J. Environ. Radioactivity, Vol. 31 No. 1, pp. 87-102,
1996
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Radon Concentrations in Surface Air and Vertical
Atmospheric Stability of the Lower Atmosphere
C. Duefias,
Radioactivity
M. Phez,
M. C. Ferniindez
& J. Carretero
Environmental Laboratory, Department of Applied Physics, Faculty of
Sciences, University of Mglaga, 29071 Mglaga, Spain
(Received 20 April 1994; accepted 20 October 1994)
ABSTRACT
The atmospheric air Rn concentrations have been measured in two
sampling points in Spain, These points show very different geographic and
climatological characteristics. Hourly measurements were taken over a
period of nearly one year. The study has been carried out on the results of
both sampling points and contains the following aspects: (a) influence of
the origin of air masses; (b) statistical analyses and Fourier series;
(c) relationship between atmospheric air Rn concentrations and meteorological parameters; (d) relationship between atmospheric air Rn concentrations and Pasquill and Turner’s stability indexes. These studies show
atmospheric Rn dependency on the vertical stability of the lower atmosphere. The obtained results imply a more effective use of Rn as a tracer in
areas with a continental climate rather than a coastal one.
INTRODUCTION
High accumulation of pollutants in the lowest levels of the atmosphere
always occur during periods of particularly stable atmospheric conditions.
Consequently, the degree of atmospheric stability is one of the most
important parameters to be taken into account in a polluted area.
Generally, the difficulty involved in measuring atmospheric stability
causes it to become unelevaluated. To achieve sufficient surveillance of
certain pollutants requires many detectors, distributed around the pollu87
88
C. Due6as et al.
ted area, especially in those located in the environment of the main source
of emission, since the areas most affected by the pollution might be
different due to wind direction.
Several researchers: Fontan et al. (1976), Hosler (1968), Israel et al.
(1966) Malakhov et al. (1966), Moses et al. (1960), Mattson (1970) and
Rotman (1973), have revealed that concentration of a gas such as Radon
(Rn) in the air is mainly dependent on the vertical stability of atmospheric
conditions. Therefore, surveillance of Rn concentrations may allow information on the vertical atmospheric stability in a polluted area.
Radon sources are found in the Earth’s crust. Released from material
containing Ra-226, once liberated from the mineral grain in a lesser
proportion to that expected from the total Ra-226 concentration, is
incorporated into the ground air pores, then transported to the surface by
diffusion or interstitial fluid flow. On arrival it is incorporated in turn into
the atmosphere in which it decays, emitting the solid descendants Ra A,
Ra B, etc. Consequently, a volumetric source of Rn produces a surface
source known as ‘Rn exhalation’. The determination of this exhalation has
been studied by numerous investigators: Pearson and Jones (1965), Clements and Wilkening (1974), Dueiias et al. (1982) and others, with large
discrepancies among the obtained conclusions. Pearson and Jones (1965)
assert that the fluctuations in Rn exhalation can be generally negligible
when using Rn as a tracer for vertical diffusion.
As the Ra-226 content in the ocean is less than that in the earth’s crust
by a factor of 103, Rn exhalation in the ocean is a non-contributory factor
in comparison with continents. Therefore, an air mass situated over a
continent is located with Rn, to be unloaded later by dispersion and decay
as it passes over the ocean.
In summary, it can be said that the concentration of the atmospheric Rn
depends on:
(a)
(b)
(c)
The vertical atmospheric stability.
The Rn source.
The time of an air mass spent over a continent.
Considering a continental situation, the variable (c) exerts only a weak
influence on the fluctuation of the concentration close to the ground,
Birot (1970). Measurements taken by Druilhet (1974) during periods of
atmospheric stability show that there is no appreciable variation in the Rn
source.
