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Residence time of tropospheric aerosols in association with radioactive nuclides
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Applied Radiation and Isotopes 64 (2006) 93–100 www.elsevier.com/locate/apradiso
Residence time of tropospheric aerosols in association with radioactive nuclides C. Papastefanou Oak Ridge National Laboratory, Environmental Sciences Division Oak Ridge, TN 37831, USA Received 3 April 2005; received in revised form 12 June 2005; accepted 12 July 2005
Abstract The residence time of atmospheric aerosol particles is a function of various removal processes, such as dry deposition by impaction, diffusion, sedimentation and resuspension as well as wet deposition by rain drops (precipitation scavenging). Estimation of the mean-residence time of atmospheric aerosols could be based on measurements of the activities and ratios of activities of cosmic-ray produced radionuclides, such as 7Be and the radioactive decay products of radon-222 emanated from soil into the atmosphere, such as 210Pb, 210Bi and 210Po. It was found that a mean value of about 8 days could be applied to aerosol particles in the lower atmosphere below precipitation cloud levels as resulted by the application of two different methods, i.e. the 7Be-associated atmospheric aerosols and the radon decay product aerosols at two different locations, i.e. at Thessaloniki, Greece 401380 N 221580 E with dry (precipitation free) climate and at Oak Ridge, Tennessee, USA 351580 N 841170 W with high precipitation (wet climate), roughly at similar temperate latitudes, but the first one at East longitude and the other at West longitude, respectively. r 2005 Elsevier Ltd. All rights reserved. Keywords: Beryllium-7; Lead-210; Bismuth-210; Polonium-210; Residence time; Tropospheric aerosols
1. Introduction The residence time of atmospheric aerosol particles in the lower atmosphere assuming that the air in the troposphere is considered as a well-mixed reservoir (closed system) is a function of various removal processes, the most important being: (a) dry deposition by impaction, diffusion and sedimentation, and (b) wet deposition by rain drops (precipitation scavenging) Corresponding author at: Present address. Aristotle University of Thessaloniki, Atomic and Nuclear Physics Laboratory, Thessaloniki 54124, Greece. Tel: +30 2310 998 005; fax: +30 2310 998 058. E-mail address: [email protected].
as a result of processes occurring both within and below the rain cloud. There could be variations in the removal rates at different continental locations of the globe (Lehmann and Sittkus, 1959), over the oceans (Koch et al., 1996), and at high altitudes of the atmosphere (Martell, 1970; Moore et al., 1973) due to changes in meteorological conditions. There is also a dependence of the tropospheric aerosol residence time on the latitude (Ehhalt, 1973; Balkanski et al., 1993; Koch et al., 1996). The residence time of atmospheric aerosol particles can be estimated by means of radioactive nuclides as tracers, which become attached to aerosol particles and are removed with them as they are scavenged by precipitation or undergo dry fallout (Warneck, 1988). Several methods have been used for estimating the mean residence time of atmospheric aerosol particles. These include measurements of the activities and ratios of (i) cosmic-ray produced radionuclides, such as 7Be (Tl/2 ¼ 53.3 days) (Shapiro
0969-8043/$ - see front matter r 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.apradiso.2005.07.006
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and Forbes-Resha, 1976; Winkler et al., 1998; Yu and Lee, 2002) and (ii) radioactive decay products of radon, 222Rn and thoron, 210Rn which emanate from continental surfaces into the atmosphere, such as 210Pb (T1/2 ¼ 22.3 years), 210Bi (Tl/2 ¼ 5.01 days) and 210Po (Tl/2 ¼ 138.38 days) (Francis et al., 1970; Poet et al., 1972; Lambert et al., 1982, 1983; Marley et al., 2000; Baskaran and Shaw, 2001). However, there is disagreement between the derived values of the residence times due to various processes, including the fact that they refer to different portions of the atmosphere, e.g. cosmic-ray produced isotopes refer to upper troposphere or lower stratosphere, such as 7Be, and radon decay products, such as 210Pb, 210Bi, and 210Po that refer to lower troposphere and also due to the existence of different sources for some isotopes, such as 210Po. This paper is dealing with the residence time of tropospheric aerosols as determined by different methods based on the measurements of the activities of radioactive nuclides which are associated with the atmospheric aerosol particles. It is also an update overview of the existing in the literature data, regarding the residence time of tropospheric aerosols.
2. Experimental methods and techniques The collection of atmospheric aerosol particles was carried out with high-volume jet air samplers, type TFIA-2 of Staplex having glass-fiber filters type TFAGF
810 of Staplex, highly retentive for particulate material, 20.32 cm 25.40 cm (800 1000 ) in dimensions and 99.28% collection efficiency for submicron particles as small as 0.3 mm and over. The air-flow rate of these samplers was regulated from 1.7 m3 min1 (60 cfm) to 1.92 m3 min1 (68 cfm), (average 1.84 m3 min1 or 65 cfm). The length of each collection period was 24 h. The size fractionation of atmospheric aerosol particles was carried out with aerosol cascade impactors, type Andersen 2000, as follows: (i) the 1-ACFM design was operated at air-flow rate of 28 l min1 (1 ft3 min1). Its stages had effective cut-off diameters (ECD) of 0.4, 0.7, 1.1, 2.1, 3.3, 4.7, 7.0 and 11.0 mm, (ii) the low-pressure modification which alters the impactor’s operation by increasing the resolution in the submicron region, involved a regulated air-flow rate of 3 l min1, five lowpressure (114 mmHg that is the absolute pressure downstream of the critical orifice) stages for the submicron region and eight atmospheric pressure stages for separating aerosol particles above 1.4 mm. The ECDs of the low-pressure stages were 0.08, 0.11, 0.23, 0.52. and 0.90 mm, whereas for the upper stages they were 1.4, 2.0, 3.3, 6.6, 10.5, 15.7, 21.7 and 35.0 mm (iii) the highvolume cascade impactors had a regulated air-flow rate either of about 0.57 m3 min1 (20 cfm) or 1.13 m3 min1 (40 cfm) and the ECDs were 0.41, 0.73, 1.4, 2.1, 4.2 and 10.2 mm for the 20 cfm configuration or 0.49, 0.95, 1.5, 3.0 and 7.2 mm for the 40 cfm configuration at the standard temperature and pressure atmospheric conditions (2501 and 760 mmHg). The stainless steel plates
Fig. 1. A typical gamma-ray spectrum of air filter obtained by a Ge detector.
