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Variations of 7Be and 210Pb (1992-1999) In Air At a Sampling Station On The Mediterranean Coast
Applied Physics (APHYS 2003) A. M6ndez-Vilas (Ed.) 9 2005 Elsevier Ltd. All rights reserved
95
Variations of 7Be and 21~ (1992-1999) In Air At a Sampling Station On The Mediterranean Coast C. DUEI~AS, M.C. FERNANDEZ, J. CARRETERO*, E. LIGER* AND S. C A l q E T E
Department of Applied Physics I, University of Mdtlaga, Campus de Teatinos s/n, 29071 Department of Applied Physics II, University of Mdtlaga, Campus de Teatinos s/n, 29071. M A L A G A , S P A I N . Abstract. The activity concentrations in air of 7Be and 21~ have been investigated at Mfilaga (SouthEast Spain) from 1992 to 1999. In this period, the measurements yielded an average 7Be activity concentrations at ground level of 4.16 o10.3 Bq m3 and an average of 0.54 ~ 3 Bq m3 for 2~~ The variation of the data with time was studied by time series analysis and seasonal patterns were identified. Activity concentrations of 7Be and 21~ were greatly affected by the meteorological conditions showing pronounced differences between seasons. Maximum activity concentrations in air were systematically observed in spring-summer of each year for 7Be and in summer for 21~ Minimum activity concentrations were observed in autumn for 7Be and in autumn-winter for 2~~ In the long term, annual variations of approximately a factor of 1.7 were observed in 7Be activity concentrations and a factor of 1.8 for 21~ The study reveals that much of the variability in the concentration activities is explained by temperature, rainfall, relative humidity and wind speed. 1.
INTRODUCTION
Long-term measurements of cosmogenic and atmospheric radionuclides such as 7Be and 21~ provide important data in studying global atmospheric processes and comparing environmental impact of radioactivity from man-made sources to natural ones (1) y (2). 7Be and 21~ are natural radionuclide tracers of aerosols originating over a range of altitudes in the atmosphere. 7Be (Tv2 = 53 days) is produced by cosmic rays impact on nitrogen and oxygen atoms in the stratosphere and the troposphere. 21~ (%/2 = 22.3 years) decays from 222Rn (Tv2 = 3.8 days) which is emitted from continents. Taken together, 7Be and 21~ yield information about vertical motions in the atmosphere and the scavenging of aerosols. Several features make 7Be and 21~ highly suitable tracers for improving general circulation model (GCM) aerosol simulation (3). Their sources are known, global in extent and relatively steady in time. The altitudinally distinct sources of 7Be and 21~ suggest that they may allow assessment of the relative importance of stratospheric and tropospheric transport pathways. We report eight years measurements (1992-1999) of 7Be and 21~ concentrations in surface air. Using these data, the present research was undertaken with the following principal goals: a) To study the variations of the data and b). To analyse seasonal patterns and to identify the main meteorological parameters that are responsible for the variations of these concentrations.
2 .MATERIAL AND METHODS Aerosol samples were collected weekly in cellulose nitrate filters, 47 mm diameter (collection efficiency 99.99% for 0.8 /~m pore size) with an air sampler
96
(RADECO, mod.AVS-28A) at a flow rate of 30 L min -1. The air sampler was lodge in an all weather sampling station and situated 10m above the ground, on the roof of the Faculty of Science building, University of Mfilaga (4~ 28'8"W; 36~ Measurements of 7Be and 21~ in each sample were carried out by non-destructive.-ray spectrometry by means of its 477.6 keV and 46 keV.-ray respectively, using a REGe-detector made by CANBERRA (relative efficiency about 30% to the efficiency of a 3"x3" NaI(T1) at 25 cm distance; resolution 2 keV for 1.33 MeV.-ray of 6~ 3. RESULTS AND DISCUSSION
3.1Characterization of data The results from individual measurements of 7Be and 21~ concentrations were analyzed to derive the statistical estimates characterizing the distributions. Table 1 provides arithmetic mean (AM) and related statistical information such as standard deviation (SD), geometric mean (GM), dispersion factor of geometric mean (DF), maximum and minimum values and coefficient of variation (CV). These values are given in mBq m -3.
