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Journal of Environmental Radioactivity 102 (2011) 500e513
Contents lists available at ScienceDirect
Journal of Environmental Radioactivity
journal homepage: www.elsevier.com/locate/jenvrad
Po-210 and Pb-210 as atmospheric tracers and global atmospheric Pb-210
fallout: a Review
M. Baskaran*
Department of Geology, Wayne State University, Detroit, MI-48202, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 20 May 2010
Received in revised form
24 September 2010
Accepted 17 October 2010
Available online 18 November 2010
Over the past w5 decades, the distribution of 222Rn and its progenies (mainly 210Pb, 210Bi and 210Po) have
provided a wealth of information as tracers to quantify several atmospheric processes that include: i)
source tracking and transport time scales of air masses; ii) the stability and vertical movement of air
masses iii) removal rate constants and residence times of aerosols; iv) chemical behavior of analog
species; and v) washout ratios and deposition velocities of aerosols. Most of these applications require
that the sources and sink terms of these nuclides are well characterized.
Utility of 210Pb, 210Bi and 210Po as atmospheric tracers requires that data on the 222Rn emanation rates
is well documented. Due to low concentrations of 226Ra in surface waters, the 222Rn emanation rates
from the continent is about two orders of magnitude higher than that of the ocean. This has led to
distinctly higher 210Pb concentrations in continental air masses compared to oceanic air masses. The
highly varying concentrations of 210Pb in air as well the depositional fluxes have yielded insight on the
sources and transit times of aerosols. In an ideal enclosed air mass (closed system with respect to these
nuclides), the residence times of aerosols obtained from the activity ratios of 210Pb/222Rn, 210Bi/210Pb, and
210
Po/210Pb are expected to agree with each other, but a large number of studies have indicated
discordance between the residence times obtained from these three pairs. Recent results from the
distribution of these nuclides in size-fractionated aerosols appear to yield consistent residence time in
smaller-size aerosols, possibly suggesting that larger size aerosols are derived from resuspended dust.
The residence times calculated from the 210Pb/222Rn, 210Bi/210Pb, and 210Po/210Pb activity ratios published
from 1970’s are compared to those data obtained in size-fractionated aerosols in this decade and possible
reasons for the discordance is discussed with some key recommendations for future studies.
The existing global atmospheric inventory data of 210Pb is re-evaluated and a ‘global curve’ for the
depositional fluxes of 210Pb is established. A current global budget for atmospheric 210Po and 210Pb is also
established. The relative importance of dry fallout of 210Po and 210Pb at different latitudes is evaluated.
The global values for the deposition velocities of aerosols using 210Po and 210Pb are synthesized.
Ó 2010 Elsevier Ltd. All rights reserved.
Keywords:
Radon exhalation rate
Global Pb-210 fallout
Residence time of aerosols
Global mass balance for Pb-210 and Po-210
Deposition velocities of aerosols
Washout ratios
1. Introduction
There are more than 30 radionuclides that are produced in the
atmosphere, with half-lives ranging from less than a second (214Po)
to 1.4 106 yrs (10Be). Of these, the daughter products of 222Rn
include 11 isotopes (7 from the major decay pathway; Fig. 1) that are
continuously produced in the atmosphere. Of the 6 elements (Po, Bi,
At, Tl, Hg and Pb) that are produced in the decay of 222Rn, the
branching ratio of the decay that goes through 214Hg, 206Tl and 210Tl
are small (<0.1%, Fig. 1). More than 99% of the 222Rn in the
atmosphere are derived from emanation mainly from the
continents. Once the 222Rn escapes from the rocks and minerals in
* Tel.: þ1 313 577 3262.
E-mail address: [email protected].
0265-931X/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.jenvrad.2010.10.007
the Earth’s upper crust, it embarks on its journey in the atmosphere
via diffusion and advection. On its pathway, some of the 222Rn
undergoes radioactive decay. The atmospheric 222Rn is not removed
by either physical or chemical means due to its inert nature. The
mean life of 222Rn (s ¼ 5.53 days) is generally comparable to the
transit time of air masses across major continents and/or ocean but
much shorter compared to the mixing time scale of the atmosphere
and hence, it is widely dispersed in the atmosphere. The activities of
the long-lived daughters, 210Pb, 210Bi and 210Po in the atmosphere to
a large extent are governed by their production rates, rates of decay
and removal by scavenging by atmospheric aerosols.
Radon-222 and its progenies in the atmosphere have been
widely utilized as powerful tracers to quantify atmospheric
processes that include: i) source tracking and transport (within and
between troposphere and stratosphere) time scales of air masses,
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M. Baskaran / Journal of Environmental Radioactivity 102 (2011) 500e513
501
Fig. 1. Radon-222 and its decay chain.
including the stability and vertical movement of air masses;
ii) removal rate constants and residence times of aerosols; iii)
deposition velocities and washout ratios of aerosols; iv) sources of
continental dust in an air mass; v) flux to and exchange between
environmental systems of other gaseous species (e.g., CH4, Hg0);
and vi) processes of attachment of metal ions to atmospheric
aerosols (e.g., Feely and Seitz, 1970; Prospero and Carlson, 1970;
Bressan et al., 1973; Turekian et al., 1977, 1989; Wilkniss and
Larson, 1984; Moriizumi et al., 1996; Turekian and Graustein,
2003; Obrist et al., 2006; Church and Sarin, 2008 and the references cited). Furthermore, some of these nuclides (e.g., 210Pb) can
serve as a surrogate for other atmospheric components (e.g.,
atmospheric sulfate, its precursor SO2 is a gas similar to 210Pb’s
precursor 222Rn; e.g., Mattsson, 1988; a large amount of stable Pb
was injected into the atmosphere in the past, from fossil fuel
combustion and use of leaded gasoline, as a volatile compound;
mercury which exists as a gaseous element; hydrocarbons). Most of
these applications require that the sources and sink terms of these
nuclides are well characterized.
In this article, we review the utility of 210Po and 210Pb as
atmospheric tracers. The global database for the depositional fluxes
of atmospheric 210Pb is evaluated and a ‘global fallout’ curve for
210
Pb is established. The factors and processes that cause spatial
and temporal variations of 222Rn and 210Pb fluxes are addressed.
The residence times calculated on bulk and size-fractionated
aerosols are compared and the possible causes for the discordance
of residence times obtained from different pairs (210Pb/222Rn,
210
Bi/210Pb and 210Po/210Pb) are discussed. We evaluate the importance of resuspended dust and how its contribution can lead to the
discordant residence time of aerosols obtained between 210Bi/210Pb
and 210Po/210Pb pairs.
2. A Brief review of sampling and analysis of
210
Po and
210
Pb
2.1. Sampling
For the bulk air sampling, large-volume pump with filters have
been commonly used. For the measurements of 210Pb and 210Po in
air, air filters such as Whatman 41 filters have been commonly
used. Although widely used, Whatman filters do not quantitatively
collect the sub-micrometer aerosols containing 210Pb and 210Po.
Collection efficiency of Whatman 41 filters for 210Pb in the marine
boundary layer has been found in the range of 70e80%, with w25%
of the 210Pb in the first filter was found in the second filter (back-up
filter) (Stafford and Ettinger, 1972; Turekian and Cochran, 1981;
Turekian et al., 1989). This is in contrast to a paired Gelman Type
A filter which has 99þ % capture efficiency for submicron aerosols
(Turekian and Cochran, 1981). Thus, when a single Whatman filter
is used, the activity of 210Pb and likely 210Po is about 20e30% lower
than the absolute concentrations. Problems associated with impact
collectors in collecting size-fractionated aerosols have been
encountered. Particle bounce-off and re-entrainment of particles
have been observed to cause overlap in particle collection efficiency
between successive stages of the impactor (Burton et al., 1973;
Dzubay et al., 1976). In the collection of precipitation samples, the
precipitation collector should be acidified to prevent the loss of
210
Po and 210Pb onto the surfaces of the collector.
2.2. Analysis of samples
210
Po analysis is commonly conducted using alpha spectrometric
methods while 210Pb can be conducted by alpha (via its granddaughter, 210Po), beta, or gamma spectrometric methods. The
advantage of alpha spectrometry is the high-energy resolution
(18e20 keV for an active area of 450 mm2, with a background of w6
counts per day; 0.8 to 2 counts per day in the 200 keV envelop area
for 209Po, or 208Po or 210Po peaks), low background (less than
0.05 counts/hr/cm2) in the energy range of 3e8 MeV. Many of the
currently used solid-state detectors (such as PIPS detectors, Canberra Inc) have a minimum active thickness of greater than 140 mm
which is sufficient for full absorption of alpha particles of up to
15 MeV, with a background of 3 to 10 counts per day in the 210Po or
209
Po or 208Po envelop region (in an envelope area of w200 keV,
corresponding to 0.038e0.115 mBq), and relatively high absolute
efficiency (w30%) and these features make a-spectrometric technique attractive for 210Po assay. When Fe3þ, Cr6þ, and other oxidants
are not reduced to lower valence states, they often get plated and
results in thick sources with poor plating efficiency and low resolution. Often in such cases, 210Po (5.304 MeV) and 209Po (4.881 MeV)
cannot be resolved (or 210Po and 208Po (5.115 MeV)). The minimum
detectable activity for 210Pb via 210Po by alpha spectrometry is
affected by the level of chemical (or reagent) blank which arises from
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M. Baskaran / Journal of Environmental Radioactivity 102 (2011) 500e513
contamination from reagents, and sample and source preparation
steps. For relatively new alpha detectors, the background of
0.03e0.09 counts per hour (cph) corresponds to 0.03e0.08 mBq
(assuming detector efficiency of 30%; Baskaran, in press).
