Temporal changes of 7Be and 210Pb concentrations in surface air at temperate latitudes (40°N)

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Applied Radiation and Isotopes 63 (2005) 277–284 www.elsevier.com/locate/apradiso

Temporal changes of 7Be and 210Pb concentrations in surface air at temperate latitudes (401N) A. Ioannidou, M. Manolopoulou, C. Papastefanou Physics Department, Nuclear Physics and Elementary Particle Physics Division, Aristotle University of Thessaloniki, Thessaloniki 54124, Greece Received 10 December 2004; received in revised form 20 February 2005; accepted 16 March 2005

Abstract Atmospheric concentrations of 7Be and 210Pb were measured for 15 years (1987–2001) in ground-level air at Thessaloniki, Northern Greece (401380 N, 221580 E). Mean activity concentrations of 7Be and 210Pb were 5.02 mBq m3 and 664 mBq m3, respectively, characteristic of the latitude of 401N. Monthly atmospheric concentrations of 7Be showed a strong seasonal trend with the highest values being observed in the summer and the lowest in the winter period. Multiple regression analysis of the data of 7Be concentrations and a number of meteorological parameters revealed that the sunspot number and temperature are the most significant parameters affecting the concentrations of 7 Be in surface air. The observed strong positive correlation between the mean monthly concentrations of 7Be and the temperature confirms that the increased rate of vertical transport within the troposphere, especially during the warm months, has as a result to carry down to the surface layer air masses enriched in 7Be. Highest values of the mean monthly atmospheric concentrations of 210Pb were observed in the autumn and lowest in the spring period. The positive correlation that was observed between 210Pb and 7Be concentrations during the summer months suggests that these two radionuclides could be used together as tracers of environmental processes. r 2005 Elsevier Ltd. All rights reserved. Keywords: Beryllium-7; Lead-210; Radionuclide transport

1. Introduction Particle reactive radionuclides such as 7Be and 210Pb have been used as atmospheric tracers for studying environmental processes such as cloud scavenging and precipitation (Koch et al., 1996; Liu et al., 2001), aerosol transit and residence times in the troposphere (Papastefanou and Ioannidou, 1995; Winkler et al., 1998), aerosol deposition velocities (Young and Silker, 1980;

Corresponding author. Tel.: +30 2310 998202; fax: +30 2310 998058. E-mail address: [email protected] (A. Ioannidou).

Crecelius 1981; Turekian et al, 1983; Lujaniene, 2003), and the fate of pollutants (Papastefanou and Ioannidou, 1996). Beryllium-7 is a relative short lived (T 1=2 ¼ 53:3 d) naturally occurring radionuclide of cosmogenic origin which is formed in the upper troposphere and lower stratosphere by spallation reactions of light atmospheric nuclei of nitrogen and oxygen with cosmic rays (Lal et al., 1958) and its flux to the Earth’s surface has a latitudinal dependence (Lal and Peters, 1967). 7Be production has negligible dependence on season and longitude, and varies with the 11-year solar cycle (Ho¨tzl et al., 1991; Ioannidou and Papastefanou, 1995; Megumi et al., 2000).

