Atmospheric radioactive nuclide deposition on the coast of the Maowei Sea, northern Beibu Gulf, China

The natural radioisotopes ⁷Be, ²¹⁰Pb, and ²¹⁰Po, with different half-lives, are all particle-reactive and serve as natural tracers to study sources and transportation of sediments, sedimentation rates, and sediment chronology. Atmospheric deposition of these radioisotopes is the premise and foundation of their tracing application. The Maowei Sea is a semi-closed bay along the Beibu Gulf, which is an important gulf in the northwest of the South China Sea, but the atmospheric deposition of the abovementioned radioisotopes has not been systematically reported along the coast. In this research paper, the atmospheric depositional fluxes of ⁷Be, ²¹⁰Pb, and ²¹⁰Po were observed over a period from June 2018 to December 2021 on the coast of the Maowei Sea. The annual atmospheric depositional fluxes (Bq m^-2 yr^-1) of ⁷Be, ²¹⁰Pb, and ²¹⁰Po on the coast of the Maowei Sea were 496.80, 201.72, and 58.08, respectively. The distributions for ⁷Be and ²¹⁰Pb depositional fluxes during a whole year (years 2019 and 2021) followed a bimodal pattern, with one peak from February to April and another peak from August to October, while the distribution for ²¹⁰Po depositional flux showed only one peak during the year of 2019 and another during 2021. The deposition flux and activity of ⁷Be showed a strong positive correlation with the deposition flux and activity of ²¹⁰Pb, respectively; the deposition flux and activity of ²¹⁰Po also showed positive correlations with the deposition fluxes and activities of ²¹⁰Pb and ⁷Be, respectively, indicating a similarly scavenging behavior from the atmosphere. A Pearson correlation matrix was used to illustrate the factors influencing the atmospheric depositions and found that precipitation, air quality index (AQI), and PM (both PM_2.5 and PM₁₀) were the major factors that influenced the deposition of these three radionuclides. Precipitation had significant positive correlations with the deposition fluxes of all three radionuclides, indicating that, for these radionuclides, rainfall was the main scavenging way from the atmosphere. The observations for specific single rainfall events and their air mass backward trajectory analyses showed that the air masses movement during the rainfall may be another important factor that impacted the depositional fluxes for ⁷Be, ²¹⁰Pb, and ²¹⁰Po.

1 Introduction

Radionuclides widely exist in the environment and are involved in various biogeochemical processes. Because of different sources and half-lives, nuclides of the natural decay chains are widely applied to atmospheric research, oceanography, and marine geology (Du et al., 2008). ⁷Be, ²¹⁰Pb, and ²¹⁰Po are natural radionuclides that have been widely used as tracers and chronometers in aquatic, sedimentary, and atmospheric systems (e.g. McNeary and Baskaran, 2007; Du et al., 2010; Chen et al., 2016; Su et al., 2017; Du et al., 2020). ⁷Be (T_1/2 = 53.3 days), ²¹⁰Pb (T_1/2 = 22.3 years), and ²¹⁰Po (T_1/2 = 138 days) have different half-lives, but they are all particle-reactive. Thus, they can be used as natural tracers to study the source and transportation of sediments, sedimentation rates, sediment chronology, atmospheric depositional fluxes, and residence time of atmospheric aerosols (e.g. Turekian et al., 1977; Zou et al., 1982; McKee et al., 1983; DeMaster et al., 1985; Qian et al., 1986; Feng et al., 1999a; Feng et al., 1999b; Fornes et al., 1999; Baskaran and Swarzenski, 2007; McNeary and Baskaran, 2007; Du et al., 2010; Huang et al., 2010; Shan, 2010; Baskaran, 2011; Dai et al., 2011; Huang et al., 2013; Du et al., 2016; Du et al., 2020).

