δ

Log R - S-(TMD)

•

R= S + (TMD)"'

1,250

I

1,000

750

1963

«I..IIIM

1965

1966

Figure 4. Measured average monthly radon concentration vs. values predicted from stability and total moisture detention in soil



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RADIONUCLIDES

1962

1963

1964

1965

I N

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1966

Figure 5. Difference between measured average monthly radon concentrations and values predicted from stability and total mois ture detention in soil all but two of the 50 values within less than 200 p C i . / c u . meter, and the average error i n prediction is 20%, compared with an average 58% error if the over-all average were predicted for every month. It would be highly unreasonable to ascribe all variability in radon concentrations at a given location solely to variations i n stability and soil moisture content, and contribution from the other cited factors would be expected to improve these predictions. Differences between predicted and measured averages are especially noticeable during high radon con centrations, in that predicted values vary less from year to year than do the measured values. The high degree of correlation, however, indi cates that the monthly average radon concentrations i n air were reason ably well predicted by monthly TMD and atmospheric stability data. Conclusion Monthly average R n concentrations in air during 1959-1966, measured daily at 8:30 a.m. by counting α-particles emitted by the short lived P o daughter in 24-hour samples of airborne particles, showed an annual maximum in August-October and minimum in February-Apr il. 2 2 2

214



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445

The maximum eight-year average was 830 p C i . / c u . meters i n October, and the minimum was 180 p C i . / c u . meters i n March. Values measured at 3:00 p.m. during 1959-1962 showed the same annual pattern, but monthly averages were only 0.41 as high as morning values. The average ratio of radon concentrations computed from radon-daughter measure ments to directly measured radon concentrations during 1965 was 0.87. Average monthly morning concentrations of radon i n air increased with average monthly values of atmospheric stability during the 12 hours before filter collection, but there was appreciable scatter of values about lines of best fit. The squared correlation coefficient was 0.53 for a linear relation between radon concentration and stability. Lower correlations applied to stability values averaged over periods longer or shorter than 12 hours. Precipitation during the 12-hour period before collection decreased radon concentrations. Average monthly morning radon concentration varied inversely with soil moisture content, expressed in terms of Thornthwaite's total moisture detention. The two variables were highly correlated, the squared corre lation coefficient being 0.84. B y relating radon concentration i n air to both T M D and stability, the squared correlation coefficient increased to 0.89. A n alternative mathematical relation, the logarithm of the radon concentration vs. TMD and stability, showed identical correlation. Acknowledgment W e thank the Cincinnati Station of the U . S. Weather Bureau for making data available to us, and D a v i d Smith of this laboratory for discussing the effect of meteorological parameters. Literature Cited (1) Gold, S., Barkhau, H. W., Shleien, B., Kahn, B., "The Natural Radiation Environment," J. A. S. Adams and W. M. Lowder, Eds., p. 369, Uni versity of Chicago Press, Chicago, 1964. (2) Hosler, C. R., Monthly Weather Rev. 94, 89 (1966). (3) Hosler, C. R., Lockhart, Jr., L. B., J. Geophys. Res. 70, 4537 (1965). (4) Kraner, H. W., Schroeder, G. L., Evans, R. D., "The Natural Radiation Environment," J. A. S. Adams and W. M. Lowder, Eds., p. 191, Uni versity of Chicago Press, Chicago, 1964. (5) Lindeken, C. L., University of California, Lawrence Radiation Labora tory, UCRL-50007-66-1, 41 (1966). (6) Lindeken, C. L., J. Geophys. Res. 73, 2823 (1968). (7) Lockhart, Jr., L. B., "The Natural Radiation Environment," J. A. S. Adams and W. M. Lowder, Eds., p. 331, University of Chicago Press, Chicago, 1964. (8) Moses, H., ANL-6398, 86 (1961). (9) Moses, H., Lucas, Jr., H. F., Zerbe, G. Α., J. Air Poll. Control Assoc. 13, 12 (1963). (10) Pearson, J. E., Jones, G. E., J. Geophys. Res. 70, 5279 (1965).


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(11) Schroeder, G. L., Kraner, H. W., Evans, R. D., J. Geophys. Res. 70, 471 (1965). (12) Servant, J., CEA-R 2434 (1964). (13) Shleien, B., Am. Ind. Hygiene Assoc. J. 24, 180 (1963). (14) Tanner, A. B., "The Natural Radiation Environment," J. A. S. Adams and W. M. Lowder, Eds., p. 161, University of Chicago Press, Chicago, 1964. (15) Thornthwaite, C. W., Mather, J. R., Drexel Institute of Technology, Publ. Climatology 8, (1) (1955). (16) Thornthwaite, C. W. et al., Drexel Institute of Technology, Publ. Climatology 17, (3) (1964). June 16, 1968.


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Relation of Radon Concentration in the Atmosphere to Total Moisture

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