Measurements of atmospheric Rn concentrations have been carried out
in two different points of Spain: Malaga (MA) and Valladolid (VA), see
Fig. 1. The air samples were collected at height of 1.5m. The geographic
coordinates of MA are 4”29’58” of longitude west and 36’40’30” of lati-
Radon concentrations in surface air and atmospheric stability of lower atmospheres
10”
8”
6”
4”
2”
0”
2”
89
4”
42”
40”
38”
38”
36
‘18”
8”
Fig. 1.
6”
Map showing
4”
the sampling
16”
14”
2”
stations:
MBlaga and Valladolid
tude north and those of VA are 4”44’36” of longitude west and 41”38’40”
of latitude north. VA is the capital of the province of the same name and it
is in the northwest zone of the Iberian Peninsula. The city lies in a valley
surrounded by distant bare hills and it is at an altitude of 728m. The
climate is dry and there are extreme temperatures. MA is also the capital
of the province of the same name and is in the south east zone of the
Iberian Peninsula on the Mediterranean coast and it is surrounded by
mountains to the north. The climate in MA is moderate, between
temperature and warm and it has a low rainfall.
The measurement periods of the atmospheric Rn were carried out at
different times and in different seasons, allowing us to contrast the use of
Rn tracer as a measurement of atmospheric stability for reasons of very
different climates.
In the two sampling points: MA and VA, the atmospheric Rn concentrations have been obtained with apparatus of continuous measurements.
The study has been carried out with the results of both sampling points
and the following aspects have been studied:
1.
2.
3.
4.
Influence of the origin of air masses.
Statistical analyses: Fourier series.
Relationships between atmospheric Rn concentrations and specific
meteorological variables.
Relationships between atmospheric air Rn concentrations and stability indices.
C. Duellas et al.
90
EXPERIMENTAL
METHODS
The atmospheric air Rn concentrations have been studied using two
different methods: electrostatic precipitation in VA and filtration of
atmospheric air in MA.
Electrostatic precipitation
The method is based on the well-known technique of electrostatic collection of the decay products of Rn onto metallic wires and plates. The
components used are shown schematically in Fig. 2: (1) Pink Schneider
filter paper for retention of atmospheric aerosols. (2) Decay chamber to
eliminate atmospheric Rn with a capacity of 50 litres. (3) The electrostatic
pecipitation chamber is a closed and light-proof hemispheric container of
16 litre capacity. The scintillator contained in the chamber is a thin layer
of ZnS (Ag) crystals, metallised with 210mm in diameter and placed on
the diametrical plane of a container. In order to collect the short-life
radon daughter ions produced inside the chamber, a very strong electric
field of about - 1500 V is applied between the scintillator and the chamber
wall. The scintillator is optically coupled with a 1lOmm photomultiplier.
Due to the geometrical shape of this system, the electric field is radial,
minimizing the transit time of ions inside the chamber.
Components (4), (7), (8), (9) and (10) are a standard electronic contiguration for the alpha count. The atmospheric air is then pumped in
through the chamber at a constant flow of 5 litres/min by a membrane
pump (6) after passing through a flow-meter (5).
The positive RaA ions are thus electrostatically collected on the scintillator,
where their subsequent alpha decays and those of RaC’ scintillations amplified accordingly. These are then counted by a scale and digitally printed.
Fig. 2. Diagram of the device employed for measured atmospheric radon by electrostatic
precipitation:
(1) pink Schneider filter paper; (2) decay chamber; (3) electrostatic
precipitation chamber; (4), (7), (8), (9), (10) standard electronic configuration;
(5) flow-meter; (6)
membrane pump.
Radon concentrations in surface air and atmospheric stability of lower atmospheres
91
The calibration was carried out by comparison with a counter, already
calibrated, Dueiias et a!. (1993). A series of several comparisons has given
an average efficiency for the counter of 24 f 0.5%. The sensitivity is
limited by the background value. The background is 10 counts/h, for a
volume of 16 litres, therefore the minimum concentration of detectable Rn
is 0.17 Bq/m3, roughly evaluated as the quotient between the background
count and the volume.
