Radon and thoron decay product and 210Pb measurements at Jungfraujoch, Switzerland

Pergamon

Atmospheric Environment Vol. 29, No. 5, pp. 607-616, 1995 Copyright © 1995 Elsevier Science Ltd Printed in Great Britain. All rights reserved 1352 2310/95 $9.50 + 0.00

1352-2310 (94) 00195-2

R A D O N A N D THORON DECAY PRODUCT A N D 21°pb MEASUREMENTS AT JUNGFRAUJOCH, SWITZERLAND H. W. G ~ , G G E L E R , * D. T. J O S T , U. B A L T E N S P E R G E R M. S C H W l K O W S K I Paul Scherrer Institut, CH-5232 Villigen, Switzerland

and

and P. S E I B E R T Institat fiir Meteorologic und Geophysik, Universit/it Wien, A-1190 Wien, Austria (First received 3 January 1994 and in final form 23 June 1994) Abstract--During one year, the atmospheric activity concentrations of the short-lived "radon" (222Rn)and "thoron" (22°Rn) decay products were measured at Jungfraujoch (3450 m a.s.l.). The measurements were performed using a modified epiphaniometer, with a time resolution of 1 h. In addition, also the long-lived radon decay product 21°Pb was measured with a time resolution of one month. Strong seasonal variations of all activity concentrations were found, the average summer values being a factor of ten higher than the average winter values. During summer time the activity concentrations of the radon and thoron decay products showed, pronounced diurnal variations. During winter these diurnal variations were absent, and the activity conc,~ntrations were close to free tropospheric background values. From the ratio between the activity concentrations of the radon decay products 214pb and 21°pb a mean residence time of aerosol particles in the al:mosphere of about six days could be estimated. The average activity concentrations of the radon and thoro:a decay products at Jungfraujoch were found to be related to the regional soil temperatures at a near-by low altitude Swiss Midland site and to the static stability of the air mass between this Midland site and the Jungfraujoch. Furthermore, a significant correlation was found between the radon and thoron decay product activity concentrations and synoptic situations with higher than average activity concentrations for air flows from NE to SW and lower than average concentrations for air flows from W to N. Key word index: Radon decay products, thoron decay products, 21°pb, Jungfraujoch, radionuclides, free troposphere, atmospheric transport, aerosols, residence time.

1. I N T R O D U C T I O N

mental radon decay products can be measured via 214po. The measured signal is then representative for "Radon" (i.e. the isotope 222Rn of radon) and the sum of all precurser nuclides (21Spo ' 214pb ' 214Bi) "thoron" (the isotope ~:2°Rn of radon) enter the atmos- which are attached to aerosol particles. It was shown phere from the earth's surface where they are pro- that for outdoor conditions the short-lived radon duced via radioactive decay of their precursors 2 3 8 U decay products are attached to aerosol particles for and 232Th, respectivelLy. In the atmosphere these nun u m b e r concentrations /> 10 a cm -3 (Whittlestone, clides decay to non-gaseous progenies. For 222R__._nn 1990). Due to the short half-life of the radon decay (half-life, T1/2=3.8d) these products are 21sp__9.o products relative to that of radon, the measured 214po (3.05 min), 214pb (26.8 min), 214Bi (19.9 min), 214P__QO activity is a representative measure for the radon (164 ps), 21°pb (22.3 yr), 21°Bi (5.0 d), 21°p0 (138.4 d) concentrations at the sampling site. and 2°6T1 (4.2 min), and for 22°Rn (55.6 s) they are Due to the very short half-life of thoron of about 216po (0.15 s), 212pb (:10.64 h), 212Bi (60.6 min), 2t2po 1 min this nuclide is measurable only in the lowest (0.3/~s) and 2°STl (3.1 min) (the underlined nuclides atmospheric layers above ground. Nevertheless, a are decaying by s-emission with >i 10% probability, signal of its source strength can be found at larger all others are fl-emitters). distances via its relatively long-lived decay product If air is pumped through a particle filter which is 212pb. The activity of 212pb can be determined via ~continuously monito:red by an or-detector environ- measurements of its daughter and grand-daughter nuclides 212Bi (E~= 6.05MeV; 36%) and 212po (8.78 MeV; 64%), respectively. Both radon and thoron are injected into the at* Also at Institut ffir Anorganische Chemic, Universit~it Bern, CH-3000 Bern, Switzerland. mosphere mainly from the soil, i.e. over the continents. 607

608

H.W.G.~,GGELER et al.