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Lead-210, a long-lived radionuclide (T 1=2 ¼ 22:6 y) is produced in the atmosphere near ground level by the decay of its precursor 222Rn (T 1=2 ¼ 3:82 d), an inert noble gas, which is released from the ground to the atmosphere predominantly from continental surfaces. Once 7Be and 210Pb are formed, they rapidly associate primarily with submicron-sized aerosol particles. Both radionuclides participate in the formation and growth of the accumulation mode aerosols (0.07–2 mm diameter), which is a major reservoir of pollutants in the atmosphere. Following their production by gas-phase nuclear transformation, these isotopes condense on the aerosol particles, growing by condensation of non-radioactive species and the fate of 7Be and 210Pb will become the fate of the carrier aerosols (Bondietti et al., 1988). By measurements in ground-level air, using cascade impactors, Papastefanou and Ioannidou, (1995) estimated the AMAD of 7Be of 0.9 mm resulting in a mean residence time of about 7–9 days. Similar results were reported by Yu and Lee (2002). Gru¨ndel and Porstendo¨rfer (2004) found that the long-lived 210Pb is absorbed almost all on aerosol particles in the accumulation mode with an average AMAD value of 0.55 mm, while a greater AMAD value of 0.702 mm was measured for 7Be aerosols. In the case of 7Be the bigger particle size are probably due to the formation in the upper region of the atmosphere. Since 7Be is of cosmogenic origin and its production rate is high in the upper troposphere and decreases with atmospheric depth, its concentrations in air increase with altitude. On the contrary, the concentrations of 210 Pb in air decrease with elevation from the ground, due to its higher production rate in the lower troposphere. The unique features of these two radionuclides with altitudinal distinct sources, suggest that they may allow assessment of the relative importance of stratospheric and tropospheric transport pathways to the levels of concentrations in surface air (Rehfeld and Heimann, 1995; Dibb et al., 2003) and make them ideal tools to depict transport processes in the whole atmosphere (Brost et al., 1991; Feichter et al., 1991; Dibb et al., 1992; Koch et al., 1996). Improved understanding of their atmospheric distributions obtained from detailed measurements will facilitate refinement and validation of global circulation models. In the temperate zones with very dry climate at east longitudes in the European Continent of the Northern Hemisphere, there is a lack of sufficient data of parallel measurements of 7Be and 210Pb in ground-level air. The main objective of this work is to examine the sources and transport mechanisms of these two radionuclides, which in combination with meteorological and climatological data would help us to elucidate the causes of temporal changes of their concentrations. For this purpose, a program for monitoring 7Be and 210Pb in ground-level air for almost 15 years since July 1987 was

implemented in the region of Thessaloniki, Northern Greece (401380 N, 221580 E).

2. Instrumentation Beryllium-7 and lead-210 concentrations were measured by performing air sampling, using Staplex highvolume air samplers with Staplex type TFAGF 810 glass-fiber filters 800  1000 and having 99.28% collection efficiency for particles as small as 0.3 mm. This design involves a regulated air-flow rate of 1.7–1.92 m3 min1 (60–68 ft3 min1). The length of each collection period was 24 h. Air samplings were carried out on the roof (20 m height) of the Faculty of Science building, University of Thessaloniki at Thessaloniki, Greece. After the air-sampling procedure, the filters are folded and compressed by means of hydraulic press at up to 3 t to obtain a cylinder 5.8 cm in diameter and 2 mm in height. All the samples were measured for 7Be activity (Eg ¼ 477 keV) using a high resolution (1.9 keV at 1.33 MeV), high-efficiency (42%), low-background HPGe detector and for 210Pb (Eg ¼ 46:50 keV) using a Ge planar detector with active area 2000 mm2, thickness 20 mm, energy resolution (FWHM) 400 eV at 5.9 keV or 700 eV at 122 keV, Be window 0.5 mm thin. The average total uncertainty for 7Be measurements was 8%, while for 210Pb was 10%. For 7Be the accumulation time for the gspectra was varied from 8  104 s up to 2  105 s, depending on the 7Be concentration in air, while for the 210 Pb spectra the accumulation time was 2  105 s.

3. Results and discussion 3.1. 7Be and

210

Pb concentration levels in surface air

Aerosol sampling for measuring activity concentrations of 7Be and 210Pb started on July 1987. The activity concentrations of 7Be (mBq m3) and 210Pb (mBq m3) for the years 1987 to 2001 are summarized in Table 1, where the arithmetic mean, the standard deviation and the range of both radionuclides are given. Mean activity concentrations of 7Be and 210Pb in surface air were 5.0272.49 mBq m3 and 6647350 mBq m3,

Table 1 Mean activity concentrations of 7Be (mBq m3) and 210Pb (mBq m3) for the years 1987–2001 at Thessaloniki, Greece (401380 N, 221580 E) Isotope

Mean7S.D.