Filtration
The components used are shown schematically in Fig. 3: (1) Decay
chamber with capacity of 2m3 to eliminate atmospheric Rn. (2) Pink
Schneider filter paper for retention of atmospheric aerosols. (3) Decay
chamber of capacity 1.5 m3 where the atmospheric Rn was disintegrated.
(4) The detection unit consisting of a light-proof container where a filter
paper (5) retains solid daughter generates of Rn while the atmospheric air
circulates through it. (3) The scintillator contained in the detection unit is
a thin layer of ZnS (Ag) crystals, deposited on a perpex disc of 210cm in
diameter. The scintillator is optically coupled with a 110mm photomultiplier. (8) is the electronic configuration that carries out the alpha
count. Atmospheric air is pumped at a low constant flow of 12m3/h by a
vacuum pump (6) after passing through a rotameter (7).
The calibration was carried out by comparison with a Luas-type flask,
previously calibrated in Chilton (England). A series of several comparisons has given an average efficiency of 17%. The background count was 4
counts/min for a volume of 1.5 m3, therefore the minimum concentration
of detectable Rn was 0.04Bq/m3, roughly evaluated as the quotient
between the background count and the volume.
Signal
Fig. 3. Diagram of the device employed for measured atmospheric radon by filtration: (1)
decay chamber; (2) pink Schneider filter paper; (3) decay chamber; (4) detection unit; (5)
filter paper; (6) vacuum pump; (7) rotameter; (8) electronic configuration.
92
C. Dueiias et al.
EXPERIMENTAL
RESULTS
Influence of the origin of air mass
The count of Rn in air mass depends on its origin, history and stability.
The source of the air mass can either be continental or maritime. As the
value of the Rn exhalation from the ocean is 1000 times less than that of
the continents, the Rn concentration in continental air is typically much
greater than in maritime air.
The results obtained in MA are displayed first. Figure 4 shows the wind
rose at MA that gives the frequency distribution simultaneously with wind
speed and direction. This wind rose has been constructed using the intervals of wind speed shown in Table 1.
Figure 4 shows that the frequent winds at MA correspond to the directions NW and SE. This result is analogous to the study carried out by
Ortega and Sanchez (1976) on the climatology of Malaga. The direction
NW and SE occur for land-sea and sea-land breezes, respectively. The
direction NW dominates during the night and the direction SE during the
day. The change of direction of NW to SE is produced between 0:900 and
13:OOh and the change of direction SE to NW is preceded by a long period
of calm, between 19:00 and 23:00 h.
The average concentrations corresponding to the different wind directions are represented in the histograms of Fig. 4. In this histogram, the Rn
concentrations are greater for winds from the land (NW, NWW) than for
those from the sea (S, SE, SW).
Figure 5 shows the wind rose at VA that gives the frequency distribu-
Fig. 4.
The wind rose and histograms of the average concentration
different wind directions in MA.
corresponding
to the
Radon concentrations in surface air and atmospheric stability of lower atmospheres
93
TABLE 1
Wind speed
mls
v<l
3>v21
7>v>3
Vd7
Calm
Weak
Moderate
Strong
tion simultaneously of wind speed and direction. This wind rose has also
been constructed using the intervals of wind speed shown in Table 1. In
Fig. 5 the frequent winds at VA corresponding to the direction N and NE
can be observed. This behaviour is similar to the study achieved by
Roldan (1985) on climatology of Valladolid. The average Rn concentrations corresponding to the different wind directions are represented in the
histogram of Fig. 5. In this histogram, the Rn concentrations are smaller
for winds from SW direction as Roldan (1985) claims the winds from this
direction predominate when it rains. The behaviour of atmospheric air Rn
concentration from SW in VA is analogous to that obtained by Duefias
(1974) in the same location sampling station. There is a clear dependence,
in a wet climate, between the previous rainfall and the air Rn concentration measured by aerosols on the day following (Soto, 1978). However, in
a dry climate this dependence is not plain clearly.
