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Activity size distributions for long-lived radon decay products in aerosols collected in Barcelona (Spain)
ARTICLE IN PRESS Applied Radiation and Isotopes 67 (2009) 872–875
Contents lists available at ScienceDirect
Applied Radiation and Isotopes journal homepage: www.elsevier.com/locate/apradiso
Activity size distributions for long-lived radon decay products in aerosols collected in Barcelona (Spain) A. Camacho , I. Valles, A. Vargas, M. Gonzalez-Perosanz, X. Ortega Institut de Te`cniques Energe`tiques, Universitat Polite`cnica de Catalunya, Diagonal 647, 08028 Barcelona, Spain
a r t i c l e in f o
a b s t r a c t
Keywords: 210 Po 210 Po Activity median aerodynamic diameters Airborne particulate
The activity median aerodynamic diameters (AMADs) of long-lived radon decay product (210Pb, 210Po) in aerosols collected in the Barcelona area (Northeast Spain) during the period from April 2006 to February 2008 are presented. The 210Po mean AMAD was 420 nm, while the 210Pb mean AMAD was 500 nm. The temporal evolution of 210Pb and 210Po AMADs shows maxima in autumn and winter and minima in spring and summer. 210Pb AMAD are being used to estimate the mean-residence time of atmospheric aerosols. & 2009 Elsevier Ltd. All rights reserved.
1. Introdution The natural radioactive gas 222Rn, after being generated in the Earth’s crust by the 238U series, may exhale to the atmosphere, where it produces short-lived (218Po, 214Pb, 214Bi, 214Po) and longlived (210Pb, 210Bi, 210Po) radioactive decay products. Following formation, the radionuclides attach to the aerosol population and their fate will follow that of the carrier aerosols. 210 Pb has been used to study the description of environmental processes such as aerosol transport and residence times in the ˜ as et al., 2004; troposphere (Baskaran and Sha, 2001; Duen Papastefanou, 2006) together with aerosol deposition velocities ˜ as et al., 2005). (Duen The aim of this study is to find out the activity median aerodynamic diameters (AMADs) for long-lived radon progeny (210Pb, 210Po) in aerosols collected in Barcelona (Spain). Atmospheric aerosol size distribution basically consists of three separate modes (Papastefanou, 2008): Aitken nuclei mode (particles smaller than 0.1 mm), accumulation mode (particles between 0.1 and 2 mm) and coarse mode (particles larger than 2 mm). However, Gru¨ndel and Postendo¨rfer (2004) showed that the long-lived radon decay products 210Po and 210Pb were almost all (93–96%) adsorbed on aerosol particles in the accumulation size. The authors did not know data concerning the activity size distribution of aerosols collected in Spain. Detailed information on the activity size distributions of 210Pb and their variation with time is required to obtain information on the residence time of aerosols (Winkler et al., 1998; Papastefanou, 2006). Furthermore, a measure of the activity ratios of 210Po/210Pb can be used to
Corresponding author. Tel.: +34 934011993; fax: +34 934017149.
E-mail address: [email protected] (A. Camacho). 0969-8043/$ - see front matter & 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.apradiso.2009.01.048
estimate the apparent atmospheric residence times of the ˜ as et al., 2004). attached particles (Marley et al., 2000; Duen
2. Materials and methods 2.1. Micro-orifice uniform deposit cascade impactor (MOUDI) The MOUDI was used for aerosol sampling. This device consists of nine stages and the inlet and operates at a flow rate of 30 L min1. The effective aerodynamic cut-off diameters in mm and the size range in brackets are: 18 [24–13] (S1), 9.9 [13–7.8] (S2), 6.2 [7.8–4.8] (S3), 3.1 [4.8–2.4] (S4), 1.8 [2.4–1.3] (S5), 1.0 [1.3–0.74] (S6), 0.56 [0.74–0.41] (S7), 0.32 [0.41–0.24] (S8), 0.18 [0.24–0.13] (S9) and 0.057 [0.13–0.01] (F). Aerosol samples were collected on aluminum foil for stages S1–S9 and on a micro-fiber paper filter for stage F. Aerosol samples from MOUDI (MOUDI-filters) were collected in Barcelona (northeast coast of Spain) twice a month from April 2006 to February 2008 and an air volume of approximately 600 m3 was collected during each sampling period or campaign. After 5 days the short-lived 220Rn and 222Rn daughters are assumed to have nearly completely decayed and the long-lived 222 Rn daughters may then be identified through gross-alpha and ˜ as et al., 2004). Therefore 210Pb and gross-beta counting (Duen 210 Po activities in the MOUDI-filters were determined by measuring gross-alpha and gross-beta activities by a proportional counter (Berthold LB 770-2) after a decay time between 15 and 21 days. The counting time was usually 1400 min and minimum detectable activity (MDA) at 95% confidence level was 1.4 mBq m3 for 210Po and 2.4 mBq m3 for 210Pb. The counting uncertainty ranged from about 10% for activities for the S7–S9 filters to approximately 40% for the S1–S4 filters.