210
Pb measurements can also be made via its beta-emitting
daughter, 210Bi. Beta counting is not isotopic-specific and hence
other beta emitting radionuclides can interfere. Beta particles have
a continuous energy spectrum (zero energy up to a maximum
energy Emax). Often mylars (absorbers) are used to absorb 210Pb
betas (Emax ¼ 0.0635 MeV) and 210Bi betas (Emax ¼ 1.163 MeV) are
used to assay 210Pb activity. Multiple counting will ensure the
reliability of the data. For example, the gas-flow anti-coincidence
counter with 10 cm lead shielding Risø Counter (manufactured by
Risø National Laboratory, Roskilde, Denmark), gives a background
value of 0.15e0.20 count per minute (cpm) for a source of 25 mm
diameter. The absolute efficiency is w40%. 0.20 cpm background
will correspond to w8 mBq (assuming counting efficiency of 40%)
and hence detection limit by beta counting > an order of magnitude higher than that of alpha counting method.
Gamma-ray spectrometry has been utilized to measure 210Pb
using the 46.5 keV energy peak in aerosols and precipitation (e.g.,
McNeary and Baskaran, 2007). The sensitivity by gamma counting
is significantly lower than that of the alpha and beta counting
methods, mainly due to low branching ratio (4.5%). The distinct
advantage is that this method is non-destructive and thus there is
no need for a spike. Sample preparation is straight forward and
many other nuclides (such as 214Bi, 212Pb and 214Pb) can be simultaneously measured. The absolute efficiency of the gamma detector
also depends on its geometry (planar, closed-end coaxial and welltype). In a well detector, the sample in low volume (well-depth of
40 mm corresponds to w4 ml) is surrounded by germanium crystal
and hence the geometrical counting efficiency is high. Overall, for
small samples, the well detectors provide the best overall sensitivity compared to all the existing gamma-ray detectors. Attempts
have been made to measure 210Pb by ICP-MS, but isobaric and
molecular ionic interferences (209Bi1H, 208Bi1H2, 194Pt16O, 198Hg12C,
etc) are major impediments that need to be overcome (Huh and
Roos, 2008; Baskaran, in press). Furthermore, the atomic abundances of 210Pb are very low (detection limit in alpha spectrometry
is 0.1 mBq ¼ 1.02 105 atoms) and hence mass spectrometric
methods have lower precision than alpha spectrometric method
(Baskaran et al., 2009).
2.3. Determination of in-situ
210
Po and
210
Pb activities
One of the most commonly used methods to measure 210Po and
Pb is by the alpha spectrometry. In a typical aerosol or precipitation samples, the activities of 210Po activities are low compared to
210
Pb (with 210Po/210Pb < 0.1) and hence the timely separation of
210
Po and 210Pb as soon as possible after collection is crucial.
Calculation of the in-situ 210Po activity involves the following
corrections: i) background subtraction for the alpha spectrum for
both 209Po (added as a yield tracer; details in Mathews et al., 2007;
IAEA, 2009) and 210Po regions; ii) decay of 210Po from the time of
plating to mid-counting; iii) decay of 210Po from the time of
collection to plating; iv) in-growth correction for 210Po from the
decay of 210Pb via 210Bi; and v) decay correction for 209Po (from the
time of last assay to the time of counting). When both 210Po and
210
Pb are measured in the same sample, quantitative separation of
210
Po and 210Pb after the first plating (in-situ 210Po) is essential.
Electroplating does not quantitatively remove all Po and residual Po
(209Po and 210Po) will affect the results. For example, leaving the
solution for about a year will result in 84% of residual 210Po to decay
away, but only <1% of the residual 209Po will decay away and hence
the residual 209Po will affect the calculation of 210Pb. Electroplating
210
with additional Ag strips does not ensure removal of residual Po
from the solution. Hence the ion-exchange separation of Po and Pb
is strongly recommended. Another way to circumvent this problem
is by adding 208Po spike after the first plating. Keeping the solution
for about 2 years will ensure that about >98% of the residual 210Po
will decay away. If the stored solution is analyzed before significant
decay of residual 210Po, one can calculate the amount of residual
210
Po from the 209Po/210Po ratio in the first plated sample and the
amount of 209Po in the second plated sample (assuming that no
209
Po was added after the first plating and only 208Po was added)
and can make corrections appropriately. The corrections for the ingrowth of the 210Po and decay of 210Po and 210Pb during the time
elapsed between: i) collection e first plating e 9M HCl ionexchange column separation e ii) second plating e counting of the
Ag plates needs to be applied. Using standard Bateman’s equations,
explicit correction terms can be established for routine procedure.
Calculation of the 210Pb activity involves the following corrections: i) background subtraction for the alpha spectrum for both
209
Po and 210Po regions; ii) decay of 210Po from the time of second
plating to mid-counting; iii) in-growth factor for 210Po from 210Pb;
iv) chemical yield factor for stable Pb; iv) decay correction factor for
the decay of 209Po from the last time it was assayed to the time of
plating; and v) decay correction for 210Pb from time of collection to
2nd plating. The decay of 210Po from the time of plating to midcounting is done from the background subtracted net counts.
3. Synthesis of earlier results and observations
3.1. Radon flux to and distribution in the atmosphere
A large body of data exists on the 222Rn flux data obtained both
by direct measurements (measured directly from the collection of
air samples containing 222Rn emitted from a known area of soil)
and from the integration of 222Rn profiles in the atmosphere by
estimating the flux and assuming balance between 222Rn emanation and decay (summarized in Robbins, 1978; Turekian et al., 1977,
2003; Church and Sarin, 2008). Each of these methods have their
own limitations (such as direct measurements are from very
narrow area and assumption of steady state between emanation
and decay is questionable when maritime air mass intrudes
continental air mass, such as maritime tropical air mass mixing
with continental and continental polar air masses). The radon flux
from mineral grains is highly variable. When 226Ra undergoes
radioactive decay in a mineral grain, the parent nuclide, 222Rn,
undergoes recoil and hence it is displaced from its original location.
The recoil distance is w200 Å and hence a significant portion of
the 222Rn located within the outer skin of 200 Å will escape
from the mineral grain. However, in a typical 100 micron size grain,
the amount of 222Rn escaped should be less than 1%, but in many
cases, the radon emanation rate exceeds 10% (summarized in
Garver and Baskaran, 2004). In order to explain this, various
mechanisms have been proposed, but none of those satisfactorily
explain this experimental observation. For example, existence of
nanopores within the mineral was proposed. The nanopores form
a network of channels within the mineral and radon diffuse
through this network (Rama and Moore, 1984). Subsequent
experiments with Ar did not support the nanopore hypothesis and
rather the high emanation rate of 222Rn was attributed to the
presence of U and its daughter products in accessory minerals in
grain boundaries (Krishnaswami and Seidmann, 1988).
About 10% of the 222Rn produced from the decay of 226Ra in the
upper 1 m of the soil is released to the atmosphere (Turekian et al.,
1989). The mean diffusion length of 222Rn in soil water is w1 mm
which is about 1000 times smaller than that in the air and hence in
regions where there is frequent precipitation and the pore spaces of
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M. Baskaran / Journal of Environmental Radioactivity 102 (2011) 500e513
the soil are saturated, the 222Rn emanation is expected to be suppressed. The average concentration of 226Ra in the upper crust is
w31 mBq g1 (assuming 238U and 226Ra are in secular equilibrium;
238
U concentration ¼ 2.5 ppm; Wedepohl, 1995) while the
concentration in surface ocean water is w1.2 103 mBq g1
(average value (weighted for surface area) calculated based on
226
Ra activities in the coastal waters (3.3 mBq g1), Pacific Ocean
(1.07 mBq g1) and Atlantic Ocean (1.23 mBq g1); average values
for Pacific and Atlantic are summarized in Cochran (1992). Note
that the average oceanic value in surface waters given in this article
is significantly lower than the value reported and used by earlier
workers (e.g., Fukai and Yokoyama, 1982; Carvalho, 1995). It has
been estimated that the global 222Rn flux from continent ranges
from 1300 to 1800 Bq m2 d1, while w17 Bq m2 y1 is reported
for the oceanic areas (Samuelsson et al., 1986; Nazaroff, 1992).
Assuming an average radon escape rate of 1 atom cm2 s1 from the
land surface, Turekian and Graustein (2003) estimated the total
inventory of 222Rn in the atmosphere to be w1.5 1018 Bq.