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supplied by the manufacturer were used for collection of the aerosol particles. Either polycarbonate or glass-fiber backup filters were used to collect all particles below the 0.08 mm collection plate for the low-pressure impactors, below the 0.4 mm collection plate for the 1-ACFM impactors, and below the 0.41 or 0.49 mm collection plate for the high-volume impactors. The length of each collection period varied from 1 to 24 h for 7Be, and 210Pb, 210Bi and 210Po, depending on impactor type and objective. The activity of 7Beassociated aerosol particles was measured through its gamma-ray peak of 477.6 keV using a high-resolution (1.9 keV at 1.33 MeV of 60Co), high-efficiency (42%) low-background HP Ge detector. The uncertainty of the gamma-counting system for 7Be measurements was better than 8.5%. The activity of 210Pb- and 210Biassociated aerosol particles was measured by a lowbackground phoswitch scintillation detector system (Harsaw model TASC-12-A6) having a background of 2 cpm and efficiency higher than 40% for counting beta radiation. This system consisted of a thin CaF2 (EU)
Fig. 2. Activity aerodynamic size distribution of
7
95
primary crystal with a decay time of 0.23 ms. The samples were counted for long enough to obtain a statistical accuracy better than 5%. It must be noted that 210Pb and 210Bi were chemically separated and measured for their activities. Detailed description of the analytical method and technique was established and presented elsewhere (Jaworowski, 1963). The activity of lead-210 was also measured by a surface barrier Ge detector through its gamma peak of 46.5 keV.The activity size distribution and the activity median aerodynamic diameter (AMAD) of the aerosol particles were determined upon the measurement of the activities of the aerosol-associated radioactive nuclides. Fig. 1 shows a typical gamma-ray spectrum of air filter obtained by a Ge detector, in which the 477.6 keV—gamma peak of 7Be and 46.5 keV—gamma peak of 210Pb are clearly shown. Fig. 2 shows the activity aerodynamic size distribution of 7Be atmospheric aerosols obtained by 1-ACFM cascade impactor.
Be atmospheric aerosols obtained by 1-ACFM cascade impactor.
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Table 1 shows data for the activity median aerodynamic diameter, AMAD of atmospheric aerosol particles associated with 7Be and 210Pb atoms. Regarding the 7Be aerosols the AMAD varied from 0.76 to 1.18 mm (average 0.90 mm). The reported AMAD values for 7Be-aerosols varied from 0.33 to 1.15 mm, while for the 210Pb-aerosols the reported AMAD values varied from 0.28 to 0.77 mm. The residence time, tR is described by a formula
3. Residence time of tropospheric aerosol particles associated with the cosmic ray produced 7Be and the radon-emanated 210Pb A method for estimating the residence time of tropospheric aerosol particles associated with the cosmic-ray produced radionuclides, such as 7Be is based on the aerosol particle growth rate, that is the change of particle diameter in respect of time, which was estimated to be 0.004–0.005 mm h1 (McMurry and Wilson, 1982) and the difference between the activity median aerodynamic diameter, AMAD of a radionuclide, e.g. 7Be and the size of Aitken nuclei in the size distribution of aerosol particles, which is 0.015 mm (NRC, 1979). The AMAD for all radionuclides is in the accumulation mode of the size distribution of atmospheric aerosol particles which ranges between 0.1 and 2.0 mm (NRC, 1979).
tR ¼
AMADmean SizeAitken nuclei . Mean particle growth rate
(1)
By the same way, the residence time of atmospheric aerosol particles associated with 210Pb, a decay product of soil emanated radon 222 could be determined, because 7Be and 210Pb have common association with atmospheric aerosol particles (Dibb and Jaffrezo, 1993).
Table 1 Activity median aerodynamic diameter (AMAD)of atmospheric aerosol particles (mm) 7
210
Be
Pb
0.76–1.18 (avg. 0.90) — 0.36, 0.38, 0.39, 0.48 0.65–1.09 (avg. 0.77) 0.44–0.74 (avg. 0.57) 0.33–1.15 (avg. 0.67) 0.70
References
— 0.60, 0.61 0.63, 0.77 (avg. 0.60) — 0.56 0.28–0.74 (avg. 0.53) — 0.55
Table 2 Residence times of tropospheric aerosols derived from
7
Be and
This work Sanak et al. (1981) Bondietti and Brantley (1986) Reineking and Porstendo¨rfer (1995) Winkler et al. (1998) Yu and Lee (2002) Porstendo¨rfer and Gru¨ndel (2003)
210
Pb activities in air (days) 7
Be
210
Pb
References
Investigation
Coordinates
Period
Thessaloniki, Greece Neuherberg, Germany
401380 N 221580 E
7.4–8.9 (avg. 8.0)
—
This work
5–6
4.5
Winkler et al. (1998)
Fullerton, California
331520 N, 1171550 W
November 21, 1991–June 22, 1993 December 22, 1994–March 21, 1996 September 1973–July 1975
35.4
—
Hong Kong
221180 N, 1141100 E
2.6–11.8
—
Northern Hemisphere Tropics
301N—801N
November 26, 2001–March 08, 2002 Computed by modelling Computed by modelling Computed by modelling Calculated by modelling
Shapiro and Forbes-Resha (1976) Yu and Lee (2002)
—
5–10
—
10–15
—
5
21
9
Southern Hemisphere South to North
481130 N, 111360 E
301S—301N 301S—801S 801S—801N
Balkanski et al. (1993) Balkanski et al. (1993) Balkanski et al. (1993) Koch et al. (1996)
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Taking into account that the AMAD of aerosol particles associated with 7Be varied from 0.76 to 1.18 mm (Table 1), then, according to Eq. (1) the residence time of atmospheric aerosols will vary between 7.4 and 8.9 days (average 8.0 days) at Thessaloniki region (481380 N, 221580 E), Northern Greece, with dry (precipitation-free) climate at temperate latitude, based in twelve measurements of aerosol samplings carried out during an 1.5 year period, thus included all seasons of a year. In Table 2, the residence times of tropospheric aerosols derived from 7Be and 210Pb concentrations in air are presented. Balkanski et al. (1993) following a global threedimensional model which uses meteorological parameters, such as precipitation scavenging, found that the tropospheric residence time is a function of latitude according to the following equation (Ehhalt, 1973) tR ¼
C , F
(2)
where C is the tropospheric column of a radionuclide extending from the surface up to the model layer just below the tropopause, and F is the total depositional flux out of the column at a given latitude. Koch et al. (1996) following a three-dimensional chemical tracer model as Balkanski et al. (1993) also found that the tropospheric residence time is a function of latitude according to Eq. (2). The data of Table 2 admit residence times of tropospheric aerosols in the range 2.6–35.4 days, but crowd into two groups of values 2.6–15 days (average 8.8 days) and 21–35.4 days (average 28.2 days). The lower values are applicable only to the boundary layer near the Earth’s surface and the higher values are appropriate to the troposphere as a whole (Junge, 1963). Martell and Moore (1974) came to the opposite conclusion, namely, that the high values are due to the contribution of stratospheric aerosols, while the lower values represent the true tropospheric residence time essentially independent of altitude.
