Temporal Variations in Radionuclide Activity (7Be and 210Pb) in Surface Aerosols at a Coastal Site in Southeastern China

Received: March 15, 2019
Revised: July 9, 2019
Accepted: July 18, 2019

Download Citation: RIS|BibTeX|https://doi.org/10.4209/aaqr.2019.02.0084  

Cite this article:

Huang, D., Bao, H. and Yu, T. (2019). Temporal Variations in Radionuclide Activity (7Be and 210Pb) in Surface Aerosols at a Coastal Site in Southeastern China. Aerosol Air Qual. Res. 19: 1969-1979. https://doi.org/10.4209/aaqr.2019.02.0084

Highlights

• ²¹⁰Pb activity in Xiamen was high in the reported regions.
• The overall ⁷Be activity was related to latitude.
• Temporal variations in ⁷Be and ²¹⁰Pb were both related to temperature in 2013–2015.
• ²¹⁰Pb activity was significantly correlated with PM_2.5.

Keywords: 7 Be; 210Pb; Aerosol; Coastal city; Environmental radioactivity.

INTRODUCTION

Natural airborne radionuclides (⁷Be and ²¹⁰Pb) serve as powerful tracers for identifying and quantifying several atmospheric processes, such as the source, transport, and mixing of air masses; air mass exchanges between various atmospheric layers; and the residence times of atmospheric gasses and pollutants (Beks et al., 1998; Heikkilä et al., 2008; Papastefanou, 2009b; Poschl et al., 2010; Baskaran, 2011; Lal and Baskaran, 2012; Du et al., 2015; Zhang et al., 2015). Therefore, it is important to understand the temporal and spatial variations of natural airborne radionuclides and influencing factors.

Cosmogenic ⁷Be is formed primarily from the cosmic-ray spallation of oxygen and nitrogen in the stratosphere, troposphere, and surface of the earth (Kaste et al., 2002). ⁷Be is a particle-reactive nuclide that has a constant source with suitable activity. This nuclide has been widely used to trace the transport of suspended particles in the atmosphere and in estuaries and the open ocean (Baskaran et al., 1993; Su et al., 2003; Huang et al., 2013; Du et al., 2015). ²¹⁰Pb (τ = 22.1 a) is continuously produced from ²²²Rn in the atmosphere, and ²²²Rn emanates primarily from the land surface. ²¹⁰Pb may rapidly attach to aerosol particles, which is mainly accomplished via washout by precipitation from the atmosphere (Baskaran, 2011; Pham et al., 2012; Du et al., 2015). ⁷Be and ²¹⁰Pb can be widely used to trace the sources, transport processes, and mixing of aerosols, as ⁷Be is produced in the stratosphere and upper troposphere and ²¹⁰Pb is produced in the lower troposphere.

Earlier studies have found that ²¹⁰Pb is associated with fine particles (Schneider et al., 1983; Winkler et al., 1998). Compared to coarse particles, fine particles have a larger influence on human health, especially particles with an aerodynamic diameter of < 2.5 µm (PM_2.5) (Chen et al., 2018; Lu et al., 2019). Long-term exposure to PM_2.5 can increase the risk of respiratory and cardiovascular diseases (Wang et al., 2018). Some pollutants (e.g., ²¹⁰Pb, metals, sulfates, and polycyclic aromatic hydrocarbons) may be transported with fine atmospheric particles (Schneider et al., 1983; Hu et al., 2002; Hu et al., 2012; Zhang et al., 2018). ²¹⁰Pb and ²¹⁰Po are significant contributors to internal irradiation in humans by natural alpha emitters (Winkler and Rosner, 2000; IAEA, 2017). Therefore, understanding the factors influencing the temporal variations in ²¹⁰Pb and ⁷Be also has important implications for public health.