The long-term global 222Rn fluxes from continents range from
0.0056 atom cm2 s1 to 0.63 atom cm2 y1 (summarized in
Table 1, Turekian et al., 1977). Using the inventory of 210Pb in
a number of soil profiles in the continental United States, higher
222
Rn fluxes, 1.5e2.0 atoms cm2 s1, have also been reported
(Turekian and Graustein, 2003). The radon release rates from soils
depends on a number of factors including the concentrations of
226
Ra, and their distribution in mineral grains, physical properties
of the mineral grains, porosity and water content of the soil (which
implies the frequency and amount of precipitation), atmospheric
pressure and wind velocity. The weekly mean emanation rates of
222
Rn between 65 and 70 N in summer was higher by a factor of
2.5 than the winter and this was attributed to snow and soil
moisture dominating the winter time and significantly drier
conditions in summer (Szegvary et al., 2009). The 222Rn emanation
rates decrease with increasing latitude, from 1 atom cm2 s1 at
30 N to 0.2 atom cm2 s1 at 70 N, and was attributed to the
increased water saturation in the soils with increasing latitude
(Conan and Robertson, 2002). High radon emanation rates close to
Japan and a relatively low flux across the rest of the North Pacific
including the California coast have been reported. High radon
3.2. Concentrations of
atmosphere
210
Po and
210
Pb in surface air and upper
Since the first measurements of 210Po, 210Bi and 210Pb in aerosols
by Stewart and Burton and Stewart (1960), a number of studies have
been conducted. In both continental and marine setting, the 210Pb
concentration in air varies widely. The factors that affect the 210Pb
concentrations in air include the seasons, atmospheric pressure
variations that affect the sources of air masses as well as local radon
emanation rates, height of the atmospheric boundary layer,
temperature inversions, diurnal and seasonal variations of meteorological parameters, soil moisture content (which affect the
210
210
Pb flux around the globe.
Pb Flux
(mBq cm2 y1)
Number
of sites
Nature of samples
North
0e10
10e20
20e30
30e40
40e50
50e60
60e70
70e80
15.8 11.3
13.3 8.3
19.5 11.0
16.0 9.7
15.5 7.5
9.3 4.5
3.7 3.5
2.0 1.5
3
3
9
33
45
10
9
10
1 Coa, 1O, and 1 C
1 Coa, 2 C
4 Coa, 5 C
14 Coa, 10 C, and 9 Coa/C
6 Coa, 26 C, and 13 Coa/C
5 Coa, 4 C
2 Coa, 4 C, 3 IC
1 Coa, 2 C, 7 IC
South
0e10b
10e20
20e30
30e40
40e50
50e60
60e70
70e80
80e90
11.7
6.7
6.5 2.7
6.1 0.2
4.2
3.3
0.63 0.22
0.21 0.12
0.18 0.08
1
2
5
5
2
1
1
7
1
Latitude
emanation rates in Japan and the regions of the ‘ring of fire’ could
be due to large number of micro and macro earthquakes (>70% of
the global earthquakes) that could allow higher escape of 222Rn.
Recent studies have shown that the submarine groundwater
discharge (SGD) is relatively high in coastal waters (Burnett et al.,
2006). In most of the groundwater, 222Rn concentrations are 1e4
orders of magnitude higher than that in the surface waters and
hence 222Rn discharged through the SGD also could be a significant
source of 222Rn to the atmosphere.
It is known that Rn is not removed by precipitation scavenging
while its daughter products are. The vertical profiles of 222Rn in the
atmosphere are shown in Fig. 2 (data taken from Moore et al., 1977).
Highest concentrations of 222Rn are most commonly found in the
continental boundary layer (3e8 Bq m3) while it decreases by an
order of magnitude, to w40 mBq m3 near the tropopause (Moore
et al., 1977; Liu et al., 1984; Kritz et al., 1993). Large variations in
222
Rn concentrations in surface air have been widely reported,
depending of the sources of air mass. For example, Carvalho (1995)
reported 222Rn variations of about 2e3 orders of magnitude
(0.02 Bq m3 to w10 Bq m3) near Lisbon, Portugal, and was
attributed to changes in the relative fraction of air masses derived
from continental Europe and maritime air mass from the Atlantic
Ocean. The troposphere height changes with season at any given
place and from place to place. The tropopause serves as a barrier to
the Rn gas, except during strong convective updrafts when some of
the 222Rn can reach the lower stratosphere leading to production of
210
Pb.
16
TP-9/28
1C
2 Co
4 Co, 1 C
5 Co
2 CI
1O
IC
IC
IC
Coa: Coastal; C: Continental; O: Ocean; IC: Ice core/sheet; Only the coastal and
continental samples are included; however, in areas where the number of samples
are limited (n < 3), the ocean samples (when available) were included.
a
Data are taken from Preiss and Genthon (1997) and Baskaran and Naidu, 1995.
b
Data is from the 222Rn flux calculated for April and May (Trumbore et al., 1990).
Height from Sea Level (km)
Table 1
Compilationa of long-term
503
TP-9/13
12
TP-4/23
TP-11/1
8
4
9/28/72
4/24/73
9/13/73
1/11/73
0
0
10
20
222
30
40
3
Rn (Bq/100 m )
Fig. 2. Temporal changes in the vertical profiles of 222Rn in the atmosphere. TP stands
for tropopause. Tropopause serves as a barrier where the 222Rn activity drops off. Data
are taken from Moore et al. (1977).
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M. Baskaran / Journal of Environmental Radioactivity 102 (2011) 500e513
-3
Pb Concentration in Air (mBq m )
0.25
210
emanation rates), frequency and amount of precipitation, presence
of snow cover, etc. The long-term temporal variations of 210Pb in
aerosols collected from Detroit over a period of 18 months (weekly to
monthly sampling) varied over an order of magnitude, from
0.30 mBq m3 to 4.22 mBq m3 (Fig. 3), with a mean of 1.2 mBq m3
(n ¼ 30; McNeary and Baskaran, 2003). Monthly 210Pb concentrations measured over a period of 12 years in Belgrade (Central Serbia)
varied between 0.30 and 3.17 mBq m3. In Southeastern Spain the
210
Pb concentration varied between 0.28 and 0.92 mBq m3, with
a geometric mean of 0.52 mBq m3 and a large number of average
values fall within a range of 0.2 to 1.2 mBq m3 (0.58 mBq m3 in
New Haven, Connecticut, Turekian et al., 1983; 0.45 mBq m3 om
Tsukuba, Japan, Sato et al., 1994; 0.74 mBq m3 in Palermo, Italy,
Cannizaro et al., 1999; 0.57 mBq m3 in South Germany, Winkler and
Rosner, 2000; 0.62 mBq m3 in Grenade, Spain, Azahra et al., 2004b;
1.15 mBq m3 in Detroit, USA, McNeary and Baskaran, 2003). In
Lisbon, Portugal the 210Pb concentration varied between 0.036 and
0.524 mBq m3, with the lowest values corresponding to oceanic air
masses (Carvalho, 1995). High 210Pb activities have been reported in
Milan, Italy when prolonged anti-cyclonic conditions with high air
temperature existed (Vecchi et al., 2005). A significant correlation
between the maximum temperature and pressure and 210Pb
concentration in air has been reported (Dueñas et al., 2005). 210Pb
concentrations at Svalbard varied between 0.011 and 0.62 mBq m3
during 2001 and are lower than stations in the northern Finland. The
variations in 222Rn concentrations were attributed to the air masses
derived from the North Atlantic Ocean, Greenland and the Canadian
Arctic (Paatero et al., 2003). The concentrations of 210Pb measured
over a period of 11 days (11e21 September 2008) in Bermuda, which
is w1300 km away from the nearest landmass, varied between
0.063 mBq m3 to 0.204 mBq m3, by a factor of 3.2 (Fig. 4). Although
the 222Rn emanation rate from the ocean surface remains constant,
such variations in the 210Pb is attributed to variations in the continent-derived dust input at this oceanic site. The 210Pb concentration
in aerosols collected from the North Pacific was found to be w2 times
higher than that of the South Pacific, reflecting higher land/sea ratio
in the Northern Hemisphere compared to the Southern Hemisphere.
Generally, the concentrations of 210Pb in aerosols are high in
winter due to low mixing height and low in the summer due to
efficient mixing of the troposphere caused by solar heating. (e.g.,
Paatero et al., 1998). Based on 30-year (1967e1996) long series of
daily 210Pb measurements in Finland, Paatero et al. (1998) observed
that daily variations are due to synoptic-scale weather situations
while the seasonal variations are attributed to the efficiency of
vertical mixing of the lower atmosphere. However, there are sites
where higher 210Pb in the summer months have also been reported
(e.g., in Spain, Garcia-Talavera et al., 2001; Azahra et al., 2004a;
Dueñas et al., 2005, 2009).
The specific activities in aerosols (Bq g1 aerosols) are one to
two orders of magnitude higher than that found in the top soils. For
example, the specific activities of 210Pb in aerosols collected in
0.2
0.15
0.1
0.05
Sep/9
Sep/11
Sep/13
Sep/15
Sep/17
Sep/19
Sep/21
Date
Fig. 4. Temporal variations of 222Pb in air over a period of 11 days in Bermuda, which is
w1300 km from the nearest land. The concentration varies by a factor of 3.2 and is
attributed to highly varying dust input to Bermuda.
Detroit over 18-months period varied between 2.7 Bq g1 to
23.0 Bq g1 (McNeary and Baskaran, 2003). The efficiency of 210Pb
entrainment in to the atmospheric aerosols is very high and hence
the specific activity of 210Pb in aerosols (e.g., 210Pb activity/g of
aerosols) is usually far greater than other material commonly
found, such as Al (Turekian et al., 1989). For example, 210Pb/Al ratio
in aerosols collected in the SEAREX program was found to be 104 to
105 times higher than that in the soils (Turekian et al., 1989). The
size-fractionated aerosols collected using a 5-stage impact collector
showed that 83% of the total 210Pb was associated with <0.95 mm
(Baskaran and Church, unpublished data). This is similar to the
observation reported by Martell and Moore (1974) that 90% of 210Pb
in aerosols is associated with particles 0.3 mm.
