- Email: [email protected]
Variation of atmospheric Be-7 in relation to PM concentrations
Applied Radiation and Isotopes 78 (2013) 82–87
Contents lists available at SciVerse ScienceDirect
Applied Radiation and Isotopes journal homepage: www.elsevier.com/locate/apradiso
Variation of atmospheric Be-7 in relation to PM concentrations J.H. Chao a,n, Y.J. Chiu b, H.P. Lee a, M.C. Lee a a b
Nuclear Science and Technology Development Center, National Tsing Hua University, Hsinchu 30013, Taiwan, ROC Hydrotech Research Institute, National Taiwan University, Taipei 10617, Taiwan, ROC
H I G H L I G H T S
The 7Be and PM concentrations in air were monitored simultaneously during 1998–2011. Both PM and 7Be concentrations increased as a result of the northeasterly monsoon. Both the lowest PM and 7Be concentrations were observed in July and August. The 7Be concentrations in surface air can be predicted from the PM concentrations.
art ic l e i nf o
a b s t r a c t
Article history: Received 14 December 2012 Accepted 12 April 2013 Available online 23 April 2013
In the present study, the influences of particulate matter (PM) and seasonal monsoons on 7Be concentrations in surface air (CBe) are elucidated. The 7Be and the corresponding PM concentrations in the air were monitored simultaneously throughout a 14-year period (1998–2011) in Hsinchu, Taiwan. During the autumn and winter seasons (Oct.–Feb.), both the PM and the 7Be concentrations increased as a result of the northeasterly monsoon. In contrast, the lowest PM and 7Be concentrations were observed in July and August. This timing is due to the occurrence of southwest monsoons, which carry air masses with low PM concentrations and are associated with depleted 7Be from low latitudes. The activity concentration of 7Be in the PM (APM) was used to explain the seasonal variations of 7Be with respect to the PM concentrations. In contrast, APM is not sensitive enough to vary with the seasons. The air masses transported by the monsoons are believed to be partially mixed with the PM locally produced in Taiwan, which explains their seasonal variations. The 7Be concentrations in surface air can be experimentally predicted from the PM concentrations based on CBe (mBq/m3) ¼0.0767 PM (μg/m3) across seasons. The annual averages of the PM and 7Be concentrations are 48.1 μg/m3 and 3.7 mBq/m3, respectively. The estimated CBe was either slightly overestimated or underestimated, depending on the season. The highest deviations occurred in July and August, when CBe was underestimated by 33%. & 2013 Elsevier Ltd. All rights reserved.
Keywords: Beryllium-7 Particulate matter Monsoons Radioactivity measurement
1. Introduction Beryllium-7 (7Be) is a cosmic-ray-produced radionuclide. Once Be is formed in the troposphere, it rapidly associates with suspended particles. Prior research indicates that more than 80% of 7Be is attached to particles smaller than 1.3 μm in diameter and that less than 10% is attached to particles larger than 10 μm (Ioannidou, 2011). An activity median aerodynamic diameter (AMAD) between 0.7 and 1.2 μm was estimated for these 7Beaerosols in the atmosphere (Papastefanou and Ioannidou, 1996; Papastefanou, 2006). The aerosols are transported by deposition and scavenging by precipitation is the primary mechanism for the transport of 7Be to the Earth's surface. The mean residence 7
n
Corresponding author. Tel.: +886 35165049; fax: +886 35122660. E-mail address: [email protected] (J.H. Chao).
0969-8043/$ - see front matter & 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.apradiso.2013.04.015
lifetimes of the aerosols were estimated to be 1–2 years for the stratosphere through the tropopause (Rehfeld and Heimann, 1995; Staley, 1982), 20–35 days for the troposphere (Bleichrodt, 1978; Koch et al., 1996), and less than 10 days for the lower atmosphere below precipitation cloud levels (Papastefanou, 2006). The production rate, deposition flux and atmospheric concentration of 7Be are primarily dependent upon latitude, altitude, solar activity and meteorological conditions (Gaggeler, 1995; Koch et al., 1996; Kulan et al., 2006; Masarik and Beer, 1999; Uematsu et al., 1994; Young and Silker, 1980). Seasonal variations of 7Be in surface air were frequently observed and usually resulted from the vertical transport of air masses associated with seasonal precipitation (Doering and Akber, 2008; Gonzalez-Gomez et al., 2006; Hernandez et al., 2004; Ioannidou et al., 2005) and the seasonally occurring horizontal transport of air masses containing different concentrations of 7Be across different latitudes (Chao et al., 2012; Megumi et al., 2000; Muramatsu et al., 2008; Narazaki et al., 2003).
J.H. Chao et al. / Applied Radiation and Isotopes 78 (2013) 82–87
Because of these characteristics, 7Be has been widely used as a chemical tracer in atmospheric and environmental sciences. Both the concentration and distribution of particulate matter (PM) are influenced by a combination of processes including anthropogenic sources such as traffic, industrial emissions and re-suspension of road dust and natural sources (Smith et al., 2001; Lenschow et al., 2001). The PM concentration in the environment has been a concern for decades due to its adverse health effects on humans (Pope, 2000; Wilson and Suh, 1997). As a result, PM concentrations have been routinely monitored in Taiwan since 1976. The particulate mass concentration, usually denoted as PM10, represents the sum of all fine particles (diameter o10 μm) from different emission sources and is a major factor in evaluating the pollutant standards index (PSI) in air. On winter days, Taiwan often experiences northeasterly monsoon winds originating in central Asia. Whereas in summer, southwesterly monsoon winds are prevalent. The winter monsoon not only brings cold air, it also transports high concentrations of PM over a long distance to Taiwan (Fang and Chang, 2010; Yang, 2002), exhibits seasonal variations and contributes approximately 30 μg/m3 to the PM10 concentrations in northern and eastern Taiwan (Lin et al., 2005). At the same time, a variety of anthropogenic pollutants associated with PM of various sizes are carried by the northeasterly monsoon from continental China to Taiwan (Cheng et al., 2008; Hsu et al., 2005). Additionally, frequent Asian dust storms occurred from winter to spring in recent years, leading to increased PM10 and the accompanying pollutant concentrations (Chan and Ng, 2011; Cheng et al., 2008; Hsu et al., 2004; Lee et al., 2006). These seasonal increases in the PM concentration may further alter the 7Be concentrations and their distribution throughout the environment. A previous study indicated that both the deposition and air concentration of 7Be in Taiwan have been altered by the monsoons, resulting in higher deposition and air concentrations of 7Be occurred in winter than in summer (Chao et al., 2012). However, these seasonal variations of 7Be and PM concentrations have not been observed simultaneously, and the influence of the PM concentration on the 7Be concentration in air is not clear. The aim of this study is to investigate the relationship between 7 Be and PM concentrations in surface air. The atmospheric concentrations of 7Be and PM (monthly data) were monitored simultaneously from 1998 to 2011 in Hsinchu, Taiwan. The seasonal variations of the 7Be and the PM concentrations were compared to explain the 7Be concentrations and their distribution with respect to the PM concentration and the seasonal monsoons.