Range

7

5.0272.49 6647350

0.47–12.70 108–1982

Be Pb

210

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12

10

8

6

4

2

7

Be atmospheric concentrations (mBq m-3)

14

(a)

Jan'02

Jan'01

Jan'00

Jan'99

Jan'98

Jan'97

Jan'96

Jan'95

Jan'94

Jan'93

Jan'92

Jan'91

Jan'90

Jan'89

Jan'88

Jan'87

0

Month

2000

1500

1000

500

210

Pb atmospheric concentrations (µBq m-3)

2500

(b)

Jan'02

Jan'01

Jan'00

Jan'99

Jan'98

Jan'97

Jan'96

Jan'95

Jan'94

Jan'93

Jan'92

Jan'91

Jan'90

Jan'89

Jan'88

Jan'87

0

Month

Fig. 1. (a) Concentrations of 7Be in surface air in Thessaloniki, Greece (401380 N, 221580 E) from July 1987 since December 2001. (b) Concentrations of 210Pb in surface air in Thessaloniki, Greece (401380 N, 221580 E) from July 1987 since December 2001.

respectively. Figs. 1a and 1b show the atmospheric concentrations of 7Be and 210Pb each month, respectively for the period 1987–2001. Feely et al. (1989) reported at the same latitude in New York City (401730 N) 4.55 mBq m3 for 7Be averaging for the period 1970–1985, while McNeary and Baskaran (2003) at a site in the southwest area of Detroit, Michigan (421 250 N 831 10 E) 175 m above mean sea level at 1 m above the ground reported 4.83 mBq m3 for 7Be, averaged for the period October 1999–February 2001. It must be noted that the environmental concentration of 7Be in the temperate zones is about 3 mBq m3 in surface air (UNSCEAR, 1982), while the concentration of 7Be in the troposphere on a global scale is 12.5 mBq m3 (UNSCEAR, 2000).

Ho¨tzl and Winkler (1996) reported mean monthly values 6407250 mBq m3 for 210Pb concentrations in ground-level air at Munich–Neuherberg (481 80 N, 111350 E) for the period 1982 to 1992, while McNeary and Baskaran (2003) reported 1150 mBq m3 for 210Pb concentrations in air. Time variations of 7Be and 210Pb surface air concentrations were studied. Atmospheric processes and meteorological parameters that may contribute to these variations are discussed below. 3.2. Variability of 7Be concentrations in surface air 7 Be in air is influenced by the 11-year cycle of solar activity with its concentrations to be inversely related to

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sunspot number (Ioannidou and Papastefanou, 1994; Ioannidou and Papastefanou, 1995; Papastefanou and Ioannidou, 2004). A correlation coefficient between the yearly averaged 7Be concentrations and the sunspot number of 0.81 was found. During the years of minimum and maximum of solar activity the yearly average concentrations of 7Be present a difference of about 45% (4.2–6.1 mBq m3). O’Brien et al. (1991) reported that the amplitude modulation of the production rate of 7Be due to the 11-year solar cycle is about 40% at mid-latitudes. From Fig. 1a it is evident that the concentrations of 7 Be in surface air show strong variations due to interactions on daily or even shorter time scales between injections of air masses from above. Averaging the data of 7Be atmospheric concentrations over the 15-year period of sampling on a monthly basis, the mean monthly concentrations of 7Be in air were obtained. In Fig. 2 the periodic pattern of mean monthly 7Be concentrations in surface air is presented, which reveals a distinct annual cycle. This periodic pattern of 7Be concentrations is exactly the same as that of 7Be data corrected for the 11-year solar cycle, since averaging data over a complete solar cycle tends to remove the effects on concentrations produced by the sunspot cycle modulation on the source. The mean monthly atmospheric concentrations of 7Be in surface air varied by a factor of 2 during the year, showing a strong seasonal trend with the highest values being observed in the summer months (7.29–6.96 mBq m3) and the lowest in the winter months (2.75–4.09 mBq m3). The 7Be data series were analysed in combination with a set of meteorological parameters. Multiple regression analysis was employed to examine the relative contributions of temperature, relative humidity and the amount of precipitation to the variability of 7Be concentrations.

9 8 7 6 5 4 3 2 1

7

Be concentrations in surface air (mBq m-3)

10

0 Jan Feb Mar Apr May Jun

Jul

Aug Sep Oct Nov Dec

--

Month

Fig. 2. Mean monthly atmospheric concentrations of 7Be averaged over the period 1987–2001. The error bars represent the standard deviation of the monthly values.