Statistical analyses: Fourier series
There are 24 daily values of atmospheric
“a
air Rn concentrations
VA
2
at both
N
2
4.00
3.50
3.00
2.50
2.00
1.50
1.00
0.50
Fig. 5.
I!
SW’
I
SE
s
-Weak
I
Moderate
c: calm
I
stmng
(38%)
The wind rose and histograms of the average concentration
different wind directions in VA.
corresponding
to the
94
C. Duerias et al.
points of sampling in MA and VA. The meteorological data were supplied
by observation stations that the National Meteorological Service has
installed in each city: MA and VA. These meteorological data are
measured every 3 h. To make the readings available on an hourly basis,
they have been interpolated using techniques given by Mineur (1966).
Besides the meteorological data, we have evaluated Pasquill (1962) and
Turner’s (1964) stability indices.
Taking the hourly results of each of these variables together, they form
a time series that has been used to calculate the contents of the adjoining
organigram. These calculations have been made by elaborating the
computer programs necessary to achieve a great quantity of information
and analyze the possible interrelations that exist among the oscillations in
the time series.
The time series we make use of are: Rn concentrations, air temperature at
1 m height, wind speed, relative humidity of air (%), atmospheric pressure
(mm of Hg) and Pasquill and Turner’s stability indices. On each time series,
we carried out some statistical analyses, to find out the basic properties of
their variability and characteristics of their periodic and irregular oscillations. For each of the time series the following has been calculated: (a)
hourly mean variation (H.M.V.); (b) diurnal standard variation (D.S.V.); (c)
autocorrelation functions; (d) harmonic analysis: Fourier series; (e) the
cross-correlation between atmospheric air Rn concentrations and certain
meteorological variables and Pasquill and Turner’s stability indexes.
Figures 6, 7, 8 and 9 show the H.M.V. of different parameters in MA
and VA versus time meridian Greenwich (T.M.G.), respectively. It can be
l
Rad6n
.
Relative humidity
Air temperature
2.5
3.0
. .
2.0
1.5
1.0
II 3.5
1
t
“s
$
T;
+i
d
3.0
1O”Cl
’
0
I
I
I
I
I
I
I
I
3
6
9
12
15
18
21
24
H.M.V. TOTAL (MA)
Fig. 6.
Variation
of total H.M.V.
time (T.M.G.)
of Radon, relative
MA.
-
humidity
and air temperature
in
Radon concentrations in surface air and atmospheric stability of lower atmospheres
Rad6n
pasnuiu
Tumer
.
!
t
. . . . .
.
.
lf
. . .
.
!
A:
I
I
25
2.0
l
1.5
.
.
.
3.0
-
73
$
1.0
.
;
8
d
0.5
I
95
0.0
2
r,
0
I
3
I
6
I
9
I
I2
Variation
of H.M.V.
I
18
time (T.M.G.)
H.M.V. TOTAL (MA)
Fig. 7.
I
15
of Radon,
Pasquill
1
21
;
24
_
and Turner
stability
indices in MA.
I
7.5
-
7.0
“e
6.5
$
-
6.0
5.5
0
3
6
9
H.M.V. TOTAL (VA)
Fig. 8.
Variation
of H.M.V.
of Radon,
12
15
18
21
i
d
24
rime (T.M.G.) -
temperature,
relative humidity
and wind in VA.
observed in these figures that qualitatively the different parameters
appearing in them show a periodic behaviour. We will show the results
after treatment with Harmonic Analyses: Fourier Series.
It is known that any temporal atmospheric development of any size is
greatly influenced by the solar cycle. Studies made by Israel and Horbert
(1966); Moses et al. (1963); Hosler (1966); Israelson et al. (1973) have
shown that local variations in Rn concentrations have a periodic fluctuation that is attributed to the variations caused by the heating and subsequent cooling of the air masses located closest to the ground by the sun.
96
10
1.5
t
7.0
“a
6.5
3
6.0
i
5.3
3
8
d
i
p
6
4
5.0
2
4.5
H.M.V. TOTAL (VA)
Fig. 9.
time (T.M.CL)
h
Variation of H.M.V. of Radon, Pasquill and Turner stability indices in VA.