Concentrations of 210Pb and 210Po in two vertical profiles, one
from Limon, Colorado and the other from Scottsbluff, Nebraska
showed that the vertical concentration profiles vary widely (Fig. 5).
The activities of 210Pb in the upper troposphere remain relatively
constant within a factor of w2 (data from Moore et al., 1973). It has
14
Height from Earth's surface (km)
504
12
10
210
Pb-Limon, CO
210
Po-Limon, CO
210
Pb-Nebraska
210
Po-Nebraska
8
6
4
2
0
0
0.2
210
Pb or
210
Fig. 3. Temporal Variations of Pb in air over a period of 18 months (October 1999 to
March 2001) in Detroit, MI, USA (plot is taken from McNeary, 2002).
210
0.4
210
0.6
0.8
-3
Po Activity (mBqm )
210
Fig. 5. Vertical profiles of
Po and
Pb in air from Limon, Colorado (April 2, 1969)
and Scottsbluff, Nebraska (January 19, 1971). Data are taken from Moore et al. (1973).
Author's personal copy
M. Baskaran / Journal of Environmental Radioactivity 102 (2011) 500e513
been shown that the stratospheric 210Pb concentration varies very
little, with a mean stratospheric activity of 0.3 mBq m3 while it is
concentration in the upper troposphere is much lower
(w0.04 mBq m3; Lambert et al., 1982; Moore et al., 1973, 1977).
This is similar to the observation for 222Rn in the lower stratosphere
and upper troposphere (Kritz et al., 1993). Due to infrequent
precipitation in the upper troposphere (because cloud condensation heights are below the upper troposphere), the residence times
of aerosols are longer in the free troposphere above the cloud cover
than in the atmospheric boundary layer and hence the concentrations of 210Pb are expected to be higher in the free troposphere
above the cloud cover, in particular near the tropopause. The
variations in the 210Pb concentrations in the lower stratosphere
could result from the rates of vertical transport processes (Feely and
Seitz, 1970).
Concentrations of 210Po in surface air vary widely, ranging from
0.01 to 0.26 mBq m3 (Moore et al., 1973; Talbot and Andren, 1983;
McNeary and Baskaran, 2007). Kim et al. (2005) reported excess
210
Po in Seoul (S. Korea) and attributed this excess to anthropogenic
sources associated with burning and incineration processes.
3.3. Weighted concentrations of
210
Pb and
210
Po
A compilation of the published data of the volume-weighted
concentrations of 210Pb (¼ annual 210Pb activity deposited/annual
precipitation, Bq L1) given in Tables 2 and 3, shows that the values
vary within a factor of w6, although world-wide distribution of
210
Pb in rain (including polar regions) vary over a factor of 20
(Rangarajan et al., 1976). Variations in the activities of 210Pb by
a factor of >100 in individual rains are common. However, from the
data presented in Table 2, certain features emerge. The lowest value
is found in the west coast of USA where the amount of precipitation
is low and the depositional flux is predominantly controlled by high
fluxes in 2e3 months in a given year (Fuller and Hammond, 1983).
The maritime air mass from the Pacific brings 210Pb-depleted air
resulting in lower volume-weighted concentrations of 210Pb. The
island site Bermuda also has very low value due to dominant
maritime air mass. At different sites in the western Mediterranean
Sea, the values vary over a factor of 2, again mainly due to varying
inputs of continental and oceanic air masses. The values reported
for Ahmedabad (India), a semi-arid region where the dry fallout is
quite significant, is also low, 0.075 Bq L1 (average). Detroit (USA) is
a continental site and has the highest values, and this is attributed
to the frequency of precipitation (McNeary and Baskaran, 2003). A
plot of the volume-weighted concentrations of 210Pb and amount of
precipitation did not show any correlation, affirming the hypothesis
that the relative fraction of continental air mass and oceanic air
mass along with amount and frequency of precipitation determine
the volume-weighted concentrations of 210Pb.
The volume-weighted concentration of 210Po during the 18months period in rain samples collected in Detroit was found to be
10.3 mBq L1, which are about two orders of magnitude lower than
that of 210Pb. The volume-weighted 210Po/210Pb ratio of 0.029, is
lower than that in the precipitation reported in other places and
this is partly due to many samples having below detection limits of
210
Po in precipitation, as the samples were processed after a time
delay and the in-growth correction for 210Po was quite high
(McNeary and Baskaran, 2007). There is very limited data in the
literature to compare the volume-weighted concentrations of 210Po.
3.4. Depositional fluxes of
210
Po and
210
Pb
The three major mechanisms by which the scavenging of
tropospheric 210Pb takes place are: i) scavenging in convective
updrafts; ii) scavenging by large-scale precipitation and iii) dry
Table 2
Volume-weightedb Concentrations of
505
210
Pb reported from global published data.
Location and Time
of Collection
Rainfall
(cm)
210
Pb
weighted
conc.
(Bq L1)
Source
Los Angeles, CA
(33 56’N, 118 24’W)
Palo Alto, CA
(37 47’N, 122 120 W)
New Haven, Connecticut
(41 N)
March 1977eFeb. 1978
Bermuda (33 N)
Sept. 1977eAug. 1978
Oak Ridge, Tennessee
(35 58’N, 84 17’W)
Sept. 1982eAug. 1983
Sept. 1983eAug. 1984
Switzerland
(46.5 )
(47 N)
Norfolk, Virginia
(36 530 N; 76 18’W)
1983
1984
Bordeaux, France (44.8’N)
Paris (48.8 N)
College Station, Texas
(30 350 N; 96 220 W)
1990
1991
Galveston, Texas
(29 18’N; 94 48’W)
1989
1990
1991
Detroit, Michigan
(42 250 N; 83 1’W)
2000
Western
Mediterranean Sea
(36e44 N; 0.5e11 W)
Ahmedabad, India
(23 0’N; 72.6 E)
2000e2002
36
0.097
49
0.051
148.1
0.13
Fuller and
Hammond (1983)
Fuller and
Hammond (1983)
Turekian et al. (1983)
169.9
0.07
Turekian et al. (1983)
110
143
0.15
0.14
Olsen et al. (1985)
120
110
0.13
0.13
Dominik et al. (1987)
Schuler et al. (1991)
134
129
0.10
0.11
Todd et al. (1989)
103
68
98
146
0.06
0.16
0.13
0.15
Thomas (1988)
Thomas (1988)
Baskaran et al. (1993)
103
97
150
0.11
0.12
0.19
Baskaran et al. (1993)
91.6a
0.35
McNeary and
Baskaran (2003)
39.5e150
65
0.12e0.26
0.075
Data (9 sites)
summarized in
Garcia-Orellana
et al. (2006)
Rastogi and Sarin (2008)
a
Based on 18 months precipitation data.
Volume-weighted concentration (Bq/L) ¼ annual
precipitation (in L) in a given area.
b
210
Pb deposited (Bq)/annual
deposition. Using a global three-dimensional simulation of 210Pb on
the aerosol scavenging, the relative importance of convective
precipitation and large-scale precipitation generated by two
research groups differed considerably while the importance of dry
deposition was estimated to account for 13e14% (Feichter et al.,
1991; Balkanski et al., 1993). Monthly variations of the depositional fluxes vary by more than an order of magnitude. During
washout, concentrations of 210Pb tend to be relatively higher in the
early stages of rainfall and in short duration rainfall than the latter
stages and rainfall that persists for many days. Long-term depositional fluxes of 210Pb can be obtained from direct measurements (by
deploying atmospheric collectors, usually for short-term, less than
2e3 years) or from natural repositories (long-term over its mean
life, 33 yrs), such as lake sediments, snow fields, and soils. In
principle, one can also estimate the flux from the measured
concentration of 210Pb in surface air and deposition velocity.
Depositional fluxes obtained from undisturbed soil cores (210Pb
annual depositional flux ¼ inventories of 210Pb/mean life of 210Pb,
32.2 yr) could be affected by the vertical transport of 210Pb via
colloidal transport. Although a major portion of the atmospherically-delivered 210Pb is retained in the upper w30 cm of the soil,
some of the humic substances could form a complex with Pb and
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506
Table 3
Current Global Budgeth for
M. Baskaran / Journal of Environmental Radioactivity 102 (2011) 500e513
210
Pb and
Source
Natural
222
Rn emanation from Landa
222
Rn emanation from watersa
(Oceans, rivers, lakes)
Resuspension of soilsb
Sea salt sprayc
Volcanic activityd
Artificial
Nuclear detonations
Fossil fuel burninge
Tetraethyl lead combustionf
Dispersion of phosphate fertilizersg
and gypsum byproducts
Measured Fluxes
Northern Hemisphere
Southern Hemisphere
Total Flux
210
Po.