One important aspect is the large temporal variability of these natural radionuclides (Preiss et al., 1996). The wet atmospheric deposition fluxes of ⁷Be and ²¹⁰Pb in Xiamen were reported in 2001–2013, and the temporal variation was assessed (Jia et al., 2003; Yi et al., 2007; Wang et al., 2014; Zhang et al., 2015; Chen et al., 2016). Although a large database of the atmospheric deposition of ⁷Be or ²¹⁰Pb around the world is available, data are scarce in Asia, especially data on the factors influencing the temporal variation of ⁷Be and ²¹⁰Pb in aerosols. Here, we present weekly variations in ⁷Be and ²¹⁰Pb in aerosol particles in a coastal city in southeastern China. Our objectives were to characterize the temporal variation in radioactivity and its influencing factors. We further calculated the effective doses and evaluated their influence on public health.

MATERIALS AND METHODS

Study Area

Xiamen is a city located on the southeastern coast of China. Xiamen features a subtropical monsoon climate characterized as CFa by the Köppen climate classification (Kottek et al., 2006). The summer is hot and humid, while the winter is mild and dry. Northeastern winds from land prevail in the winter, and southeastern winds from the ocean prevail in the summer (Bao et al., 2018). The average annual temperature is 20.8°C, and the annual precipitation has been approximately 1,200 mm during the past 50 years.

Meteorological data (including daily rainfall amount, daily maximum temperature (Tmax) and daily minimum temperature (Tmin)) were obtained from https://rp5.ru/, while daily air quality data (including PM_2.5 and PM₁₀) were obtained from https://www.aqistudy.cn/historydata/. Detailed information is provided in Supplementary Fig. S1. 

Sampling and Measurement

The aerosol particles were collected on the roof of the isotopic building at the Third Institute of Oceanography in Xiamen (approximately 15 m above the ground; Fig. 1). The building is located at a distance of 50 m from Xiamen Bay, which is part of the coastline of the Taiwan Strait.

Fig. 1. Sampling location in Xiamen (southeastern coast of China). The figure was drawn using ODV software (Schlitzer, 2015).

The aerosol particles for the radioactivity assessment were collected using a super high-volume sampler (MDS-600; TRACERLAB GmbH). The air was filtered through a 60 × 60 cm Petrianov FPP-15-1.5 filter. The sampling time ranged from 14 h to 24 h, and the average flow rate was 800 m³ h^–1. Therefore, the total sample volume in a filter was 11,000–19,000 m³, with an average value of 14,000 m³.

The activity of nuclides in the filter was directly determined with an HPGe detector (BE6530; Canberra), with a 60% counting efficiency and energy resolution of 2.2 keV (at 1332 keV). The activity of nuclides ²¹⁰Pb (46.5 keV) and ⁷Be (477.6 keV) was determined for 2–3 h since most background nuclides (²¹⁴Pb, ²¹⁴Bi, etc.) decay sharply after 1 week (Wang et al., 2012). The efficiency curve was obtained by LabSOCS (Lépy et al., 2001; Bronson, 2003; Du et al., 2008). The ⁷Be activities were corrected for radioactive decay from the middle of the collection time to the middle of the counting time (Kim et al., 1998; Du et al., 2015). In addition, the minimum detection levels (MDLs) for ⁷Be and ²¹⁰Pb in the aerosols were 0.03 mBq m^–3.

Statistical Analysis

Pearson’s correlation was performed to examine the correlation between radionuclide activity and meteorological parameters. All analyses were performed using MYSTAT. 

RESULTS

Meteorological Conditions during the Sampling Period

The average monthly temperature ranged from 12.7–29.4°C from September 2013 to December 2015, and the maximum and minimum temperatures were in July and February, respectively. The average precipitation was 0.1–469.0 mm month^–1 during the sampling period. The maximum monthly precipitation occurred in August 2015, while the minimum monthly precipitation occurred in January 2014 (Supplementary Fig. S1).

During our sampling period in 2013–2015, the mean wind direction was from the north or northeast from October to December (Supplementary Fig. S2), from the east from January to March, and from the southwest or southeast from May to August. 

Temporal Variations in the Activities of ⁷Be and ²¹⁰Pb

The weekly activities of ²¹⁰Pb and ⁷Be in aerosols ranged from 0.17 to 3.31 mBq m^–3 (mean: 1.26 ± 0.78 mBq m^–3) and from 0.17 to 9.84 mBq m^–3 (mean: 4.37 ± 2.65 mBq m^–3), respectively, from Oct. 2013 to Dec. 2015 (Figs. 2(a) and 2(b)). As shown in Fig. 2, the highest and lowest activity values for ⁷Be appeared on Oct. 8, 2014, and Nov. 9, 2015, respectively, while the highest and lowest activity values for ²¹⁰Pb occurred on Jan. 2, 2014, and May 23, 2014, respectively.