TABLE 1. Statistical parameters. AM
GM
DF
SD
Max
Min
CV
Be-7
4.16
4.07
1.4
0.15
6.0
2.5
20.4
Pb-210
0.54
0.52
0.6
0.026
0.92
0.28
27.3
Normal distribution for 7Be is significant at the 0.1 level. Otherwise, 21~ concentration appears approximately log-normal (significant level less than 0.1). Assuming these types of distribution, the GM for the 21~ data and the AM for 7Be data should be used to characterize average values. A range of values of 3.5 mBq m 3 and a mean value of 4.16 mBq m -3 were found for 7Be activity. A range of 0.64 mBq m -3 and a geometric mean of 0.52. mBq m -3 were found for 21~ activity.
3.2Temporal variations and some meteorological factors affecting variation in concentrations. In Fig. 1 the box and whiskers diagrams of 7Be and 21~ concentrations along the eight years studied are represented. From a visual inspection of the data in the box and whiskers diagrams, annual changes seem to be produced and are commonly attributable to different factors such as temperature, atmospheric stability or frequency and amount of precipitation. Several studies have been performed on the relationship between the meteorological parameters and the concentration of diverse radionuclides in air.
FIGURE 1. Annual variations of the activities for the 1992-99 period.
97 In Fig. 2 the box and whisker diagrams of the maximum air temperature, relative humidity and wind speed are plotted during eight years and also the amount precipitation per year. Examining the diagrams we conclude that there are differences among the studied years, mainly concerning the precipitation and wind speed. Firstly, we observe the variations in the amounts of precipitation during the eight period. The amounts of precipitation during all years, exception 1996 and 1997 are lower than the 550 mm average of the annual rainfall in M~laga over a 56 years period while in 1996 and 1997 the total annual rainfall was higher than this average.
FIGURE 2. Box and whiskers diagrams for some meteorological parameters over the studied period
As meteorology plays an important role in the dispersion and transport of pollutants, we have performed a study to identify which meteorological parameters are strongly associated with the fluctuations of weekly concentrations. During the period of this study, meteorological data (wind speed, temperature, pressure, rainfall, and relative humidity) were supplied by the nearby weather station. First we performed a simple 9 regression of 7 Be and 210 Pb concentrations and some meteorological factors and then we carried out a multiple regression in order to determine the extent to which the variations in concentrations might be attributed to the combination of these meteorological parameters. In our analysis we used average of the maximum temperature (T), rainfall (r), average relative humidity (H), average pressure (p), average wind speed (v). In Tables 2 the correlation coefficient between 'Be and 210Pb concentrations respecti v e1y and those parameters are summarized. TABLE 2. Correlation coefficients between concentrations and meteorological factors
Be-7 Pb-210
T_max
Rainfall
Rel_Humidity
Wind Speed
Pressure
0.502
-0.459
-0.570
0.087
-0.343
0.589
-0.472
-0.211
-0.290
-0.006
The study of correlation reveals a pronounced positive correlation with the average of the maximum temperature and a negative one with the other meteorological factors.