4. Residence time of tropospheric aerosol particles associated with the radon decay product radionucldes 210 Pb, 210Bi and 210Po and their activity ratios
210
Po
a;138:38 days 206
!
Pb ðstableÞ:
The residence time, tR is described by the formula tR ¼
ð3Þ
1 ðlBi N Bi Þ=ðlPb N Pb Þ , lBi 1 ðlBi N Bi Þ=ðlPb N Pb Þ
(4)
where lBiNBi is the activity of 210Bi, lPbNPb is the activity of 210Pb in air and lBi ¼ 0.138 day1 is the decay constant of 210Bi. Eq. (4) was derived from the equation of the production and removal of radionuclides assuming a steady state equilibrium dN Bi ¼ lPb N Pb ðlBi þ lR ÞN Bi ¼ 0, dt
(5)
where lR ¼ 1/tR is the first-order rate constant for the removal of aerosols from the atmosphere by all processes, that is the inverse of residence time, tR. The ratio of the activities lBiNBi/lPbNPb in Eq. (4) varied from 0.48 to 0.68 (this work) or from 0.42 to 0.85 (Moore et al., 1972). If the activity of 210Po, lPoNPo in air is considered and lPo ¼ 5.0 103 days1is the decay constant of 210Po, then the activity ratio of 210Po and 210Pb is given by the equation lPo N Po t2 R . ¼ lPb N Pb tR þ 1=lBi tR þ 1=lPo
(6)
From which the residence time, tR is determined by tR ¼
b þ ðb2 4acÞ1=2 , 2a
(7)
where a ¼ lPb N Pb lPo N Po , 1 1 b ¼ lPo N Po , þ lBi lPo 1 , c ¼ lPo N Po lBi lPo lBi ¼ 0:138 day1 and lPo ¼ 5 103 days1 .
ð8Þ
The ratio of the activities lPoNPo/lPbNPb in Eq. (6) varied from 0.054 to 0.092 (Moore et al., 1972). The residence time, tR can also be determined through the ratio of activities of radon, 222Rn, lRnNRn and 210Pb, lPbNPb in air according to the sequential disintegrations in the alpha and beta decay scheme as 222
A method for estimating the residence time of tropospheric aerosol particles associated with radon decay product radoinuclides is based on the radioactivity of a pair of genetically related radioisotopes, such as 210Pb, 210Bi or 210Pb, 210Po according to the sequential disintegrations in the beta decay scheme, as b ;22:3 year b- ;5:10 days 210 Pb ! 210 Bi !
97
214
Rn Bi
a; 3:824 days 218
!
b ; 19:7 m 214
!
a; 3:05 m 214
Po !
a; 164 ms 210
Po !
Pb
Pb
b ; 26:8 m
!
b ; 22:3 years
!
ð9Þ
and Eq. (5) by the formula tR ¼
1 1 1 lPb N Pb , lPb ðlRn N Rn Þ=ðlPb N Pb Þ 1 lPb lRn N Rn
(10)
where lPb ¼ 8.5 105 days1 is the decay constant of 210Pb and ðlRn N Rn Þ=ðlPb N Pb Þ 1. The ratio of the activities lRnNRn/lPbNPb in Eq. (10) varied from 282 to 7700 (Moore et al., 1972).
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Peirson et al. (1966) Rangarajan (1992) Ahmed et al. (2000)
This work Poet et al. (1972), Moore et al. (1972, 1973) Marley et al. (2000) Francis et al. (1970) Fry and Menon (1962) Gavini et al. (1974) Baskaran and Shaw (2001) Baskaran and Shaw (2001) Ga¨ggeler et al. (1995) Lehmann and Sittkus (1959) Lambert et al. (1982, 1983)
Pb
40 — —
— — — — — — — — 6.5 6.77 — — — 33–66 9.6 — 2–320 (avg. 40) 11.9–32 0–38.9 1–12 (avg.6) 20 —
— 11–77 4.8–15.3 (avg. 8.2) 1.59–13.0 (avg. 5.4)
6–67 — 2.4–25.6 (avg.8.5) 3–240 (avg. 20) — — — — 8.8–10.5 7–9 — 8 4.3–12.87 (avg. 9.83)
— 2.2–3.4
210
Rn/
222
Pb
210
Po/
210
Pb
210
Bi/
210
Pb activity ratios in air (days)
210
Rn/
June 1996–August 1996 June 01, 1966–August 16, 1966 February 02, 1961–August 23, 1961 April 09, 1973–April 18, 1974 January 08, 1996–February 26, 1996 January 10, 1996–March 15, 1996 January 1991–December 1991 — 1978–1983
— October 1987–May 1988 January 1999–December 1999
35158 N, 84117 W 401010 N, 1051170 W
41.71N, 88.01W 431510 N, 891220 W 351030 N, 781540 W 351030 N, 781540 W 65.11N, 147.51W 69.51N, 141.21W 461320 N, 071590 E 471590 N, 71510 E 481520 N, 21200 E
511400 N, 51020 W 181580 N, 721500 E 281N, 301450 E
Argonne, Illinois Madison, Wisconsin Fayetteville, Arkansas Fayetteville, Arkansas Poker Flat, Alaska Eagle, Alaska Jungfraujoch, Switzerland Freiburg, Germany Gif-sur-Yvette, France
Milford Haven, Wales Bombay, India EI-Minia, Egypt
0
Coordinates
0
Period
October 15, 1984–Sepember 16, 1985 Sepember 05, 1967–January 02, 1972
222
Pb and
210
Po/
210
Pb,
210
Bi/
Oak Ridge, Tennessee Boulder Colorado
Most of the naturally occurring and man-made produced radionuclides reside in particles with diameter ranging between 0.28 and 1.18 mm, in the accumulation mode in the activity size distribution of atmospheric aerosols. In such particles a mean tropospheric aerosol residence time of about 8 days could be applied as
Investigation
5. Conclusions
210
The isotopic ratios were considered as nuclear clocks. The applicability of the radioisotope ratios depend on the steady state conditions hold at the time and place of measurements and the kind of the samples whether they are surface air or precipitation (rain or snow) for the radioisotope activity determination. Taking into account the 210Bi/ 210Pb ratios as determined in 21 measurements of aerosol samplings carried out during an annual period, thus included all seasons of a year at Oak Ridge, TN (351580 N, 841170 W) at temperate latitude with high precipitation (wet climate), the estimated residence time of tropospheric aerosols in the boundary layer varied from 4.8 to 15.3 days (average 8.2 days). In Table 3, the residence times of tropospheric aerosols derived from 210Bi/ 210Pb, 210Po/ 210Pb and 222Rn/ 210Pb, activity ratios in air are presented. The data of Table 3 admit residence times of lower values as low as 1.59 days and higher values as high as 320 days. Mostly, the