83
(HD-28, RADeCO) set to a flow rate of 40 LPM to collect total suspended particulates. The weekly samples were weighed (2006– 2011) and beta decays were counted (1998–2011) and these counts were used to estimate the PM concentrations from 1998 to 2011. 2.3. Gamma-ray counting Filters (4–5 weekly samples) were combined as a monthly sample that was counted with a germanium detector (GC3520; Canberra) connected to a multi-channel analyzer (MCA 35-plus; Canberra). The activity of 7Be was determined by detecting the emission of gamma rays at 478 keV. The spectra were analyzed with SAMPO 90 software. The 7Be activity in air (mBq/m3) was decay-corrected to the mid-point of sample collection. 2.4. Gross-beta counting Weekly air samples were measured with a gas-flow proportional counter (LB 5100; Tennelec), which had a counting efficiency of 39%. The gross-beta activity in air (Aβ), in mBq/m3, was determined by comparing to a 40K standard (0.2 g KCl or an equivalent activity of 3.24 Bq). Gamma-ray counting revealed that, aside from the naturally occurring radionuclide 40K, 232Th and 238 U, almost no other beta-emitters could be detected. As 7Be is not a beta-emitter, it has little impact on the beta counting results. Thus, the beta activity in air is totally attributed to the PM concentration, which consists primarily of the naturally occurring radionuclides 40K, 232Th and 238U. 2.5. Estimating the particulate matter (PM) concentrations in air From 2006 to 2011, the corresponding PM concentrations in the atmosphere (μg/m3) were determined by subtracting the filter weight before and after a weekly collection. The PM concentrations before 2006 can be determined retrospectively from the beta counting records. 2.6. The activity concentration of 7Be in the particulate matter The activity concentration of 7Be in the PM (Bq/g) is denoted as APM, based on the assumption that almost all 7Be is attached to PM in the atmosphere. The APM can be defined as APM ðBq=gÞ ¼
C Be 103 PM
ð1Þ
2. Materials and methods
where CBe is the atmospheric concentration of 7Be (mBq/m3).
2.1. Sampling sites
2.7. Related records
The city of Hsinchu is located in northern Taiwan. The prevailing wind direction in the region is northeasterly, and northern Taiwan experiences higher wind speeds and more days with precipitation than southern Taiwan (Lu et al., 2006; Yang, 2002). The atmospheric concentration of 7Be and the respective PM concentration in Hsinchu were monitored simultaneously from 1998 to 2011 in two monitoring stations (ST-01 and ST-02). ST-01 was set on the roof of the Radioactivity Measurement Laboratory of the National Tsing Hua University ( 24 47′ N, 120 59′ E, altitude ¼ 30 m), and ST-02 was set 100 m apart from ST-01 at an altitude of 10 m.
The concentrations of PM10 (particles with a diameter o 10 μm) were obtained from the air-quality monitoring network of the Taiwan Environmental Protection Administration (Taiwan EPA, 2011). These concentrations were recorded from 1996 to 2011 at a nearby air-quality monitoring station located in Hsinchu (24 48′ N, 120 58′E, altitude ¼ 13 m).
3. Results and discussion 3.1. The concentration of the PM based on the recorded beta activities
2.2. Sample preparation Air samples were collected once a week with a 47 mm (diameter) glass-fiber filter mounted on a high-volume air sampler
The monthly average PM concentrations from the two monitoring stations, ST-01 and ST-02, were positively correlated with their gross-beta activities (Aβ) in the air from 2006 to 2011 (Fig. 1).
84
J.H. Chao et al. / Applied Radiation and Isotopes 78 (2013) 82–87
140
(2006-2011) ST-01 ST-02
y = 57.819x 2 R = 0.6844
14 12
100 80
10 7Be(mBq/m3)
PM(μg/m3)
120
60 40 20 0 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6
8 6 4
A ( mBq/m3 )
2
Fig. 1. Correlation of the PM concentration with gross-beta activity (Aβ) in the air from 2006 to 2011.
0 200
140
100
150
80
APM(Bq/g)
PM(μg/m3)
120
60 40 20 0 140
50
120 100
0
60
98 19 99 20 00 20 01 20 02 20 03 20 04 20 05 20 06 20 07 20 08 20 09 20 10 20 11 20 12
80
19
PM(μg/m3)
100
Year
40 7
Fig. 3. Temporal evolution of the (a) Be concentration (mBq/m3) in the air and the (b) APM (Bq/g) at the monitoring station ST-01 from 1998 to 2011.
20 0 140
ST-02, respectively, which are compared to the PM10 concentrations recorded at the local air-quality monitoring station (Fig. 2(c)) (Taiwan EPA, 2011). As a whole, all of the measured PM and PM10 concentrations cycled seasonally: they are higher in the winter and lower in the summer. The average PM (ST-01), PM (ST-02) and PM10 concentrations were 47.7, 48.5 and 47.3 μg/m3, respectively. This finding suggests that the measured PM concentrations, being attributed to the suspended particulates, are comparable to the PM10 concentrations in surface air. In addition, the particles with diameters of 410 μm appear to contribute a negligible amount to the PM concentration; thus 7Be attaching to PM of 410 μm is insignificant in this regard.
PM10(μg/m3)
120 100 80 60 40 20
19
9 19 6 9 19 7 9 19 8 99 20 0 20 0 0 20 1 0 20 2 03 20 0 20 4 05 20 0 20 6 07 20 0 20 8 09 20 1 20 0 11
0
Year Fig. 2. Temporal evolution of the PM concentrations monitored (a) at ST-01 and (b) at ST-02, which are compared to (c) the PM10 records from the EPA (Taiwan EPA, 2011).
The relationship between PM and Aβ can be represented as 3
3
PMðμg=m Þ ¼ 58Aβ ðmBq=m Þ
ð2Þ
therefore, the pre-2006 (from 1998 on) PM concentrations at the two monitoring stations before 2006 (from 1998) were calculated retrospectively based on the beta counting records. 3.2. Seasonal variation of the PM concentration Changes in the monthly average PM concentrations from 1998 to 2011 are illustrated in Fig. 2(a) and (b) for the stations ST-01 and
3.3. Seasonal variation of 7Be with the PM concentration During the monitoring period (1998–2011), changes in the monthly averages of 7Be are represented in Figs. 3(a) and 4(a). The variations of 7Be in the atmosphere showed a cyclical pattern similar to those of the PM concentrations. The average APM are illustrated in Figs. 3(b), and 4(b) which were calculated based on the measured PM and 7Be concentrations (Eq. (1)) for ST-01 and ST-02, respectively. Combining the data from ST-01 and ST-02, the monthly average PM, 7Be and APM throughout the entire annual cycle is illustrated in Fig. 5. Compared to PM and 7Be, APM appears to be on a different seasonal cycle: it is higher in spring and autumn and lower in summer and winter. The monthly deviation
J.H. Chao et al. / Applied Radiation and Isotopes 78 (2013) 82–87
85
90
14
80 70
PM(μg/m3)
12
7Be(mBq/m3)
10 8
60 50 40 30 20
6
10
4
8 7Be(mBq/m3)
0
2 0
200
6 4 2 0 140
120
APM(Bq/g)
APM(Bq/g)
150
100
100 80 60 40
50
20
99 20 00 20 01 20 02 20 03 20 04 20 05 20 06 20 07 20 08 20 09 20 10 20 11 20 12
98
V
EC
T C
P
O
D
N
O
SE
L AU G
Month 19
19
JU
N JA
0
FE B M AR AP R M AY JU N
0
Year Fig. 4. Temporal evolution of the (a) 7Be concentration (mBq/m3) in the air and the (b) APM (Bq/g) at the monitoring station ST-02 from 1998 to 2011.