Due to the low concentrations of 2 3 S U and 232Th in ocean water, marine air masses are strongly depleted in these nuclides. Average exhalation rates of 222Rn are 1.5x 10 -2 B q m - 2 s -1 over continents (UNSCEAR, 1982) and about 2 x 10- 4 Bq m - 2 s- 1 over ocean water (Jacobi, 1962). Exhalation rates of 22°Rn over continents are in the order of 0.4 Bq m - 2 s- 1 (Schumann, 1972). Typical radon activity concentrations in air above the continents are about 4 Bq m - 3 whereas over the oceans the activity concentrations are only about 0.1 Bq m-3 (Lambert et al., 1990, and references therein). In addition, there is a strong vertical gradient of the radon activity concentration with values of about (4-8)x l0 -4 Bqm -3 at the tropopause (Liu et al., 1984). In recent years, radon has been used successfully for atmospheric modelling because it is a chemically inert tracer with a rather well-defined source term and lifetime. Examples are modelling of tropospheric advective transport processes (Feichter and Crutzen, 1990), troposphere-stratosphere exchange (Lambert et al., 1990), and long-range transport to Antarctic areas (Heimann et al., 1990; Lambert et al., 1990b). It was one of the purposes of this work to use continuous radon and thoron decay product monitoring at the high-alpine site Jungfraujoch (3450 m a.s.1.) as an aid for interpreting measurements of aerosol-borne chemical species. Aerosol particles are transported by long-range transport in the free troposphere as well as by convective vertical mixing with polluted air from regional sources at lower altitudes. These measurements were performed in the framework of a more general study on atmospheric processes within the European environmental project

ALPTRAC, which itself is part of EUROTRAC (Baltensperger et al., 1992; Schwikowski et al., 1992). Continuous measurements of the radon and thoron decay products (via their decay products 214po and 212Bi/212po, respectively) were made with a modified version of the epiphaniometer (G/iggeler et al., 1989). This device allows to measure aerosol-borne ~-active species continuously. In addition, 2t 0pb was measured with a time resolution of one month by the grand-daughter nuclide 21°po which decays via or-emission (Eg=5.30 MeV). 21°pb is the only long-lived (T1/2=22.3 yr) decay product of radon. Its average near-surface activity in air is about 4 x 10 -4 Bq m-3 (Rangarajan et al., 1986; HStzl and Winkler, 1987). Little is known about the atmospheric concentrations of 21°pb at higher altitudes. Only few data are available on vertical concentration profiles (Kownacka et al., 1990).

2. E X P E R I M E N T A L

The instrument used in this study resemblesvery much the epiphaniometer which was introduced recently as a new instrument for continuous aerosol monitoring (G/iggeler et al., 1989). In that device, air is pumped through a closed container which delivers 211Pb atoms at a constant rate. In the container the short-lived 211pb atoms (Tl/2=36min) attach to the aerosol particles which are then transported through a capillary to a filter station. Here the aerosol particles are deposited and the a-decay of 211Bi,a daughter nuclide of 211pb' is measured. The dimensions of the capillary are such that non-attached 211pbatoms do not reach the filter. It has been shown that the activity deposited on the filter is proportional to the so-called Fuchs surface of the aerosol particles (G/iggeleret al., 1989; Rogak et al., 1991). The epiphaniometer has been used for continuous background aerosol monitoring at remote locations such as the

180 -

214p0

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256

512

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Channel Fig. 1. ,,-Spectrum ofa 4 h measurement with the modifiedepiphaniometer at Jungfraujoch from 2rid September 1991 (total air volume 0.96 m3). The prominent peak originates from the radon decay product 214po (Eg = 7.68 MeV). Minor peaks can be attributed to the thoron decay products 212po (Eg=8.78 MeV) and to the sum of the radon decay product 21apo (Eg=6.00 MeV) and the thoron decay product 212Bi(Eg=6.05 MeV).