Since the 7Be concentrations are influenced by the 11year solar cycle modulation, the sunspot number as a contributed parameter was considered in the above analysis. The correlation coefficient for this regression was 0.63 and the adjusted R2 ¼ 0:39. The significance of the calculated correlation was statistically evaluated using t-test and F-test. The regression analysis between the four independent parameters and the 7Be concentrations was significant (F 4;168 ¼ 27:996; po0:0001). The analysis revealed that the rainfall amount had no any influence on the 7Be concentrations and that the inclusion of rainfall amount in the regression analysis did not improve the fit. The relative humidity had little impact on the 7Be activity, since it was strongly correlated with temperature (0.56) and rainfall amount (+0.45). The temperature and the sunspot number were the most significant parameters affecting the concentrations of 7Be in surface air. The partial correlation coefficients (95% of confidence level) of the 7Be— temperature and 7Be—sunspot number were (0.46, po0:0001) and (0.34, po0:0001), respectively. The adjusted R2 ¼ 0:39 indicates that the regression model explains about 40% of the 7Be variability. This means that other important effectiveness factors were not captured by the data. The results of the multiple regression analysis are presented in Table 2. However, when multiple regression analysis was performed on the mean monthly values of temperature, relative humidity, rainfall amount and 7Be concentrations in air over the time period 1987–2001, then the regression coefficient of 0.98 and the adjusted R2 of 0.96 (F 3;8 ¼ 79:84321, po0:0001) that was observed indicate that the considered model included all the important factors. The only significant parameter affecting the mean monthly concentrations of 7Be in air over the 15year period was the temperature. The strong dependence (r ¼ 0:95; po0:0001) of 7Be on the temperature (Fig. 3) reaching its higher values during the summer months was due to the increased rate of vertical transport of air masses within the troposphere during the warmer months. This vertical transport carries down to the surface layer 7Be that has been produced within the upper troposphere, where its activity concentration is higher (Feely et al., 1989). The analysis showed that the wet scavenging had no any influence on the levels of 7Be concentrations in surface air. This seems reasonable for the region of investigation, which was characterized for a long time by a relatively dry (precipitation free) climate (Papastefanou and Ioannidou, 1991) with total precipitation accumulation during 1987–2001 varying from 23.2 to 49.2 cm, averaging: 39.0 cm. However, an extremely high precipitation amount during the spring months of 2001, i.e. 51% of the annual precipitation amount had as a result a remarkable decrease of 7Be concentrations in surface air. The lowest ever observed value of

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Table 2 Results of multiple regression analysis between 7Be concentrations and temperature, T (1C), relative humidity, RH (%), rainfall amount (cm) and sunspot number

Intercept Temperature Relative humidity Rainfall amount Sunspot number

BETAa

Std. Error of BETA

Bb

Std. Error of B

t-valuesc

p-levelc

0.4796 0.1668 0.0633 0.2814

0.0724 0.0801 0.0677 0.0606

6.8657 0.1609 0.0522 0.0057 0.0129

1.9178 0.0243 0.0251 0.0061 0.0028

3.5799 6.6258 2.0833 0.9343 4.6450

0.0004 o0.0001 0.0387 0.3515 o0.0001

R ¼ 0:63243062. Adjusted R2 ¼ 0:38568203. F(4,168) ¼ 27.996; po0:00001 Std. Error of estimate: 1.9514. a BETA: the betas reflect the unique contribution of each independent variable. b B coefficients: represent the independent contribution of each independent variable to the prediction of the dependent variable after controlling for all other independent variables. c t-value with the p-level value: indicate if the relationship of each independent variable with the depend variable is statistically significant.

y=0.20595x + 1.64736 R=0.95, p<0.0001

Mean monthly atmospheric concentrations of 7Be (mBq m-3)

10 9 8 7 6 5 4 3 2 1 0 0

5

10

15

20

25

30

Mean monthly temperature, T(°C)

Fig. 3. Mean monthly atmospheric concentrations of 7Be versus temperature, T (1C). The error bars represent the standard deviation of the monthly values.