98 Temperature is the variable most strongly correlated to the activities. High temperatures are often associated to upward convection currents in the atmosphere .On the contrary, the correlation with the humidity is negative because air temperature and air humidity show an opposite behaviour. A multiple regression analysis was carried out in order to find the factors that influence the 7Be concentrations. The variables were obtained choose the maximum temperature and relative humidity, obtaining the equation: Be-7 (mBq/m 3) = (0.066 + 0.030) T_max - (0.066 + 0.024) H + (7.251 + 1.835) (1)
The validity of analysis of the regression equation were taken into account the relative error of the coefficient of each independent variable, the standard error of the estimate, the R-squared value and the p-value of regression. The R-squared for Eq.(1) is 42% and the p-value is less than 0.01. So, there is a statistically significant relationship between the variables at 99%confidence level The study of the correlations for 21~ reveals a correlation with the maximum temperature and pressure. A multiple regression analysis was carried out then in order to find the factor 7Be, we have obtained the equation 2: Pb-210 (mBq/m 3) = (0.024 + 0.005) T max + (9.7 10.5 + 1.2 10"7) P - (9.714 + 4.603) (2)
The R-squared is and the p-value is less than, so there is a statistically significant relationship between the variables at the 95% confidence level. The correlation of 7Be and 2~~ air concentrations with meteorological parameters is very difficult, mainly due to the relative short times of atmospheric reload, less than 2 days (4) y (5) compared with the sampling time. This could be the best explication for the relatively poor correlation obtained.
F I G U R E 3.- Observed and predicted data for Be-7 and Pb-210
In Fig. 2 we plotted the observed and predicted data obtained applying Eqs (1) and (2).There are a better concordance between the calculated values and those experimentally observed for 7Be than 21~ CONCLUSIONS The results presented in this paper identify several meteorological parameters which are strongly associated with the fluctuations of weekly concentrations of 7Be and 21~ These results are useful to provide information on the atmosphere from a coastal Mediterranean area where data are insufficient now days.
99
REFERENCES 1. Duefias, C., M.C. Fem~indez, J. Carretero, E. Liger and S. Cafiete. J. Geophys. Res., 106, D24,34,059-34,065 (2001). 2. Duefias, C., M.C. Fem~indez, J. Carretero, E. Liger and S. Cafiete..1. Geophys. Res., 108, D11,4336-4345 (2003). 3. Koch D.M., D.J. Jacob and W.C. Graustein J. Geophys. Res.,lO1, 18,151 (1996). 4. Bergametti,G.,Dutot A.L. Buat-M6nart, P. Losno,R. and Remoudaki,E Tellus,41B, pp.353361 (1989). 5. Caillet, S. Arpagaus, P.,Monna, F. and Dominif. Jr. Environ. Radioact.,53, pp.241-256 (2001).
95
Variations of 7Be and 21~ (1992-1999) In Air At a Sampling Station On The Mediterranean Coast C. DUEI~AS, M.C. FERNANDEZ, J. CARRETERO*, E. LIGER* AND S. C A l q E T E
Department of Applied Physics I, University of Mdtlaga, Campus de Teatinos s/n, 29071 Department of Applied Physics II, University of Mdtlaga, Campus de Teatinos s/n, 29071. M A L A G A , S P A I N . Abstract. The activity concentrations in air of 7Be and 21~ have been investigated at Mfilaga (SouthEast Spain) from 1992 to 1999. In this period, the measurements yielded an average 7Be activity concentrations at ground level of 4.16 o10.3 Bq m3 and an average of 0.54 ~ 3 Bq m3 for 2~~ The variation of the data with time was studied by time series analysis and seasonal patterns were identified. Activity concentrations of 7Be and 21~ were greatly affected by the meteorological conditions showing pronounced differences between seasons. Maximum activity concentrations in air were systematically observed in spring-summer of each year for 7Be and in summer for 21~ Minimum activity concentrations were observed in autumn for 7Be and in autumn-winter for 2~~ In the long term, annual variations of approximately a factor of 1.7 were observed in 7Be activity concentrations and a factor of 1.8 for 21~ The study reveals that much of the variability in the concentration activities is explained by temperature, rainfall, relative humidity and wind speed. 1.