222Rn and 22°Rn decay products and 21°pb measurements Jungfraujoch (3450 m a.s.l.) and the Colic Gnifetti (4450 m a.s.l.) (Baltensperger et ,d., 1991). The time resolution of this device can be as short as 5 rain (Pandis et al., 1991). The epiphaniometer allows to measure or-particles with an energy resolution of about 30 KeV; therefore it can also be used for continuous r~onitoring of natural aerosol borne ~t-active nuclides. In order to increase the sensitivity of the instrument a gas flog' rate of 4 Erain-1 instead of the commonly used 1 fmin-1 was used (G/iggeler et al., 1989). The gas flow rate in this device is monitored by a thermal mass flow meter. In addition, the 211Pb source was removed so that only activity of environmental origin was collected. The major part of natural aerosol borne a-active products are decay products of radon and thoron. The counting geometry was such that the detector faced the layer of aerosol particles deposited continuously on a Nuclepore filter. This ~Lrrangementassured that the energy resolution of the a-measurements remained unchanged even for long sampling periods because only the uppermost layer of deposited material contained the activity of short-lived nuclides. The or-activity was measured continuously. Due to the continuous flow mode, the measured quantity is the number of decaying atoms (dN) per volume of air (dV) [(dN/dt)/(dV/dt)=dN/dV; where dN/dt=activity and dV/dt = gas flow rate],, which, under equilibrium condition (produced atoms = decaying atoms), is equal to the total number of atoms per volume. The or-spectrum was analyzed every hour, representing an air volume of 0.24 m- a but the channel contents were: reset only after 4 h. Thus, it was possible to achieve a 1 h resolution during times with high concentrations, where~Ls during times with low concentrations only values integrated over 4 h gave reasonable results. A typical spectrum is shown in Fig. 1. Visible is a main or-peak from 2~4po at 7.69 MeY and minor peaks at 8.78 MeV from 212po and at 6.0MeV from 21Spo+212Bi, respectively. All three peak areas were stored in the memory of the epiphaniometer. The epiphaniometer was connected to a modem which allowed a data transfer to the Paul Scherrer Institute. Under the relatively clean air conditions at Jungfraujoch typical running times for an individual filter at 4 E min- ~ air throughput was one month. Once every month the Nuclepore filter was changed, stored for about 1 yr to achieve equilibrium between the 2lOpb and 2~Opoactivities and then measured for 2~Opo(Eet= 5.3 MeV) to determine the average monthly 21°pb activity in air. 3. RESULTSAND DISCUSSION

3.1. Monthly mean activity concentrations of the radon and thoron decay products 214pb, 212pb and 21°pb Figure 2 shows the monthly averaged activity concentrations of 2~4Pb and 212pb, deduced from the measured count-rates of 2~4P0 and 212p0, respectively, as well as the activity concentrations of 21°pb (measured off-line via 21°po) for the period January to December 1991. All activities per air volume are based on standard pressure and temperature. In case of equilibrium condition, in the atmosphere all activity concentl:ations of the short-lived radon decay products are equal and also equal to the radon activity concentration. Due to the experimental condition of a continuous collection of aerosol particles and simultaneous counting of the 7.69 MeV a-particles from 214po, the measured signal represents the total n u m b e r concentrations of all aerosol-borne shortlived radon decay products 21Spo, 2~4pb, 2~4Bi, and 214po with 10.5, 51.6, 37.9 and 6 x 10-6%, respect-