Consequently, the simple observation of hourly Rn concentrations in VA
during the series of readings show the existence of a 24 h fluctuation
period which has been corroborated by the determination of the autocorrelation coefficient with a delay variable of the time series made up of
these concentrations (see Fig. 10).
The harmonic analysis allows us to separate the oscillations observed by
using the sum of a certain number of simple waves, their frequencies being
multiples of the fundamental frequency. This treatment can be applied to
Autocorrelatiou Radon (VA)
Fig. 10.
Autocorrelation
of Radon in VA.
Radon concentrations in surface air and atmospheric stability of lower atmospheres
97
either H.M.V. or D.S.V. In this work, harmonic analysis has been applied
to H.S.V. The wave associated with the fundamental harmonic, given that
it has a period of 24 h, coinciding with that of 1 day, is called the diurnal
wave, and for the same reason, the wave associated with the second
harmonic is known as the semi-diurnal wave.
It has been confirmed that the diurnal wave contribution is fundamental
to all the time series analyzed, and the contribution from the rest of the
harmonics can be considered to be less important. This behaviour is illustrated in Tables 2 and 3. Table 2 shows the results of measurements made
in VA and Table 3 of those in MA. In the first column of both tables are
the analyzed time series; the second column shows % variance that represents the ratio between the square of the amplitude of diurnal wave (first
harmonic (Ci2) and the double of square of standard deviation (SX2),
Panofsky and Brier (1968); the third column shows the delay of the first
harmonic:
Looking at the above tables, it shows that the % variance of oscillation
is about 90% for measurements taken in VA while for those of MA, it
fluctuates between 70 and 87%. The values obtained for the phasing out
of the first harmonic or diurnal waves are very similar for both cities, thus
giving a high agreement in results. It can be concluded that the V.H.M. of
the analyzed variables at both points of measurements are well represented with good approximation by the diurnal wave and semidiurnal
TABLE 2
Valladolid
Radon
Air temperature
Relative humidity
Wind speed
Pasquill
Turner
% Variance
95
94
91
89
89
92
Delay (h)
238
5,2
4,8
-3,9
194
134
TABLE 3
Mrilaga
% Variance
Radon
Air temperature
Relative humidity
Wind speed
Pasquill
Turner
87
79
76
70
70
83
Delay (h)
236
4,2
335
-2,7
1,4
121
C. Due&s et al.
98
wave. This result is similar to that obtained by other researchers such as
Druilhet (1973), Cautenet (1974) and Gonzalez and Garzon (1978a).
Relationships between atmospheric air Rn concentrations and certain
meteorological variables
(a) Wind speed
The correlation existing between the H.M.V. of Rn and wind speed is high
in both cities although less so in Malaga and with a 3 h phase delay, which
could be attributed to the occurrence of coastal winds, which as we have
mentioned is very important for Malaga from the climatological point of
view.
Areas meteorologically disposed to weak or moderate winds are associated with elevated Rn concentrations
maintained by anticyclonic
regimes where the atmosphere has little disturbance giving rise to a strong
nocturnal vertical stability and a slowed atmospheric circulation that
allows the air mass closest to the ground to increase its Rn content. On the
other hand, areas with strong winds have greater disturbance and an
associated weakness in atmospheric stability.
This behaviour has been corroborated by several investigators, Sot0
(1978), Dueiias et al. (1990), Gonzalez and Garzon (19783), Cautenet
(1974) and others.
(b) Relative humidity and air temperature
These two parameters were analyzed simultaneously since there exists a
high level of interdependence between them, owing to increases in atmospheric temperature resulting in a decrease in atmospheric humidity and
vice versa. Therefore, and as can be seen in Figs 6 and 8, variations in
both meteorological parameters can be seen to be out of phase.