INTRODUCTION
Long-term measurements of cosmogenic and atmospheric radionuclides such as 7Be and 21~ provide important data in studying global atmospheric processes and comparing environmental impact of radioactivity from man-made sources to natural ones (1) y (2). 7Be and 21~ are natural radionuclide tracers of aerosols originating over a range of altitudes in the atmosphere. 7Be (Tv2 = 53 days) is produced by cosmic rays impact on nitrogen and oxygen atoms in the stratosphere and the troposphere. 21~ (%/2 = 22.3 years) decays from 222Rn (Tv2 = 3.8 days) which is emitted from continents. Taken together, 7Be and 21~ yield information about vertical motions in the atmosphere and the scavenging of aerosols. Several features make 7Be and 21~ highly suitable tracers for improving general circulation model (GCM) aerosol simulation (3). Their sources are known, global in extent and relatively steady in time. The altitudinally distinct sources of 7Be and 21~ suggest that they may allow assessment of the relative importance of stratospheric and tropospheric transport pathways. We report eight years measurements (1992-1999) of 7Be and 21~ concentrations in surface air. Using these data, the present research was undertaken with the following principal goals: a) To study the variations of the data and b). To analyse seasonal patterns and to identify the main meteorological parameters that are responsible for the variations of these concentrations.
2 .MATERIAL AND METHODS Aerosol samples were collected weekly in cellulose nitrate filters, 47 mm diameter (collection efficiency 99.99% for 0.8 /~m pore size) with an air sampler
96
(RADECO, mod.AVS-28A) at a flow rate of 30 L min -1. The air sampler was lodge in an all weather sampling station and situated 10m above the ground, on the roof of the Faculty of Science building, University of Mfilaga (4~ 28'8"W; 36~ Measurements of 7Be and 21~ in each sample were carried out by non-destructive.-ray spectrometry by means of its 477.6 keV and 46 keV.-ray respectively, using a REGe-detector made by CANBERRA (relative efficiency about 30% to the efficiency of a 3"x3" NaI(T1) at 25 cm distance; resolution 2 keV for 1.33 MeV.-ray of 6~ 3. RESULTS AND DISCUSSION
3.1Characterization of data The results from individual measurements of 7Be and 21~ concentrations were analyzed to derive the statistical estimates characterizing the distributions. Table 1 provides arithmetic mean (AM) and related statistical information such as standard deviation (SD), geometric mean (GM), dispersion factor of geometric mean (DF), maximum and minimum values and coefficient of variation (CV). These values are given in mBq m -3.
TABLE 1. Statistical parameters. AM
GM
DF
SD
Max
Min
CV
Be-7
4.16
4.07
1.4
0.15
6.0
2.5
20.4
Pb-210
0.54
0.52
0.6
0.026
0.92
0.28
27.3
Normal distribution for 7Be is significant at the 0.1 level. Otherwise, 21~ concentration appears approximately log-normal (significant level less than 0.1). Assuming these types of distribution, the GM for the 21~ data and the AM for 7Be data should be used to characterize average values. A range of values of 3.5 mBq m 3 and a mean value of 4.16 mBq m -3 were found for 7Be activity. A range of 0.64 mBq m -3 and a geometric mean of 0.52. mBq m -3 were found for 21~ activity.
3.2Temporal variations and some meteorological factors affecting variation in concentrations. In Fig. 1 the box and whiskers diagrams of 7Be and 21~ concentrations along the eight years studied are represented. From a visual inspection of the data in the box and whiskers diagrams, annual changes seem to be produced and are commonly attributable to different factors such as temperature, atmospheric stability or frequency and amount of precipitation. Several studies have been performed on the relationship between the meteorological parameters and the concentration of diverse radionuclides in air.
FIGURE 1. Annual variations of the activities for the 1992-99 period.
97 In Fig. 2 the box and whisker diagrams of the maximum air temperature, relative humidity and wind speed are plotted during eight years and also the amount precipitation per year. Examining the diagrams we conclude that there are differences among the studied years, mainly concerning the precipitation and wind speed. Firstly, we observe the variations in the amounts of precipitation during the eight period. The amounts of precipitation during all years, exception 1996 and 1997 are lower than the 550 mm average of the annual rainfall in M~laga over a 56 years period while in 1996 and 1997 the total annual rainfall was higher than this average.