609

ively, according to their half-lives (see e.g. Porstendfrfer, 1994). In Fig. 2 from the measured total n u m b e r of concentration the deduced activity concentration of 214pb is plotted ( = 0.516 × total n u m b e r concentration x 2fb!214). Assuming a total adsorption of all short-lived radon decay products to aerosol particles, this activity concentration is equal to the radon activity concentration. This assumption, however, might not be fulfilled at Jungfraujoch due to the low aerosol n u m b e r concentrations as well as local plate-out effects (Whittlestone, 1990; Porstend6rfer, 1994). Therefore, the 214pb activity concentrations shown in Fig. 2 should be considered only as an approximate measure (i.e. lower limit) of the local radon activity concentration. The measured 8.78 MeV ~t-events of 21Zpo, corrected by the 64% fl-decay branch of 212Bi ' correspond to the total number concentrations of 212po, 2t2Bi and 212pb with 8× 10 -1°, 8.7 and 91.3%, respectively (Porstend6rder, 1994). As for the radon case, the plotted activity concentrations of zt2pb in Fig. 2 resemble equilibrium condition in the atmosphere assuming a quantitative attachment to aerosol particles for all nuclides from the decay of 21Zpb. Also shown in Fig. 2 are the total aerosol surface concentrations as determined by an independant epiphaniometer (Baltensperger et al., 1991) and the global radiation as measured by the automatic network station of the Swiss Meteorological service at this site. Obviously, all four signals exhibit a seasonal trend with maxima in summer and minima in winter. Baltensperger et al. (1991) attributed the seasonal variation of the total aerosol surface concentration at Jungfraujoch to convective transport of polluted air masses during the summer half-year. This explanation is at least partly true for the measured seasonal variation of the radionuelides depicted in Fig. 2. In most cases, high concentrations of these nuclides can be explained by transport of air masses to the Jungfraujoch from lower altitudes (see Section 3.2). At nearby low altitude sites total aerosol concentrations such as total suspended particulate matter (TSP) concentrations do not exhibit pronounced seasonal fluctuations (BUWAL, 1992). The same observation holds for 21°pb (G/iggeler et al., 1976; von G u n t e n et al., 1993; H6tzl et al., 1987; Kolb, 1978). During winter time with no thermal convection the ambient air at Jungfraujoch resembles true background conditions of the free troposphere (Baltensperger et al., 1991). This interpretation is corroborated by measurements of S5Kr at Jungfraujoeh, which do not show any seasonal fluctuations (Weis, 1992). This nuclide is produced by nuclear fission and escapes into the troposphere from nuclear reprocessing and power plants. Like radon, this noble gas has no other sink besides its radioactive decay (half-life: 10.76 yr). Its atmospheric residence time is therefore very long and it is expected to be well mixed within the troposphere. In accordance with theoretical expectations, equal atmospheric activity concentrations of about

610

H.W. G~,GGELER et al. .c_ E

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Fig. 2. Monthly averaged values of the total aerosol surface concentrations from the epiphaniometer, the global radiation at Jungfraujoch, the deduced radon and the thoron decay product activity concentrations of 2~4pb and 212pb, respectively (see text), and the long-lived radon decay product 2t°Pb. Also shown is the ratio between the two radon decay product activity concentrations 2~4pb/2~Opb' which is a measure for the aerosol residence time in the atmosphere (see text).

1 B q m -3 of SSKr were found at Bern (540m a.s.1.) (Loosli, 1992) and at Jungfraujoch (3450m a.s.1.) (Weis, 1992). Activity concentrations of 21°pb at Jungfraujoch during the year 1991 ranged from 5 x 10- 5 Bq m - a for the winter period to about 5 x 10 -4 B q m -3 for a summer month. The average annual activity concentrations for the time period 1975-1983 at the nearby low altitude location Fribourg was about 4.5 x 10 -4 B q m -3 (von Gunten et al., 1993). At Weissfluhjoch (2450 m a.s.l.) near Davos an average 21°pb activity concentration of 8.4 x 10- 5 Bq m - 3 was measured for the time period January to March 1988

(Baltensperger et al., 1993). The winter value of about 5 x 10 -5 B q m -a is somewhat higher than the one found in Antarctica with 1.5 x 10 -5 B q m -3 (Rangarajan et al., 1986) and similar to values reported for Arctic summer air at 75°N with (7.5 + 2.8) x 10 -5 B q m -3 (Samuelsson et al., 1986). F o r the radon and thoron decay products 214pb and 2 t 2pb the seasonal variations are very similar. It is interesting to note that the maxima are shifted to late summer, if compared to both the total aerosol surface concentration as well as the global radiation. We attribute this effect to the soil temperature (see Section 33). The highest monthly mean activity concentration