210
Pb Flux
(Bq y1)
210
3.50 1016
0.095 1016
1.05 1015
0.029 1015
0.033 1016
3.7 1011
6.0 1013
0.033 1016
7.4 1011
2.4 1015
0
8.2 1012
<1011
2.0 1013
0
8.2 1012
<1011
6 1011
2.66 1016 Bq
1.11 1016 Bq
3.64 1016 y
3.82 1015 y
Po Flux
(Bq y1)
a 210
Pb flux was calculated based on fluxes given in Table 1; assuming a residence
time of 10 days, the 210Po/210Pb ratio ¼ 0.03 was used in calculating the 210Po
activity
b
The flux from resuspended dust was estimated for 210Pb by Robbins (1978);
210
Po flux was calculated assuming 210Po/210Pb AR ¼ 1.0
c 210
Pb estimate is from Robbins (1978); 210Po/210Pb ratio of 2 in the microlayer
was assumed (Bacon and Elzerman, 1980)
d
Annual flux for 210Pb and 210Po was taken from Lambert et al. (1982)
e
Burning of coal in 1970 was estimated to contribute 3.7 1012 Bq of 210Pb
(Jaworowski et al., 1972) assuming a 2% annual increase in consumption, the estimated contribution is 8.2 1012 Bq; it was assumed that 210Po and 210Pb are in
equilibrium.
f
Robbins (1978) estimated this to be 1.1 1012 Bq; its use has decreased by >90%
and hence it is estimated to contribute < 1011 Bq y1.
g 210
Pb flux from use of phosphate fertilizer was reported in Robbins (1978) and
a 3% annual increase in phosphate use was assumed; most of the 210Pb flux is
derived from higher 222Rn release and the 210Po/210Pb ratio of 0.03 is used to estimate 210Po flux.
h
Contributions from plant exudates and forest fires are basically recycled
components and are included in the atmospheric dust; contributions from cement
manufacturing, uranium ore mining, milling operations, and lead smelting are
considered negligible compared to the production from 222Rn.
could result in downward migration, although this amount is
expected to be small (<10%). Long-term depositional fluxes
obtained from the sediment cores in aqueous systems (such as
ocean and lake systems) have the inherent uncertainty of sediment
focusing or erosion as well as boundary scavenging that could
result in varying sediment inventories of 210Pb. Furthermore, in
coastal areas, the submarine groundwater discharge could
contribute high levels of 222Rn which could result in additional
210
Pb production.
The reported depositional fluxes in a continental setting in a 10
belt almost vary by an order of magnitude. For example, the
depositional flux of 210Pb in Japan, with w33 mBq cm2 y1 is about
10 times higher than the value reported for the west coast of the
United States, 3.3e5.0 mBq cm2 y1 (Fuller and Hammond, 1983).
This lower depositional flux in the west coast of the United States
can be attributed to two reasons: i) lower amount of precipitations
in those places where the data is reported (2.5 mBq cm2 y1 in San
Francisco, CA with 49 cm/yr, with very little rainfall during the
summer and early fall months, and 3.5 mBq cm2 y1 in Los
Angeles, CA with 36 cm/yr annual rainfall, with very little (<5% of
the total) rainfall during summer months; Fuller and Hammond,
1983); and ii) predominantly 210Pb-depleted oceanic air masses.
This is in contrast to the prediction based on one-dimensional
model in which vertical air column was assumed as if it is wellmixed with a globally invariant removal rate constant (constant
residence time of 210Pb-laden aerosol) (Turekian et al., 1977).
Although both areas are tectonically-active, 222Rn emanation rates
are likely elevated in both places, but the relative proportion of
maritime and continental air masses along with the differences in
the amount and frequency of precipitation could be the key factors
for the differences in the depositional fluxes.
It has been shown that the atmospheric depositional flux is
strongly correlated with the amounts of precipitation (Nevissi,
1985; Baskaran et al., 1993; Kim et al., 1998; Caillet et al., 2001;
Garcia-Orellana et al., 2006). However, there are other places
where such correlation was not found (e.g., Texel and Groningen,
Netherlands, Beks et al., 1998). Beks et al. (1998) reported that the
variations in the annual depositional flux at two sites in
Netherlands appear to be correlated to the number of heavy rains
and thunderstorms. It was shown that 4% of the total rainy days
account for w25 to 30% of the total annual deposition rate of 210Pb
(McNeary and Baskaran, 2003). The higher depositional fluxes
during thunderstorm may be related to the variations in the cloud
condensation height for normal rainfall and thunderstorms as well
as weather systems/type of air masses (continental versus maritime) in which thunderstorms frequently occur. It was shown that
precipitation-normalized depositional fluxes of 210Pb are generally
higher during spring and summer seasons, and this is likely due to
frequency of rainfall (Baskaran, 1995).
The correlation between 210Po specific activity and precipitation
was found to be weaker than that of 210Pb and was attributed to the
possible presence of volatile Po present in the atmosphere which
could escape from scavenging by the water droplets (McNeary and
Baskaran, 2007). Although the 210Po depositional flux data are
limited, further studies are needed to address the importance of
volatile Po in the atmospheric scavenging of Po. The activity ratios of
210
Po/210Pb in the bulk precipitation were found to be slightly higher
than the aerosols, indicating possible presence of 210Po that are not
completely scavenged by precipitation condensation (McNeary and
Baskaran, 2007). The 210Po/210Pb ratios in precipitation samples also
vary considerably: 0 to 0.15 (mean: 0.05, San Francisco, USA, Fuller
and Hammond, 1983), 0.11 to 0.42 (mean: 0.24, western North
Atlantic, Hussain et al., 1998), 0.005 to 0.284 (mean: 0.072 in Detroit,
USA, McNeary and Baskaran, 2007). A plot of the 210Po/210Pb ratios in
precipitation versus 210Po/210Pb ratios (data taken from McNeary
and Baskaran, 2007) in aerosols indicates considerable scatter in
the plot (Fig. 6) and the activity ratios in aerosols vary in short time
scales compared to the period of collection of the precipitation (2 to
5 weeks).
Depositional flux of 210Pb at any given site depends on the 222Rn
concentration in the air and the scavenging efficiency by rain.
Global depositional flux of 210Pb integrated over 10 latitude belt for
the Northern and Southern Hemisphere are given in Table 1 (Fig. 7).
Most of the data for this plot is taken from Preiss and Genthon
(1997). Only data from continents, coastal oceans and ice cores/
sheets are included in plotting this figure. Due to sediment
focusing/erosion issues, the inventories from lakes and oceans
(except coastal oceans where no other data available) are not
included in the plot. The 210Pb flux at high latitudes (>70 N and S)
is less than 10% of the fallout of the tropical and sub-tropical
regions, and this is likely due to low radon emanation rates and low
amounts of precipitation (Table 1). Overall, the depositional flux in
the Southern Hemisphere is distinctly lower than that in
the Northern Hemisphere, mainly due to larger landmass in the
Northern Hemisphere. Although there are isolated areas where the
210
Pb depositional flux is higher than that reported in Table 1 (such
as coastal areas of Japan), overall, the depositional flux is significantly lower than 20 mBq cm2 y1. It has been shown that there
are very little variations in the atmospheric depositional fluxes of
210
Pb (based on the concentrations of 210Pb in lichens in Sweden
and Finland) in the northern Europe (north of 60 N, Kauranen and
Author's personal copy
M. Baskaran / Journal of Environmental Radioactivity 102 (2011) 500e513
0.2
Expected Line
0.1
210
Po/
210
Pb ratio in precipitation
0.3
0
0
0.05
210
0.1
0.15
0.2
210
Po/
Pb ratio in aerosols
Fig. 6. 210Po/210Pb ratios in precipitation versus aerosols. If there is uniform washout of
the atmospheric aerosols, then, we expect the line to fall on a straight line given above.
There is considerable scatter indicating that there is large-scale heterogeneity in the
samples. The aerosol collection is over 24 h while the precipitation samples integrate
much longer (weeks to a month). Data are from McNeary and Baskaran (2007).
Miettinen, 1969). The 210Pb flux in ice caps in Antarctica is about 20
times lower than that in Greenland and this is attributed to the
differences in the release rates of 222Rn in the surrounding areas
(Turekian et al., 1977).
3.5. Dry depositional flux
The contribution of dry depositional flux from the resuspended
dust, mainly derived from electrostatic or gravitational settling, to
the bulk (dry þ wet) depositional flux varies widely, depending on
a number of factors including rural versus urban areas, proximity to
the point of emission of aerosols near factories, amounts and
frequency of precipitation, etc. However, the 210Pb in air derived
from the decay of 222Rn is attached to accumulation mode aerosols
Fig. 7. Global atmospheric depositional fluxes of 210Pb at different latitude belts (10
belt). This figure represents 167 different sites around the globe. Details of the sources
of data are given in Table 1. Majority of the data is taken from the compiled data set by
Preiss and Genthon (1997).
507
and are not much affected by gravitational settling. Globally, it is
estimated that the dry fallout is w10% of the bulk fallout. Using
global three-dimensional simulation of 210Pb, Balkanski et al. (1993)
estimated the dry deposition to represent 14% of the global sink.
However, in semi-arid areas and areas where the amount of
precipitation is low (such as sub-tropical deserts that extend from
w20 e30 latitude and in large continental regions of the middle
latitudes; southern California, Arizona, etc), it is important to note
that dry depositional flux could be >50% of the bulk depositional
flux and in those areas, the dry fallout could be more important than
the wet fallout. For example, in Ahmedabad, a high-dust semi-arid
region in western India, the dry fallout of 210Pb accounted for 24 to
44% (mean: 35%) over a period of 3 years (2000e2002, Rastogi and
Sarin, 2008). There is only a limited amount of data that compares
the simultaneous measurements of 210Po and 210Pb in aerosols and
precipitation samples. In a recent study in Southeast Michigan, the
210
Po/210Pb activity ratios in most of the bulk precipitation and
aerosol samples was found to vary between 0.01 and 0.20, but in dry
precipitation samples, this ratio was much higher (0.05 to 0.64,
mean ¼ 0.33) (McNeary and Baskaran, 2007). Thus, the dry deposition samples are highly biased by resuspended dust and will yield
higher 210Po/210Pb ratios in the dry fallout. As discussed in section
3.8, the 210Po/210Pb ratios vary depending on the size of the aerosols
and hence 210Po/210Pb ratios in dry fallout samples collected at
different altitudes above the ground in a site could shed light on the
importance of aerosol particle size and resuspended dust on the
210
Po/210Pb ratios in dry fallout.