from the annual average values for PM (48.1 μg/m3), 7 Be (3.7 mBq/m3) and APM (76.7 Bq/g3) are also illustrated in Fig. 6. Lower concentrations of PM and 7Be were simultaneously observed during the summer (Jun.–Sept.), and higher concentrations of PM and 7Be were found in the other seasons. An unusually high 7Be concentration occurring in the spring (Mar.–Apr.) is referred to as the spring peak (Chao et al., 2012). During this period, stratospheric air intruding into the troposphere may enhance the 7Be concentration in the air and the respective APM value. During the autumn and winter (Oct.–Feb.), the PM concentrations rapidly increased as a result of the northeasterly monsoon, whereas the 7Be concentrations varied within a steady range of 4.2–5.3 mBq/m3. Thus, the APM was inversely related to the PM concentration and reached a higher level in October (92.2 Bq/g) and a lower level (63.4 Bq/g) in January, when the PM concentration reaches its annual maximum (70 μg/m3). 3.4. Correlating 7Be with the PM concentration Overall, the 7Be concentrations (CBe) were linearly related to the respective PM concentrations at the two stations (ST-01 and ST-02) (Fig. 7). This relationship can be expressed as C Be ðmBq=m3 Þ ¼ 0:0767PMðμg=m3 Þ
ð3Þ
therefore, the average APM is 76.7 (Bq/g). The strong positive correlation (r2 ¼0.6776) between the 7Be and PM concentrations
Fig. 5. Seasonal variations of (a) PM, (b) 7Be and (c) APM from 1998 to 2011. The average values for PM, 7Be and APM are 48.1 (μg/m3), 3.7 (mBq/m3) and 76.7 (Bq/g), respectively.
implies that the 7Be concentration can be predicted from the PM concentration in the atmosphere. However, the predicted CBe is either slightly overestimated or underestimated depending on the seasonal variation of APM (Figs. 5(c) and 6(c)). The coefficient of variation (CV) represents the degree of dispersion for the monitoring data. The CV is ordered as follows: 7 Be (51%) 4PM (42%) 4APM (30%). Compared with the 7Be and PM concentrations, the APM is not sensitive enough to vary seasonally in the same way. This can be explained by the fact that the PM brought to Taiwan by the monsoons are mixed with the PM produced locally, causing 7Be to be nearly uniformly associated with PM. It is particularly noteworthy that the 7Be concentration reached its lowest value (1.2 mBq/m3) in July and August, 68% lower than the annual average (3.7 mBq/m3), whereas the APM during that period was 50 Bq/g, only 35% lower than its annual average (76.7 Bq/g). The APM is used to describe the mixing degree of the air masses and the distribution of 7Be with respect to PM. Research indicates that the winter monsoon accounts for a 60–80% impact, or 30 μg/m3, of the PM concentration in northern Taiwan during the winter (Lin et al., 2005). This effect is reduced as the local emissions increase and the seasons vary. 3.5. The influence of seasonal monsoons on the 7Be concentration The average 7Be concentration in Hsinchu in the winter (Nov.–Feb.) is 4.7 mBq/m3, whereas it is 1.9 mBq/m3 in the summer (Jun.–Sept.), and even lower (1.2 mBq/m3) in July and August. The large variation in 7Be between winter and summer can be
86
J.H. Chao et al. / Applied Radiation and Isotopes 78 (2013) 82–87
The annual average 7Be concentration in the atmosphere is 3.7 mBq/m3 in Hsinchu, comparable to the global average of the 7 Be concentration at surface level of approximately 3–5 mBq/m3 (Gaggeler, 1995). The 7Be concentrations and the respective APM can be altered by both natural and anthropogenic sources of PM, especially in urban areas, where sources such as traffic, industrial emissions and the re-suspension of road dust must be considered. The annual average APM is 76.7 Bq/g in Hsinchu, which can be seen as an indicator to describe the 7Be concentrations in the environment.
60 40
Δ PM(%)
20 0 -20 -40 -60 80
4. Conclusions
60
In this study, the PM concentration was introduced to explain the seasonal variations of the 7Be concentration in the air. Seasonal monitoring results revealed that the 7Be concentrations in surface air were influenced by both the seasonal monsoons and the accompanying PM concentrations. The northeasterly monsoon, from middle latitudes in autumn and winter, not only carried higher-PM air masses, but was also associated with higher 7Be concentrations than those brought by the southwesterly monsoon from the low latitudes in the summer. Although both the PM and the 7Be originate independently from different latitudes, they join and move together to Taiwan. By mixing with local 7Be-attached PM, the activity concentration of 7Be in the PM (APM) remained relatively steady over the changing seasons. This characteristic allows the 7Be concentrations to be estimated by monitoring the PM concentrations in surface air.
Δ7Be(%)
40 20 0 -20 -40 -60
Δ (APM)(%)
-80 40 20 0 -20 -40 -60
JA
N FE B M AR AP R M AY JU N JU L AU G SE P O C T N O V D EC
Acknowledgments
Month Fig. 6. Monthly deviation (%) relative to the averages for (a) PM (b) 7Be and (c) APM from 1998 to 2011.
References
20
ST-01 ST-02
18
7Be(mBq/m3)
16 14 12
y = 0.0767x 2
R = 0.6776
10 8 6 4 2 0
This study was financially supported by the National Science Council of the Republic of China (Taiwan) under Contract no. 100-2221-E-113-MY2.
0
20
40
60
80
100
120
140
PM Concentration(µg/m3) Fig. 7. Correlation of 7Be with the PM concentration in the air for the monitoring stations ST-01 and ST-02.
attributed not only to the seasonal monsoons, which horizontally transport air masses between latitudes, but also to the associated PM concentration and corresponding 7Be. Taiwan experiences the southwest monsoon in summer (Jun.–Sept.), when air masses with low PM and 7Be concentrations move together from low latitude areas. Conversely, high concentrations of PM and 7Be were coincidently observed in winter. This correlation resulted from the horizontal transport of air masses from middle latitudes by the northeast monsoon, which carries high PM and 7Be concentrations from continental China.