2.92 mBq m3, for the spring period, suggests that high amounts of rainfall should affect the concentration levels of 7Be in surface air. The highest 7Be activity concentrations during the warm season in the region of investigation were attributed to more efficient vertical transport of air masses in the warm season. A phenomenon that advocates to the high observed values during summer is the elevation of the tropopause during the warm summer months for midlatitudes (Gustafson et al., 1961; Gerasopoulos et al., 2001). For geomagnetic latitudes over l ¼ 401N, the elevation of tropopause during the warm summer months and the vertical transport of air masses within the troposphere are stronger (Parker, 1962). In order to confirm that phenomenon, the meteorological data provided by the Greek Meteorolo-

gical Service (EMY) were analysed for the corresponding time period. The analysis gave that the tropopause reaches its higher level, (up to 18 km, average 12 km), during the summer period, especially during July and August. On the other hand, during the winter months, where the temperatures are low, the atmosphere is more ‘‘stable’’, the tropopause height is lower and therefore the concentrations of 7Be in surface air are low i.e. more than 50% of the 7Be concentrations in air are less than 3 mBq m3. Finally, the sporadic high values of 7Be concentrations observed during springtime (Fig. 1), ranging between 8.5 and 11 mBq m3, were probably connected with stratosphere-to-troposphere exchange (Gustafson et al., 1961; Dutkiewicz and Husain, 1985). The combined effects of high 7Be production rates in the stratosphere, about 70%, (Lal and Peters, 1967) and the relatively rapid removal of aerosol-associated species from the troposphere, produce stratospheric 7Be concentrations about an order of magnitude higher than those just below the tropopause (Bhandari et al., 1966). Consequently, the stratosphere to troposphere exchange (STE) of air masses results in a significant increase of 7 Be in the troposphere during the spring period. 3.3. Variability of

210

Pb concentrations in surface air

Lead-210 is the first long-lived (T 1=2 ¼ 22:6 y) decay product of 222Rn, the first gas in the U-238 decay chain. Although 222Rn is mainly exhaled from the land surfaces, convective transport of radon-rich air from the boundary layer can reach the upper troposphere or even the stratosphere (Lambert et al., 1982; Kritz et al. 1993). Since atmospheric 222Rn is chemically inert and unscavenged and as a result is not removed from the atmosphere by physical or chemical means, and its halflife (T 1=2 ¼ 3:82 d) is much less than the mixing time of

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might reflect on high washout, since in the region of Thessaloniki, the higher amount of precipitation (32% of total annual precipitation) occurred during the spring months. The emanation of radon is strongly diminished when the soil is saturated with water, resulting in less production of 210Pb near ground-level air. The observed values of 210Pb concentrations during the summer period showed a weak increase, although the higher air mixing within the troposphere during the warm summer months was expected to deplete the 210Pb concentrations in ground-level air, since its concentrations in the air decrease with elevation from the earth’s surface. A slight positive correlation coefficient (r ¼ 0:40, po0:00854, 44 points) was observed between the monthly atmospheric concentrations of 210Pb and the temperature during the warm and dry summer months, which probably reflects the influence of temperature in increasing 210Pb production through its parent 222Rn increased exhalation from the ground. For the same period, a positive correlation coefficient (0.62, po0:0001, 44 points) between the monthly atmospheric concentrations of 210Pb and 7Be (Fig. 5) was found also. It should be noted that in order to compare the concentrations in air of these two radionuclides, the 7 Be concentrations were corrected by taking into account the 11-year solar cycle that reflects only on the 7Be values but not on the 210Pb values. In a first approximation the positive correlation between the monthly atmospheric concentrations of these two radionuclides during the summer period was unexpected due to their different production source terms. The observed relationship suggests that, for entirely different and unrelated reasons the major features in the atmospheric concentration patterns of 7Be and 210Pb during summer in the region of investigation turn out to be similar enough so that these two radionuclides could be used together as atmospheric tracers, since environmental

1500 1400

Pb concntrations in surface air (µBq m-3)

1300 1200 1100 1000 900 800 700 600 500 400 300 200 100 0 Jan Feb Mar Apr May Jun