ARTICLE IN PRESS A. Camacho et al. / Applied Radiation and Isotopes 67 (2009) 872–875
The AMAD were calculated by the Grapher V-5 software using the data of the measured activity in each stage (Ai) after modifying it taking into account the total activity (A) in the campaign (DR ¼ Ai/A) and the range size of the impactor stage ½D log Dp ¼ log size range ¼ logðDphigh =Dplow Þ. These data were approximated by a log-normal distribution characterized by the AMAD and geometric standard deviations (sg) (Gru¨ndel and Postendo¨rfer, 2004).
2.2. High volume filters Airborne particulate sampling was carried out at the same location by pumping air through G3 polypropylene filters (high-volume filters) with a surface area of 44 44 cm using an ASS-500 sampler pump station. The filters were changed weekly and the average weekly volume usually ranged from 70 000 to 100 000 m3. 210 Pb activity in the high-volume filters was measured with intrinsic germanium detectors with a counting time longer than two days and at approximately 15 days of elapsed time. The counting uncertainty was 14% and the 210Pb MDA at 95% confidence level was 9 mBq m3. Statistical analyses were carried out using SPSS V-15 software.
3. Results and discussion 3.1. Validation of the gross-alpha (210Po) and gross-beta (210Pb) results In order to validate that the gross-beta results were produced by 210Pb and the gross-alpha results by 210Po some actions were done. The 210Pb content measured by gamma counting in highvolume filters collected at the same place and period was compared with the gross-beta activities in the MOUDI filters. Table 1 shows the groups of data to be compared. The sum of gross-beta activity in MOUDI samples was obtained by the sum of the beta activities on all impactor stages and on the back-up filter in each campaign. Application of the T-test for independent samples to the data presented in Table 1 reported a significance value (2-tailed) of 0.944 (greater than 0.05) which concludes that no statistically significant differences between groups exist. On the other hand, comparison of mean gross-beta activities and 210 Pb content measured by gamma spectrometry at each stage was done. As 210Pb content in stages was very low, some campaigns needed to accumulate in order to obtain activities that could be measured. Table 2 shows data to be compared. The application of the T-test for dependent samples to data reported a significance value (2-tailed) of 0.544 which concludes that no statistically significant differences between groups exist.
Table 1 Sum of gross beta activities in MOUDI filters and filters.
210
Pb activities in high volume
Campaign
Sum of gross-beta activity (MOUDI filters, mBq m3)
210 Pb activity (high volume filters, mBq m3)
C-16 C-17 C-18 C-19 C-20 C-21
533 459 591 523 301 500
492 497 505 601 298 539
Table 2 Mean gross-beta activities and
873
210
Pb activities in stages of MOUDI filters.
Campaign
Stage
Gross–beta activity (mBq m3)
210 Pb activity (mBq m3)
C1–C10
S7 S8 S9 F
226 93.1 31.3 40.5
260 76.4 34.8 33.9
C11–C13
S4 S5 S6 S7 S8 S9 F
12.8 16.5 50.7 150 195 151 58.3
5.5 8.8 21.0 103 137 91.8 44.8
C14–C17
S4 S5 S6 S7 S8 S9 F
11.4 16.2 97.3 230 138 36.7 46.6
45.9 32.0 159 286 195 118 57.7
To ensure that gross-alpha activities were produced by 210Po, the growth of the gross-alpha activity with time in all the stages was measured. The growth of activity shows a half life of 138.4 days for stages S6 to F which indicates that in these stages alpha activities were produced by 210Po. Further studies will be done to find out which radionuclides produced alpha activities in stages S1–S5. 3.2. Gross-alpha (210Po) and gross-beta (210Pb) activities in MOUDI filters Twenty-one sets of MOUDI-filters were obtained in Barcelona (Spain) from April 2006 to February 2008. Table 3 shows the mean, minima and maxima activities, standard deviation and number of data (n) for 210Po and 210Pb for each stage for the sampling period. Furthermore, the sum of gross-alpha and grossbeta activities are shown, which were calculated by summing the mean value from each stage. 