lower tR values were resulted from the 210Bi/ 210Pb activity ratios and higher tR values from the 210Po/ 210Pb activity ratios. Low tR values were also resulted from the 222Rn/ 210Pb activity ratios. Poet et al. (1972) concluded that longer apparent residence time values for tR based on the 210Po/ 210Pb activity ratios, than for tR based on the 210Bi/ 210Pb activity ratios. The difference may be explained by the presence of a mixture of aerosols of various apparent ages, of which older aerosols (those of greater age) contribute most of 210Po. On the other hand, the solid products of radon-222 decay, that is 210Pb, 210Bi and 210Po might arise from sources other than radioactive decay within the atmosphere. Poet et al. (1972) showed that up to 85% of 210Po in the atmosphere is of terrestrial origin, and the vertical profile of 210Po was found to differ appreciably from that expected from the decay of 222Rn. They also found that the mean aerosol residence time increases with altitude within the troposphere less than by a factor of 3 (Moore et al., 1973). Lambert et al. (1979) indicated that volcanic gases are very rich in long-lived radon decay products, especially in 210Po relative to 210Pb. Soil particles are the most likely contributors, since a part of the tropospheric aerosols originates at the Earth’s surface. Coal burning and forests fires presumably are additional sources of radionuclides
References
C. Papastefanou / Applied Radiation and Isotopes 64 (2006) 93–100
Table 3 Residence times of tropospheric aerosols derived from
98
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resulted by the application of two different methods, i.e. the 7Be-associated atmospheric aerosols and the radon decay product aerosols at two different locations, i.e. at Thessaloniki, Greece 401380 N 221580 E with dry (precipitation-free) climate and at Oak Ridge, TN, USA 351580 N 841170 W with high precipitation (wet climate), roughly at similar temperate latitudes, but the first one at East longitude and the other at West longitude, respectively. It has been reported that the residence time of atmospheric aerosols in the lower troposphere below precipitation cloud level increases with the tropospherere by a factor of 3 or less and there is a dependence on the latitude.
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Environment VI, 5–9 June 1995, Montreal, Canada. Book of Abstracts. Clarkson University, Potsdam, NY, p. 199. Sanak, J., Gaudry, A., Lambert, G., 1981. Size distribution of 210Pb aerosols over oceans. Geophys. Res. Lett. 8 (10), 1067–1069. Shapiro, M.H., Forbes-Resha, J.L., 1976. Mean residence time of 7Be-bearing aerosols in the troposphere. J. Geophys. Res. 81 (15), 2647–2649.
Warneck, P., 1988. Chemistry of the natural atmosphere. Academic Press, Inc., San Diego, USA. Winkler, R., Dietl, F., Franck, G., Tschiersch, J., 1998. Temporal variation of 7Be and 210Pb size distribution in ambient aerosol. Atmos. Environ. 32 (6), 983–991. Yu, K.N., Lee, L.Y.L., 2002. Measurement of atmospheric 7Be properties using high-efficiency gamma spectroscopy. Appl. Radiat. Isot. 57, 941–946.
Applied Radiation and Isotopes 64 (2006) 93–100 www.elsevier.com/locate/apradiso
Residence time of tropospheric aerosols in association with radioactive nuclides C. Papastefanou Oak Ridge National Laboratory, Environmental Sciences Division Oak Ridge, TN 37831, USA Received 3 April 2005; received in revised form 12 June 2005; accepted 12 July 2005
Abstract The residence time of atmospheric aerosol particles is a function of various removal processes, such as dry deposition by impaction, diffusion, sedimentation and resuspension as well as wet deposition by rain drops (precipitation scavenging). Estimation of the mean-residence time of atmospheric aerosols could be based on measurements of the activities and ratios of activities of cosmic-ray produced radionuclides, such as 7Be and the radioactive decay products of radon-222 emanated from soil into the atmosphere, such as 210Pb, 210Bi and 210Po. It was found that a mean value of about 8 days could be applied to aerosol particles in the lower atmosphere below precipitation cloud levels as resulted by the application of two different methods, i.e. the 7Be-associated atmospheric aerosols and the radon decay product aerosols at two different locations, i.e. at Thessaloniki, Greece 401380 N 221580 E with dry (precipitation free) climate and at Oak Ridge, Tennessee, USA 351580 N 841170 W with high precipitation (wet climate), roughly at similar temperate latitudes, but the first one at East longitude and the other at West longitude, respectively. r 2005 Elsevier Ltd. All rights reserved. Keywords: Beryllium-7; Lead-210; Bismuth-210; Polonium-210; Residence time; Tropospheric aerosols
1. Introduction The residence time of atmospheric aerosol particles in the lower atmosphere assuming that the air in the troposphere is considered as a well-mixed reservoir (closed system) is a function of various removal processes, the most important being: (a) dry deposition by impaction, diffusion and sedimentation, and (b) wet deposition by rain drops (precipitation scavenging) Corresponding author at: Present address. Aristotle University of Thessaloniki, Atomic and Nuclear Physics Laboratory, Thessaloniki 54124, Greece. Tel: +30 2310 998 005; fax: +30 2310 998 058. E-mail address: [email protected].
