Temporal variation of 7Be and 210Pb size distributions in ambient aerosol

Atmospheric Environment Vol. 32, No. 6, pp. 983—991, 1998( 1998 Elsevier Science Ltd

All rights reserved. Printed in Great Britain1352—2310/98 $19.00#0.00PII: S1352–2310(97)00333–6

TEMPORAL VARIATION OF 7Be AND 210Pb SIZEDISTRIBUTIONS IN AMBIENT AEROSOL

R. WINKLER, F. DIETL, G. FRANK and J. TSCHIERSCHGSF - National Research Center for Environment and Health, Institute of Radiation Protection,

D-85764 Neuherberg, Germany

(First received 23 December 1996 and in final form 11 July 1997. Published March 1998)

Abstract—The size distributions of the cosmogenic 7Be and of the long-lived radon progeny 210Pb inambient aerosols were measured continuously from December 1994 to the end of March 1996 inground-level air at a semi-rural location in south Germany. Aerosol sampling was performed at a height of4 m above ground with a low-pressure cascade impactor of the Berner type covering the size range from0.06 to 16 km and simultaneously with an high-volume sampler. Each sampling period was 10 d. Activitiesof 7Be and 210Pb were measured by gamma spectrometry and aerosol mass was determined gravimetrically.In all experiments the activity distributions of 7Be as well as of 210Pb were unimodal (log-normal) andassociated with submicron aerosols of about 0.5—0.6 km aerodynamic diameter. On average, the activitymedian diameters of 7Be (AMD: 0.57 km) and of 210Pb (AMD: 0.53 km) have been found to be significantlylower than the average mass median diameter (MMD: 0.675 km) and higher or at most equal than therespective surface median diameter (SMD: 0.465 km) of the aerosols: SMD)AMD

P"210(AMD

B%7(MMD. Variation of the atmospheric processes during the study period resulted in a variability of theactivity median diameter between 0.44 to 0.74 km for 7Be and from 0.28 to 0.74 km for 210Pb. While in thewinter months (October to April) a difference between the activity distributions of 210Pb (AMD: 0.595 km)and 7Be (AMD: 0.59 km) was not detectable, in summer, 210Pb was associated with significantly smalleraerosols (AMD: 0.43 km) than 7Be (AMD: 0.52 km). Comparing the activity median diameters observed insummer with those in winter, on average significantly lower diameters were found in summer pointing toshorter residence times in the summer months. ( 1998 Elsevier Science Ltd. All rights reserved.

Key word index : 7Be, 210Pb, aerosol size distribution, air activity concentrations, impactor, time series,residence time.

1. INTRODUCTION

While 7Be is a relatively short-lived (t1@2

"53.3 d)naturally occurring radionuclide of cosmogenic origin(Lal and Peters, 1962), the long-lived 210Pb (t

1@2"

22.3 yr) is a progeny of the primordial 238U—226Ra-decay chain. In opposition to 7Be, it is mainly pro-duced in the atmosphere near ground level by thedecay of the rare gas 222Rn emanating from soil, andwhose short-lived decay products 218Po, 214Pb, 214Biand 214Po. Following the production of 7Be by gas-phase nuclear transformations in the stratosphere andthe upper troposphere and the formation of 210Pbafter the decay of 222Rn to low vapour pressureprogeny in ground-level air, the radionuclides attachon the aerosol population and their fate will becomethe fate of the carrier aerosols. The size distribution ofthe radionuclides on the aerosol particles is the resultof a combination of atmospheric processes like, e.g.coagulation of ultrafine particles, fog and clouddroplet formation, evaporation and condensation,washout, rainout and sedimentation and of contribu-tions of, e.g. dust storms and combustion products to

the tropospheric aerosol mixture. The final size distri-bution then reflects all these interdependent processes.7Be as well as 210Pb have therefore been used instudying the description of environmental processessuch as aerosol transport and residence times inthe troposphere (e.g. Martell and Moore, 1974; Papas-tefanou and Ioannidou, 1995), aerosol deposition vel-ocities (Young and Silker, 1980) and aerosol trappingabove ground vegetation (Bondietti et al., 1984). Be-cause their sources are known, global in extent, andrelatively steady in time these tracers also build anideal tool to depict transport processes in the wholeatmosphere and to test the models ability of reproduc-ing these (Rehfeld and Heimann, 1995; Koch et al.,1996). Because it is of purely outdoor origin, 7Be hasalso been used as a tracer in experiments examiningthe ingress of aerosols into buildings (Roed and Can-nel, 1987). In contrast to 7Be, the radon progeny210Pb is not exclusively produced in the outdooratmosphere, and depending on the indoor radon con-centration increased 210Pb concentrations have beenobserved indoors (Haninger et al., 1996). Conse-quently, a mixture of 210Pb attached to indoor and