210Po levels ranged from 1.6 to 15.8 mBq m3 and 210Pb activities ranged from 3.5 to 368 mBq m3. Maximum activities for 210Po and 210Pb were detected for the S7 stage (76.2% of the campaigns) and for the S8 stage (19.0% of the campaigns). A wide range of 210Pb activities were measured in all the stages and campaigns with most of the 210Pb occurring in the fineaerosol fractions in contrast to 210Po activities which were similar in all the stages and campaigns. Sum of 210Po activities in surface air were observed to be lower by a factor of 10 than sum of 210Pb concentrations as found for other sites (Baskaran and Sha, 2001; ˜ as et al., 2004). Sum of 210Po and 210Pb activities were similar Duen to gross-alpha and gross-beta activities measured at other ˜ as et al., 2004; Vecchi et al., locations (Winkler et al., 1998; Duen 2005). The activity ratios of 210Po/210Pb were approximately 0.29 for coarse particles and about 0.07 for fine-aerosols. The high ratio in larger aerosols is presumably due to contributions from resuspended soil particles (Marley et al., 2000). 3.3. AMAD results The measured activity size distribution of the aerosols can be approximated by a sum of three log-normal distributions. But because on average, more than 85% of the total gross-alpha and
ARTICLE IN PRESS 874
Table 3 Results of Stage
S1 S2 S3 S4 S5 S6 S7 S8 S9 F
A. Camacho et al. / Applied Radiation and Isotopes 67 (2009) 872–875
210
Po and
210
Pb activities in size-fractionated aerosols from Barcelona (Spain).
Gross alpha (210Po) activity (mBq m3)
Gross beta (210Pb) activity (mBq m3)
Ratio
Mean
Standard deviation
Minima
Maxima
n
Mean
Standard deviation
Minima
Maxima
n
210
3.2 1.86 2.4 2.6 2.8 4.6 8.4 7.2 6.5 5.1
1.0 0.02 0.8 0.6 0.9 1.8 4.2 4.4 4.1 1.4
2.0 1.85 1.6 1.9 1.6 1.9 3.7 2.9 1.6 3.8
5.2 1.88 3.7 3.5 4.6 7.1 14.3 15.8 14.4 7.6
15 2 10 7 10 14 16 16 8 6
9.2 5.3 6.5 11.3 16.4 98.7 198 119 53.8 49.4
4.5 1.6 2.1 2.3 4.2 48.2 78.4 61.8 48.0 20.2
4.2 3.5 3.7 7.6 7.3 35.5 76.0 29.5 10.0 16.9
19.4 9.2 12.5 16.8 23.2 237 368 208 177 90.4
19 16 21 21 21 21 21 21 21 19
0.348 0.351 0.369 0.230 0.171 0.047 0.042 0.061 0.121 0.103
Sum of gross-alpha activities
44.7
Sum of gross-beta activities
Table 4 210 Po and
0.079
210
Campaign
C1 C2 C3 C4 C5 C6 C7 C8 C9 C10 C11 C12 C13 C14 C15 C16 C17 C18 C19 C20 C21
Pb AMAD in aerosols collected in Barcelona (Spain). Sampling period
210
210
Start
Finish
AMAD (mm)
sg
AMAD (mm)
sg
24-4-06 22-6-06 2-10-06 17-10-06 3-11-06 20-11-06 20-12-06 8-1-07 23-1-7 8-2-07 3-5-07 22-5-07 27-7-07 3-10-07 18-10-07 31-10-07 14-11-07 12-12-07 27-12-07 10-1-08 23-1-08
12-5-06 14-7-06 16-10-06 2-11-06 17-11-06 5-12-06 4-1-07 22-1-07 7-2-07 23-2-07 21-5-07 5-6-07 6-8-07 17-10-07 31-10-07 14-11-07 28-11-07 27-12-07 10-1-08 23-1-08 7-2-08
0.38 0.45 0.41 – 0.55 0.57 0.52 0.45 – 0.42 0.37 0.24 0.33 0.36 0.36 0.44 0.44 0.38 0.34 – 0.56
1.72 1.91 1.46 – 1.69 1.76 1.40 1.59 – 1.57 1.70 2.21 1.97 1.53 1.45 1.58 1.78 2.35 2.00 – 2.26
0.42 0.52 0.51 0.57 0.59 0.70 0.60 0.60 0.80 0.60 0.34 0.29 0.32 0.40 0.47 0.53 0.56 0.45 0.33 0.32 0.52
1.70 1.67 1.49 1.46 1.54 1.58 1.56 1.57 2.05 1.59 1.92 1.85 1.93 1.64 1.64 1.58 1.72 1.96 1.54 1.48 1.85
Po
Pb
210
Pb. Subscript ‘‘a’’ refers to accumulation
gross-beta activities are associated with particles in the accumulation mode, data were approximated by a log-normal distribution. Fig. 1 shows the aerodynamic size distribution of grossbeta activities obtained for the C21 campaign. Table 4 shows the calculated AMAD and the geometric standard deviation for 210Po and 210Pb during each campaign. The 210Po mean AMAD was 420 nm and showed a range from 240 to 570 nm. 210Pb mean AMAD was 500 nm with a range from 290 to 800 nm. 210Pb AMAD measured in Barcelona (Spain) was similar to that obtained in USA or Germany (Winkler et al., 1998; Marley et al., 2000; Postendorfer et al., 2000; Gru¨ndel and Postendo¨rfer, 2004). In some campaigns experimental data from beta counting can be approximated by the sum of two log-normal distributions showing the accumulation mode and the coarse mode, because a small, but significant, fraction of the beta activity (average value 8%) was attached to coarse particles (41000 nm) as was explained by Postendo¨rfer et al. (2000). It should be commented that in January 2008 the coarse mode was obtained in all the campaigns due to the incorporation of particles produced in the demolition of some surrounding buildings.