⁷Be is a typical radionuclide produced by cosmic rays bombarding nitrogen and oxygen atoms in the atmosphere (Lal et al., 1958). The main factors affecting its generation are the activity cycles of sunspots, altitude, and latitude. (Lal and Peters, 1967). Approximately 70% of ⁷Be in the atmosphere is generated at the bottom of the stratosphere and 30% at the top of the troposphere. Once generated, ⁷Be will be tightly adsorbed to the atmospheric particles, and the residence time of particles in the stratosphere is approximately 1 year, which is much longer than the half-life of ⁷Be (Dutkiewicz and Husain, 1985). Therefore, most of the ⁷Be produced in the stratosphere does not reach the troposphere except during spring (mostly in the mid-latitudes), resulting in ⁷Be that sinks to the Earth’s surface coming mainly from the troposphere. After its generation in the atmosphere, ⁷Be will be quickly adsorbed to the submicron particles and arrive at the earth’s surface by wet and dry deposition and participate in a series of geochemical processes (Wallbrink and Murray, 1994; Zhu and Olsen, 2009). At the same time, ⁷Be suffers from its self-radioactive decay. Due to the particle’s active and clear source, ⁷Be is widely used to trace atmospheric processes and geochemical processes, such as particulate transport over a time scale of approximately 200 days (Zheng et al., 2010; Shi et al., 2011; Huang, 2012).

While ²¹⁰Pb is produced by radioactive decay from its progenitor, ²²²Rn, which is a noble gas isotope in the ²³⁸U decay chain, emanates primarily from rocks and minerals on the land surface including continent and ocean, and escapes into the atmosphere (Moore et al., 1973; Baskaran et al., 1993; Du et al., 2020). ²¹⁰Po is generated from ²¹⁰Pb via ²¹⁰Bi (T_{1/2 =} 5.0 _d). Similar to ⁷Be, both ²¹⁰Pb and ²¹⁰Po are easily adsorbed by atmospheric particles of sub-micron scale, and then reach the Earth’s surface via dry and wet deposition and are finally buried in sediments (Peirson et al., 1966; Baskaran, 2011). In the absence of ²¹⁰Po from sources other than the radioactive decay of atmospheric ²²²Rn-derived ²¹⁰Pb, the ²¹⁰Po/²¹⁰Pb activity ratio (AR) usually can be utilized to determine the age of aerosols, particles, snow, melt pond, ice core, etc. (Moore et al., 1973; Robbins, 1978; Nozaki et al., 1997; Marley et al., 2000; Baskaran and Santschi, 2002; Huang, 2012; Du et al., 2016; Baskaran and Krupp, 2021).

Although ⁷Be, ²¹⁰Pb, and ²¹⁰Po have different sources, they are all atmospheric fallout radioactive isotopes. The prerequisite for their tracer application is a deep understanding of their atmospheric deposition characteristics and deposition fluxes. In China, there have been some reports on the atmospheric deposition of ⁷Be, ²¹⁰Pb, and ²¹⁰Po (Table 1), but these studies mainly focused on the eastern coastal areas of China, and there was less data on the study of the three isotopes simultaneously. As an important bay in the northwest of the South China Sea, the atmospheric deposition flux of radioisotopes along the coast of the Beibu Gulf is also worth attention. This is of great significance for a deeper understanding of the source-sink pattern of radioactive isotopes in the South China Sea. However, studies on the atmospheric deposition of ⁷Be, ²¹⁰Pb, and ²¹⁰Po along the Beibu Gulf are very limited and have not been reported previously. The Maowei Sea is a typical subtropical bay facing the Beibu Gulf (Figure 1) and is located in the southern part of Qinzhou, Guangxi, China (Figure 1), which is one of the land-sea node cities in the southward channel of “the Belt and Road” initiative. Thus, according to “the Belt and Road” national strategy and regional advantages, it is necessary to pay more attention to the atmospheric environment on the coast of the Maowei Sea. Besides, this study site is suitable for investigating the boundary layer outflow of continental pollution to the Beibu Gulf region. Therefore, in this study, the depositional characteristics of ⁷Be, ²¹⁰Pb, and ²¹⁰Po were studied during a long period on the coast of the Maowei Sea to reveal the atmosphere-sea surface interactions in the Beibu Gulf. The present study is also the first observation of the radioactive nuclides’ depositional fluxes along the Beibu Gulf in southern China.

TABLE 1

Table 1 A summary of atmospheric depositional fluxes of ⁷Be, ²¹⁰Pb, and ²¹⁰Po in China.

FIGURE 1

Figure 1 Location of Guangxi Province (A, B), study site, and monitoring stations of air quality and meteorological factors along Beibu Gulf, Guangxi (C). 20-year (2000-2019) average of daily and monthly precipitation at the Qinzhou site (D) from National Data Center for Meteorological Sciences. The wind-rose diagram from June 2018 to December 2021 for the Qinzhou site (E, data from https://www.weatheronline.co.uk/).