Fig. 2. Weekly variations in (a) ⁷Be activity, (b) ²¹⁰Pb activity, and (c) ⁷Be/²¹⁰Pb ratio in aerosols in Xiamen.

Based on the weekly activities of ⁷Be and ²¹⁰Pb, the average monthly activities of ⁷Be and ²¹⁰Pb ranged from 1.18 to 7.62 mBq m^–3 and from 0.34 to 2.57 mBq m^–3, respectively. The highest monthly ⁷Be (7.62 ± 0.97 mBq m^–3) and ²¹⁰Pb (2.57 ± 0.49 mBq m^–3) activities were observed in Oct. 2013 and Dec. 2013, respectively. The monthly ⁷Be activity decreased from October to September, while the monthly variation in ²¹⁰Pb showed a clear “U” shape, with the lowest value appearing in June–July (Fig. 3). A similar pattern was observed in other coastal regions (Vecchi et al., 2005; Dueñas et al., 2011).

Fig. 3. Monthly variations in ⁷Be and ²¹⁰Pb activity and the ⁷Be/²¹⁰Pb ratio in aerosols in Xiamen.

The ratio of ⁷Be/²¹⁰Pb ranged from 2.19 to 8.17, with an average value of 4.00 (Figs. 2(c) and 3). More than half of the samples had a ⁷Be/²¹⁰Pb ratio in the range of 2–4. No significant seasonal trend was observed for this ratio.

DISCUSSION

The ⁷Be and ²¹⁰Pb Activity Levels in Xiamen: A Comparison with Other Regions

The ²¹⁰Pb activity in aerosols may be controlled by the presence of significant land masses within the prevailing wind direction, resuspension of dust, and the wet deposition and vertical mixing of air (IAEA, 2017). Preiss et al. (1996) collected ²¹⁰Pb activities in surface air around the world, and the annual average activity of ²¹⁰Pb was 0–1.0 mBq m^–3. Due to the large land masses, the highest ²¹⁰Pb activities were observed in the subtropical and temperate latitudes of the Northern Hemisphere (IAEA, 2017). Table 1 summarizes the ²¹⁰Pb activity observed in some previous studies. The ²¹⁰Pb activity in Xiamen was comparable with that in Detroit and Belgrade and higher than the values reported for other regions. The high activity of ²¹⁰Pb may be related to local sources. The ²²⁶Ra (parent of ²²²Rn) activity in Fujian Province (Xiamen is one coastal city of Fujian Province) in the surface soil (54 Bq kg^–1) is especially high in China (36.6 Bq kg^–1) (MEP, 2014), which is related to the magmatic bedrock in Fujian Province (Zhuo et al., 2008a; MEP, 2014; Huang and Yu, 2017). Corresponding to the higher ²²⁶Ra activities in the soil in southeastern China, the ²²²Rn flux in southeastern China is generally higher than the average flux for all of China and around the world (UNSCEAR, 2000; Zhuo et al., 2008b).

The production rates of ⁷Be vary with latitude, as the transport of cosmogenic particles, production rates, and their transport pathways are affected by the intensity of the geomagnetic field (Liu et al., 2001; Papastefanou, 2009a). Du et al. (2015) concluded that wet ⁷Be depositional fluxes increase with latitude. The depositional fluxes of ⁷Be at 23 stations around the world are summarized in Fig. 4. A significant relationship between the annual mean activity of ⁷Be and latitude can be observed in the global samples (R² = 0.21, n = 21 (excluding Waliguan and Edinburgh), p < 0.01; Fig. 4(a)). However, due to large differences in geographical and meteorological conditions among those stations, the correlation coefficient suggests that latitude explains approximately 20% of the variation. To better elucidate the influence of latitude on the ⁷Be activity, the ⁷Be activity data from 8 coastal stations with similar elevations and geomagnetic fields along the eastern coast of China were selected. As shown in Fig. 4(b), the relationship between the depositional flux of ⁷Be and latitude along the eastern coast of China was much more significant than that observed globally (R² = 0.89, p < 0.001, n = 8), suggesting that the overall 7Be deposition in coastal regions is strongly controlled by latitude.