983

outdoor aerosol, respectively, can be assumed to con-tribute to the radiation dose to man. Information onthe size distribution of 210Pb is needed nevertheless toassess the deposition ratios and radiation dose distri-bution for inhaled 210Pb in the respiratory system.Generally, to understand and to model transport andremoval of aerosol attached radionuclides, detailedinformation on the activity size distributions and theirvariation with time is required.

However, very few activity size distributionmeasurements have been performed systematicallyover a full annual cycle to study variations associatedwith different meteorological conditions. Within theframe of a long-term study on the behavior ofradionuclides in the atmosphere the present study wastherefore started in December 1994 and the activitysize distributions of 7Be and 210Pb were measuredcontinuously for each decade until March 1996.

2. EXPERIMENTAL

2.1. Sampling

Size-selective aerosol sampling is performed with a nine-stage low-pressure cascade impactor of the Berner type (LPI30/0.06/2, Hauke KG; Berner and Lurzer, 1980) with a nom-inal volumetric flow rate of 28.9 l min~1. The effective aero-dynamic cutoff diameters are 0.06, 0.12, 0.25, 0.5, 1.0, 2.0, 4.0,8.0 and 16 km. The aerosols are collected on impactionsubstrates stamped from aluminum foil of about 15 kmthickness. The air inlet is at a height of 4 m above ground onthe flat roof of a 2.5 m high instruments container. In orderto avoid overloading, less than about 8 mg of particulatematerial is collected at any stage within the sampling period.Each collection period is 10 d. Simultaneously, total dust iscollected at the same location with an high-volume sampler(ASS-500, CLRP Warsaw) at an air flow rate of 700 m3 h~1on organic fiber filters (Viledon FA2311). The samplingstation is located on a cleared field on the grounds of theGSF research center at Neuherberg (490 m above sea level,48°13@N, 11°36@E), about 10 km north of the city of Munich.This location can be characterized as being typical of semi-rural locations in south Germany. The natural environment,mostly grassland, is uncultivated with some woodland to thenorth. The prevalent wind is from SWW and the averageannual sum of precipitation is about 850 mm.

2.2. Measurement

After the end of collection, the collecting substrates arecarefully removed from the impactor and are reweighed forevaluation of the size distribution of the particle mass. Thenthe aluminum substrates are folded, put into minivials madeof polyethylene and are measured for 7Be (477 keV) and210Pb (46.5 keV) activity at near 4n geometry with a highresolution, low-background HPGe well detector (peak effi-ciency 70% at 46.5 keV and 19% at 477 keV, respectively).The counting time is at least 90,000 s and minimum detect-able activity at 95% confidence level is 0.02 Bq 7Be and0.01 Bq 210Pb. The statistical uncertainty ranges from about5% for the activities on the impactor stages 3 and 4 to about30% near the detection limit (impactor stages 2 and 6). Themaximum systematic uncertainty is estimated to be $10%.For the determination of total aerosol mass the filters fromthe HiVol sampler are reweighed after a storage time of 1 dafter the end of sampling. For direct gamma spectrometrythe dust-loaded filters are compressed to provide suitablegeometry for measurement.

2.3. Data evaluation

From the 7Be and 210Pb activities measured on eachimpactor stage the aerosol-attached activity size distribu-tions are approximated by log-normal distributions. Therespective cumulative plots, where the value of the ordinateindicates the cumulated activity less than the given aerosolsize, are used to read the 50% diameter (d

50) and to calculate

the geometric standard deviation (GSD) from d84.1

and d50

.The mass related size distributions obtained by reweighing

the exposed impaction substrates in most cases did not fit anunimodal distribution. Therefore, an inversion procedurebased on the Twomey (1975) algorithm has been applied forsize distribution analysis that takes into account the separ-ation characteristics of the impactor type used in this work(Winklmayr et al., 1990). For statistical analysis, only themost frequent mode (major mode) of maximally three aero-sol size modes resulting by the application of the aboveinversion procedure was used. For this, among the modesthat one has been chosen as the major mode when the ratioof its frequency and the frequency of the second frequentmode was larger or equal to 1.5 and provided that they werebetween 0.2 and 12 km which corresponds the mean dia-meter of the second and the last but one impactor stages,respectively.