0.60
Po-210 AMAD Pb-210 AMAD
0.50 AMAD (µm)
Fig. 1. Activity size distribution of mode and ‘‘c’’ to coarse mode.
567.6
Po/210Pb
0.40 0.30 0.20 0.10 0.00
Autumn
Winter
Spring
Summer
Season Fig. 2. Seasonal variation of
210
Po and
210
Pb AMAD.
The temporal evolution of 210Pb and 210Po AMADs shows maxima in autumn and winter and minima in spring and summer (Winkler et al., 1998). AMAD results (mean, minimum, and maximum) versus the season are presented in Fig. 2.
ARTICLE IN PRESS A. Camacho et al. / Applied Radiation and Isotopes 67 (2009) 872–875
875
3.4. Future applications
Acknowledgment
The mean residence time (MRT) of aerosols collected in Barcelona are estimated by using two methods: the 222Rn daughter activity ratio method (Marley et al., 2000; Baskaran ˜ as et al., 2004) and the AMAD method and Sha, 2001; Duen (Papastefanou, 2006). Preliminary results show an MRT of 22.5 days by the activity ratio method and an MRT of 15.4 days by the AMAD method.
This work has been supported by the Spanish Education Science Ministry (MEC) through the research project CGL200504182/CLI.
4. Conclusions
Data comparison between results obtained using different
counting or sampling devices confirm that for stages S4 to F the gross-beta activities of the MOUDI-filters were from 210Pb radionuclide. Also data confirm that for stages S6 to F grossalpha activities were from 210Po. Maxima activities for 210Po and 210Pb were detected for the 560 mm cut-off diameter stage (76.2% of the campaigns) and for the 320 mm cut-off diameter stage (19% of the campaigns). The activity ratios of 210Po/210Pb were approximately 0.29 for coarse particles and about 0.07 for fine aerosols. The high ratio in larger aerosols is presumably due to contributions from resuspended soil particles. The 210Po mean AMAD was 420 nm, while the 210Pb mean AMAD was 500 nm. These values were similar to those obtained at other sites. In January 2008 the coarse mode was obtained in all the campaigns due to the incorporation of particles produced in the demolition of some surrounding buildings. The temporal evolution of 210Pb and 210Po AMADs shows maxima in autumn and winter and minima in spring and summer.