222Rn and Z2°Rndecay products and 21°pb measurements observed in 1991 for 214pb is about 1.3 Bqm -3. The highest daily mean amounts to 3.6 Bq m-3. The lowest monthly mean tor 214pb is about 0.1 Bqm -3. Unfortunately, there is a lack of measurements for radon at nearby low altitude locations in Switzerland. For e.g. Heidelberg, Germany, the radon activity concentration is about 5 Bqm -3, with no seasonal trend (Volpp, 1984). An increased exhalation rate during summer, as we assume to occur, could be compensated by an enhanced vertical mixing in this season, The average activity concentrations of the thoron decay product 2X2pb were very low, about 7 x 10 -3 Bqm -3 in summer and 1 x 10 -3 Bqm -3 in winter. The l~Lighest daily mean was 3.6 x 10 -2 Bqm -3. For this nuclide no information exists about its concentration pattern over Europe. In Fig. 2 also the ratios between the monthly mean values of 2,4pb and z ~Opb activity concentrations are

611

depicted. They vary from about 1000 to 10,000. These values can be transformed to average residence times z for aerosol particles in the atmosphere by the approximation

z~[A(2*°Pb)/A(214po)]

x ~'(Pb-21 -l O)

~(pbl_210) = In 2/T1/2 (Pb-210) Based on the data shown in Fig. 2 (bottom), resulting values for z are between 1 and 12 d (average value 6 d), in reasonable agreement with the typically observed average aerosol residence times in the atmosphere of about one week (Papastefanou and Bondietti, 1991; Rangarajan and Eapen, 1990). In Antarctic air a 222Rn to Zl°pb ratio of 1000 (Lambert, 1990a) and in surface air in India of 6000 was found (Rangarajan et al., 1986). In the latter study it was shown that for maritime air, i.e. during monsoon periods, this ratio was considerably lower than during periods with with

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Fig. 3. Diurnal variations of the radon (a) and thoron (b) decay product activity concentrations for each month during the year 1991. Most data arc 4 h mean values, except for July-September for 214pb with 1 h mean values (see text).

H.W. G,~GGELER et al.

612

prevailing continental air masses. In a review of ratios m e a s u r e d worldwide, the highest value found was 11,000 (Lima, Peru), a n d the lowest one was 53 (USSR ship, A r a b i a n sea 0 - 1 0 ° N ) ( R a n g a r a i a n et al., 1975).

3.2. Diurnal variations of the 2 ~CPb activity concentrations As a n example, Fig. 3 top a n d b o t t o m show the diurnal variations of the activity c o n c e n t r a t i o n s of

Table 1. Partial correlations coefficients between monthly means (MM) and pentad means (PM) of measured radon and thoron decay product activity concentrations at Jungfraujoch and the temperature gradient between Jungfraujoch and the Bern-Liebefeld (GT), the air temperature at Jungfraujoch (T JFJ), and a set of air (+) and soil ( - ) temperatures at Bern-Liebefeld (T + 2 m, + 5 cm, - 5 cm, - 20 cm, - 50 cm, T - 100 cm) during the year 1991

Radon (MM) Radon (PM) Thoron (MM) Thoron (PM)

GT

TJFJ

T+2m

T+5cm

T-5cm

T-20cm

T-50cm

T-100cm

0.563 0.586 0.530 0.450

0.854 0.661 0.822 0.706

0.946 0.803 0.908 0.772

0.942 0.807 0.902 0.775

0.955 0.839 0.913 0.804

0.957 0.840 0.918 0.810

0.955 0.838 0.912 0.818

0.943 0.830 0.907 0.816

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102 Day of the Year Fig. 4. Pentad means of the logarithm ofthe radon (top) and thoron (bottom) decay product activity concentrations at Jungfraujoch during the year 1991. The solid line indicates values estimated by a linear regression which are based on the soil temperature at Bern-Liebefeid (100 cm depth) and the temperature gradient between Jungfraujoch and Bem-Liebefeld (see text).