3.6. Depositional velocities of aerosols
The deposition velocity (Vd) can be used to transform concentrations of a species (Cs) in air to its flux (F) to the Earth’s surface.
The deposition velocity for any nuclide is calculated from the
following equation:
Vd ¼ F=Cs
(1)
The advantage of using 210Po or 210Pb are the following: i) Both
of these nuclides are primarily derived from Earth’s surface and
thus the deposition velocity obtained for 210Po and 210Pb can be
used for other species such as Hg; ii) The distribution of 210Po and
210
Pb in aerosols are likely similar to other particle-reactive
contaminants of interest and therefore can be used to determine
the fluxes of these contaminants to the Earth’s surface; and iii) the
activities of 210Po in aerosol and precipitation samples can be easily
measured. If 210Po and 210Pb are attached to similar aerosols, then,
the deposition velocities should be similar. A plot of the deposition
velocities obtained using 210Po vs. 210Pb is shown in Fig. 8. A slope of
1.78 indicates that the Vd (210Po) is significantly higher than that of
Vd (210Pb). This may be due to more volatile nature of 210Po that
results in higher escape of 210Po from the earth’s surface. A large
variations in the deposition velocity based on 210Pb, between
0.1 cm s1 to 5.0 cm s1 have been reported, although the average
values agree within a factor of w3 (0.6 to 1.9 cm s1; Turekian et al.,
1983; Graustein and Turekian, 1986; Todd et al., 1989; Anand and
Rengarajan, 1990; Beks et al., 1998; Hussain et al., 1998; Winkler
and Rosner, 2000; McNeary and Baskaran, 2003, 2007; Dueñas
et al., 2005). There was no significant correlation between the
total deposition velocities derived from either 210Pb or 210Po and
the total particulate matter collected onto the filter which suggests
that the aerosol mass does not control the amount of 210Pb scavenged by aerosols (McNeary and Baskaran, 2003; Dueñas et al.,
2005). McNeary and Baskaran (2003) proposed that only a small
portion of the aerosols scavenges effectively 210Pb from the atmosphere and a major portion of the aerosols do not actively
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508
M. Baskaran / Journal of Environmental Radioactivity 102 (2011) 500e513
case the washout ratio is expected to be lower. There is very
limited data in the literature on the washout ratios based on 210Po
and more studies are needed to address this issue.
10
y = -0.0848 + 1.78x R= 0.68
8
210
-1
Po (cm s )
3.8. Residence times of aerosols
6
4
2
0
0
1
2
210
3
4
-1
Pb (cm s )
210
Fig. 8. Deposition velocity based Po versus 210Pb in Detroit, Michigan, USA (data are
taken from McNeary and Baskaran, 2007).
participate in the removal of these nuclides from the air mass. This
hypothesis can be tested with a simple calculation: if we assume
clean background air to contain aerosol number concentration of
103 particles cm3, and the activity of 210Pb to be 1.2 mBq m3
(average over 18 months for Detroit, USA), then there are 1.2 106
atoms m3 present while the number of aerosols is w109 particles
m3. Thus, three orders of magnitude of higher aerosol particles are
available than the number of 210Pb ions (at the time of production,
it is in the ionic state) and hence only w0.1% of the aerosol particles
will have 210Pb atom (assuming one adsorbed 210Pb atom/aerosol; if
particles had more than one atom/particle, then, < 0.1% of the
aerosol particles would have adsorbed 210Pb atoms). Detailed sizefractionated aerosol study coupled with chemical characterization
of the aerosol particles and laboratory experimentation to quantify
the sorption behavior of different aerosols is needed.
3.7. Washout ratios
The washout ratio, W, that relates the average concentration of
Pb or 210Po in surface level precipitation to its average concentration in unwashed surface level air, is calculated as follows:
The residence time of tropospheric aerosols is expected to vary
with latitude. In low latitudes (0e20 N/S) due to high amounts of
precipitation, the scavenging of aerosols is rapid in the boundary
layer than above it and hence the residence time of aerosols are
expected to be shorter. The disequilibrium between 222Rn, 210Pb,
210
Bi and 210Po (i.e., 210Pb/226Ra, 210Bi/210Pb and 210Po/210Pb activity
ratios) have been utilized to determine the residence time of
aerosols, with often discordant values resulting from these pairs. In
a well-mixed, isolated atmospheric box where the rate of supply of
222
Rn is constant, the rate of change of decay product concentrations is given by:
dNd =dt ¼ lp Np ld Nd lr Nd
where Np and Nd denote concentrations (atom/m3) of parent and
daughter products, lp and ld are their respective decay constants,
and lr is the first-order removal rate constant. Equation (1)
assumes the following: i) the short-lived daughters (218Po, 214Pb
and 214Bi) are in equilibrium with 222Rn which implies that the
time scales that we are tracing is much longer than 3 h and the
system is in steady state during this time scale (during which
these daughters reach secular equilibrium with 222Rn); ii) The
source of 210Pb, 210Bi and 210Po in the air are derived from the
decay of 222Rn in a parcel of air and that parcel of air does not mix
with any other air masses that have different 222Rn and its progenies; and iii) the removal follows a first-order kinetics. Rearranging this equation for 210Pb/222Rn and 210Bi/210Pb yields the
following equation:
srðPbRnÞ ¼ ðAPb =ARn Þ sPb =ð1 APb =ARn Þ
(3)
srðBiPbÞ ¼ ABi sBi =ðAPb ABi Þ
(4)
Because of the intermediate daughter (210Bi) in the decay
between 210Pb and 210Po, the temporal relationship between the
activities of 210Po and 210Pb is given as follows (Moore et al., 1973):
210
W ¼ rCrain =Cair
where r is the density of air at standard conditions (1.2 kg m3 at
20 C and 0.76 m Hg) and Crain and Cair are the radionuclide
activities in bulk deposition (in Bq kg1) and surface air (in
Bq m3), respectively. This calculation is based on the assumption
that the specific 210Pb or 210Po content of the air in the precipitating cloud is the same as that measured at the surface level in
aerosol. However, if another air mass is intruding from an adjacent
area and the precipitation is derived from that air mass, then, this
value could result in considerable error. The average washout
ratios based on 210Pb is reported to vary between 30e2500 (mean:
780, Northern Wisconsin, Talbot and Andren, 1983; 215 Southeastern Virginia, Todd et al., 1989; 30 for the Western North
Atlantic, Hussain et al., 1998; 637 for Detroit, Michigan, McNeary
and Baskaran, 2003; 1908 for southeastern Spain, Dueñas et al.,
2005). There is only one set of data exist for the washout ratio
based on 210Po. It varied between 68 and 4522 (mean: 762 for
Detroit, Michigan) implying that the mean value is comparable to
that of 210Pb. Because of the volatile nature of 210Po, volatile
species of Po can stay in the air for longer periods of time in which
(2)
srðPoPbÞ ¼
1=2 2a
b þ b2 4ac
(5)
where a ¼ APb APo; b ¼ APo (sBi þ sPo); and c ¼ APo (sBisPo)
If the system (with all its members) is closed with respect to
their inputs and outputs and the inputs and outputs are governed
by the radioactive decay and first-order removal (scavenging,
gravitational settling, removal by coagulation, etc), then we
anticipate the residence time obtained by both 210Bi/210Pb and
210
Po/210Pb methods to be similar. But discrepancies obtained by
these two methods are frequent, due to deviations from these
ideal conditions. The advantages of 210Bi method are: i) the mean
life of 210Bi (7.2 d) is comparable to the mean residence time of
aerosols and water vapor in the lower atmosphere and is less
sensitive to extraneous sources than is 210Po (e.g., Papastefanou
and Bondietti, 1991); ii) volatile nature of 210Po (Hussain et al.,
1995; Le Cloarec et al., 1995; Nho et al., 1996, 1997; Kim et al.,
2000; Su and Hu, 2002) could result in additional sources of
210
Po to the atmosphere which could alter the residence time
based on 210Po/210Pb pair (such as large amounts of 210Po are
released to the atmosphere from major volcanic events); iii) in
surface air sampling, finite amount of resuspended dust which
will have 210Po/210Pb activity ratio of w1.0 (higher 210Po/210Pb
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M. Baskaran / Journal of Environmental Radioactivity 102 (2011) 500e513
ratio was found in dry depositional flux samples; McNeary and
Baskaran, 2007) which will drastically affect 210Po/210Pb residence times. The only disadvantage with 210Bi/210Pb is the timesensitive nature of this pair. It has been well-documented that the
210
Bi/210Pb-based residence time is always lower than the
210
Po/210Pb-based residence times (e.g., residence times obtained
by both pairs are summarized in Papastefanou, 2009). If all the
210
Po originates from the decay of 222Rn, then, the diverse residence times should represent mixing of air masses with different
aerosol residence times. Possible additional sources contributing
to the measured concentrations of the progenies are the reason
why discordant residence times have been reported. More
recently, Lozano et al. (in press) pointed that the decay of in-situ
210
Bi could contribute to 210Po and in studies where only 210Po and
210
Pb are measured and reported, the correction for this component of 210Po should be applied.