FIGURE 2. Box and whiskers diagrams for some meteorological parameters over the studied period
As meteorology plays an important role in the dispersion and transport of pollutants, we have performed a study to identify which meteorological parameters are strongly associated with the fluctuations of weekly concentrations. During the period of this study, meteorological data (wind speed, temperature, pressure, rainfall, and relative humidity) were supplied by the nearby weather station. First we performed a simple 9 regression of 7 Be and 210 Pb concentrations and some meteorological factors and then we carried out a multiple regression in order to determine the extent to which the variations in concentrations might be attributed to the combination of these meteorological parameters. In our analysis we used average of the maximum temperature (T), rainfall (r), average relative humidity (H), average pressure (p), average wind speed (v). In Tables 2 the correlation coefficient between 'Be and 210Pb concentrations respecti v e1y and those parameters are summarized. TABLE 2. Correlation coefficients between concentrations and meteorological factors
Be-7 Pb-210
T_max
Rainfall
Rel_Humidity
Wind Speed
Pressure
0.502
-0.459
-0.570
0.087
-0.343
0.589
-0.472
-0.211
-0.290
-0.006
The study of correlation reveals a pronounced positive correlation with the average of the maximum temperature and a negative one with the other meteorological factors.
98 Temperature is the variable most strongly correlated to the activities. High temperatures are often associated to upward convection currents in the atmosphere .On the contrary, the correlation with the humidity is negative because air temperature and air humidity show an opposite behaviour. A multiple regression analysis was carried out in order to find the factors that influence the 7Be concentrations. The variables were obtained choose the maximum temperature and relative humidity, obtaining the equation: Be-7 (mBq/m 3) = (0.066 + 0.030) T_max - (0.066 + 0.024) H + (7.251 + 1.835) (1)
The validity of analysis of the regression equation were taken into account the relative error of the coefficient of each independent variable, the standard error of the estimate, the R-squared value and the p-value of regression. The R-squared for Eq.(1) is 42% and the p-value is less than 0.01. So, there is a statistically significant relationship between the variables at 99%confidence level The study of the correlations for 21~ reveals a correlation with the maximum temperature and pressure. A multiple regression analysis was carried out then in order to find the factor 7Be, we have obtained the equation 2: Pb-210 (mBq/m 3) = (0.024 + 0.005) T max + (9.7 10.5 + 1.2 10"7) P - (9.714 + 4.603) (2)
The R-squared is and the p-value is less than, so there is a statistically significant relationship between the variables at the 95% confidence level. The correlation of 7Be and 2~~ air concentrations with meteorological parameters is very difficult, mainly due to the relative short times of atmospheric reload, less than 2 days (4) y (5) compared with the sampling time. This could be the best explication for the relatively poor correlation obtained.
F I G U R E 3.- Observed and predicted data for Be-7 and Pb-210
In Fig. 2 we plotted the observed and predicted data obtained applying Eqs (1) and (2).There are a better concordance between the calculated values and those experimentally observed for 7Be than 21~ CONCLUSIONS The results presented in this paper identify several meteorological parameters which are strongly associated with the fluctuations of weekly concentrations of 7Be and 21~ These results are useful to provide information on the atmosphere from a coastal Mediterranean area where data are insufficient now days.
99
REFERENCES 1. Duefias, C., M.C. Fem~indez, J. Carretero, E. Liger and S. Cafiete. J. Geophys. Res., 106, D24,34,059-34,065 (2001). 2. Duefias, C., M.C. Fem~indez, J. Carretero, E. Liger and S. Cafiete..1. Geophys. Res., 108, D11,4336-4345 (2003). 3. Koch D.M., D.J. Jacob and W.C. Graustein J. Geophys. Res.,lO1, 18,151 (1996). 4. Bergametti,G.,Dutot A.L. Buat-M6nart, P. Losno,R. and Remoudaki,E Tellus,41B, pp.353361 (1989). 5. Caillet, S. Arpagaus, P.,Monna, F. and Dominif. Jr. Environ. Radioact.,53, pp.241-256 (2001).