References Baskaran, M., Shaw, G.E., 2001. Residence time of arctic haze aerosols using the concentrations and activity ratios for 210Po, 210Pb and 7Be. Journal of Aerosol Science 32, 443–452. ˜ as, C., Ferna´ndez, M.C., Carretero, J., Liger, E., Can ˜ ete, S., 2004. Long-term Duen variation of the concentrations of long-lived Rn descendants and cosmogenic 7 Be and determination of the MRT of aerosols. Atmospheric Environment 38, 1291–1301. ˜ as, C., Ferna´ndez, M.C., Carretero, J., Liger, E., Can ˜ ete, S., 2005. Deposition Duen velocities and washout ratios on a coastal site (southeastern Spain) calculated 7 210 from Be and Pb measurements. Atmospheric Environment 39, 6897–6908. Gru¨ndel, M., Postendo¨rfer, J., 2004. Differences between the activity size distributions of the different natural radionuclide aerosols in outdoor air. Atmospheric Environment 38, 3723–3728. Marley, N.A., Gaffney, J.S., Drayton, P.J., Cunningham, M.M., Orlandini, K.A., Paode, R., 2000. Measurement of 210Pb, 210Po and 210Bi in size-fractionated atmospheric aerosols: an estimate of fine-aerosol residence times. Aerosol Science and Technology 32, 569–583. Papastefanou, C., 2006. Residence time of tropospheric aerosol in association with radioactive nuclides. Applied Radiation and Isotopes 64, 93–100. Papastefanou, C., 2008. Atmospheric aerosol particles. In: Baxter, M.S. (Ed.), Radioactivity in the Environment, vol. 12, p. 5. Postendo¨rfer, J., Zock, Ch., Reineking, A., 2000. Aerosol size distribution of the radon progeny in outdoor air. Journal of Environmental Radioactivity 51, 37–48. Vecchi, R., Marcazzan, G., Valli, G., 2005. Seasonal variation of 210Pb activity concentration in outdoor air of Milan (Italy). Journal of Environmental Radioactivity 82, 251–266. Winkler, R., Dietl, F., Frank, G., Tschiersch, J., 1998. Temporal variation of 7Be and 210 Pb size distributions in ambient aerosol. Atmospheric Environment 32 (6), 983–991.
Contents lists available at ScienceDirect
Applied Radiation and Isotopes journal homepage: www.elsevier.com/locate/apradiso
Activity size distributions for long-lived radon decay products in aerosols collected in Barcelona (Spain) A. Camacho , I. Valles, A. Vargas, M. Gonzalez-Perosanz, X. Ortega Institut de Te`cniques Energe`tiques, Universitat Polite`cnica de Catalunya, Diagonal 647, 08028 Barcelona, Spain
a r t i c l e in f o
a b s t r a c t
Keywords: 210 Po 210 Po Activity median aerodynamic diameters Airborne particulate
The activity median aerodynamic diameters (AMADs) of long-lived radon decay product (210Pb, 210Po) in aerosols collected in the Barcelona area (Northeast Spain) during the period from April 2006 to February 2008 are presented. The 210Po mean AMAD was 420 nm, while the 210Pb mean AMAD was 500 nm. The temporal evolution of 210Pb and 210Po AMADs shows maxima in autumn and winter and minima in spring and summer. 210Pb AMAD are being used to estimate the mean-residence time of atmospheric aerosols. & 2009 Elsevier Ltd. All rights reserved.
1. Introdution The natural radioactive gas 222Rn, after being generated in the Earth’s crust by the 238U series, may exhale to the atmosphere, where it produces short-lived (218Po, 214Pb, 214Bi, 214Po) and longlived (210Pb, 210Bi, 210Po) radioactive decay products. Following formation, the radionuclides attach to the aerosol population and their fate will follow that of the carrier aerosols. 210 Pb has been used to study the description of environmental processes such as aerosol transport and residence times in the ˜ as et al., 2004; troposphere (Baskaran and Sha, 2001; Duen Papastefanou, 2006) together with aerosol deposition velocities ˜ as et al., 2005). (Duen The aim of this study is to find out the activity median aerodynamic diameters (AMADs) for long-lived radon progeny (210Pb, 210Po) in aerosols collected in Barcelona (Spain). Atmospheric aerosol size distribution basically consists of three separate modes (Papastefanou, 2008): Aitken nuclei mode (particles smaller than 0.1 mm), accumulation mode (particles between 0.1 and 2 mm) and coarse mode (particles larger than 2 mm). However, Gru¨ndel and Postendo¨rfer (2004) showed that the long-lived radon decay products 210Po and 210Pb were almost all (93–96%) adsorbed on aerosol particles in the accumulation size. The authors did not know data concerning the activity size distribution of aerosols collected in Spain. Detailed information on the activity size distributions of 210Pb and their variation with time is required to obtain information on the residence time of aerosols (Winkler et al., 1998; Papastefanou, 2006). Furthermore, a measure of the activity ratios of 210Po/210Pb can be used to
Corresponding author. Tel.: +34 934011993; fax: +34 934017149.