222Rn and 22°Rn decay products and 21°Pb measurements

2t4pb and 212pb l%r 1991. For the summer period, typical diurnal variations with a peak in the afternoon are observed for 2t*pb. For 212pb, this pattern is much less pronounced. This is expected because of the long half-life of 10.6 h for 2 t 2Pb, causing a smoothingout of the daily variLations. Daily variations similar to those for 21*pb we:re also found for the total aerosol surface concentrations (Baltensperger et al., 1991). During summer time the well-pronounced daily variations are caused by thermal convection which is suppressed by the strong atmospheric stability of the boundary layer during the winter period. This means that during the winter period free tropospheric air prevails at Jungfraujoch. Feichter and Crutzen (1990) modelled the vertical transport of radon by convective processes in clouds. The global distribuLtion was simulated for a summer (July) and a winter (January) month. The resulting mean activities of radon were about 4 x 10- 2 Bq m - 3 in July and 2 x 10-2 Bq m - 3 in January, for the area of Jungfraujoch and a level of 700 hPa (about 50 hPa below Jungfraujoclh). Both values are smaller than those observed (see Fig. 2). However, the discrepancy is higher in summer than in winter, most likely caused by the elevated mixing layer over the Alps during this time period.

613

gional-scale and even long-range transport, the origin of the measured activity concentrations can be expected to be the soils of the Swiss Midland or other parts of the European continent. Transport to Jungfraujoch thus involves a vertical and a horizontal component. The vertical transport can be accomplished by large-scale vertical air motions or by vertical mixing in the convective boundary layer, either near Jungfraujoch or anywhere else along the horizontal transport path. The intensity of regional-scale vertical mixing can be parametrized by the static stability of the air below Jungfraujoch. As a first approximation it can be expressed by the vertical temperature gradient between Jungfraujoch and a station in the Swiss Midland. This temperature gradient has a seasonal variation similar to that of the temperature itself (high stability in winter, low stability in summer). In the following, we try to separate the two components being responsible for the variations of radon and thoron decay product activity concentrations at Jungfraujoch, first, the seasonal variation of temperature and static stability, and, second, the horizontal transports due to different synoptic situations. The statistical analysis of the seasonal variations is based on monthly and pentad (5 d) means of the common logarithms of the radon and thoron daughter activity concentrations, in order to smooth out synop3.3. Radon and thoron progenies as air mass tracers tic processes with a short time scale. The logarithms Since the source of radon and thoron is the soil, were taken to achieve more well-behaved statistics, these species should be useful to distinguish between because the original data are distributed approxmaritime and continental air masses, respectively, or imately log-normal. The temperature of the Swiss between free tropospheric and boundary layer air Midland is represented by measurements at Bernmasses. Since the 'vicinity of Jungfraujoch is mostly Liebefeld (46°56'N, 7°25'E, 585 m a.s.l., distance from covered by snow and ice (preventing a local emanation Jungfraujoch 60 km). Temperature data of Jungfrauof radon and thoron), and the half-life of radon or the joch and Bern-Liebefeld were taken from the data base thoron progeny 212pb is long enough to allow re- of the automatic network of the Swiss Meteorological

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Fig. 5. Daily moans of the logarithm of the radon decay product activity concentrations at Jungfraujoch during the year 1991. The solid line is a fifth-order polynomial fit which was used to eliminate the seasonal variation.

614

H.W. G~,GGELER

Service (SMA), and are supplied as hourly mean values. Monthly mean values of the temperature gradients between the Jungfraujoch air temperature at 2 m above ground and a set of air ( + ) and soil ( - ) temperatures at Bern-Liebefeld ( + 2 m, + 5 cm, - 5 cm, - 2 0 cm, - 5 0 cm, - 100 cm) were calculated, skipping those days where radon data were missing. March 1991 was not included in the following computations, since there were only 3 d with data. The

et al.

partial correlation coefficients between the monthly means of the logarithm of the radon and thoron decay product activity concentrations at Jungfraujoch and the temperature data are given in Table 1. While the temperature gradient explains only 32% (=0.5632 ) of the variance, the air temperature at Bern-Liebefeld accounts for about 89% (=0.9462 ) and the soil temperature (at 5 cm depth) at Bern-Liebeid for 91% ( = 0.9552) of the radon variance. Monthly means of air

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Fig. 6. Mean values and 80% confidence intervals for daily mean activity concentrations of radon (a) and thoron (b) decay products as a function of the synoptic patterns according to Steinacker (1991). Besides the main air flow directions the two additional categories V (variable flow) and WG (weak pressure gradient) are shown (for details see text). The activities are given in a normalized form (mean value = 0 and standard = 1) of the residuals from the solid curve depicted in Fig. 5. The solid line is shown to guide the eye.