Fig. 4. The latitudinal variation in annual depositional fluxes of ⁷Be (mBq m⁻³) at (a) stations around the world and (b) stations in coastal cities in China. The stations are (1) Beijing (Zhu et al., 2014); (2) Qingdao (Yang et al., 2013); (3) Haiyan (Jiang, 1995); (4) Hangzhou (Jiang, 1994); (5) Xiamen (this study); (6) Guangzhou (Yang et al., 2011); (7) Shenzhen; (8) Shenzhen (Song et al., 2003); (9) Edinburgh (Likuku, 2006); (10) Waliguan (Zheng et al., 2005); (11) Palermo (Cannizzaro et al., 2004); (12) Yamagata (Kikuchi et al., 2009); (13) Granada (Azahra et al., 2004); (14) El Arenosillo, El Carmen, and La Rabida (Lozano et al., 2011); (15) Lanzhou (Wu et al., 2011); (16) Xi’an (Chang et al., 2008); (17) Málaga (Gordo et al., 2015); (18) Daejeon (Chae et al., 2011); (19) Islamabad (Ali et al., 2011); (20) Kuwait (Al-Azmi et al., 2001); and (21) Guiyang (Lee et al., 2004).

The ⁷Be/²¹⁰Pb activity ratios in aerosols are significantly lower than those in bulk precipitation samples (2.06–40.0, averaging 10.9 ± 2.3) (Chen et al., 2016). First, this may be attributed to the difference in the scavenging functions of ⁷Be and ²¹⁰Pb. Most of the ⁷Be is removed by precipitation at ground level, while larger amounts of ²¹⁰Pb are produced from the decay of ²²²Rn, which can attach to aerosols and be removed by the dry depositional process (McNeary and Baskaran, 2003). 

Factors Influencing the Temporal Variations in ⁷Be and ²¹⁰Pb Activities

During our sampling period, the weekly ⁷Be and ²¹⁰Pb activities varied by a factor of 59 and 19, respectively. This variation is much larger than that in earlier observations in Granada, Spain (Azahra et al., 2004). As shown in Fig. 2, the variations in ⁷Be and ²¹⁰Pb in some months (e.g., May 2014, Sep. 2014, and Sep. 2015) were comparable to their annual variations, suggesting that ⁷Be and ²¹⁰Pb are highly dynamic. The activities of ²¹⁰Pb and ⁷Be were significantly correlated during our sampling period (Fig. 5). A significant correlation between ⁷Be and ²¹⁰Pb (R² = 0.53) was also observed during wet deposition (Chen et al., 2016), which indicates that to some degree the variations in ⁷Be and ²¹⁰Pb are controlled by similar processes. However, the correlation coefficient between the weekly ⁷Be and ²¹⁰Pb values in our samples (R² = 0.46) suggests that they were also affected by different factors related to differences in their sources.

Fig. 5. Linear correlation coefficients of weekly ⁷Be and ²¹⁰Pb activities with PM_2.5, PM₁₀ and temperature. All the correlations are significant at the 0.01 level (2-tailed).

Overall, the temporal distributions of ⁷Be and ²¹⁰Pb in the surface air are determined by the source and scavenging processes in the air, which are in turn influenced by several factors, such as regional precipitation, humidity, and air temperature. Here, we correlated the 7Be and ²¹⁰Pb activities with environmental parameters (e.g., total suspended particulate concentration (TSP), PM_2.5, PM₁₀, Tmax, Tmin, and precipitation) to explore the factors influencing the temporal variations in ⁷Be and ²¹⁰Pb.

Factors Influencing the Weekly Variation in ²¹⁰Pb Activities

Variation in ²¹⁰Pb activity can be produced by variations in the local radon emanation rates, the height of the atmospheric boundary layer, meteorological parameters, temperature inversions, precipitation, and soil moisture content (Suzuki et al., 1999; Wan et al., 2005; Du et al., 2008; Baskaran, 2016). Significant correlations were found between ²¹⁰Pb and temperature, TSP, PM_2.5, and PM₁₀ (Fig. 5).