The cross-correlation
coefficient Rn-relative humidity is positive,
showing the direct relation between these magnitudes, which is coherent.
Cross-Correlation
Cross-correlation
TABLE 4
Between Atmospheric
and Wind Speed
Concentrations
Radon
Delay (hours)
Wind speed
-0,83
Valladolid (0)
Wind speed
-0,61
Mglaga (-3)
Radon concentrations in surface air and atmospheric stability of lower atmospheres
Cross-Correlation
TABLE 5
Between the Atmospheric Concentrations,
Air Temperature
Relative Humidity
Cross-correlation
Relative humidity
Air temperature
Radon
0,81
-0,72
Valladolid
Radon
0,67
-0,63
Mglaga
99
and
City
High relative humidity is usually associated with periods when wind
speeds are low and the atmosphere is stable. The low winds and atmospheric stability are probably the most important factors resulting in
increased Rn concentration. Moreover Tanner (1980) suggests that the
diffusion coefficient of gas in the air decreases as humidity increases,
which produces an increase in atmospheric Rn concentrations above the
ground. These results are analogous to those obtained by Duefias et al.
(1990) Gonzalez and Garzon (1978b) and Cautenet (1974).
The other meteorological parameters that have not been used for evaluating the Pasquill and Turner stability indices have also been analyzed,
but the results obtained do not help to explain the variations of concentrations of Rn. For instance, with the pressure that causes the weak daily
oscillations, we have obtained insignificant results compared to other
meteorological variables analyzed.
Relationships between atmospheric Rn concentrations and vertical stability
of the atmosphere
The stability of the atmosphere was established by two criteria: Pasquill
and Turner’s stability categories. Stability near the ground is dependent
primarily upon net radiation and wind speed. Without the influence of
clouds, insolation (incoming radiation) during the day is dependent upon
solar altitude, which depends on the time of day and the time of year.
When clouds are present, their cover and thickness decrease incoming and
outgoing radiation. In this system insolation is estimated by solar altitude
and modified by existing conditions of total cloud cover and cloud ceiling
height. At night, estimates of outgoing radiation are made by considering
cloud cover alone.
Pasquill’s stability index was specified in terms of wind speed, the
amount of cloud present, time of day and time of year. Applying these
criteria, Pasquill established different types of stability from type A (very
unstable) to type F (moderately unstable) and D+ which correspond to a
neutral atmosphere with light winds (< 2m/s).
C. Duecas et al.
100
Cross-Correlation
TABLE 6
Between the Atmospheric Concentrations
Stability Categories
and Pasquill and Turner
Cross-correlation
I. Pasquill
I. Turner
City and delay
Radon
0,82
0.81
Valladolid (0)
Radon
0,51
0.68
MBlaga (1)
Turner’s stability classes are as follows: (1) extremely unstable;
(2) unstable; (3) slightly unstable; (4) neutral; (5) slightly stable; (6) stable;
(7) extremely stable. The stability depends on wind speed and net radiation. The net radiation index ranges from 4, highest positive net radiation
to -2, highest negative net radiation. Instability occurs with high positive
net radiation and low wind speed, stability with high negative net radiation, light winds and neutral conditions with cloudy skies or high wind
speeds.
On examining the correlation coefficients, we conclude that these coefticients are especially good for Turner’s index, although in MA there is a
1 h delay time to the H.M.V.
CONCLUSION
Two sets of experimental equipment have been used to continuously
measure the temporal development of Rn in the air at two different
sampling points, Malaga and Valladolid.
The variation of D.S.V. of Rn concentrations, meteorological parameters, and the Pasquill and Turner stability indices, adjust according to
the variation shown by the diurnal wave. The correlation existing between
wind, temperature, relative humidity and Rn concentration provides that
there is a concordance between the Rn concentration and parameters
linked to meteorological factors. The correlation coefficients between Rn
concentrations, meteorological parameters and the stability indices are
higher in VA than in MA, which shows a more effective use of Rn as a
tracer in areas with continental climates rather than coastal ones.
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