Bleichrodt, J.F., 1978. Mean tropospheric residence time of cosmic-ray-produced 7 Be at north temperate latitudes. J. Geophys. Res. 83, 3058–3062. Chan, C.C., Ng, H.C., 2011. A case-crossover analysis of Asian dust storms and mortality in the downwind areas using 14-year data in Taipei. Sci. Total Environ. 410–411, 47–52. Chao, J.H., Chiu, Y.J., Lee, H.P., Lee, M.C., 2012. Deposition of beryllium-7 in Hsinchu, Taiwan. Appl. Radiat. Isot. 70, 415–422. Cheng, M.T., Chou, W.C., Chio, C.P., Hsu, S.C., Su, Y.R., Kuo, P.H., Tsuang, B.J., Lin, S.H., Chou, C.C.K., 2008. Compositions and source apportionments of atmospheric aerosol during Asian dust storm and local pollution in central Taiwan. J. Atmos. Chem. 61, 155–173. Doering, C., Akber, R., 2008. Beryllium-7 in near-surface air and deposition at Brisbane, Australia. J. Environ. Radioact. 99, 461–467. Fang, G.C., Chang, S.C., 2010. Atmospheric particulate (PM10 and PM2.5) mass concentration and seasonal variation study in the Taiwan area during 2000– 2008. Atmos. Res 98, 368–377. Gaggelez, H.W., 1995. Radioactivity in the atmosphere. Radiochim. Acta 70/71, 345–353. Gonzalez-Gomez, C., Azahra, M., Lopez-Penalver, J.J., Camacho-Garcia, A., Bardouni, T.El., Boukhal, H., 2006. Seasonal variability in 7Be depositional fluxes at Granada, Spain. Appl. Radiat. Isot 64, 228–234. Hernandez, C.M.A., Aguila, H.C., Asencio, M.D., Caravaca, A.M., 2004. Reconstruction of 137Cs signal in Cuba using 7Be as tracer of vertical transport processes in the stmosphere. J. Environ. Radioact. 75, 133–142. Hsu, S.C., Liu, S.C., Lin, C.Y., Hsu, R.T., Huang, Y.T., Chen, Y.W., 2004. Metal compositions of PM10 and PM2.5 aerosols in Taipei during 2002. Terr. Atmos. Ocean Sci. 15, 925–948. Hsu, S.C., Liu, S.C., Jeng, W.L., Lin, F.J., Huang, Y.T., Lung, S.C.C., Liu, T.H., Tu, J.Y., 2005. Variations of Cd/Pb and Zn/Pb ratios in Taipei aerosols reflecting long-range transport or local pollution emissions. Sci. total Environ. 347, 111–121. Ioannidou, A., Manolopoulou, M., Papastefanou, C., 2005. Temporal changes of 7Be and 210Pb concentrations in surface air at temperate latitudes (401 N). Appl. Radiat. Isot. 63, 277–284.
J.H. Chao et al. / Applied Radiation and Isotopes 78 (2013) 82–87 Ioannidou, A., 2011. Activity size distribution of 7Be in association with trace metals in the urban aera of the city of Thessaloniki, Greece. Atmos. Environ. 45, 1286–1290. Koch, D.M., Jacob, D.J., Graustein, W.C., 1996. Vertical transport of tropospheric aerosols as indicated by Be and Pb in a chemical tracer model. J. Geophys. Res. 101, 18651–18666. Kulan, A., Aldahan, A., Possnert, G., Vintersved, I., 2006. Distribution of 7Be in surface air of Europe. Atmos. Environ. 40, 3855–3868. Lee, C.T., Chuang, M.T., Chan, C.C., Cheng, T.J., Huang, S.L., 2006. Aerosol characteristics from the Taiwan aerosol supersite in the Asian yellow-dust periods of 2002. Atmos. Environ. 40, 3409–3418. Lenschow, P., Abraham, H.J., Kutzner, K., Lutz, M., Preub, J.D., Reichenbacher, W., 2001. Some ideas about the source of PM10. Atmos. Environ. 35 (Suppl. 1), s23–s33. Lin, C.Y., Liu, S.C., Chou, C.C., Huang, S.J., Liu, C.M., Kuo, C.H., Young, C.Y., 2005. Longrange transport of aerosols and their impact on the air quality of Taiwan. Atmos. Environ. 39, 6066–6076. Lu, H.C., Chang, C.L., Hsieh, J.C., 2006. Classification of PM10 distributions in Taiwan. Atmos. Environ. 40, 1452–1463. Muramatsu, H., Yoshizawa, S., Abe, T., Ishii, T., Wada, N., Horiuchi, Y., Kanekatsu, R., 2008. Variation of 7Be concentration in surface air at Nagano, Japan. J. Radioanal. Nucl. Chem. 275, 299–307. Masarik, J., Beer, J., 1999. Simulation of particle fluxes and cosmogenic nuclide production in the earth's atmosphere. J. Geophys. Res D104 (10), 12099–13012. Megumi, K., Matsunami, T., Ito, N., Kiyoda, S., Mizohata, A., Asano, T., 2000. Factors, especially sunspot number, causing variations in surface air concentrations and depositions of 7Be in Osaka, Japan. Geophys. Res. Lett. 27, 361–364.
87
Narazaki, Y., Fujitaka, K., Igarashi, S., Ishikawa, Y., Fujinami, N., 2003. Seasonal variation of 7Be deposition in Japan. J. Radioanal. Nucl. Chem. 256, 489–496. Papastefanou, C., 2006. Residence time of tropospheric aerosols in association with radioactive nuclides. Appl. Radiat. Isot. 64, 93–100. Papastefanou, C., Ioannidou, A., 1996. Beryllium-7 aerosols in ambient air. Environ. Intern 22, 123–130. Pope, C.A., 2000. Review: epidemiological basis for particulate air pollution health standards. Aerosol Sci. Tech. 32, 4–14. Rehfeld, S., Heimann, M., 1995. Three dimensional atmospheric transport simulation of the radioactive tracers, 210Pb, 7Be, 10Be and 90Sr. J. Geophys. Res. 100, 26141–26161. Smith, S., Stribley, F.T., Milligan, P., Barratt, B., 2001. Factors influencing measurements of PM10 during 1995–1997 in London. Atmos. Environ. 35, 4651–4662. Staley, D.O., 1982. Strontium-90 in surface air and stratosphere: some interpretations of the 1963–75 data. J. Atmos. Sci. 99, 1571–1590. Taiwan Environmental Protection Administration (Taiwan EPA), 2011. Executive Yuan R.O.C. 〈http://tagm.epa.gov.tw//tagm/zh-tw/YearlyDataDownload.aspx〉. Uematsu, M., Duce, R.A., Prospero, J.M., 1994. Atmospheric beryllium-7 concentrations over the Pacific Ocean. Geophys. Res. Lett. 21, 561–564. Wilson, W.E., Suh, H.H., 1997. Fine particles and coarse particles: concentration relationships relevant to epidemiologic studies. J. Air Waste Manage. Assoc. 47, 1238–1249. Yang, K.L., 2002. Spatial and seasonal variation of PM10 mass concentrations in Taiwan. Atmos. Environ. 36, 3403–3411. Young, J.A., Silker, W.B., 1980. Aerosol deposition velocities on the Pacific and Atlantic Oceans calculated from 7Be measurements. Earth Planet. Sci. Lett. 50, 92–104.