The effects of addition of equal amounts of 210Bi and 210Po from
sources other radioactive decay of 210Pb in the atmosphere can be
evaluated as follows: If an air parcel with a finite initial amount of
210
Pb is 5 days old and all the 210Bi and 210Po in that air parcel are
derived from the decay of this initial 210Pb, then, the expected
activity ratios of 210Bi/210Pb and 210Po/210Pb are 0.5 and 0.010,
respectively. If there is a 10% additional input of 210Pb from the
atmospheric dust, with 210Bi and 210Pb in secular equilibrium, then,
the apparent residence times calculated using 210Bi/210Pb and
210
Po/210Pb will yield 5.7 d and 29 d, respectively. Thus, a smaller
contribution of 210Po will affect the 210Po/210Pb-based residence
time drastically. Note that the 210Po/210Pb ratio in wet precipitation
is generally < 0.1 and thus the uppermost soil layer will likely have
210
Po/210Pb activity ratio < 1.0, but in about 2 years this ratio will be
close to w1, assuming that there is no escape of volatile 210Po from
this layer.
The residence times obtained on the bulk as well as size-fractionated aerosols are given in Fig. 9. The gross discrepancy
between the 210Po and 210Bi-based residence times in the bulk
aerosol samples is evident (data taken from Poet et al., 1972). On
the other hand, in the size-fractionated aerosol samples, there is
a strong correlation between 210Po and 210Bi-based residence
times (data taken from Gaffney et al., 2004). Thus, it appears that
the extraneous 210Po is primarily confined to the larger-size
509
aerosols. Wallner et al. (2002) reported 210Po/210Pb ratios ranging
between 0.6 and 2.5 in >1 mm-sized aerosols collected with
impactor while in sizes with <2 mm, this ratio was lower. One
another possibility is the input of aerosols from the lower
stratosphere. The residence time of aerosols in the whole stratosphere is w1e2 years and hence the 210Po/210Pb AR in stratospheric aerosols is expected to be w1.0. However, the input of
stratospheric air to the lower tropospheric air is likely negligible.
From earlier studies during the time when nuclear-weaponsderived 90Sr was present in the stratosphere, based on 90Sr/210Pb
ratios in the aerosols, it was found that there was no stratospheric
source for these nuclides (Moore et al., 1976). Turekian et al.
(1989) reported that in about half of the aerosol samples
collected at the New Zealand site and in the North Pacific westerlies showed no trend in 210Po/210Pb ratios with increasing size,
but in the remaining samples, there was some increasing trend in
>2 mm samples. Using modeling approach, it was estimated that
the residence time of aerosols produced at 9-km altitude is about
four times longer than that for aerosol produced in the lowest
500 m of atmosphere (Balkanski et al., 1993). From vertical profiles
of 222Rn, 210Pb, 210Bi and 210Po, Moore et al. (1973) observed that
the mean residence time of aerosols increased with altitude
within a factor 3 the troposphere and the mean aerosol residence
time in the lower stratosphere is about 1e2 months (Fig. 10). The
vertical profiles of the residence time obtained by both 210Bi/210Po
and 210Po/210Pb pair is given in Fig. 10. The discordancy between
210
Po/210Pb and 210Bi/210Pb residence time was attributed to the
210
Po derived from resuspended material (Poet et al., 1972; Moore
et al., 1973). More systematic study is required to address the
importance of volatile Po in the atmosphere and how it affects the
determination of the residence time. There are specific cases
where the residence time of aerosols could be longer than the
mean life of 210Bi (e.g., arctic haze) where 210Po will likely serve as
a suitable tracer (Baskaran and Shaw, 2001). For example, very low
concentrations of 222Rn (37 to 74 mBq m3) in air associated with
arctic haze, fog and clouds was attributed to isolation of this arctic
haze from the continental source for 3e4 weeks (Wilkniss and
Larson, 1984). Temporal evolutions of 210Po/210Pb ratios from the
210
210
Po/
0
Pb-based Residence Time (d)
20
40
60
80
100
14
12
Height (km)
10
8
6
4
Bi/Pb - Limon, CO
Po/Pb - Nebraska
Po/Pb - Limon, CO
Bi/Pb - Nebraska
2
0
0
10
210
Bi/
Fig. 9. Comparison of the residence times based on 210Po/210Pb and 210Bi/210Pb in bulk
(Poet et al., 1972) and size-fractionated aerosol (Marley et al., 2000) samples from
several sites in North America. Strong correlation in size-fractionated samples indicates that the additional sources of 210Po are likely confined to the courser fractions.
20
30
40
50
210
Pb-based Residence Time (d)
Fig. 10. Vertical profiles of residence times of aerosols from Limon, Colorado (April 2,
1969) and Scottsbluff, Nebraska (January 19, 1971) based on 210Bi/210Pb and 210Po/210Pb
pairs Data are taken from Moore et al. (1973).
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510
M. Baskaran / Journal of Environmental Radioactivity 102 (2011) 500e513
time of the formation of haze to its disappearance need to be
evaluated to determine if 210Po is a better atmospheric tracer.
the dust are picked-up along its transit from the place of origin to
the place of dust collection.
3.10. Global atmospheric inventory of
3.9. Importance of resuspended dust on the residence time of
aerosols
Wind-blown dust is the single largest contributor to the global
particulate burden of the troposphere, except sporadic release of
massive volcanic eruptions (Andrea, 1995). It has been shown that
there is a very strong correlation between airborne concentrations
of Al and total suspended particulate matter (collected using
Whatman 41) when the ground is not covered with snow (e.g.,
Talbot and Andren, 1983). A major portion of these dusts are
derived from dust storms in sub-tropical highs at latitudes near 30
where most of the deserts occur. The ‘dust belt’ includes deserts in
North Africa, Central and South Asia, the Middle East and China.
Saharan dusts have been documented to have been transported to
the Caribbean and Florida and dust transported across the Pacific
from Asia has been detected in the west coast of USA (Prospero,
1999; Husar et al., 2001; Griffin et al., 2002; Bollhöfer and
Rosman, 2001) and yellow dust storm from China was reported in
S. Korea (Kim et al., 1998). Analysis of 210Pb on dust collected at
different places in the Mediterranean Sea (central, eastern and
western) showed that the dust deposition accounted for 1.2 to
3.5 mg cm2 y1, with 210Pb activities ranging between 0.39 to
9.7 mBq/mg, with a mean value of 4.2 mBq/mg. The highly varying
(spatially and temporally) amounts of Saharan dust is estimated to
contribute >50% of the total 210Pb flux in sites where the precipitation is low, with expected low 210Pb fallout (Garcia-Orellana et al.,
2006). At a site north of Barcelona in the western Mediterranean
Sea, Garcia-Orellana et al. (2006) estimated the dry atmospheric
fallout to represent w16% of the total atmospheric deposition of
210
Pb.
In the determination of the tropospheric residence times of
aerosols, it is assumed that 210Po, 210Bi and 210Po are derived from
the decay of 222Rn and the contribution from atmospheric dust is
not included. However, the fine dust is likely contributing a significant portion of the 210Pb and 210Po to the rain. Finer particulate
matter (<10 mm) that can be carried by wind to longer distances
likely will have 210Po/210Pb and 210Bi/210Pb activity ratios of w1.0.
However, as the dust traverses through the atmosphere it could
pick-up additional 210Po and 210Pb, with ratios 210Po/210Pb < 0.1.
Thus, the wind-blown dust when collected at sites far away from
the place of origin will likely have 210Po/210Pb AR < 1, but its value
will depend on the trajectory distance of the air mass. Back
trajectory analysis combined with the spatial sampling from the
source region of the dust storm (such as from Sahara along its
trajectory) as well as sampling below the dust cloud (to quantify
input from surface soils along the trajectory) for 210Po, 210Pb, and
210
Bi is expected to yield better insight on how the 210Po/210Pb
ratios in aerosols vary with distance from the source. As was discussed earlier, even a small amount of dust (5e10%) could significantly affect the residence time obtained by 210Po as opposed to
210
Bi pair. It has been shown that recycling of 10Be of from continents to the atmosphere has resulted in significant variations in the
concentrations of 10Be (atoms/g aerosol) and Heikkilä et al. (2008)
estimated that recycled 10Be from atmospheric dust could
contribute up to 10e20% of the bulk concentration. In order to
quantify the amount of dust contribution to the 210Po and 210Pb, it is
important to measure index elements such as Al or Ti to quantify
the terrigenous dust by direct measurements on aerosols. In principle, the Al/210Po, Al/210Bi and Al/210Pb ratios in the dust can be
used to make corrections for dust-derived 210Po, 210Bi and 210Pb,
although there is uncertainty on how much 210Po, 210Bi and 210Pb in
210
Po and
210
Pb
The last global mass balance for 210Pb was done by Robbins
(1978) and a number of changes have taken place since then.
Those changes include: i) virtually no 210Pb derived from nuclear
weapons test in the atmosphere; ii) increased phosphate fertilizer
and fossil fuel usage; iii) elimination of leaded gasoline in most
countries in the world resulting in the decrease in the release of
210
Pb and 210Po to the atmosphere; iv) better characterization of
atmospheric 210Pb production rates and removal rates; and v)
better characterization of other sources. Furthermore, we have
a very large data set on the depositional fluxes of 210Pb for the
whole Earth (Preiss and Genthon, 1997). Using the present day
information, a revised mass balance is made.