as a result of processes occurring both within and below the rain cloud. There could be variations in the removal rates at different continental locations of the globe (Lehmann and Sittkus, 1959), over the oceans (Koch et al., 1996), and at high altitudes of the atmosphere (Martell, 1970; Moore et al., 1973) due to changes in meteorological conditions. There is also a dependence of the tropospheric aerosol residence time on the latitude (Ehhalt, 1973; Balkanski et al., 1993; Koch et al., 1996). The residence time of atmospheric aerosol particles can be estimated by means of radioactive nuclides as tracers, which become attached to aerosol particles and are removed with them as they are scavenged by precipitation or undergo dry fallout (Warneck, 1988). Several methods have been used for estimating the mean residence time of atmospheric aerosol particles. These include measurements of the activities and ratios of (i) cosmic-ray produced radionuclides, such as 7Be (Tl/2 ¼ 53.3 days) (Shapiro
0969-8043/$ - see front matter r 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.apradiso.2005.07.006
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and Forbes-Resha, 1976; Winkler et al., 1998; Yu and Lee, 2002) and (ii) radioactive decay products of radon, 222Rn and thoron, 210Rn which emanate from continental surfaces into the atmosphere, such as 210Pb (T1/2 ¼ 22.3 years), 210Bi (Tl/2 ¼ 5.01 days) and 210Po (Tl/2 ¼ 138.38 days) (Francis et al., 1970; Poet et al., 1972; Lambert et al., 1982, 1983; Marley et al., 2000; Baskaran and Shaw, 2001). However, there is disagreement between the derived values of the residence times due to various processes, including the fact that they refer to different portions of the atmosphere, e.g. cosmic-ray produced isotopes refer to upper troposphere or lower stratosphere, such as 7Be, and radon decay products, such as 210Pb, 210Bi, and 210Po that refer to lower troposphere and also due to the existence of different sources for some isotopes, such as 210Po. This paper is dealing with the residence time of tropospheric aerosols as determined by different methods based on the measurements of the activities of radioactive nuclides which are associated with the atmospheric aerosol particles. It is also an update overview of the existing in the literature data, regarding the residence time of tropospheric aerosols.
2. Experimental methods and techniques The collection of atmospheric aerosol particles was carried out with high-volume jet air samplers, type TFIA-2 of Staplex having glass-fiber filters type TFAGF
810 of Staplex, highly retentive for particulate material, 20.32 cm 25.40 cm (800 1000 ) in dimensions and 99.28% collection efficiency for submicron particles as small as 0.3 mm and over. The air-flow rate of these samplers was regulated from 1.7 m3 min1 (60 cfm) to 1.92 m3 min1 (68 cfm), (average 1.84 m3 min1 or 65 cfm). The length of each collection period was 24 h. The size fractionation of atmospheric aerosol particles was carried out with aerosol cascade impactors, type Andersen 2000, as follows: (i) the 1-ACFM design was operated at air-flow rate of 28 l min1 (1 ft3 min1). Its stages had effective cut-off diameters (ECD) of 0.4, 0.7, 1.1, 2.1, 3.3, 4.7, 7.0 and 11.0 mm, (ii) the low-pressure modification which alters the impactor’s operation by increasing the resolution in the submicron region, involved a regulated air-flow rate of 3 l min1, five lowpressure (114 mmHg that is the absolute pressure downstream of the critical orifice) stages for the submicron region and eight atmospheric pressure stages for separating aerosol particles above 1.4 mm. The ECDs of the low-pressure stages were 0.08, 0.11, 0.23, 0.52. and 0.90 mm, whereas for the upper stages they were 1.4, 2.0, 3.3, 6.6, 10.5, 15.7, 21.7 and 35.0 mm (iii) the highvolume cascade impactors had a regulated air-flow rate either of about 0.57 m3 min1 (20 cfm) or 1.13 m3 min1 (40 cfm) and the ECDs were 0.41, 0.73, 1.4, 2.1, 4.2 and 10.2 mm for the 20 cfm configuration or 0.49, 0.95, 1.5, 3.0 and 7.2 mm for the 40 cfm configuration at the standard temperature and pressure atmospheric conditions (2501 and 760 mmHg). The stainless steel plates
Fig. 1. A typical gamma-ray spectrum of air filter obtained by a Ge detector.
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supplied by the manufacturer were used for collection of the aerosol particles. Either polycarbonate or glass-fiber backup filters were used to collect all particles below the 0.08 mm collection plate for the low-pressure impactors, below the 0.4 mm collection plate for the 1-ACFM impactors, and below the 0.41 or 0.49 mm collection plate for the high-volume impactors. The length of each collection period varied from 1 to 24 h for 7Be, and 210Pb, 210Bi and 210Po, depending on impactor type and objective. The activity of 7Beassociated aerosol particles was measured through its gamma-ray peak of 477.6 keV using a high-resolution (1.9 keV at 1.33 MeV of 60Co), high-efficiency (42%) low-background HP Ge detector. The uncertainty of the gamma-counting system for 7Be measurements was better than 8.5%. The activity of 210Pb- and 210Biassociated aerosol particles was measured by a lowbackground phoswitch scintillation detector system (Harsaw model TASC-12-A6) having a background of 2 cpm and efficiency higher than 40% for counting beta radiation. This system consisted of a thin CaF2 (EU)
Fig. 2. Activity aerodynamic size distribution of
7
95
primary crystal with a decay time of 0.23 ms. The samples were counted for long enough to obtain a statistical accuracy better than 5%. It must be noted that 210Pb and 210Bi were chemically separated and measured for their activities. Detailed description of the analytical method and technique was established and presented elsewhere (Jaworowski, 1963). The activity of lead-210 was also measured by a surface barrier Ge detector through its gamma peak of 46.5 keV.The activity size distribution and the activity median aerodynamic diameter (AMAD) of the aerosol particles were determined upon the measurement of the activities of the aerosol-associated radioactive nuclides. Fig. 1 shows a typical gamma-ray spectrum of air filter obtained by a Ge detector, in which the 477.6 keV—gamma peak of 7Be and 46.5 keV—gamma peak of 210Pb are clearly shown. Fig. 2 shows the activity aerodynamic size distribution of 7Be atmospheric aerosols obtained by 1-ACFM cascade impactor.
Be atmospheric aerosols obtained by 1-ACFM cascade impactor.