222Rn and 22°Rn decay products and 21°pb measurements

615

next step, the seasonal variation was removed by fitting a fifth-order polynomial to the series of daily values (Fig. 5), which was then subtracted. The residuals were then normalized to have a mean value of zero and a standard deviation of one. Finally, mean values of these normalized concentrations were calculated for each of the synoptic flow patterns as defined by Steinacker (1991). These patterns describe the main flow direction at the 850 hPa level in the Alpine region during each day. There are 8 distinct flow types according to the wind directions (N, NE, E, etc.), one class "variable" where the main flow direction is inhomogeneous in time or space (often in connection with a frontal passage), and one class "weak gradient" for situations with small wind velocities and pressure gradients. Unfortunately, the latter two classes account for about half of all days, and there were only LGRN =G,.96 + 1.09 GT + 0.070 TS, (1) between 5 and 17 cases for the distinct flow patterns. The results are shown in Fig. 6. Higher than normal where LGRN is computed from the radon activity concentrations in 10- 3 Bq m - 3, GT is expressed in K radon daughter activity concentrations are observed per 100 m and TS in °C. The multiple correlation is with flows from NE to SW, the maximum occurring 0.97 (explaining 94% of the variance). The small with easterly flow, and with variable flow situations. improvement by -~tdding the temperature gradient Lower than normal activity concentrations occur with emphasizes the importance of the thermal influence on flow from W to N, and with variable flow directions; monthly radon concentrations. the minimum concentrations is found for NW flows. The same calculations were also made for the means This shows that the radon daughter activity concenover 5 d (pentad m,~ans). The result is still very good tration is a significant signal of maritime or continenwith a multiple correlation coefficient of 0.86. Ob- tal origin of the air masses. The thoron results do not differ from those of radon served and computed values are depicted in Fig. 4. These results show 1chaton a time scale where synoptic within the error bars, with the exception of northerly processes are smoothed out the radon concentrations flows having especially low thoron concentrations. at Jungfraujoch are mainly determined by the regional temperature and the static stability. Acknowledgements--Free access to the Forschungstation The simplest explanation of the high correlation Jungfraujoch is highly appreciated as well as support and found between the soil temperature in the Swiss help in controlling the epiphaniometer set-ups by Mr and Mrs Kuster and Mr and Mrs B/irtschi. We are thankful to Mr Midland and the radon decay product activity con- M. Emmenegger for technical assistance and to Mr W. centration at Jungfraujoch would be to assume a N/igeli for support during installation work at Jungfraujoch. dependence of the radon exhalation rate on the soil Careful reading of the manuscript by Prof. H. R. von Gunten temperature. A seasonal variation of the radon con- is highly appreciated. Meteorological data were generously centration in the soil which is well correlated with provided by the Schweizerische Meteorologische Anstalt (SMA). This work was part of the program ALPTRAC, temperature is described in the literature, though the which itself is a subproject of EUROTRAC and was supmechanisms respon:~ible for these observations are not ported by the Swiss National ScienceFoundation, the Swiss Bundesamt fiir Bildung und Wissenschaften and by the yet fully understood (Heinicke, 1987). The results for thoron (Table 1) are generally sim- Austrian Fonds zur F6rderung der wissenschaftlichenForschung under grant P7809-GEO (ALPTRAC-SNOWMET). ilar, with somewhal lower correlation coefficients (the multiple correlatio~t coefficient is 0.93 for monthly and 0.82 for pentad means). The influence of the stability REFERENCES on pentad means is less pronounced than in the case of radon, explaining only 20% instead of 34% of the Baltensperger U., G~iggelerH. W., Jost D. T., Emmenegger variance. The reason for this observation is not clear. M. and N~igeliW. (1991) Continuous aerosol monitoring However, the correlation with the temperature at with the epiphaniometer. Atmospheric Environment 25A, 629-634. Jungfraujoch (pentad means) is better for thoron than for radon (0.71 ins'read of 0.66). This could be evid- Baltensperger U., Schwikowski M., G~iggelerH. W. and Jost D. T. 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