The surface size distributions have been determined fromthe impactor data by dividing the aerosol masses on theimpactor stages by the mean diameters of the respectiveimpactor stages. It is obvious, however, that spherical par-ticles have to be assumed for the conversion from mass tosurface size distribution.

By applying the Chi-square goodness of fit test it waschecked, whether the data are distributed to a normal (Gaus-sian) frequency distribution. The nonparametric U-test ofWilcoxon, Mann and Whitney and the Wilcoxon signedranks test were used for the comparison of two data sets. Todetect whether a correlation between two data sets is signifi-cant, the nonparametric Spearman correlation was applied(Spearman correlation coefficient o

SP). Cross-correlation

analysis was used to detect a lag between two time series. Toemphasize the average course of a time series a smoothingprocedure (simple moving average, five terms) has beenapplied to the original data.

3. RESULTS AND DISCUSSION

In Fig. 1 the time series of the 7Be air concentrationand of the respective activity median diameter (AMD)are shown from the third decade of December 1994 tothe end of March 1996. The above smoothing proced-ure has been applied to the original data to elucidatethe average courses. As can be seen in Table 1, theAMD ranged from 0.44 km in the third decade ofJanuary 1995 to 0.74 km in the second decade ofJanuary 1996 while the 7Be air concentration variedbetween 1.4 mBq m~3 in the second decade ofDecember 1995 to 6.7 mBq m~3 in the first decade ofAugust 1995. Summary statistics of 7Be air concentra-tions and of the aerosol size association of thisradionuclide are presented at the end of Tables 1 and2. Due to its origin, the seasonal behavior of thecosmogenic 7Be is characterized by maximum airconcentrations in the summer months at this location(Hotzl and Winkler, 1987; Hotzl et al., 1991). As canbe seen in the figure, on average, during the period ofhigh 7Be air concentrations, i.e. in the summer, rela-tively low values of the AMD (0.45—0.52 km) have

984 R. WINKLER et al.

Fig. 1. Time series of 7Be activity concentration and activitymedian diameter (AMD) in ground-level air from December1994 to the end of March 1996 at Munich-Neuherberg

(summer period: May—September).

been observed. From other locations activity mediandiameters have been reported to range between 0.29and 0.50 km at Oak Ridge, Tennessee, U.S.A. (Bon-dietti et al., 1987, 1988), 0.54—1.18 km at Roskilde,Denmark (Lange, 1994), 0.65—1.09 km at Gottingen,Germany (Reineking and Porstendorfer, 1995) and0.76—1.18 km at Thessaloniki, Greece (Papastefanouand Ioannidou, 1995). Due to limited sample sizes,seasonal variations were, however, not detectable.

The time series of 210Pb air concentrations and ofthe corresponding AMD values are presented inFig. 2. Again the smoothed curves were included inthe figures. The summary statistics for the air con-centration and the aerosol size association of 210Pbare given at the end of Tables 1 and 2. As can be seenin Table 1, air concentrations ranged from 0.13to 1.48 mBq m~3. The average concentration was0.42 mBq m~3. This average is slightly lower than thelong-term average of 0.64 mBq m~3 obtained in aninvestigation on the 210Pb air concentration atground level at Munich-Neuherberg (Hotzl andWinkler, 1996). In that study, a cyclic seasonal pat-tern with high 210Pb concentrations in winter hasbeen identified. The AMD values of 210Pb rangedfrom 0.28 to 0.74 km (Table 1). Only few data are avail-able for comparison: AMD(0.3 km at Boulder,

Colorado, U.S.A. (Moore et al., 1980), 0.28—0.49 km atOak Ridge, Tennessee, U.S.A. (Bondietti et al., 1987,1988),(0.58 km at rural New Jersey, U.S.A. (Knuthet al., 1983), 0.56 km at Gottingen, Germany (Reinek-ing and Porstendorfer, 1995) and about 0.6 km overoceans (Sanak et al., 1981). The 210Pb data at OakRidge, though limited, suggested that the summerAMD is larger than the winter AMD at that location.