Contents lists available at SciVerse ScienceDirect
Applied Radiation and Isotopes journal homepage: www.elsevier.com/locate/apradiso
Variation of atmospheric Be-7 in relation to PM concentrations J.H. Chao a,n, Y.J. Chiu b, H.P. Lee a, M.C. Lee a a b
Nuclear Science and Technology Development Center, National Tsing Hua University, Hsinchu 30013, Taiwan, ROC Hydrotech Research Institute, National Taiwan University, Taipei 10617, Taiwan, ROC
H I G H L I G H T S
The 7Be and PM concentrations in air were monitored simultaneously during 1998–2011. Both PM and 7Be concentrations increased as a result of the northeasterly monsoon. Both the lowest PM and 7Be concentrations were observed in July and August. The 7Be concentrations in surface air can be predicted from the PM concentrations.
art ic l e i nf o
a b s t r a c t
Article history: Received 14 December 2012 Accepted 12 April 2013 Available online 23 April 2013
In the present study, the influences of particulate matter (PM) and seasonal monsoons on 7Be concentrations in surface air (CBe) are elucidated. The 7Be and the corresponding PM concentrations in the air were monitored simultaneously throughout a 14-year period (1998–2011) in Hsinchu, Taiwan. During the autumn and winter seasons (Oct.–Feb.), both the PM and the 7Be concentrations increased as a result of the northeasterly monsoon. In contrast, the lowest PM and 7Be concentrations were observed in July and August. This timing is due to the occurrence of southwest monsoons, which carry air masses with low PM concentrations and are associated with depleted 7Be from low latitudes. The activity concentration of 7Be in the PM (APM) was used to explain the seasonal variations of 7Be with respect to the PM concentrations. In contrast, APM is not sensitive enough to vary with the seasons. The air masses transported by the monsoons are believed to be partially mixed with the PM locally produced in Taiwan, which explains their seasonal variations. The 7Be concentrations in surface air can be experimentally predicted from the PM concentrations based on CBe (mBq/m3) ¼0.0767 PM (μg/m3) across seasons. The annual averages of the PM and 7Be concentrations are 48.1 μg/m3 and 3.7 mBq/m3, respectively. The estimated CBe was either slightly overestimated or underestimated, depending on the season. The highest deviations occurred in July and August, when CBe was underestimated by 33%. & 2013 Elsevier Ltd. All rights reserved.
Keywords: Beryllium-7 Particulate matter Monsoons Radioactivity measurement
1. Introduction Beryllium-7 (7Be) is a cosmic-ray-produced radionuclide. Once Be is formed in the troposphere, it rapidly associates with suspended particles. Prior research indicates that more than 80% of 7Be is attached to particles smaller than 1.3 μm in diameter and that less than 10% is attached to particles larger than 10 μm (Ioannidou, 2011). An activity median aerodynamic diameter (AMAD) between 0.7 and 1.2 μm was estimated for these 7Beaerosols in the atmosphere (Papastefanou and Ioannidou, 1996; Papastefanou, 2006). The aerosols are transported by deposition and scavenging by precipitation is the primary mechanism for the transport of 7Be to the Earth's surface. The mean residence 7
n
Corresponding author. Tel.: +886 35165049; fax: +886 35122660. E-mail address: [email protected] (J.H. Chao).
0969-8043/$ - see front matter & 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.apradiso.2013.04.015
lifetimes of the aerosols were estimated to be 1–2 years for the stratosphere through the tropopause (Rehfeld and Heimann, 1995; Staley, 1982), 20–35 days for the troposphere (Bleichrodt, 1978; Koch et al., 1996), and less than 10 days for the lower atmosphere below precipitation cloud levels (Papastefanou, 2006). The production rate, deposition flux and atmospheric concentration of 7Be are primarily dependent upon latitude, altitude, solar activity and meteorological conditions (Gaggeler, 1995; Koch et al., 1996; Kulan et al., 2006; Masarik and Beer, 1999; Uematsu et al., 1994; Young and Silker, 1980). Seasonal variations of 7Be in surface air were frequently observed and usually resulted from the vertical transport of air masses associated with seasonal precipitation (Doering and Akber, 2008; Gonzalez-Gomez et al., 2006; Hernandez et al., 2004; Ioannidou et al., 2005) and the seasonally occurring horizontal transport of air masses containing different concentrations of 7Be across different latitudes (Chao et al., 2012; Megumi et al., 2000; Muramatsu et al., 2008; Narazaki et al., 2003).
J.H. Chao et al. / Applied Radiation and Isotopes 78 (2013) 82–87
Because of these characteristics, 7Be has been widely used as a chemical tracer in atmospheric and environmental sciences. Both the concentration and distribution of particulate matter (PM) are influenced by a combination of processes including anthropogenic sources such as traffic, industrial emissions and re-suspension of road dust and natural sources (Smith et al., 2001; Lenschow et al., 2001). The PM concentration in the environment has been a concern for decades due to its adverse health effects on humans (Pope, 2000; Wilson and Suh, 1997). As a result, PM concentrations have been routinely monitored in Taiwan since 1976. The particulate mass concentration, usually denoted as PM10, represents the sum of all fine particles (diameter o10 μm) from different emission sources and is a major factor in evaluating the pollutant standards index (PSI) in air. On winter days, Taiwan often experiences northeasterly monsoon winds originating in central Asia. Whereas in summer, southwesterly monsoon winds are prevalent. The winter monsoon not only brings cold air, it also transports high concentrations of PM over a long distance to Taiwan (Fang and Chang, 2010; Yang, 2002), exhibits seasonal variations and contributes approximately 30 μg/m3 to the PM10 concentrations in northern and eastern Taiwan (Lin et al., 2005). At the same time, a variety of anthropogenic pollutants associated with PM of various sizes are carried by the northeasterly monsoon from continental China to Taiwan (Cheng et al., 2008; Hsu et al., 2005). Additionally, frequent Asian dust storms occurred from winter to spring in recent years, leading to increased PM10 and the accompanying pollutant concentrations (Chan and Ng, 2011; Cheng et al., 2008; Hsu et al., 2004; Lee et al., 2006). These seasonal increases in the PM concentration may further alter the 7Be concentrations and their distribution throughout the environment. A previous study indicated that both the deposition and air concentration of 7Be in Taiwan have been altered by the monsoons, resulting in higher deposition and air concentrations of 7Be occurred in winter than in summer (Chao et al., 2012). However, these seasonal variations of 7Be and PM concentrations have not been observed simultaneously, and the influence of the PM concentration on the 7Be concentration in air is not clear. The aim of this study is to investigate the relationship between 7 Be and PM concentrations in surface air. The atmospheric concentrations of 7Be and PM (monthly data) were monitored simultaneously from 1998 to 2011 in Hsinchu, Taiwan. The seasonal variations of the 7Be and the PM concentrations were compared to explain the 7Be concentrations and their distribution with respect to the PM concentration and the seasonal monsoons.