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Table 1 shows data for the activity median aerodynamic diameter, AMAD of atmospheric aerosol particles associated with 7Be and 210Pb atoms. Regarding the 7Be aerosols the AMAD varied from 0.76 to 1.18 mm (average 0.90 mm). The reported AMAD values for 7Be-aerosols varied from 0.33 to 1.15 mm, while for the 210Pb-aerosols the reported AMAD values varied from 0.28 to 0.77 mm. The residence time, tR is described by a formula
3. Residence time of tropospheric aerosol particles associated with the cosmic ray produced 7Be and the radon-emanated 210Pb A method for estimating the residence time of tropospheric aerosol particles associated with the cosmic-ray produced radionuclides, such as 7Be is based on the aerosol particle growth rate, that is the change of particle diameter in respect of time, which was estimated to be 0.004–0.005 mm h1 (McMurry and Wilson, 1982) and the difference between the activity median aerodynamic diameter, AMAD of a radionuclide, e.g. 7Be and the size of Aitken nuclei in the size distribution of aerosol particles, which is 0.015 mm (NRC, 1979). The AMAD for all radionuclides is in the accumulation mode of the size distribution of atmospheric aerosol particles which ranges between 0.1 and 2.0 mm (NRC, 1979).
tR ¼
AMADmean SizeAitken nuclei . Mean particle growth rate
(1)
By the same way, the residence time of atmospheric aerosol particles associated with 210Pb, a decay product of soil emanated radon 222 could be determined, because 7Be and 210Pb have common association with atmospheric aerosol particles (Dibb and Jaffrezo, 1993).
Table 1 Activity median aerodynamic diameter (AMAD)of atmospheric aerosol particles (mm) 7
210
Be
Pb
0.76–1.18 (avg. 0.90) — 0.36, 0.38, 0.39, 0.48 0.65–1.09 (avg. 0.77) 0.44–0.74 (avg. 0.57) 0.33–1.15 (avg. 0.67) 0.70
References
— 0.60, 0.61 0.63, 0.77 (avg. 0.60) — 0.56 0.28–0.74 (avg. 0.53) — 0.55
Table 2 Residence times of tropospheric aerosols derived from
7
Be and
This work Sanak et al. (1981) Bondietti and Brantley (1986) Reineking and Porstendo¨rfer (1995) Winkler et al. (1998) Yu and Lee (2002) Porstendo¨rfer and Gru¨ndel (2003)
210
Pb activities in air (days) 7
Be
210
Pb
References
Investigation
Coordinates
Period
Thessaloniki, Greece Neuherberg, Germany
401380 N 221580 E
7.4–8.9 (avg. 8.0)
—
This work
5–6
4.5
Winkler et al. (1998)
Fullerton, California
331520 N, 1171550 W
November 21, 1991–June 22, 1993 December 22, 1994–March 21, 1996 September 1973–July 1975
35.4
—
Hong Kong
221180 N, 1141100 E
2.6–11.8
—
Northern Hemisphere Tropics
301N—801N
November 26, 2001–March 08, 2002 Computed by modelling Computed by modelling Computed by modelling Calculated by modelling
Shapiro and Forbes-Resha (1976) Yu and Lee (2002)
—
5–10
—
10–15
—
5
21
9
Southern Hemisphere South to North
481130 N, 111360 E
301S—301N 301S—801S 801S—801N
Balkanski et al. (1993) Balkanski et al. (1993) Balkanski et al. (1993) Koch et al. (1996)
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Taking into account that the AMAD of aerosol particles associated with 7Be varied from 0.76 to 1.18 mm (Table 1), then, according to Eq. (1) the residence time of atmospheric aerosols will vary between 7.4 and 8.9 days (average 8.0 days) at Thessaloniki region (481380 N, 221580 E), Northern Greece, with dry (precipitation-free) climate at temperate latitude, based in twelve measurements of aerosol samplings carried out during an 1.5 year period, thus included all seasons of a year. In Table 2, the residence times of tropospheric aerosols derived from 7Be and 210Pb concentrations in air are presented. Balkanski et al. (1993) following a global threedimensional model which uses meteorological parameters, such as precipitation scavenging, found that the tropospheric residence time is a function of latitude according to the following equation (Ehhalt, 1973) tR ¼
C , F
(2)
where C is the tropospheric column of a radionuclide extending from the surface up to the model layer just below the tropopause, and F is the total depositional flux out of the column at a given latitude. Koch et al. (1996) following a three-dimensional chemical tracer model as Balkanski et al. (1993) also found that the tropospheric residence time is a function of latitude according to Eq. (2). The data of Table 2 admit residence times of tropospheric aerosols in the range 2.6–35.4 days, but crowd into two groups of values 2.6–15 days (average 8.8 days) and 21–35.4 days (average 28.2 days). The lower values are applicable only to the boundary layer near the Earth’s surface and the higher values are appropriate to the troposphere as a whole (Junge, 1963). Martell and Moore (1974) came to the opposite conclusion, namely, that the high values are due to the contribution of stratospheric aerosols, while the lower values represent the true tropospheric residence time essentially independent of altitude.
4. Residence time of tropospheric aerosol particles associated with the radon decay product radionucldes 210 Pb, 210Bi and 210Po and their activity ratios
210
Po
a;138:38 days 206
!
Pb ðstableÞ:
The residence time, tR is described by the formula tR ¼
ð3Þ
1 ðlBi N Bi Þ=ðlPb N Pb Þ , lBi 1 ðlBi N Bi Þ=ðlPb N Pb Þ
(4)
where lBiNBi is the activity of 210Bi, lPbNPb is the activity of 210Pb in air and lBi ¼ 0.138 day1 is the decay constant of 210Bi. Eq. (4) was derived from the equation of the production and removal of radionuclides assuming a steady state equilibrium dN Bi ¼ lPb N Pb ðlBi þ lR ÞN Bi ¼ 0, dt
(5)
where lR ¼ 1/tR is the first-order rate constant for the removal of aerosols from the atmosphere by all processes, that is the inverse of residence time, tR. The ratio of the activities lBiNBi/lPbNPb in Eq. (4) varied from 0.48 to 0.68 (this work) or from 0.42 to 0.85 (Moore et al., 1972). If the activity of 210Po, lPoNPo in air is considered and lPo ¼ 5.0 103 days1is the decay constant of 210Po, then the activity ratio of 210Po and 210Pb is given by the equation lPo N Po t2 R . ¼ lPb N Pb tR þ 1=lBi tR þ 1=lPo
(6)
From which the residence time, tR is determined by tR ¼
b þ ðb2 4acÞ1=2 , 2a
(7)
where a ¼ lPb N Pb lPo N Po , 1 1 b ¼ lPo N Po , þ lBi lPo 1 , c ¼ lPo N Po lBi lPo lBi ¼ 0:138 day1 and lPo ¼ 5 103 days1 .