Because both ⁷Be and ²¹⁰Pb are particle-reactive radionuclides, they should in theory be affected by particle concentrations. As shown in Fig. 5, weekly ²¹⁰Pb was significantly correlated with PM_2.5 and PM₁₀. This correlation has not been reported before. The correlation coefficient suggests that PM_2.5 can explain more than 50% of the variation in ²¹⁰Pb in aerosols. Interestingly, the correlation was more significant between ²¹⁰Pb and PM_2.5 (and PM₁₀) than between ²¹⁰Pb and TSP, which is consistent with an earlier study that indicated that the ²¹⁰Pb activity is higher in fine particles than in coarse particles (Winkler et al., 1998).A significant negative correlation was observed between weekly ²¹⁰Pb and temperature (Fig. 5). This finding contrasts with observations in an inland region (Azahra et al., 2004) and is consistent with an earlier study in a coastal region (Wan et al., 2005), which further suggests that temperature plays an important role in affecting the atmospheric deposition of ²¹⁰Pb along coasts. On the one hand, the release of ²²²Rn is normally low in June–August when the temperature is high, whereas it is high in December and January when temperatures are low in the Northern Hemisphere (Cheng et al., 2001; Zahorowski et al., 2005). On the other hand, the scavenging process may also play a role. Wet deposition (i.e., rainout and washout) is the predominant mechanism contributing to the total deposition (and scavenging) (Kikuchi et al., 2009; Laguionie et al., 2014). An earlier study found that the wet deposition fluxes of ²¹⁰Pb were positively correlated with rainfall precipitation at the same location (Wang et al., 2014; Chen et al., 2016). The distribution of rainfall has distinct wet and dry periods. The wet period is from May to October, with higher temperatures. As rainfall increases, more ²¹⁰Pb may be scavenged, and the activity of ²¹⁰Pb in aerosols decreases. In our study, average ²¹⁰Pb showed a significant negative relationship with average precipitation (Fig. 6(a)) on the monthly scale, which further indicates that the scavenging process also affects ²¹⁰Pb in aerosol particles.

Fig. 6. The relationship (a) between monthly ²¹⁰Pb activities (mBq m^–3) and monthly precipitation (mm) and (b) between monthly ⁷Be activities (mBq m^–3) and precipitation (mm).

Temperature also explained the lower ²¹⁰Pb observed in Nov. and Dec. 2015 compared to that during the same months in 2013 and 2014, as the maximum temperature was much higher than that in 2013 and 2014 (Supplementary Fig. S1). Overall, compared to the fine particle concentration, temperature and precipitation played less significant roles in the dry deposition of ²¹⁰Pb.

The temporal variation in ²¹⁰Pb activities in aerosols may also be related to the wind direction. From October to December in 2014–2015, a large fraction of wind was from the north (northeastern China). From April to September, the winds were southeasterly and derived from the South China Sea and tropical oceans (Supplementary Fig. S2). Then, from October to December, the study area was primarily affected by air masses transported from northeastern China (land). ²¹⁰Pb is produced from ²²²Rn, which primarily emanates from the land surface. Therefore, the higher ²¹⁰Pb activities from October to December than from April to September (Fig. 3) could also be partly attributed to differences in the source of the aerosols.

Factors Influencing the Weekly Variation in ⁷Be Activities

The temporal variations in ⁷Be activities in air samples depend upon three processes: 1) the transport of stratospheric air into the troposphere during the late winter and early spring seasons, 2) the increased vertical transport rate of ⁷Be in warmer months (from the upper troposphere to the middle and lower troposphere and into the surface air), and 3) the variation in the rate of washout for aerosols (McNeary and Baskaran, 2003).

A significant correlation was also found between ⁷Be and PM_2.5 (and PM₁₀), although with a lower correlation coefficient than that with ²¹⁰Pb. Since ⁷Be is also a particle-reactive radionuclide, the particle concentration is expected to influence ⁷Be, and the weaker significant correlation might be because ⁷Be has different sources than ²¹⁰Pb.