Jul

Aug Sep Oct Nov Dec

--

Month

210

210

Pb concentrations in surface air (µBq m-3)

the atmosphere, its concentrations are greatest near the land surface and decrease with both altitude and distance from land. As a result the main source of 210 Pb in the free troposphere is a quiescent ascent of its gaseous 222Rn parent from the ground and upward transport of subspecies mobilized by resuspension and motion with the atmospheric masses, while its concentrations will be highest near surface. The observed periodic pattern of mean monthly concentrations of 210Pb in surface air over the 15-year period (Fig. 4) is generally different from that of 7Be. Given the magnitude of the standard deviations, it appears that there is actually little seasonality, indicating however higher mean monthly atmospheric concentrations of 210Pb during the autumn period (707– 932 mBq m3) and lowest during the spring (479– 529 mBq m3). Year-to-year variations in the atmospheric concentration pattern (Fig. 1b) did not significantly contradict the general trend observed for mean monthly concentrations through the years 1987–2001. The maximal values were observed during October–November each year, while the minima during March–April. Factors such as atmospheric pressure variations, temperature inversions, diurnal or seasonal variations in meteorological conditions, precipitations accumulation, soil moisture and ground coverage by snow and ice are known to affect the emanation rate of 222Rn from ground surface and thus the concentrations of 210Pb in groundlevel air (Turekian et al., 1977; Lambert et al., 1982; Feichter et al., 1991). The higher values of 210Pb during autumn might be attributed to the frequent inversion conditions of the surface layers, resulting in a build-up of radon and its decay products in ground-level air, while the relative low values during the winter months might be due to the low emanation of radon from the frozen or snow-covered soil (Ho¨tzl and Winkler, 1987). The minimal values of 210Pb concentrations during the spring

1600

y=78.05249x + 154.54465 R=0.62, p<0.0001

1400 1200 1000 800 600 400 200 0 0

2

4

6

8

10

12

14

7

Fig. 4. Mean monthly atmospheric concentrations of 210Pb averaged over the period 1987–2001. The error bars represent the standard deviation of the monthly values.

Be concentrations in surface air (mBq m-3)

Fig. 5. Monthly atmospheric concentrations of 7 Be, for the summer months.

210

Pb versus

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processes have a greater effect on their temporal atmospheric concentrations than do source processes. No correlation was found between the 210Pb concentrations in surface air and temperature, T (1C) (except summer period), the relative humidity, RH (%) and rainfall amount (cm). However, the very interesting and rare occasion of an extremely high precipitation amount of 51% during the spring period of 2001 had resulted in an extremely low atmospheric concentration of 210Pb. The lowest ever observed value of 210Pb 250 mBq m3, suggests that high amounts of rainfall, especially during spring period should affect the concentrations of 210Pb in surface air.

4. Conclusions Fifteen years of 7Be and 210Pb atmospheric concentrations measured at Thessaloniki, Greece were used together with meteorological data in order to study the mechanisms that govern their concentrations levels at temperate latitudes (401N). 7Be and 210Pb, two radionuclides of different origin, showed a different periodic pattern of atmospheric concentrations in surface air during a year. Monthly atmospheric concentrations of 7 Be showed strong seasonal trends with the highest values being observed in the warm summer and the lowest in the cold winter period. Highest values of the mean monthly atmospheric concentrations of 210Pb were observed in the autumn and lowest in the spring period. The highest observed 7Be values during summer period were correlated with the elevation of tropopause during the warm summer months and the vertical removal of air masses within the troposphere, which are both stronger during that period. The observed strong positive correlation coefficient between the mean monthly activity concentrations of 7Be, averaged over the period 1987–2001, and the temperature, T (1C), confirms that the increased rate of vertical transport within the troposphere, especially during the warmest months, has as a result to carry down to the surface layer air masses enriched in 7Be. The 210Pb maxima in autumn were attributed to the frequent inversion conditions of the surface air layers, resulting in a build-up (enrichment) of radon and its decay products in ground-level air. The slightly increased values during the warm and dry summer months probably reflected the influence of temperature in increasing 210Pb production through its parent 222Rn increased exhalation from the ground. The positive correlation that was observed between 210Pb and 7Be only for the summer period implies that their atmospheric removal behaviours are relatively similar during this period for the region of investigation, suggesting that these two radionuclides could be used together as tracers of environmental processes.

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Acknowledgements The authors are grateful to Professor Dr. Ya. Goutsidou for providing data from the Laboratory of Meteorology and Climatology Aristotle University of Thessaloniki, Greece.

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