2 Study site

The Maowei Sea belongs to the inner Qinzhou Bay, with an average water depth of ~2.5 m. The surface area of the Maowei Sea is ~135 km² with two major rivers (Maoling River and Qin River) discharging into the bay from the north. The tides of the Maowei Sea are irregular and diurnal, with a wide range of 0.05-4.04 m (Chang et al., 2015). The Maowei Sea is located in the south of Qinzhou City and is an important aquaculture bay for the life of people in Qinzhou. Qinzhou is an important city in the Beibu Gulf urban agglomeration. It is located on the southwest coast of Guangxi at 21°35′-22°41′ N and 107°72′-109°56′ E adjacent to the Maowei Sea (Figure 1). Qinzhou is bordered by the Beibu Gulf in the south and thousands of mountains in the northwest, hence it is affected both by marine and land air masses. In the monsoon region of southeast Asia, Qinzhou is strongly influenced by solar radiation and the monsoon circulation. The climate is classified as tropical maritime monsoon, and the rainy season is between May and September. A figure for the 20-year (from 2000 to 2019) average of daily and monthly precipitation at the Qinzhou site (Figure 1C) from the National Data Center for Meteorological Sciences is shown in Figure 1D. The wind-rose diagram during our sampling period for the Qinzhou site (Figure 1E, data from https://www.weatheronline.co.uk/) showed that the dominant wind direction during the sampling period was south during the summer, north during the fall and winter, and south and north during the spring.

3 Sampling and methods

Samples were collected from June 2018 to December 2021 from the roof of the Ocean Building at Beibu Gulf University, which was built on the coast of the Maowei Sea, at ~20 m above ground level (Figure 1C, site named MWS, 21.8975°N, 108.6083°E, ~25 m above mean sea level). At this height, the contribution of ⁷Be, ²¹⁰Pb, and ²¹⁰Po from resuspended dust from the local ground area is assumed negligible (Du et al., 2015). Wet and dry depositional samples were collected at monthly intervals by a rainwater collector (a steel container with a ceramic painting surface) with a surface area of 1.44 m². At the same time, single rainfall events in the rainy season during the year 2019 were also collected to identify the contributions of the single rainfall events and analyze the influence of the climate factors.

After the samples were collected, the dilute HNO₃ was used to rinse the surface of the container to prevent the loss of ⁷Be, ²¹⁰Pb, and ²¹⁰Po by adsorption onto the container walls. The daily precipitation data were obtained from the National Meteorological Information Center (https://data.cma.cn/). Co-precipitation of ⁷Be, ²¹⁰Pb, and ²¹⁰Po from rainwater was obtained by Fe(OH)₃ (Du et al., 2008; Du et al., 2015). Briefly, after collection, the rainwater samples were acidified to pH~2 with concentrated HNO₃. Then 1 mg stable Pb²⁺ (in the form of Pb(NO₃)₂) and 1 mg Be²⁺ (in the form of BeSO₄) were added for the determination of chemical yield. A known amount of iron ion (5 mg Fe³⁺ per liter sample) in the form of FeCl₃ was added to the solution. Then, 10 hours later, the pH of the solution was adjusted to ~8 by adding ammonium hydroxide. The Fe(OH)₃ precipitation was allowed to settle, and the supernatant and precipitate were separated by centrifugation. The residue was freeze-dried before measurement. As has been reported before (Du et al., 2008), the co-precipitate efficiency of this method was approximately 96% and 99% for ⁷Be and ²¹⁰Pb, respectively.

The activities of ⁷Be and ²¹⁰Pb in the samples were measured by an HPGe gamma-ray detector system (50% relative efficiency, Canberra GX5019) after the dried residue was quantitatively transferred into a plastic box (70×35 mm). The activities of ²¹⁰Pb (46.5 KeV, 4.25%) and ⁷Be (477.6 KeV, 10.5%) were obtained from the counting of the samples for 24 h. The efficiency calibration curve was obtained by LabSOCS with efficiency uncertainties of less than 7% for ⁷Be and 10% for ²¹⁰Pb (Bronson, 2003; Wu et al., 2009). During the deployment period, ⁷Be was continuously added by wet and dry precipitation from the first day of collection, but at the same time, it also underwent radioactive decay. Thus, the ⁷Be depositions were corrected for radioactive decay from the middle time of the collection date to the mid-counting time (Kim et al., 1998). Since there was a relatively long half-life, the correction of radioactive decay for ²¹⁰Pb was negligible.