Synthesis of global 210Pb depositional flux from >140 sites on
Earth’s surface indicates that there is highly varying depositional
fluxes at various latitudes and using one ‘global average’ value
would not be accurate. Using the average value of the atmospheric
depositional fluxes of 210Pb (obtained from atmospheric collectors,
coastal sediment cores, soil profiles in continents, ice cores; Table 1,
Fig. 7) for each of the latitude belt, the global atmospheric depositional flux curve based on 147 sites for 210Pb is established (Fig. 7).
A finite amount of 210Po in the atmosphere is produced from the
decay of 210Pb (via 210Bi). One of the extraneous sources for 210Po is
the volcanic gases. Due to its volatility, 210Po is highly enriched in
volcanic gases with 210Po/210Pb activity ratios up to 600 (57 to 614) in
the Stromboli’s plume have been reported (Gauthier et al., 2000).
The 210Bi/210Pb and 210Po/210Bi ratios in the same plume varied
between 5 and 92 and 1.5 and 18.9, respectively. Volcanic gas
emissions are highly patchy, both spatially and temporally. There are
about 60 of the Earth’s 550 historically-active volcanoes erupting
each year (excluding those under the ocean floor; Simkin et al.,
1994). The concentrations of 210Po in air were extremely high in
weeks and in some cases months before major eruption (Nevissi,
1984; Sheng and Kuroda, 1985; Su and Hu, 2002). Shortly after the
eruption stops, over three orders of magnitude decrease in the
activity of 210Po in air even at >1300 km from the eruption site has
been reported (Su and Hu, 2002). Depositional fluxes of 210Po 2e3
days before Mayon’s eruption was reported to be 1.7 mBq cm2 d1,
which is about 2e3 orders of magnitude higher than reported when
there is no volcanic eruption-derived 210Po present (Su and Hu,
2002). Bennett et al. (1982) estimated the contribution of 210Po to
the atmosphere from the May 18th, 1980 eruption of Mt. St. Helens
to be in the range of 1.0 1013 Bq to w17 1013 Bq, which is
significantly higher than that supplied from coal burning, phosphate
fertilizer production and lead smelting. Lambert et al. (1982) estimated a global volcanic 210Po output of 2.4 1015 Bq y1 of 210Po,
with a median 210Po/210Pb activity ratio of 40, resulting in a 210Pb
contribution of 6.0 1013 Bq y1. The volcanic eruption is episodic
and is not in steady state, and hence it is difficult to estimate the
annual flux.
Using the 222Rn emanation rate of 0.75 atom cm2 s1 (Wilkening
et al., 1975) from all the continental area (1.50 1018 cm2), the corresponding 210Pb flux is estimated to be 3.50 1016 Bq y1 (1 atom
222
Rn cm2 s1 ¼ 31.08 mBq 210Pb cm2 y1), under the assumption
that the upward flux of 222Rn equals the downward flux of 210Pb
(210Pb flux ¼ 222Rn flux X mean life of 222Rn/mean life of 210Pb).
However, 24% of the land area is covered by glaciers and permafrost
with negligible 222Rn release. This 0.75 atom cm2 s1 for the whole
land area corresponds to 1.00 atom cm2 y1 from the productive
areas (1.12 1018 cm2). 222Rn emanation rate from surface waters
(ocean, rivers and lakes) is 0.088 atom cm2 s1 (Wilkening
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M. Baskaran / Journal of Environmental Radioactivity 102 (2011) 500e513
and Clements, 1975). This corresponds to 210Pb flux of
0.095 1016 Bq y1 (area ¼ 3.50 1018 cm2). One another importance source is the atmospheric dust. The atmospheric dust contribution varies both spatially and temporally, with episodic events
contributing to most of the atmospheric dust. Moore et al. (1976)
estimated as much as 60% of the atmospheric 210Po is derived from
the atmospheric dust. Using the surface flux of 210Po obtained by
Moore et al. (1976), Robbins (1978) estimated the 210Pb global flux
(assuming 210Po and 210Pb are in equilibrium) from resuspended dust
to be 0.033 1016 Bq y1. The 210Po/210Pb AR in the very top surface
soils is less than 1, mainly due to low atmospheric deposition of 210Po,
with 210Po/210Pb values generally <0.1. However, the 210Po is anticipated to reach secular equilibrium in about 2 years. When fine dust
from surface soils is resuspended with less certain 210Po/210Pb ratios,
it is likely that as the dust moves through the atmosphere, it will
scavenge additional 210Pb, with 210Po/210Pb ratios < 0.01 to 0.10.
Thus, the assumption that 210Po and 210Pb are in equilibrium could
underestimate the dust-derived flux estimate for 210Pb.
Extensive use of phosphate fertilizers could contribute to the
release of 222Rn. Phosphate ores have much higher concentrations
of 238U (20 to 300 ppm corresponding to 0.25 to 3.75 Bq g1) and its
daughter products, including 226Ra. Resuspension and radon
exhalation from the widespread dispersal of phosphate fertilizers
in land areas around the globe along with the gypsum byproducts
of fertilizer production has been estimated to contribute
3.7e7.4 1012 Bq y1 (Moore et al., 1976; Robbins, 1978). Assuming
a global increase of 3% in the consumption of phosphate fertilizers
with similar 226Ra concentrations, could result in an atmospheric
flux of 10e20 1012 Bq y1 in year 2010. Retention of atmospheric
210
Pb on to plant leaf surfaces could alter the distribution of 210Pb in
the atmosphere, although this source may not affect the global
mass balance estimates. Forest fires are confined to selected regions
and also are sporadic and it is unlikely to contribute any significant
amounts of 210Pb or 210Po. Use of leaded gasoline through the
combustion of tetraethyl lead in the globe has decreased drastically
(>95% since 1970s) and hence its contribution to the atmospheric
210
Pb or 210Po budget is likely negligible. Evidences of 210Pb derived
from weapons testing, possibly derived from the 208Pb (2n, g)
reaction with the lead structural materials in the nuclear devices,
are discussed in Robbins (1978). Since the cessation of nuclear
weapons testing, this component is likely negligible. Even in
historical records, this component would have decreased to w20%
of the total delivered from weapons testing. 210Pb and 210Po derived
from fossil fuel consumption is another significant source. It was
estimated that w3.7 1012 Bq y1 of 210Pb is derived from the
burning of coal which has high concentration of 210Pb compared to
other fossil fuel (Jaworowski et al., 1972). In several countries usage
of peat in power plants as fuel could be a significant source of
natural radionuclides. The amount of 210Po and 210Pb derived from
these sources are expected to be significantly lower than that of the
fossil fuels. The average increase in the use of coal is 2.0% (from
4129 million short tons in 1980 to 7075 million short tons in 2007;
http://www.eia.doe.gov/aer/txt/ptb1115.html, downloaded on 15
May 2010). This corresponds to 8.2 1012 Bq/yr of 210Pb in year
2010, assuming that the amount of 210Pb/210Po released to the
atmosphere per unit mass of coal has remained constant during the
past 40 years.
It has been shown that the ocean microlayers are highly
enriched in 210Po (Bacon and Elzerman, 1980; Heyraud and Cherry,
1983). It is widely known that dimethyl sulfide is formed in the
ocean waters and released to the atmosphere. As a metalloid in the
sulfur-selenium-tellurium group XVI, Po can form a volatile
dimethyl polonide and could be released to the atmosphere
(summarized in Church and Sarin, 2008).
511
4. Summary and future direction
The daughter products of 222Rn, in particular, 210Po and 210Pb,
can be more effectively used as atmospheric tracers in the future,
provided we improve our understanding of their sources and
pathways. Recent results of the residence time of aerosols calculated using 210Po/210Pb and 210Bi/210Pb in the finer fraction of the
size-fractionated aerosols seem to yield consistent values and more
studies are needed to validate this observation. Presence of atmospheric Po in the volatile form and its effect on the first-order
scavenging by aerosols needs to be assessed. Release of 210Po to the
atmosphere from ocean surface layers in the form of methyl polonide and other volatile forms of Po needs to be quantified. It has
been proposed that only a small fraction of the aerosol particles in
the atmosphere actively participate in the removal of Po, Pb and
other particle-reactive species. Chemical characterization of the
aerosol particles and their role in the removal of Po and Pb needs to
be investigated. Now that some of the sources of 210Po and 210Pb to
the atmosphere are eliminated or diminished (such as nuclear
weapons, leaded gasoline), we can quantify other sources and
pathways of 210Po (such as plant exudates, fertilizers, forest fires,
volcanic activity, etc) better.
Some linkages between oscillations of surface 210Pb concentrations in the air in Southern Finland and state of the northeastern
part of the Atlantic Ocean have been observed, with the warmer
surface water in the North Atlantic resulting in low 210Pb concentrations. Higher amounts of warm and saline-water in the North
Sea have been linked to enhanced cyclonic activity and low 210Pb air
concentration. More studies linking surface water temperature of
the source area to the continental 210Pb concentrations will provide
insight on the long-term environmental changes and coupling
between the oceans and continents.
Acknowledgements
The work synthesized in this manuscript was partially supported by NSF Grant (OCE-0851032). We thank the two anonymous
reviewers for their thoughtful and thorough review of the earlier
version of this manuscript.
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