Radon Concentrations in Surface Air and Vertical Atmospheric … · 2007-06-06 · Rotman (1973), have revealed that concentration of a gas such as Radon (Rn) in the air is mainly

ELSEVIER

J. Environ. Radioactivity, Vol. 31 No. 1, pp. 87-102, 1996

Copyright 0 1996 Elsevier Science Limited Printed in Ireland. All rights reserved

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0265-931X(95)00058-1

Radon Concentrations in Surface Air and Vertical Atmospheric Stability of the Lower Atmosphere

C. Duefias, M. Phez, M. C. Ferniindez & J. Carretero

Radioactivity 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 pollu-

87

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), Clem- ents 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) The vertical atmospheric stability. (b) The Rn source. (c) 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 89

40”

38”

36

10” 8” 6” 4” 2” 0” 2” 4”

‘18” 16” 14”

8” 6” 4” 2”

42”

38”

Fig. 1. Map showing the sampling 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. Influence of the origin of air masses. 2. Statistical analyses: Fourier series. 3. Relationships between atmospheric Rn concentrations and specific

meteorological variables. 4. Relationships between atmospheric air Rn concentrations and stabi-

lity indices.

90 C. Duellas et al.

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 collec- tion 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 contig- uration 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 ampli- fied 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.


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:OO h 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 corresponding to the

different wind directions in MA.


TABLE 1

Wind speed mls

Calm v<l

Weak 3>v21

Moderate 7>v>3

Strong Vd7

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 air Rn concentrations at both

“a 2 2

4.00

3.50

3.00

2.50

2.00

1.50

1.00

0.50 I !

VA N

SW’ I SE

s -Weak I Moderate I stmng

c: calm (38%)

Fig. 5. The wind rose and histograms of the average concentration corresponding to the different wind directions in VA.

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

. .

II 1

1O”Cl ’ I I I I I I I I

0 3 6 9 12 15 18 21 24

H.M.V. TOTAL (MA) time (T.M.G.) -

3.0 2.5

t

2.0 - “s

1.5 $

1.0 T; +i

3.5 d

3.0

Fig. 6. Variation of total H.M.V. of Radon, relative humidity and air temperature in MA.


Rad6n

pasnuiu Tumer

t 3.0

I . . . . . 25

.

A: . I . ! l 2.0 -

. . . l f ! 73

. . 1.5 $ .

. 1.0 ; . 8

0.5 d

I 0.0

2

r, I I I I I I 1 ; 0 3 6 9 I2 15 18 21 24

H.M.V. TOTAL (MA) time (T.M.G.) _

Fig. 7. Variation of H.M.V. of Radon, Pasquill and Turner stability indices in MA.

I 7.5 -

7.0 “e

$ 6.5 -

6.0 i d

5.5

0 3 6 9 12 15 18 21 24

H.M.V. TOTAL (VA) rime (T.M.G.) -

Fig. 8. Variation of H.M.V. of Radon, 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

8

d 6

i

p 4

2

H.M.V. TOTAL (VA) time (T.M.CL) h

1.5 t

7.0 - “a

6.5 3

6.0 i

5.3 3

5.0

4.5

Fig. 9. 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.


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 illu- strated 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 repre- sents 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 % Variance Delay (h)

Radon 95 238 Air temperature 94 5,2

Relative humidity 91 4,8 Wind speed 89 -3,9

Pasquill 89 194 Turner 92 134

TABLE 3

Mrilaga % Variance Delay (h)

Radon 87 236 Air temperature 79 4,2

Relative humidity 76 335 Wind speed 70 -2,7

Pasquill 70 1,4 Turner 83 121

98 C. Due&s et al.

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 (1978), Dueiias et al. (1990), Gonzalez and (1974) and others.

(b) Relative humidity and air temperature

several investigators, Sot0 Garzon (19783), Cautenet

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.

TABLE 4 Cross-Correlation Between Atmospheric Concentrations

and Wind Speed

Cross-correlation Radon Delay (hours)

Wind speed

Wind speed

-0,83 Valladolid (0)

-0,61 Mglaga (-3)


TABLE 5 Cross-Correlation Between the Atmospheric Concentrations, Relative Humidity and

Air Temperature

Cross-correlation Relative humidity Air temperature City

Radon 0,81

Radon 0,67

-0,72 Valladolid

-0,63 Mglaga

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 eval- uating 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).

100 C. Duecas et al.

TABLE 6 Cross-Correlation Between the Atmospheric Concentrations and Pasquill and Turner

Stability Categories

Cross-correlation I. Pasquill I. Turner City and delay

Radon

Radon

0,82 0.81 Valladolid (0)

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 coef- ticients 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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Documents

Radon Concentrations in Surface Air and Vertical Atmospheric … · 2007-06-06 · Rotman (1973), have revealed that concentration of a gas such as Radon (Rn) in the air is mainly