83
(HD-28, RADeCO) set to a flow rate of 40 LPM to collect total suspended particulates. The weekly samples were weighed (2006– 2011) and beta decays were counted (1998–2011) and these counts were used to estimate the PM concentrations from 1998 to 2011. 2.3. Gamma-ray counting Filters (4–5 weekly samples) were combined as a monthly sample that was counted with a germanium detector (GC3520; Canberra) connected to a multi-channel analyzer (MCA 35-plus; Canberra). The activity of 7Be was determined by detecting the emission of gamma rays at 478 keV. The spectra were analyzed with SAMPO 90 software. The 7Be activity in air (mBq/m3) was decay-corrected to the mid-point of sample collection. 2.4. Gross-beta counting Weekly air samples were measured with a gas-flow proportional counter (LB 5100; Tennelec), which had a counting efficiency of 39%. The gross-beta activity in air (Aβ), in mBq/m3, was determined by comparing to a 40K standard (0.2 g KCl or an equivalent activity of 3.24 Bq). Gamma-ray counting revealed that, aside from the naturally occurring radionuclide 40K, 232Th and 238 U, almost no other beta-emitters could be detected. As 7Be is not a beta-emitter, it has little impact on the beta counting results. Thus, the beta activity in air is totally attributed to the PM concentration, which consists primarily of the naturally occurring radionuclides 40K, 232Th and 238U. 2.5. Estimating the particulate matter (PM) concentrations in air From 2006 to 2011, the corresponding PM concentrations in the atmosphere (μg/m3) were determined by subtracting the filter weight before and after a weekly collection. The PM concentrations before 2006 can be determined retrospectively from the beta counting records. 2.6. The activity concentration of 7Be in the particulate matter The activity concentration of 7Be in the PM (Bq/g) is denoted as APM, based on the assumption that almost all 7Be is attached to PM in the atmosphere. The APM can be defined as APM ðBq=gÞ ¼
C Be 103 PM
ð1Þ
2. Materials and methods
where CBe is the atmospheric concentration of 7Be (mBq/m3).
2.1. Sampling sites
2.7. Related records
The city of Hsinchu is located in northern Taiwan. The prevailing wind direction in the region is northeasterly, and northern Taiwan experiences higher wind speeds and more days with precipitation than southern Taiwan (Lu et al., 2006; Yang, 2002). The atmospheric concentration of 7Be and the respective PM concentration in Hsinchu were monitored simultaneously from 1998 to 2011 in two monitoring stations (ST-01 and ST-02). ST-01 was set on the roof of the Radioactivity Measurement Laboratory of the National Tsing Hua University ( 24 47′ N, 120 59′ E, altitude ¼ 30 m), and ST-02 was set 100 m apart from ST-01 at an altitude of 10 m.
The concentrations of PM10 (particles with a diameter o 10 μm) were obtained from the air-quality monitoring network of the Taiwan Environmental Protection Administration (Taiwan EPA, 2011). These concentrations were recorded from 1996 to 2011 at a nearby air-quality monitoring station located in Hsinchu (24 48′ N, 120 58′E, altitude ¼ 13 m).
3. Results and discussion 3.1. The concentration of the PM based on the recorded beta activities
2.2. Sample preparation Air samples were collected once a week with a 47 mm (diameter) glass-fiber filter mounted on a high-volume air sampler
The monthly average PM concentrations from the two monitoring stations, ST-01 and ST-02, were positively correlated with their gross-beta activities (Aβ) in the air from 2006 to 2011 (Fig. 1).
84
J.H. Chao et al. / Applied Radiation and Isotopes 78 (2013) 82–87
140
(2006-2011) ST-01 ST-02
y = 57.819x 2 R = 0.6844
14 12
100 80
10 7Be(mBq/m3)
PM(μg/m3)
120
60 40 20 0 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6
8 6 4
A ( mBq/m3 )
2
Fig. 1. Correlation of the PM concentration with gross-beta activity (Aβ) in the air from 2006 to 2011.
0 200
140
100
150
80
APM(Bq/g)
PM(μg/m3)
120
60 40 20 0 140
50
120 100
0
60
98 19 99 20 00 20 01 20 02 20 03 20 04 20 05 20 06 20 07 20 08 20 09 20 10 20 11 20 12
80
19
PM(μg/m3)
100
Year
40 7
Fig. 3. Temporal evolution of the (a) Be concentration (mBq/m3) in the air and the (b) APM (Bq/g) at the monitoring station ST-01 from 1998 to 2011.
20 0 140
ST-02, respectively, which are compared to the PM10 concentrations recorded at the local air-quality monitoring station (Fig. 2(c)) (Taiwan EPA, 2011). As a whole, all of the measured PM and PM10 concentrations cycled seasonally: they are higher in the winter and lower in the summer. The average PM (ST-01), PM (ST-02) and PM10 concentrations were 47.7, 48.5 and 47.3 μg/m3, respectively. This finding suggests that the measured PM concentrations, being attributed to the suspended particulates, are comparable to the PM10 concentrations in surface air. In addition, the particles with diameters of 410 μm appear to contribute a negligible amount to the PM concentration; thus 7Be attaching to PM of 410 μm is insignificant in this regard.
PM10(μg/m3)
120 100 80 60 40 20
19
9 19 6 9 19 7 9 19 8 99 20 0 20 0 0 20 1 0 20 2 03 20 0 20 4 05 20 0 20 6 07 20 0 20 8 09 20 1 20 0 11
0
Year Fig. 2. Temporal evolution of the PM concentrations monitored (a) at ST-01 and (b) at ST-02, which are compared to (c) the PM10 records from the EPA (Taiwan EPA, 2011).
The relationship between PM and Aβ can be represented as 3
3
PMðμg=m Þ ¼ 58Aβ ðmBq=m Þ
ð2Þ
therefore, the pre-2006 (from 1998 on) PM concentrations at the two monitoring stations before 2006 (from 1998) were calculated retrospectively based on the beta counting records. 3.2. Seasonal variation of the PM concentration Changes in the monthly average PM concentrations from 1998 to 2011 are illustrated in Fig. 2(a) and (b) for the stations ST-01 and
3.3. Seasonal variation of 7Be with the PM concentration During the monitoring period (1998–2011), changes in the monthly averages of 7Be are represented in Figs. 3(a) and 4(a). The variations of 7Be in the atmosphere showed a cyclical pattern similar to those of the PM concentrations. The average APM are illustrated in Figs. 3(b), and 4(b) which were calculated based on the measured PM and 7Be concentrations (Eq. (1)) for ST-01 and ST-02, respectively. Combining the data from ST-01 and ST-02, the monthly average PM, 7Be and APM throughout the entire annual cycle is illustrated in Fig. 5. Compared to PM and 7Be, APM appears to be on a different seasonal cycle: it is higher in spring and autumn and lower in summer and winter. The monthly deviation
J.H. Chao et al. / Applied Radiation and Isotopes 78 (2013) 82–87
85
90
14
80 70
PM(μg/m3)
12
7Be(mBq/m3)
10 8
60 50 40 30 20
6
10
4
8 7Be(mBq/m3)
0
2 0
200
6 4 2 0 140
120
APM(Bq/g)
APM(Bq/g)
150
100
100 80 60 40
50
20
99 20 00 20 01 20 02 20 03 20 04 20 05 20 06 20 07 20 08 20 09 20 10 20 11 20 12
98
V
EC
T C
P
O
D
N
O
SE
L AU G
Month 19
19
JU
N JA
0
FE B M AR AP R M AY JU N
0
Year Fig. 4. Temporal evolution of the (a) 7Be concentration (mBq/m3) in the air and the (b) APM (Bq/g) at the monitoring station ST-02 from 1998 to 2011.