Analogous to 7Be, minimum activity mediandiameters were observed for 210Pb from May to Sep-tember (see Figs 1 and 2). This suggests a positivecorrelation between 7Be-AMD and the 210Pb-AMDwhich was indeed observed (o

SP"0.474, p(0.01, two-

tailed). Cross-correlation analysis between these twotime series revealed a correlation coefficient which issignificant at lag 0 (0.452, p(0.05), but not significantfor other lags, i.e. both series are in phase, but the datain one decade of the first series are not related to thedata in the next decade of the second series.

To test whether there is a statistical significantdifference between the activity size distributions in thesummer months and those observed in winter, thenonparametric º-test and the Wilcoxon signed rankstest were applied to the data. For simplicity, wedenote the months May—September as ‘‘summer’’, andthe months October—April as ‘‘winter’’, respectively.These summer months correspond to the period whenthe monthly mean air temperature was higher than#12°C. On average, about 60% of the annual sum ofprecipitation is deposited during these five months, i.e.the mean monthly sum of precipitation in this periodis about twice the mean monthly precipitation in thewinter period. As well as for 7Be as for 210Pb it hasbeen found that the AMD values in summer are lowerthan those in winter:

7Be-AMDSUMMER

(7Be-AMDWINTER

and

210Pb-AMDSUMMER

(210Pb-AMDWINTER

.

The significance level was 0.01 (two-tailed) for therespective differences.

To compare the activity size distributions of 7Beand 210Pb, we applied again the U-test and the Wil-coxon signed ranks test. Taking the full sample sizeinto account (Table 2), on average, the AMD of 210Pb(0.53 km) was significantly lower (p(0.05, two-tailed)than that of 7Be (0.57 km):

AMDP"210

(AMDB%7

.

However, if the summer and winter data of bothdistributions (Table 2) are compared separately, a dif-ference between the distributions of 210Pb and 7Be inwinter was no longer detectable (p'0.05, two-tailed),whereas in summer 210Pb was associated with signifi-cantly (p(0.001, two-tailed) smaller aerosols than 7Be.

Since the observed activity size distributions reflectthe sum of the atmospheric processes to which theaerosols were subjected, it is of interest to considerother aerosol parameters and also the meteorologicalconditions prevailing during the study. Therefore, in

Size distributions in ambient aerosol 985

Table 1. Activity concentrations and activity median aerodynamic diameters (AMD) of 7Beand 210Pb from December 1994 to the end of March 1996 at Munich-Neuherberg (7Be and

210Pb activities are referred to the mid of each collection period)

Collection start 7Be 210Pband stop date

Activity conc. AMD Activity conc. AMDDD/MM/YY (mBq m~3) (km) (mBq m~3) (km)

22/12/94 2.3 0.59 0.55 0.5430/12/94 3.0 0.57 0.51 0.5409/01/95 2.0 0.45 0.43 0.5920/01/95 2.8 0.44 0.17 0.6030/01/95 3.8 0.45 0.27 0.5210/02/95 3.1 0.59 0.16 0.3220/02/95 2.9 0.56 0.13 —01/03/95 3.2 0.56 0.17 —10/03/95 3.3 0.62 0.34 0.7220/03/95 2.5 0.64 0.17 —31/03/95 4.2 0.66 0.31 0.6710/04/95 2.9 — 0.22 —20/04/95 3.2 0.60 0.34 0.4402/05/95 6.6 0.55 0.48 0.4410/05/95 3.1 0.57 0.22 0.4219/05/95 4.1 0.56 0.37 0.3930/05/95 2.8 0.62 0.25 0.4409/06/95 3.1 0.52 0.22 0.3120/06/95 4.9 0.50 0.32 0.3330/06/95 6.2 0.46 0.47 0.4310/07/95 5.6 0.45 0.43 0.4820/07/95 4.9 0.51 0.50 0.4831/07/95 6.7 0.50 0.64 0.3910/08/95 4.1 0.52 0.52 0.5421/08/95 3.6 0.54 0.46 0.4231/08/95 4.0 0.51 0.34 0.3811/09/95 3.7 0.60 0.51 0.4720/09/95 2.5 0.53 0.28 0.4802/10/95 2.6 0.62 0.61 0.5310/10/95 3.1 0.64 1.48 0.6120/10/95 4.0 0.58 0.78 0.5830/10/95 3.7 0.57 0.32 0.4110/11/95 1.6 0.63 0.43 0.6220/11/95 2.5 0.72 0.82 0.7030/11/95 1.7 0.45 0.71 0.5911/12/95 1.4 0.58 0.54 0.6120/12/95 2.5 0.55 0.35 0.2830/12/95 1.8 0.72 0.63 0.7410/01/96 2.8 0.74 0.90 0.5922/01/96 2.4 — 1.29 —30/01/96 2.8 0.70 0.67 0.6612/02/96 2.5 0.51 0.27 —20/02/96 3.2 0.52 0.64 0.6429/02/96 3.3 0.66 0.34 0.6111/03/96 2.3 0.69 0.75 0.6021/03/96 4.0 — 0.27 —