ð8Þ
The ratio of the activities lPoNPo/lPbNPb in Eq. (6) varied from 0.054 to 0.092 (Moore et al., 1972). The residence time, tR can also be determined through the ratio of activities of radon, 222Rn, lRnNRn and 210Pb, lPbNPb in air according to the sequential disintegrations in the alpha and beta decay scheme as 222
A method for estimating the residence time of tropospheric aerosol particles associated with radon decay product radoinuclides is based on the radioactivity of a pair of genetically related radioisotopes, such as 210Pb, 210Bi or 210Pb, 210Po according to the sequential disintegrations in the beta decay scheme, as b ;22:3 year b- ;5:10 days 210 Pb ! 210 Bi !
97
214
Rn Bi
a; 3:824 days 218
!
b ; 19:7 m 214
!
a; 3:05 m 214
Po !
a; 164 ms 210
Po !
Pb
Pb
b ; 26:8 m
!
b ; 22:3 years
!
ð9Þ
and Eq. (5) by the formula tR ¼
1 1 1 lPb N Pb , lPb ðlRn N Rn Þ=ðlPb N Pb Þ 1 lPb lRn N Rn
(10)
where lPb ¼ 8.5 105 days1 is the decay constant of 210Pb and ðlRn N Rn Þ=ðlPb N Pb Þ 1. The ratio of the activities lRnNRn/lPbNPb in Eq. (10) varied from 282 to 7700 (Moore et al., 1972).
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Peirson et al. (1966) Rangarajan (1992) Ahmed et al. (2000)
This work Poet et al. (1972), Moore et al. (1972, 1973) Marley et al. (2000) Francis et al. (1970) Fry and Menon (1962) Gavini et al. (1974) Baskaran and Shaw (2001) Baskaran and Shaw (2001) Ga¨ggeler et al. (1995) Lehmann and Sittkus (1959) Lambert et al. (1982, 1983)
Pb
40 — —
— — — — — — — — 6.5 6.77 — — — 33–66 9.6 — 2–320 (avg. 40) 11.9–32 0–38.9 1–12 (avg.6) 20 —
— 11–77 4.8–15.3 (avg. 8.2) 1.59–13.0 (avg. 5.4)
6–67 — 2.4–25.6 (avg.8.5) 3–240 (avg. 20) — — — — 8.8–10.5 7–9 — 8 4.3–12.87 (avg. 9.83)
— 2.2–3.4
210
Rn/
222
Pb
210
Po/
210
Pb
210
Bi/
210
Pb activity ratios in air (days)
210
Rn/
June 1996–August 1996 June 01, 1966–August 16, 1966 February 02, 1961–August 23, 1961 April 09, 1973–April 18, 1974 January 08, 1996–February 26, 1996 January 10, 1996–March 15, 1996 January 1991–December 1991 — 1978–1983
— October 1987–May 1988 January 1999–December 1999
35158 N, 84117 W 401010 N, 1051170 W
41.71N, 88.01W 431510 N, 891220 W 351030 N, 781540 W 351030 N, 781540 W 65.11N, 147.51W 69.51N, 141.21W 461320 N, 071590 E 471590 N, 71510 E 481520 N, 21200 E
511400 N, 51020 W 181580 N, 721500 E 281N, 301450 E
Argonne, Illinois Madison, Wisconsin Fayetteville, Arkansas Fayetteville, Arkansas Poker Flat, Alaska Eagle, Alaska Jungfraujoch, Switzerland Freiburg, Germany Gif-sur-Yvette, France
Milford Haven, Wales Bombay, India EI-Minia, Egypt
0
Coordinates
0
Period
October 15, 1984–Sepember 16, 1985 Sepember 05, 1967–January 02, 1972
222
Pb and
210
Po/
210
Pb,
210
Bi/
Oak Ridge, Tennessee Boulder Colorado
Most of the naturally occurring and man-made produced radionuclides reside in particles with diameter ranging between 0.28 and 1.18 mm, in the accumulation mode in the activity size distribution of atmospheric aerosols. In such particles a mean tropospheric aerosol residence time of about 8 days could be applied as
Investigation
5. Conclusions
210
The isotopic ratios were considered as nuclear clocks. The applicability of the radioisotope ratios depend on the steady state conditions hold at the time and place of measurements and the kind of the samples whether they are surface air or precipitation (rain or snow) for the radioisotope activity determination. Taking into account the 210Bi/ 210Pb ratios as determined in 21 measurements of aerosol samplings carried out during an annual period, thus included all seasons of a year at Oak Ridge, TN (351580 N, 841170 W) at temperate latitude with high precipitation (wet climate), the estimated residence time of tropospheric aerosols in the boundary layer varied from 4.8 to 15.3 days (average 8.2 days). In Table 3, the residence times of tropospheric aerosols derived from 210Bi/ 210Pb, 210Po/ 210Pb and 222Rn/ 210Pb, activity ratios in air are presented. The data of Table 3 admit residence times of lower values as low as 1.59 days and higher values as high as 320 days. Mostly, the lower tR values were resulted from the 210Bi/ 210Pb activity ratios and higher tR values from the 210Po/ 210Pb activity ratios. Low tR values were also resulted from the 222Rn/ 210Pb activity ratios. Poet et al. (1972) concluded that longer apparent residence time values for tR based on the 210Po/ 210Pb activity ratios, than for tR based on the 210Bi/ 210Pb activity ratios. The difference may be explained by the presence of a mixture of aerosols of various apparent ages, of which older aerosols (those of greater age) contribute most of 210Po. On the other hand, the solid products of radon-222 decay, that is 210Pb, 210Bi and 210Po might arise from sources other than radioactive decay within the atmosphere. Poet et al. (1972) showed that up to 85% of 210Po in the atmosphere is of terrestrial origin, and the vertical profile of 210Po was found to differ appreciably from that expected from the decay of 222Rn. They also found that the mean aerosol residence time increases with altitude within the troposphere less than by a factor of 3 (Moore et al., 1973). Lambert et al. (1979) indicated that volcanic gases are very rich in long-lived radon decay products, especially in 210Po relative to 210Pb. Soil particles are the most likely contributors, since a part of the tropospheric aerosols originates at the Earth’s surface. Coal burning and forests fires presumably are additional sources of radionuclides
References
C. Papastefanou / Applied Radiation and Isotopes 64 (2006) 93–100
Table 3 Residence times of tropospheric aerosols derived from
98
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resulted by the application of two different methods, i.e. the 7Be-associated atmospheric aerosols and the radon decay product aerosols at two different locations, i.e. at Thessaloniki, Greece 401380 N 221580 E with dry (precipitation-free) climate and at Oak Ridge, TN, USA 351580 N 841170 W with high precipitation (wet climate), roughly at similar temperate latitudes, but the first one at East longitude and the other at West longitude, respectively. It has been reported that the residence time of atmospheric aerosols in the lower troposphere below precipitation cloud level increases with the tropospherere by a factor of 3 or less and there is a dependence on the latitude.