In contrast to ²¹⁰Pb, the variation in ⁷Be was more strongly related to temperature (Fig. 5). In other regions, the stratosphere-troposphere exchange of air masses was found to increase with decreasing temperature (Feely et al., 1989; Baskaran, 1995; Du et al., 2015), and the deposition of ⁷Be was high in winter and spring. In Xiamen, the monthly ⁷Be activity in winter and spring (October–April) was higher than that in summer and autumn (May–September) because of the intense vertical exchange of air masses in winter and spring. In addition, the correlation between ⁷Be and temperature may also be related to the seasonal changes in the wind direction and the wet scavenging process (i.e., precipitation). From October to April of the following year, the winds are northeasterly and derive from high-latitude regions of the northern Asian continent (e.g., northern China, the Korean Peninsula, and Japanese Islands) (Supplementary Fig. S2); from May to September, the winds are southeasterly and derive from lower-latitude regions in southwestern China and the South China Sea (Supplementary Fig. S2). Correspondingly, higher ⁷Be activity was observed from October to April of the following year, which is mainly due to the northeastern monsoon. Conversely, lower ⁷Be activity was observed from May to September, which is mainly due to the southeastern monsoon. Variations in the rate of aerosol washout may also affect the temporal variations in ⁷Be activities. The negative correlation between monthly ⁷Be and precipitation is also significant (which further confirms the influence of the scavenging process on ⁷Be in aerosols; Fig. 6(b)).

Environmental Implications

As observed in our study, both ⁷Be and ²¹⁰Pb showed a significant correlation with PM_2.5. The average breathing rate for adults is 22.2 m³ d^–1 (General Administration of Quality Supervision of the People’s Republic of China, 2000). The effective dose coefficients for the ingestion of ⁷Be and ²¹⁰Pb for members of the public were 5.5 × 10^–11 and 5.6 × 10^–6 Sv/Bq, respectively (Eckerman et al., 2012). The committed effective doses of ⁷Be and ²¹⁰Pb for Xiamen citizens were 1.9 × 10^–3 and 5.7 × 10¹ μSv, respectively. Most internal irradiation doses via ²¹⁰Pb inhalation exist on bone surfaces and lungs. Fine atmospheric particles may be deposited within the lungs. Although the effective dose only accounts for 5.7% of the effective dose limit (1 mSv yr^–1), the effective dose needs to be considered when evaluating the air quality since only two radionuclides were considered here. The significant correlations of ²¹⁰Pb and ⁷Be with atmospheric particles suggest that radionuclides can also be pollutants impacting human health and should be considered in air quality models, especially in regions and seasons with higher PM_2.5 levels. 

CONCLUSIONS

The atmospheric deposition of radionuclides has significant implications for the environment. By monitoring radionuclides in surface aerosols in Xiamen, a coastal city in southeastern China, we discovered a high level of ²¹⁰Pb activity, as well as a strong correlation between ⁷Be activity and latitude in the coastal region. The temporal variations in both radionuclides were influenced by their sources and scavenging processes. Overall, the ²¹⁰Pb activity exhibited a significant positive correlation with the PM_2.5 concentration and a weaker negative correlation with the air temperature (due to the release of ²²²Rn) and precipitation (scavenging). The ⁷Be activity, on the other hand, was primarily influenced by the temperature (specifically, in mid-latitude regions, the stratosphere-troposphere exchange of air masses increases as the temperature decreases and as the northeasterly wind from high-latitude regions strengthens) and precipitation.

The effective dose coefficient for ingested ⁷Be and ²¹⁰Pb was estimated to account for approximately 5.7% of the dose limit in Xiamen. The relationship between ²¹⁰Pb and PM_2.5 (and to a lesser extent, between ⁷Be and PM_2.5) suggests that radionuclides should be considered in air quality models, especially for areas and seasons with high PM_2.5 concentrations as well as for regions at high latitudes.

ACKNOWLEDGMENTS

This research was supported by the Natural Science Foundation of China (41306073 and 41706078), the Science Foundation of the Fujian Province (2017J05065), the Public Science and Technology Research Funds Projects of Ocean (201505005-1), the Scientific Research Foundation of the Third Institute of Oceanography (Nos. 2013011 and 2018029), and the Ministry of Natural Resources.