Mean 3.33 0.57 0.47 0.52Median 3.10 0.57 0.43 0.53Range 1.40—6.70 0.44—0.74 0.13—1.48 0.28—0.74

Fig. 3 the time series of the major mode of the aerosolmass size distributions are shown together with theaerosol concentration time series. Generally, theatmospheric aerosol size distribution at this locationhas been found to follow a trimodal distribution ex-pected for condensation-derived aerosols. The resultsare summarized in Table 3. Similar distributions havebeen found, e.g. by John et al. (1990). However, be-

cause, on average, more than 92% of the total 7Beactivity and more than 89% of the total 210Pb activityare associated with particles larger than about 0.2 kmand smaller than 2 km, for comparison with the masssize or surface size distribution only the major modehas been considered. The corresponding averagevalues are 0.675 km for the MMD and 0.465 km forthe SMD, respectively.


Table 2. Summary of aerodynamic size association of 7Be and 210Pb and of aerosol size distribution (major mode)from December 1994 to the end of March 1996 at Munich-Neuherberg (AMD, activity median diameter; MMD,mass median diameter; SMD, surface median diameter; GSD, geometric standard deviation; Summer,

May—September, Winter, October—April)

Sample Median 95% Conf. limitsize of median

7Be AMD (km) GSD AMD (km) GSDAll data 43 0.57 1.9 0.53—0.60 1.8—2.1Summer 15 0.52 2.0 0.50—0.56 1.9—2.2Winter 28 0.59 1.8 0.56—0.64 1.7—2.1

210Pb AMD (km) GSD AMD (km) GSDAll data 39 0.53 2.3 0.44—0.59 1.9—2.5Summer 15 0.43 2.3 0.39—0.48 1.9—2.5Winter 24 0.595 2.3 0.54—0.62 1.7—2.7

Aerosol mass (major mode) MMD (km) GSD MMD (km) GSDAll data 44 0.675 1.6 0.60—0.71 1.6—1.7Summer 15 0.60 1.6 0.58—0.68 1.5—1.7Winter 29 0.70 1.7 0.62—0.76 1.6—1.8

Aerosol surface (major mode) SMD (km) GSD SMD (km) GSDAll data 44 0.465 1.6 0.44—0.49 1.6—1.7Summer 15 0.44 1.6 0.38—0.55 1.5—1.7Winter 29 0.47 1.6 0.42—0.52 1.6—1.7

Fig. 2. Time series of 210Pb activity concentration andactivity median diameter (AMD) in ground-level air fromDecember 1994 to the end of March 1996 at Munich-

Neuherberg (summer period: May—September).

Comparing the MMD and SMD with the aboveactivity size distributions of the radionuclides, it isevident that the median of the activity size distribu-tions varies between the MMD and the SMD. By

Fig. 3. Time series of aerosol concentration (TSP) and massmedian diameter (MMD, major mode) of the aerosol inground-level air from December 1994 to the end of March 1996at Munich-Neuherberg (summer period: May—September).

statistical analysis (U-test and Wilcoxon signed rankstest) it has been found that, on average, both activitymedian diameters are significantly lower than theMMD and larger or equal to the SMD. Therefore, the


Table 3. Summary of aerosol mass size distribution (calculated accordingto Winklmayr et al., 1990) from December 1994 to the end of March 1996at Munich-Neuherberg (MMD, mass median diameter; GSD, geometric

standard deviation)

1st Mode 2nd Mode 3rd Mode

Rel. mass conc. (%) 64 24 12MMD (km)