After collection, samples for ²¹⁰Po were analyzed in the laboratory following the description made by Zhong et al. (2019); Zhong et al. (2020). Briefly, the water samples were immediately filtered through 0.45 μm membrane filters and stirred vigorously for 30 minutes to remove the residual ²²²Rn gas. The filtered samples were acidified immediately to pH 1~2 with HNO₃ and soon after spiked with ~1 dpm of ²⁰⁹Po. Then, Po was coprecipitated with Fe(OH)₃. After settling and centrifugation, the supernatant was discarded and the precipitate was dissolved. After adjusting pH to 1.5~2 and adding 1 ml 20% hydroxylamine hydrochloride, 1 ml 25% Citrate sodium, and 0.3 g Ascorbic acid, Po isotopes were auto-deposited onto nickel disks immersed in the solution. The ²⁰⁹Po and ²¹⁰Po were measured using a low background α-spectrometer (Alpha Analyst, AlphEnsembol-8, TE). The treatment method of particulate ²¹⁰Po on the filter membrane was digested on the heating plate after adding HCl for acidification and ~1 dpm ²⁰⁹Po (weigh the spike to get the extract amount) as a tracer. Because of the black carbon in the sample, the sample could never be digested clearly, so used a filter membrane to filter and collect the solution. Then, the solution was treated the same as the dissolved precipitate for the dissolved ²¹⁰Po.

The estimation of depositional fluxes for ⁷Be, ²¹⁰Pb, and ²¹⁰Po in this study was based on the variations of nuclides’ activities with time (Jia et al., 2003; Bi, 2013). During the sampling period for a specific sample, the radioactive nuclides were added through wet and dry precipitation continuously, but the radioactive self-decay happened at the same time. Thus, the activity of a specific radioactive nuclide over time can be expressed as:

$\begin{array}{ll}\frac{da}{dt}=sf-\lambda a& (1)\end{array}$

Where, A is the activity of a specific nuclide; t is the elapsed time; S is the surface area of the rainwater collector; F is the depositional flux of a specific nuclide; λ is the decay constant of the specific nuclide. Assuming the depositional flux was stable during the sampling period for the monthly sample/single rainfall sample, the activity of nuclide in the collector after a collecting time of t₁ can be expressed by:

$\begin{array}{ll}{A}_{t_1}=\frac{SF}{\lambda }\times (1-e_{}^{-\lambda t_1})& (2)\end{array}$

If the sample was measured at t₂, then the measured activity can be given as:

$\begin{array}{ll}{A}_{t_2}=A_{t_1}\times e_{}^{-\lambda t_2}=\frac{SF}{\lambda }\times (1-e_{}^{-\lambda t_1})e_{}^{-\lambda t_2}& (3)\end{array}$

Then, the depositional flux can be given as:

$\begin{array}{ll}F=\frac{A_{t_2}\times \lambda \times e_{}^{\lambda t_2}}{S\times (1-e_{}^{-\lambda t_1})}& (4)\end{array}$

Equation (4) can be used to estimate the monthly depositional flux for ⁷Be. However, for ²¹⁰Pb, the correction for its radioactive self-decay can be negligible because of its long half-life. Therefore, equation (4) for ²¹⁰Pb can be simplified as the following equation.

$\begin{array}{ll}F=\frac{A_{t_2}}{S\times t_1}& (5)\end{array}$

For ²¹⁰Po, in addition to its self-decay, it was necessary to correct the contribution from its parent. Because of the short half-life of ²¹⁰Bi (T_{1/2 =} 5 days), it could be considered that ²¹⁰Pb and ²¹⁰Bi were in equilibrium during the sampling period. Thus,

$\begin{array}{ll}\frac{d{A}_{Po}}{dt}=S{F}_{Po}-{\lambda }_{Po}{A}_{Po}+{\lambda }_{Po}{A}_{Pb}& (6)\end{array}$

Then, the depositional flux for ²¹⁰Po can be given as:

$\begin{array}{ll}{F}_{Po}=\frac{{\lambda }_{Po}}{S\times (1-e_{}^{-{\lambda }_{Po}{t}_1})}\left[A_{Po}-\frac{S{F}_{Pb}}{{\lambda }_{Pb}}(1-e_{}^{-{\lambda }_{Po}{t}_1})-\frac{{\lambda }_{Po}S{F}_{Pb}}{({\lambda }_{Po}-{\lambda }_{Pb}){\lambda }_{Pb}}(e_{}^{-{\lambda }_{Po}{t}_1}-e_{}^{-{\lambda }_{Pb}{t}_1})\right]& (7)\end{array}$

The volume-weighted activity of a specific nuclide (A_V) in the atmosphere can be estimated based on the total amount of the nuclide divided by the rainfall data. Thus, the volume-weighted activity can be expressed as:

$\begin{array}{ll}{A}_V=\frac{F\times t_1}{P}& (8)\end{array}$

where, P is the precipitation during the sampling period (mm).

4 Results and discussion

4.1 Precipitation and air quality

During the study period, the mean annual total precipitation was 2765 mm, and the average air quality index (AQI) showed an excellent air quality rating. The average concentrations (μg/m³) for PM_2.5 and PM₁₀ in the air during the sampling period were 26.49 and 46.78, respectively. No significant differences (p>0.05) were found among different sampling years for all these factors (precipitation, AQI, PM_2.5, PM₁₀). As seen in Figure 2, the monthly precipitation varied contrary to the levels of PM_2.5 (r=-0.624, p<0.01), PM₁₀ (r=-0.622, p<0.01), and AQI (r=-0.633, p<0.01), and AQI varied consistent with the levels of PM_2.5 (r=-0.991, p<0.01) and PM₁₀ (r=-0.987, p<0.01). This indicated that the precipitation might be the major factor influencing the scavenging of PM in the study area and affected the air quality further.

FIGURE 2

Figure 2 Monthly variation of precipitation, AQI, and concentrations of PM_2.5 and PM₁₀ during the sampling period.