from the annual average values for PM (48.1 μg/m3), 7 Be (3.7 mBq/m3) and APM (76.7 Bq/g3) are also illustrated in Fig. 6. Lower concentrations of PM and 7Be were simultaneously observed during the summer (Jun.–Sept.), and higher concentrations of PM and 7Be were found in the other seasons. An unusually high 7Be concentration occurring in the spring (Mar.–Apr.) is referred to as the spring peak (Chao et al., 2012). During this period, stratospheric air intruding into the troposphere may enhance the 7Be concentration in the air and the respective APM value. During the autumn and winter (Oct.–Feb.), the PM concentrations rapidly increased as a result of the northeasterly monsoon, whereas the 7Be concentrations varied within a steady range of 4.2–5.3 mBq/m3. Thus, the APM was inversely related to the PM concentration and reached a higher level in October (92.2 Bq/g) and a lower level (63.4 Bq/g) in January, when the PM concentration reaches its annual maximum (70 μg/m3). 3.4. Correlating 7Be with the PM concentration Overall, the 7Be concentrations (CBe) were linearly related to the respective PM concentrations at the two stations (ST-01 and ST-02) (Fig. 7). This relationship can be expressed as C Be ðmBq=m3 Þ ¼ 0:0767PMðμg=m3 Þ
ð3Þ
therefore, the average APM is 76.7 (Bq/g). The strong positive correlation (r2 ¼0.6776) between the 7Be and PM concentrations
Fig. 5. Seasonal variations of (a) PM, (b) 7Be and (c) APM from 1998 to 2011. The average values for PM, 7Be and APM are 48.1 (μg/m3), 3.7 (mBq/m3) and 76.7 (Bq/g), respectively.
implies that the 7Be concentration can be predicted from the PM concentration in the atmosphere. However, the predicted CBe is either slightly overestimated or underestimated depending on the seasonal variation of APM (Figs. 5(c) and 6(c)). The coefficient of variation (CV) represents the degree of dispersion for the monitoring data. The CV is ordered as follows: 7 Be (51%) 4PM (42%) 4APM (30%). Compared with the 7Be and PM concentrations, the APM is not sensitive enough to vary seasonally in the same way. This can be explained by the fact that the PM brought to Taiwan by the monsoons are mixed with the PM produced locally, causing 7Be to be nearly uniformly associated with PM. It is particularly noteworthy that the 7Be concentration reached its lowest value (1.2 mBq/m3) in July and August, 68% lower than the annual average (3.7 mBq/m3), whereas the APM during that period was 50 Bq/g, only 35% lower than its annual average (76.7 Bq/g). The APM is used to describe the mixing degree of the air masses and the distribution of 7Be with respect to PM. Research indicates that the winter monsoon accounts for a 60–80% impact, or 30 μg/m3, of the PM concentration in northern Taiwan during the winter (Lin et al., 2005). This effect is reduced as the local emissions increase and the seasons vary. 3.5. The influence of seasonal monsoons on the 7Be concentration The average 7Be concentration in Hsinchu in the winter (Nov.–Feb.) is 4.7 mBq/m3, whereas it is 1.9 mBq/m3 in the summer (Jun.–Sept.), and even lower (1.2 mBq/m3) in July and August. The large variation in 7Be between winter and summer can be
86
J.H. Chao et al. / Applied Radiation and Isotopes 78 (2013) 82–87
The annual average 7Be concentration in the atmosphere is 3.7 mBq/m3 in Hsinchu, comparable to the global average of the 7 Be concentration at surface level of approximately 3–5 mBq/m3 (Gaggeler, 1995). The 7Be concentrations and the respective APM can be altered by both natural and anthropogenic sources of PM, especially in urban areas, where sources such as traffic, industrial emissions and the re-suspension of road dust must be considered. The annual average APM is 76.7 Bq/g in Hsinchu, which can be seen as an indicator to describe the 7Be concentrations in the environment.
60 40
Δ PM(%)
20 0 -20 -40 -60 80
4. Conclusions
60
In this study, the PM concentration was introduced to explain the seasonal variations of the 7Be concentration in the air. Seasonal monitoring results revealed that the 7Be concentrations in surface air were influenced by both the seasonal monsoons and the accompanying PM concentrations. The northeasterly monsoon, from middle latitudes in autumn and winter, not only carried higher-PM air masses, but was also associated with higher 7Be concentrations than those brought by the southwesterly monsoon from the low latitudes in the summer. Although both the PM and the 7Be originate independently from different latitudes, they join and move together to Taiwan. By mixing with local 7Be-attached PM, the activity concentration of 7Be in the PM (APM) remained relatively steady over the changing seasons. This characteristic allows the 7Be concentrations to be estimated by monitoring the PM concentrations in surface air.
Δ7Be(%)
40 20 0 -20 -40 -60
Δ (APM)(%)
-80 40 20 0 -20 -40 -60
JA
N FE B M AR AP R M AY JU N JU L AU G SE P O C T N O V D EC
Acknowledgments
Month Fig. 6. Monthly deviation (%) relative to the averages for (a) PM (b) 7Be and (c) APM from 1998 to 2011.
References
20
ST-01 ST-02
18
7Be(mBq/m3)
16 14 12
y = 0.0767x 2
R = 0.6776
10 8 6 4 2 0
This study was financially supported by the National Science Council of the Republic of China (Taiwan) under Contract no. 100-2221-E-113-MY2.
0
20
40
60
80
100
120
140
PM Concentration(µg/m3) Fig. 7. Correlation of 7Be with the PM concentration in the air for the monitoring stations ST-01 and ST-02.
attributed not only to the seasonal monsoons, which horizontally transport air masses between latitudes, but also to the associated PM concentration and corresponding 7Be. Taiwan experiences the southwest monsoon in summer (Jun.–Sept.), when air masses with low PM and 7Be concentrations move together from low latitude areas. Conversely, high concentrations of PM and 7Be were coincidently observed in winter. This correlation resulted from the horizontal transport of air masses from middle latitudes by the northeast monsoon, which carries high PM and 7Be concentrations from continental China.