Median 0.67 3.6 0.1295% c.l. 0.60—0.71 3.1—4.0 0.10—0.19Range 0.43—0.95 1.4—4.8 0.08—0.25

GSDMedian 1.63 1.75 1.50Range 1.37—2.49 1.38—4.41 1.34—1.84

Table 4. Spearman rank correlations of aerosol mass median diameter (MMD), surface mediandiameter (SMD), 7Be- and 210Pb-activitity median diameter (AMD) and precipitation (all data:from December 1994 to end of March 1996, Summer: May—September, Winter:

October—April)

AMD (7Be) AMD (210Pb) PrecipitationAll data All data All data

Summer/Winter Summer/Winter Summer/Winter

MMD ## ### !

#/## nd/# nd/!!

SMD ## # ndnd/## #/nd nd/nd

AMD (7Be) ## ndnd/## nd/nd

AMD (210Pb) !!

nd/nd

Note: #, positive correlated, p(0.05, ##, p(0.01; ###, p(0.001; !, negative corre-lated, p(0.05, !!, p(0.01; p, significance level, two-tailed; nd, correlation not detectable(p'0.05).

following order can be derived:

SMD)AMDP"210

(AMDB%7

(MMD.

The activity size distribution for radionuclides ofatmospheric origin like 7Be and 210Pb is determinedmainly by two different processes: attachment ofradionuclides on aerosol particles and transformationof the atmospheric aerosol. Attachment of radioactiveatoms and ions on aerosol was investigated early.According to theory (e.g. Baust, 1967; Lassen andRau, 1960) the attachment process is controlled bydiffusion taking into account electrostatic forces andgas kinetics. For small particles the attachment isproportional to d2, the particle surface. Only for largeaerosol particles (diameter d'1 km) the attachtmentwill approximate a dependence proportional to d.Calculations of Baust (1967) show that about 90% ofthe natural activity should be initially attached toparticles smaller than d"0.5 km. Results of measure-ments of Lassen and Rau (1960) and Porstendorferet al. (1979) agree well with the diffusion attachment

model and confirm the predicted dependence on d.Transformation of the atmospheric aerosol, however,change the activity size distribution. The correspond-ing processes such as coagulation and deposition havebeen studied widely (e.g. Seinfeld, 1986; Warneck,1988). In a first approximation, the activity size distri-bution should therefore coincide with the surface sizedistribution as long as the aerosols are newly formed.In contrast, the coincidence between surface andactivity distribution should vanish for aged aerosolswith time for transformation.

Taking the full sample size (Table 2) into account,statistical analysis (nonparametric Spearman cor-relation) revealed significant positive correlationsbetween MMD and 7Be-AMD, MMD and210Pb-AMD, SMD and 7Be-AMD, and SMD and210Pb-AMD, respectively (Table 4). By cross-correla-tion analysis of the respective time series, significantcorrelation coefficients were found at lag 0. At otherlags, however, a correlation was not detectable. Thisfinding reflects the fact that, on balance, the aerosolswith their characteristic size distributions were


Fig. 4. Seasonal behavior of precipitation and median aero-dynamic diameters of aerosol mass, particle surface, 7Be and210Pb activity. During the summer months (May—Septem-ber) with high values of precipitation the median diametersdecrease. The decrease is most effective for 210Pb with itssource mainly in ground-level air. In summer, the AMD of210Pb is close to the SMD of ambient aerosol, indicating

newly formed aerosols.

influenced at the same time prevalently by the sameatmospheric processes.

However, there are also processes in the atmo-sphere which discriminate according to the differ-ent source area of the considered aerosol and mayaffect the respective size distribution differently. Thesedifferences can be observed when shorter periodsare examined, e.g. seasons. In the summer period anhigher amount of precipitation was observed (Fig. 4).Wet deposition should affect the 210Pb aerosol, whichis formed mainly in ground-level air, to a larger extentthan the 7Be aerosol which originates from the strato-sphere and the upper troposphere. Because of the sizedependence of the scavenging process, this shouldaffect also the AMD. According to Fig. 4, lower me-dian diameters were found during the summer monthsboth for 7Be and 210Pb. But the decrease of the AMDof 210Pb is much larger and coincides even with theSMD, which is not the case in the winter period. Fromthese observations, one could conclude that the sea-sonal variation of the AMD at a location is connectedwith the seasonal distribution of the rainfall at thatlocation. In fact, Bondietti et al. (1987, 1988) observed

for a location (Oak Ridge) with higher amounts ofrainfall in winter as compared to summer the lowestAMD values for 210Pb in the winter months.