4.2 Atmospheric deposition of ⁷Be, ²¹⁰Pb, and ²¹⁰Po

The calculation of monthly deposition fluxes for ⁷Be, ²¹⁰Pb, and ²¹⁰Po were based on equations (4), (5), and (7), respectively. During our sampling period, the monthly deposition fluxes (Bq m^-2 month^-1) of ⁷Be varied from 4.16 to 103.58 with a mean of 41.40 (n=38). No significant difference (p>0.05) was observed among different sampling years. The annual ⁷Be deposition flux was estimated to be 496.80 Bq m^-2 yr^-1, which was within the low level of the global reported data range (409-5300 Bq m^-2 yr^-1 from 19.0°N to 47.7°N, Du et al., 2015). In China, this observation result was close to the reported data in Guangzhou but lower than other sites with a higher latitude than this study region (Table 1). The atmospheric deposition ⁷Be flux was expected to have a latitudinal variation since it originated from the interaction between cosmic rays and the atmosphere. Some studies found this to be the case (Turekian et al., 1983; McNeary and Baskaran, 2003; Usoskin and Kovaltsov, 2008; Du et al., 2015; Zhong, 2020). Our study site was at a low latitude of 21.9°N but higher than the deposition flux in Spain (36-37°N), indicating that in addition to its different production rate with latitude, other factors, such as air convection, precipitation, and residence time of airborne particles, might affect the atmospheric deposition ⁷Be flux. From June 2018 to December 2021, the atmospheric depositional ²¹⁰Pb fluxes (Bq m^-2 month^-1) varied from 1.92 to 42.82 with an average of 16.81 (n=38) on the Maowei Sea coast. A significant difference (p<0.05) was found between different sampling years (June 2018-July 2019 and June 2020-July 2021). The mean deposition flux was higher during June 2018-July 2019 than in June 2020-July 2021. As is widely known, the COVID outbreak reached epidemic proportions at the end of 2019. The ²¹⁰Pb in the atmosphere mainly comes from the emanation of ²²²Rn, 99% of which in the atmosphere originates from the continent (Baskaran, 2011). Thus, the lower deposition flux in the later year may be due to reduced human and terrestrial activities caused by the COVID-19 pandemic and which had not yet been fully restored. As reported, the GDP decreased by 6.8% from the last quarter of 2019 to the first quarter of 2020, and the unemployment rate of residents increased to 6.2% in February 2020 (Xia et al., 2020). Whereas, the urban pollutants were reduced especially for the concentrations of PM_2.5 and PM₁₀ with a decrease of 48% and 47%, respectively (Wu et al., 2020). The mean annual ²¹⁰Pb deposition flux was estimated to be 201.72 Bq m^-2 yr^-1, which was among the global ²¹⁰Pb deposition flux range (59-840 Bq m^-2 yr^-1, Du et al., 2015; Zhong, 2020). In China, the ²¹⁰Pb deposition flux on the Maowei Sea Coast was close to the average reported value (255 Bq m^-2 yr^-1, Table 1). A significant linear relationship was found between the deposition fluxes of ²¹⁰Pb and ⁷Be (r=0.887, p<0.01, n=38), indicating a similar atmospheric environment behavior path for ²¹⁰Pb and ⁷Be (Jia et al., 2003; McNeary and Baskaran, 2003). During the study period, the atmospheric deposition fluxes (Bq m^-2 month^-1) of ²¹⁰Po ranged from ~0 to 34.35 with an average of 4.84 (n=38). Similarly, a significant difference (p<0.05) was found for ²¹⁰Po deposition flux between different sampling years (June 2018-July 2019 and June 2020-July 2021). A much lower ²¹⁰Po deposition flux was obtained during June 2020-July 2021 than during June 2018-July 2019. Since the ²¹⁰Po in the atmosphere mainly comes from the decay of ²¹⁰Pb, the COVID-19 pandemic might also have influenced the production of ²¹⁰Po, consequently influencing its deposition flux. The mean annual ²¹⁰Po deposition flux was estimated to be 58.08 Bq m^-2 yr^-1, which was among the global ²¹⁰Po deposition flux range (11.40-83.46 Bq m^-2 yr^-1, Zhong, 2020), but higher than ever reported data in China (11.40-20.60 Bq m^-2 yr^-1, Table 2). In general, the atmospheric deposition of ²¹⁰Po showed a defect compared to ²¹⁰Pb, which may be due to the short residence time of ²¹⁰Pb in the atmosphere, which makes it difficult to produce enough ²¹⁰Po. Linear relationships were found between the deposition fluxes of ²¹⁰Po and ⁷Be (r=0.449, p<0.01, n=38) and between ²¹⁰Po and ²¹⁰Pb (r=0.310, p=0.058, n=38), but they were not significant, indicating a different atmospheric environment behavior of ²¹⁰Po.

TABLE 2

Table 2 Depositional fluxes and activities of ⁷Be, ²¹⁰Pb, and ²¹⁰Po measured in the single rainfall events on the coast of the Maowei Sea during 2019.

The temporal variations of monthly deposition fluxes for ⁷Be, ²¹⁰Pb, and ²¹⁰Po during the study period are shown in Figure 3A. Due to the special circumstance caused by COVID-19, no data was available from January to May 2020. From Figure 3A, we can see that the highest ⁷Be deposition flux value was observed in April 2021 and the lowest value was in November 2019; it was also much lower in November 2020. The highest ²¹⁰Pb deposition flux value was observed in August 2018 and the lowest value was in November 2020. The highest monthly deposition flux of ²¹⁰Po was shown in February 2019. Except for February, March, and April 2019, the deposition flux of ²¹⁰Po was much lower than 10 Bq m^-2 month^-1 and even near zero in several months, which indicated that ²¹⁰Po mainly came from the decay of ²¹⁰Pb and the residence time of ²¹⁰Pb in the atmosphere was short, making it difficult to produce enough ²¹⁰Po. The high deposition flux in February, March, and April may be due to the special high-humidity weather that is experienced in Guangxi, China from February to April. The special high-humidity weather is called “Huinan Tian” in Chinese and usually occurs after a long-term cold air in spring. During this, temperature and humidity increase suddenly, atmospheric pressure decreases suddenly, and the air becomes saturated (Luo et al., 2015; Huang et al., 2020). Under the influence of “Huinan Tian”, water vapor in the air condenses into small water drops, which can carry more ⁷Be, ²¹⁰Pb, and ²¹⁰Po during each rainfall event because of a relatively larger surface area per unit mass. Such cases are very similar to the plum rain season in Shanghai, China (Yang et al., 2005; Du, et al., 2015).