Bleichrodt, J.F., 1978. Mean tropospheric residence time of cosmic-ray-produced 7 Be at north temperate latitudes. J. Geophys. Res. 83, 3058–3062. Chan, C.C., Ng, H.C., 2011. A case-crossover analysis of Asian dust storms and mortality in the downwind areas using 14-year data in Taipei. Sci. Total Environ. 410–411, 47–52. Chao, J.H., Chiu, Y.J., Lee, H.P., Lee, M.C., 2012. Deposition of beryllium-7 in Hsinchu, Taiwan. Appl. Radiat. Isot. 70, 415–422. Cheng, M.T., Chou, W.C., Chio, C.P., Hsu, S.C., Su, Y.R., Kuo, P.H., Tsuang, B.J., Lin, S.H., Chou, C.C.K., 2008. Compositions and source apportionments of atmospheric aerosol during Asian dust storm and local pollution in central Taiwan. J. Atmos. Chem. 61, 155–173. Doering, C., Akber, R., 2008. Beryllium-7 in near-surface air and deposition at Brisbane, Australia. J. Environ. Radioact. 99, 461–467. Fang, G.C., Chang, S.C., 2010. Atmospheric particulate (PM10 and PM2.5) mass concentration and seasonal variation study in the Taiwan area during 2000– 2008. Atmos. Res 98, 368–377. Gaggelez, H.W., 1995. Radioactivity in the atmosphere. Radiochim. Acta 70/71, 345–353. Gonzalez-Gomez, C., Azahra, M., Lopez-Penalver, J.J., Camacho-Garcia, A., Bardouni, T.El., Boukhal, H., 2006. Seasonal variability in 7Be depositional fluxes at Granada, Spain. Appl. Radiat. Isot 64, 228–234. Hernandez, C.M.A., Aguila, H.C., Asencio, M.D., Caravaca, A.M., 2004. Reconstruction of 137Cs signal in Cuba using 7Be as tracer of vertical transport processes in the stmosphere. J. Environ. Radioact. 75, 133–142. Hsu, S.C., Liu, S.C., Lin, C.Y., Hsu, R.T., Huang, Y.T., Chen, Y.W., 2004. Metal compositions of PM10 and PM2.5 aerosols in Taipei during 2002. Terr. Atmos. Ocean Sci. 15, 925–948. Hsu, S.C., Liu, S.C., Jeng, W.L., Lin, F.J., Huang, Y.T., Lung, S.C.C., Liu, T.H., Tu, J.Y., 2005. Variations of Cd/Pb and Zn/Pb ratios in Taipei aerosols reflecting long-range transport or local pollution emissions. Sci. total Environ. 347, 111–121. Ioannidou, A., Manolopoulou, M., Papastefanou, C., 2005. Temporal changes of 7Be and 210Pb concentrations in surface air at temperate latitudes (401 N). Appl. Radiat. Isot. 63, 277–284.
J.H. Chao et al. / Applied Radiation and Isotopes 78 (2013) 82–87 Ioannidou, A., 2011. Activity size distribution of 7Be in association with trace metals in the urban aera of the city of Thessaloniki, Greece. Atmos. Environ. 45, 1286–1290. Koch, D.M., Jacob, D.J., Graustein, W.C., 1996. Vertical transport of tropospheric aerosols as indicated by Be and Pb in a chemical tracer model. J. Geophys. Res. 101, 18651–18666. Kulan, A., Aldahan, A., Possnert, G., Vintersved, I., 2006. Distribution of 7Be in surface air of Europe. Atmos. Environ. 40, 3855–3868. Lee, C.T., Chuang, M.T., Chan, C.C., Cheng, T.J., Huang, S.L., 2006. Aerosol characteristics from the Taiwan aerosol supersite in the Asian yellow-dust periods of 2002. Atmos. Environ. 40, 3409–3418. Lenschow, P., Abraham, H.J., Kutzner, K., Lutz, M., Preub, J.D., Reichenbacher, W., 2001. Some ideas about the source of PM10. Atmos. Environ. 35 (Suppl. 1), s23–s33. Lin, C.Y., Liu, S.C., Chou, C.C., Huang, S.J., Liu, C.M., Kuo, C.H., Young, C.Y., 2005. Longrange transport of aerosols and their impact on the air quality of Taiwan. Atmos. Environ. 39, 6066–6076. Lu, H.C., Chang, C.L., Hsieh, J.C., 2006. Classification of PM10 distributions in Taiwan. Atmos. Environ. 40, 1452–1463. Muramatsu, H., Yoshizawa, S., Abe, T., Ishii, T., Wada, N., Horiuchi, Y., Kanekatsu, R., 2008. Variation of 7Be concentration in surface air at Nagano, Japan. J. Radioanal. Nucl. Chem. 275, 299–307. Masarik, J., Beer, J., 1999. Simulation of particle fluxes and cosmogenic nuclide production in the earth's atmosphere. J. Geophys. Res D104 (10), 12099–13012. Megumi, K., Matsunami, T., Ito, N., Kiyoda, S., Mizohata, A., Asano, T., 2000. Factors, especially sunspot number, causing variations in surface air concentrations and depositions of 7Be in Osaka, Japan. Geophys. Res. Lett. 27, 361–364.
87
Narazaki, Y., Fujitaka, K., Igarashi, S., Ishikawa, Y., Fujinami, N., 2003. Seasonal variation of 7Be deposition in Japan. J. Radioanal. Nucl. Chem. 256, 489–496. Papastefanou, C., 2006. Residence time of tropospheric aerosols in association with radioactive nuclides. Appl. Radiat. Isot. 64, 93–100. Papastefanou, C., Ioannidou, A., 1996. Beryllium-7 aerosols in ambient air. Environ. Intern 22, 123–130. Pope, C.A., 2000. Review: epidemiological basis for particulate air pollution health standards. Aerosol Sci. Tech. 32, 4–14. Rehfeld, S., Heimann, M., 1995. Three dimensional atmospheric transport simulation of the radioactive tracers, 210Pb, 7Be, 10Be and 90Sr. J. Geophys. Res. 100, 26141–26161. Smith, S., Stribley, F.T., Milligan, P., Barratt, B., 2001. Factors influencing measurements of PM10 during 1995–1997 in London. Atmos. Environ. 35, 4651–4662. Staley, D.O., 1982. Strontium-90 in surface air and stratosphere: some interpretations of the 1963–75 data. J. Atmos. Sci. 99, 1571–1590. Taiwan Environmental Protection Administration (Taiwan EPA), 2011. Executive Yuan R.O.C. 〈http://tagm.epa.gov.tw//tagm/zh-tw/YearlyDataDownload.aspx〉. Uematsu, M., Duce, R.A., Prospero, J.M., 1994. Atmospheric beryllium-7 concentrations over the Pacific Ocean. Geophys. Res. Lett. 21, 561–564. Wilson, W.E., Suh, H.H., 1997. Fine particles and coarse particles: concentration relationships relevant to epidemiologic studies. J. Air Waste Manage. Assoc. 47, 1238–1249. Yang, K.L., 2002. Spatial and seasonal variation of PM10 mass concentrations in Taiwan. Atmos. Environ. 36, 3403–3411. Young, J.A., Silker, W.B., 1980. Aerosol deposition velocities on the Pacific and Atlantic Oceans calculated from 7Be measurements. Earth Planet. Sci. Lett. 50, 92–104.