References Ahmed, A.A., Mohammed, A., All, A.A., El-Hussein, A., Barakat, A., 2000. A study on aerosol residence time in ElMinia, Egypt. J. Aerosol Sci. 31 (S1), 470–471. Balkanski, P.J., Jacob, D.J., Gardner, G.M., Graustein, W.C., Turekian, K.K., 1993. Transport and residence times of tropospheric aerosols inferred from a global three-dimensional simulation of Pb. J. Geophys. Res. 98 (011), 20573–20586. Baskaran, M., Shaw, G.E., 2001. Residence time of arctic haze aerosols using the concentrations and activity ratios of 210Po, 210Pb and 7Be. J. Aerosol Sci. 32, 443–452. Bondietti, E.A., Brantley, J.N., 1986. Characteristics of Chernobyl radioactivity in Tennessee. Nature 322, 313–314. Dibb, J.E., Jaffrezo, J-L., 1993. Beryllium-7 and Lead-210 in aerosol and snow in the Dye 3 Gas, Aerosol and Snow Sampling Program. Atmos. Environ. 27A, 2751–2760. Ehhalt, D.H., 1973. Turnover times of 137Cs and HTO in the troposhere and removal rates of natural particles and water vapor. J. Geophys. Res. 78, 7076–7086. Francis, C.W., Chesters, G., Haskin, L.A., 1970. Determination of 210Pb mean residence time in the atmosphere. Environ. Sci. Technol. 4 (7), 586–589. Fry, L.M., Menon, K.K., 1962. Determination of the tropospheric residence time of lead-210. Science 137, 994–995. Ga¨ggeler, H.W., Jost, D.T., Baltensperger, U., Schwikowski, M., 1995. Radon and thoron decay product and 210Pb measurements at Jungfraujoch, Switzerland. Atmos. Environ. 29 (5), 607–616. Gavini, K.B., Beck, J.N., Kuroda, P.K., 1974. Mean residence times of the long-lived radon daughters in the atmosphere. J. Geophys. Res. 79, 4447–4452. Jaworowski, Z., 1963. The determination of lead-210 and bismuth-210 in biological materials. Nukleonika 8 (5), 333–340. Junge, C.E., 1963. Air Chemistry and Radioactivity. Academic Press, New York. Koch, D.M., Jacob, D.J., Graustein, W.C., 1996. Vertical transport of tropospheric aerosols as indicated by 7Be and 210Pb in a chemical tracer model. J. Geophys. Res. 101 (013), 18651–18666. Lambert, G., Buisson, A., Sanak, J., Ardouin, B., 1979. Modification of the atmospheric polonium-210 to lead-210
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ratio by volcanic emissions. J. Geophys. Res. 84 (011), 6980–6986. Lambert, G., Polian, G., Sanak, J., Ardouin, B., Buisson, A., Jegou, A., Leroulley, J.C., 1982. Cycle du radon et de ses descendants: Application a 1’etude des echanges troposphere-stratosphere. Ann. Geophys. 38, 497–531. Lambert, G., Sanak, J., Polian, G., 1983. Mean residence time of submicrometer aerosols in the global troposphere. In: Pruppacher, H.R., Semonin, R.G., Slinn, W.G.N. (Eds.), Precipitation Scavenging, Dry Deposition and Resuspension. Elsevier, New York, pp. 1353–1359. Lehmann, L., Sittkus, A., 1959. Bestimmung von Aerosolverweilzeiten aus den RaD und RaF-Gehalt der atmospharischen Luft und des Niederschlages. Naturwissenschaft 46, 9–10. Marley, N.A., Gaffney, O.S., Drayton, P.J., Cunningham, M.M., Orlandini, K.A., Paode, R., 2000. Measurement of 210Pb, 210Po and 210Bi in size-fractionated atmospheric aerosols: An estimate of fine-aerosol residence times. Aerosol Sci. Technol. 32, 569–583. Martell, E.A., 1970. Transport patterns and residence times for atmospheric trace constituents vs altitudes. In: Radionuclides in the Environment. ACS Symposium Series No.93, American Chemical Society, Washington, DC, pp. 138–157. Martell, E.A., Moore, H.E., 1974. Tropospheric aerosol residence times: A critical review. J. Rech. Atmos. 8, 903–910. McMurry, P.H., Wilson, J.C., 1982. Growth laws for the formation of secondary ambient aerosols: implications for chemical conversion mechanisms. Atmos. Environ. 16, 121–134. Moore, H.E., Poet, S.E., Martell, E.A., 1972. Tropospheric aerosol residence times indicated by radon and radondaughter concentrations. In: Adams, J.A.S., Lowder, W.M., Gessel, T.F. (Eds.), Natural Radiation Environment II. CONF-720805-P2. Technical Information Center/US Department of Energy, Washington, DC, pp. 775–786. Moore, H.E., Poet, S.E., Martell, E.A., 1973. 222Rn, 210Pb, 210Bi and 210Po profiles and aerosol residence times Versus altitudes. J. Geophys. Res. 78 (30), 7065–7075. National Research Council (NRC), 1979. Airborne Particles. University Park Press, Baltimore, USA. Peirson, D.H., Cambray, R.S., Spicer, G.S., 1966. Lead-210 and polonium-210 in the atmosphere. Tellus 18, 427–433. Poet, S.E., Moore, H.E., kartell, E.A., 1972. Lead-210, bismuth-210 and polonium-210 in the atmosphere: accurate ratio measurement and application to aerosol residence time determination. J. Geophys. Res. 77 (33), 6515–6527. Porstendo¨rfer, J., Gru¨ndel, M., 2003. Comparison of the activity size distribution of the radionuclide aerosols in outdoor air. In: Dresden Symposium on Radiation Protection, 3–7 March 2003, Dresden, Germany. Book of Abstracts, Dresden University of Technology, Dresden, Germany, p. 24. Rangarajan, C., 1992. A study of the mean residence time of the natural radioactive aerosols in the planetary boundary layer. J. Environ. Radioactivity 15, 193–206. Reineking, A., Porstendo¨rfer, J., 1995. Time variations of size distribution of aerosol-attached activities of 212Pb, 210Pb and 7Be in the outdoor atmosphere. In: Natural Radiation
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