Separate statistical analysis of summer and winterdata is summarized in Tables 2 and 4. In fact, as for210Pb, a positive correlation between MMD andAMD was merely detectable for the winter data(o

SP"0.440, p(0.05), however not for the summer

data. In contrast to this finding, the summer data of210Pb-AMD were significantly positive correlatedwith the summer-SMD (o

SP"0.579, p(0.05), while

for the respective winter data a correlation was notdetectable. A different behavior has been observed for7Be: summer as well as winter data are correlated withthe respective MMD, however not with the SMD.The strong negative correlation between precipitationand aerosol behavior (Table 4) confirms the well-known importance of wet deposition processes forresidence times of aerosols in the atmosphere.

Residence times of 7Be aerosols (qR) were cal-

culated, e.g. by Papastefanou and Ioannidou (1995)from the AMD of 7Be and a mean aerosol growth rateof 0.004—0.005 km h~1 (McMurry and Wilson, 1982)by dividing the difference between the measuredAMD of 7Be aerosols (AMD

.%!/) and the 0.015 km

diameter of Aitken nuclei (AMDA*5,%/

) by the meangrowth rate (MGR) according to the equation

qR"

AMD.%!/

—AMDA*5,%/

MGR.

From their results they estimated a mean residencetime of 7—9 d for 7Be aerosols in ground-level airat the Thessaloniki Region (Greece) at sea level.Applying this equation to the average AMD of7Be (0.57 km) and of 210Pb (0.53 km) observed at ourlocation, mean residence times of 4—5 d for 210Pb and5—6 d for 7Be can be estimated, slightly shorter for210Pb in summer (3—4 d) and somewhat higher for 7Beand 210Pb in winter (6—7 d). This is very similar to thevalues found for 7Be at Sarakina and Mt Hortiatis byPapastefanou and Ioannidou (1995) at a height of250 m above sea level ( 6—7 d) and 1000m above sealevel (5—6 d), respectively. By an independent method,Papastefanou and Bondietti (1991) calculated, fromthe 210Bi/210Pb ratio, a mean residence time of about8 d for 210Pb aerosols at Oak Ridge National Labor-atory, Tennessee, U.S.A. For 210Pb the residence timeof 4—5 d of the present study would also fit the resultsof Koch et al. (1996), who used 7Be and 210Pb asaerosol tracers in a three-dimensional chemical tracermodel in order to study aerosol transport and re-moval in the atmosphere. From 7Be/210Pb concen-tration ratios they calculated mean troposphericresidence times of 5—6 d for 210Pb at a latitude ofabout 50° N. The respective residence times of 7Beaerosols, however, calculated by these authors was10—12 d, which is higher by a factor of about 2 thanthe 5—7 d estimated in the present work.


4. CONCLUSIONS

Continuous aerosol sampling over a long period(16 months) showed that, in general, the activity me-dian diameters of 210Pb and 7Be are in the same sizerange. Nevertheless, on average, the AMD of 210Pb(0.53 km) was significantly smaller than that of 7Be(0.57 km). The activity median diameters rangedbetween the mass median and the surface mediandiameter of the ambient aerosol, indicating that bothradionuclides are involved in the transformation pro-cess of the tropospheric aerosol after formation in thestratosphere and upper troposphere (7Be) or in theground-level air (210Pb). The study of seasonal effectsshowed, however, a stronger decrease of the AMD of210Pb compared to 7Be in the summer period. TheAMD of 210Pb is very close to the SMD of ambientaerosol in summer which is typical for a newly formedaerosol. The strong negative correlation between pre-cipitation and the AMD of 210Pb in summer suggeststhat wet deposition might be responsible for the shortresidence time of 210Pb in this period. For the wholesampling period, residence times of 4—6 d could beestimated for 210Pb and 7Be. For 210Pb in the sum-mer period an average residence time of 3—4 d wasestimated, significantly shorter than the average resi-dence time of 6—7 d in the winter period.

Acknowledgement—The authors are grateful to K. Bunzl forvaluable discussions and to K. Aehlig and A. Pliml fortechnical assistance.

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Temporal variation of 7Be and 210Pb size distributions in ambient aerosol