E-mail address: [email protected] (A. Camacho). 0969-8043/$ - see front matter & 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.apradiso.2009.01.048
estimate the apparent atmospheric residence times of the ˜ as et al., 2004). attached particles (Marley et al., 2000; Duen
2. Materials and methods 2.1. Micro-orifice uniform deposit cascade impactor (MOUDI) The MOUDI was used for aerosol sampling. This device consists of nine stages and the inlet and operates at a flow rate of 30 L min1. The effective aerodynamic cut-off diameters in mm and the size range in brackets are: 18 [24–13] (S1), 9.9 [13–7.8] (S2), 6.2 [7.8–4.8] (S3), 3.1 [4.8–2.4] (S4), 1.8 [2.4–1.3] (S5), 1.0 [1.3–0.74] (S6), 0.56 [0.74–0.41] (S7), 0.32 [0.41–0.24] (S8), 0.18 [0.24–0.13] (S9) and 0.057 [0.13–0.01] (F). Aerosol samples were collected on aluminum foil for stages S1–S9 and on a micro-fiber paper filter for stage F. Aerosol samples from MOUDI (MOUDI-filters) were collected in Barcelona (northeast coast of Spain) twice a month from April 2006 to February 2008 and an air volume of approximately 600 m3 was collected during each sampling period or campaign. After 5 days the short-lived 220Rn and 222Rn daughters are assumed to have nearly completely decayed and the long-lived 222 Rn daughters may then be identified through gross-alpha and ˜ as et al., 2004). Therefore 210Pb and gross-beta counting (Duen 210 Po activities in the MOUDI-filters were determined by measuring gross-alpha and gross-beta activities by a proportional counter (Berthold LB 770-2) after a decay time between 15 and 21 days. The counting time was usually 1400 min and minimum detectable activity (MDA) at 95% confidence level was 1.4 mBq m3 for 210Po and 2.4 mBq m3 for 210Pb. The counting uncertainty ranged from about 10% for activities for the S7–S9 filters to approximately 40% for the S1–S4 filters.
ARTICLE IN PRESS A. Camacho et al. / Applied Radiation and Isotopes 67 (2009) 872–875
The AMAD were calculated by the Grapher V-5 software using the data of the measured activity in each stage (Ai) after modifying it taking into account the total activity (A) in the campaign (DR ¼ Ai/A) and the range size of the impactor stage ½D log Dp ¼ log size range ¼ logðDphigh =Dplow Þ. These data were approximated by a log-normal distribution characterized by the AMAD and geometric standard deviations (sg) (Gru¨ndel and Postendo¨rfer, 2004).
2.2. High volume filters Airborne particulate sampling was carried out at the same location by pumping air through G3 polypropylene filters (high-volume filters) with a surface area of 44 44 cm using an ASS-500 sampler pump station. The filters were changed weekly and the average weekly volume usually ranged from 70 000 to 100 000 m3. 210 Pb activity in the high-volume filters was measured with intrinsic germanium detectors with a counting time longer than two days and at approximately 15 days of elapsed time. The counting uncertainty was 14% and the 210Pb MDA at 95% confidence level was 9 mBq m3. Statistical analyses were carried out using SPSS V-15 software.
3. Results and discussion 3.1. Validation of the gross-alpha (210Po) and gross-beta (210Pb) results In order to validate that the gross-beta results were produced by 210Pb and the gross-alpha results by 210Po some actions were done. The 210Pb content measured by gamma counting in highvolume filters collected at the same place and period was compared with the gross-beta activities in the MOUDI filters. Table 1 shows the groups of data to be compared. The sum of gross-beta activity in MOUDI samples was obtained by the sum of the beta activities on all impactor stages and on the back-up filter in each campaign. Application of the T-test for independent samples to the data presented in Table 1 reported a significance value (2-tailed) of 0.944 (greater than 0.05) which concludes that no statistically significant differences between groups exist. On the other hand, comparison of mean gross-beta activities and 210 Pb content measured by gamma spectrometry at each stage was done. As 210Pb content in stages was very low, some campaigns needed to accumulate in order to obtain activities that could be measured. Table 2 shows data to be compared. The application of the T-test for dependent samples to data reported a significance value (2-tailed) of 0.544 which concludes that no statistically significant differences between groups exist.
Table 1 Sum of gross beta activities in MOUDI filters and filters.
210
Pb activities in high volume
Campaign
Sum of gross-beta activity (MOUDI filters, mBq m3)
210 Pb activity (high volume filters, mBq m3)
C-16 C-17 C-18 C-19 C-20 C-21
533 459 591 523 301 500
492 497 505 601 298 539
Table 2 Mean gross-beta activities and
873
210
Pb activities in stages of MOUDI filters.
Campaign
Stage
Gross–beta activity (mBq m3)
210 Pb activity (mBq m3)
C1–C10
S7 S8 S9 F
226 93.1 31.3 40.5
260 76.4 34.8 33.9
C11–C13
S4 S5 S6 S7 S8 S9 F
12.8 16.5 50.7 150 195 151 58.3
5.5 8.8 21.0 103 137 91.8 44.8
C14–C17
S4 S5 S6 S7 S8 S9 F
11.4 16.2 97.3 230 138 36.7 46.6
45.9 32.0 159 286 195 118 57.7
To ensure that gross-alpha activities were produced by 210Po, the growth of the gross-alpha activity with time in all the stages was measured. The growth of activity shows a half life of 138.4 days for stages S6 to F which indicates that in these stages alpha activities were produced by 210Po. Further studies will be done to find out which radionuclides produced alpha activities in stages S1–S5. 3.2. Gross-alpha (210Po) and gross-beta (210Pb) activities in MOUDI filters Twenty-one sets of MOUDI-filters were obtained in Barcelona (Spain) from April 2006 to February 2008. Table 3 shows the mean, minima and maxima activities, standard deviation and number of data (n) for 210Po and 210Pb for each stage for the sampling period. Furthermore, the sum of gross-alpha and grossbeta activities are shown, which were calculated by summing the mean value from each stage. 210Po levels ranged from 1.6 to 15.8 mBq m3 and 210Pb activities ranged from 3.5 to 368 mBq m3. Maximum activities for 210Po and 210Pb were detected for the S7 stage (76.2% of the campaigns) and for the S8 stage (19.0% of the campaigns). A wide range of 210Pb activities were measured in all the stages and campaigns with most of the 210Pb occurring in the fineaerosol fractions in contrast to 210Po activities which were similar in all the stages and campaigns. Sum of 210Po activities in surface air were observed to be lower by a factor of 10 than sum of 210Pb concentrations as found for other sites (Baskaran and Sha, 2001; ˜ as et al., 2004). Sum of 210Po and 210Pb activities were similar Duen to gross-alpha and gross-beta activities measured at other ˜ as et al., 2004; Vecchi et al., locations (Winkler et al., 1998; Duen 2005). The activity ratios of 210Po/210Pb were approximately 0.29 for coarse particles and about 0.07 for fine-aerosols. The high ratio in larger aerosols is presumably due to contributions from resuspended soil particles (Marley et al., 2000). 3.3. AMAD results The measured activity size distribution of the aerosols can be approximated by a sum of three log-normal distributions. But because on average, more than 85% of the total gross-alpha and
ARTICLE IN PRESS 874
Table 3 Results of Stage
S1 S2 S3 S4 S5 S6 S7 S8 S9 F
A. Camacho et al. / Applied Radiation and Isotopes 67 (2009) 872–875
210
Po and
210
Pb activities in size-fractionated aerosols from Barcelona (Spain).
Gross alpha (210Po) activity (mBq m3)
Gross beta (210Pb) activity (mBq m3)
Ratio
Mean
Standard deviation
Minima
Maxima
n
Mean
Standard deviation
Minima
Maxima
n
210
3.2 1.86 2.4 2.6 2.8 4.6 8.4 7.2 6.5 5.1
1.0 0.02 0.8 0.6 0.9 1.8 4.2 4.4 4.1 1.4
2.0 1.85 1.6 1.9 1.6 1.9 3.7 2.9 1.6 3.8
5.2 1.88 3.7 3.5 4.6 7.1 14.3 15.8 14.4 7.6
15 2 10 7 10 14 16 16 8 6
9.2 5.3 6.5 11.3 16.4 98.7 198 119 53.8 49.4
4.5 1.6 2.1 2.3 4.2 48.2 78.4 61.8 48.0 20.2
4.2 3.5 3.7 7.6 7.3 35.5 76.0 29.5 10.0 16.9
19.4 9.2 12.5 16.8 23.2 237 368 208 177 90.4
19 16 21 21 21 21 21 21 21 19
0.348 0.351 0.369 0.230 0.171 0.047 0.042 0.061 0.121 0.103
Sum of gross-alpha activities
44.7
Sum of gross-beta activities
Table 4 210 Po and
0.079
210
Campaign
C1 C2 C3 C4 C5 C6 C7 C8 C9 C10 C11 C12 C13 C14 C15 C16 C17 C18 C19 C20 C21
Pb AMAD in aerosols collected in Barcelona (Spain). Sampling period
210
210
Start
Finish
AMAD (mm)
sg
AMAD (mm)
sg
24-4-06 22-6-06 2-10-06 17-10-06 3-11-06 20-11-06 20-12-06 8-1-07 23-1-7 8-2-07 3-5-07 22-5-07 27-7-07 3-10-07 18-10-07 31-10-07 14-11-07 12-12-07 27-12-07 10-1-08 23-1-08
12-5-06 14-7-06 16-10-06 2-11-06 17-11-06 5-12-06 4-1-07 22-1-07 7-2-07 23-2-07 21-5-07 5-6-07 6-8-07 17-10-07 31-10-07 14-11-07 28-11-07 27-12-07 10-1-08 23-1-08 7-2-08
0.38 0.45 0.41 – 0.55 0.57 0.52 0.45 – 0.42 0.37 0.24 0.33 0.36 0.36 0.44 0.44 0.38 0.34 – 0.56
1.72 1.91 1.46 – 1.69 1.76 1.40 1.59 – 1.57 1.70 2.21 1.97 1.53 1.45 1.58 1.78 2.35 2.00 – 2.26
0.42 0.52 0.51 0.57 0.59 0.70 0.60 0.60 0.80 0.60 0.34 0.29 0.32 0.40 0.47 0.53 0.56 0.45 0.33 0.32 0.52
1.70 1.67 1.49 1.46 1.54 1.58 1.56 1.57 2.05 1.59 1.92 1.85 1.93 1.64 1.64 1.58 1.72 1.96 1.54 1.48 1.85
Po
Pb
210
Pb. Subscript ‘‘a’’ refers to accumulation
gross-beta activities are associated with particles in the accumulation mode, data were approximated by a log-normal distribution. Fig. 1 shows the aerodynamic size distribution of grossbeta activities obtained for the C21 campaign. Table 4 shows the calculated AMAD and the geometric standard deviation for 210Po and 210Pb during each campaign. The 210Po mean AMAD was 420 nm and showed a range from 240 to 570 nm. 210Pb mean AMAD was 500 nm with a range from 290 to 800 nm. 210Pb AMAD measured in Barcelona (Spain) was similar to that obtained in USA or Germany (Winkler et al., 1998; Marley et al., 2000; Postendorfer et al., 2000; Gru¨ndel and Postendo¨rfer, 2004). In some campaigns experimental data from beta counting can be approximated by the sum of two log-normal distributions showing the accumulation mode and the coarse mode, because a small, but significant, fraction of the beta activity (average value 8%) was attached to coarse particles (41000 nm) as was explained by Postendo¨rfer et al. (2000). It should be commented that in January 2008 the coarse mode was obtained in all the campaigns due to the incorporation of particles produced in the demolition of some surrounding buildings.
0.60
Po-210 AMAD Pb-210 AMAD
0.50 AMAD (µm)
Fig. 1. Activity size distribution of mode and ‘‘c’’ to coarse mode.
567.6
Po/210Pb
0.40 0.30 0.20 0.10 0.00
Autumn
Winter
Spring
Summer
Season Fig. 2. Seasonal variation of
210
Po and
210
Pb AMAD.
The temporal evolution of 210Pb and 210Po AMADs shows maxima in autumn and winter and minima in spring and summer (Winkler et al., 1998). AMAD results (mean, minimum, and maximum) versus the season are presented in Fig. 2.
ARTICLE IN PRESS A. Camacho et al. / Applied Radiation and Isotopes 67 (2009) 872–875
875
3.4. Future applications
Acknowledgment
The mean residence time (MRT) of aerosols collected in Barcelona are estimated by using two methods: the 222Rn daughter activity ratio method (Marley et al., 2000; Baskaran ˜ as et al., 2004) and the AMAD method and Sha, 2001; Duen (Papastefanou, 2006). Preliminary results show an MRT of 22.5 days by the activity ratio method and an MRT of 15.4 days by the AMAD method.
This work has been supported by the Spanish Education Science Ministry (MEC) through the research project CGL200504182/CLI.
4. Conclusions
Data comparison between results obtained using different
counting or sampling devices confirm that for stages S4 to F the gross-beta activities of the MOUDI-filters were from 210Pb radionuclide. Also data confirm that for stages S6 to F grossalpha activities were from 210Po. Maxima activities for 210Po and 210Pb were detected for the 560 mm cut-off diameter stage (76.2% of the campaigns) and for the 320 mm cut-off diameter stage (19% of the campaigns). The activity ratios of 210Po/210Pb were approximately 0.29 for coarse particles and about 0.07 for fine aerosols. The high ratio in larger aerosols is presumably due to contributions from resuspended soil particles. The 210Po mean AMAD was 420 nm, while the 210Pb mean AMAD was 500 nm. These values were similar to those obtained at other sites. In January 2008 the coarse mode was obtained in all the campaigns due to the incorporation of particles produced in the demolition of some surrounding buildings. The temporal evolution of 210Pb and 210Po AMADs shows maxima in autumn and winter and minima in spring and summer.
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