FIGURE 3

Figure 3 The temporal distribution pattern of monthly deposition fluxes for ⁷Be, ²¹⁰Pb, and ²¹⁰Po (A, unit: Bq·m^-2·month^-1); and the distribution of seasonal deposition fluxes for ⁷Be, ²¹⁰Pb, and ²¹⁰Po (B, unit: Bq·m^-2·season^-1).

This special weather made the atmospheric deposition much slower in order to produce more ²¹⁰Po in the air. The deposition fluxes of ⁷Be, ²¹⁰Pb, and ²¹⁰Po varied a lot in the same month of different years, with a maximum coefficient of variation of 100%, 101%, and 149% for ⁷Be, ²¹⁰Pb, and ²¹⁰Po, respectively, and a minimum coefficient of variation of 13%, 8%, and 36% for ⁷Be, ²¹⁰Pb, and ²¹⁰Po, respectively. The monthly deposition flux varied most in November for both ⁷Be and ²¹⁰Pb, and in December for ²¹⁰Po; and varied least in April for both ⁷Be and ²¹⁰Pb, and in August for ²¹⁰Po. The overall variation trends for ⁷Be, ²¹⁰Pb, and ²¹⁰Po deposition fluxes showed high values from June to October and from February to April, and low values from November to February. If we analyze the data from January 2019 to December 2019, and from January 2021 to December 2021, it becomes clear that the distribution of ⁷Be and ²¹⁰Pb deposition fluxes followed a bimodal pattern, with one peak from February to April and another peak from August to October. While the distribution of ²¹⁰Po deposition flux was similar to an irregular parabola, with the peak showing from August to October 2019 and from February to April 2021.

The distribution of seasonal deposition fluxes for ⁷Be, ²¹⁰Pb, and ²¹⁰Po are shown in Figure 3B. The seasonal deposition fluxes ranged from 52.18 to 245.93 (mean=128.97, n=12) Bq m^-2 season^-1 for ⁷Be, from 23.48 to 109.48 (mean=52.36, n=12) Bq m^-2 season^-1 for ²¹⁰Pb, and from 0.36 to 47.78 (mean=15.18, n=12) Bq m^-2 season^-1 for ²¹⁰Po. From Figure 3B, we can see that the highest seasonal deposition flux was observed in summer 2018 for both ⁷Be and ²¹⁰Pb, whereas the highest seasonal value for ²¹⁰Po was observed in winter 2018. The lowest seasonal deposition flux was observed in the fall of 2020 for both ⁷Be and ²¹⁰Pb and in the winter of 2020 for ²¹⁰Po. We can still find that the seasonal variation during each year showed the highest value in summer, followed by spring, and the lowest value was seen during fall for ²¹⁰Pb and ⁷Be. This was different from the reported data in Shanghai that showed the highest deposition fluxes in winter and spring for ²¹⁰Pb and ⁷Be, respectively, and the lowest in summer for both nuclides (Du et al., 2015). The seasonal variation for ²¹⁰Po was not obvious, and the maximum and minimum values may occur simultaneously in the same season of different years. The seasonal deposition fluxes of ²¹⁰Pb and ⁷Be also varied during the same season of different years, with a maximum coefficient of variation of 45.34% being reported in the summer for ²¹⁰Pb and 48.37% for ⁷Be in the winter.

The total deposition fluxes of ⁷Be, ²¹⁰Pb, and ²¹⁰Po on the coast of the Maowei Sea were, respectively, 484, 199, and 99.4 Bq m^-2 yr^-1 for 2019 and 520, 190, and 26.2 Bq m^-2 yr^-1 for 2021. No significant difference was observed for ⁷Be and ²¹⁰Pb. Atmospheric input of ⁷Be, ²¹⁰Pb, and ²¹⁰Po on the Maowei Sea water surface could be estimated to be 6.53×10¹⁰, 2.69×10¹⁰, and 1.34×10¹⁰ Bq yr^-1 in 2019 and 7.02×10¹⁰, 2.56×10¹⁰, and 0.35×10¹⁰ Bq yr^-1 in 2021 by multiplying the surface area of the Maowei Sea. If we consider the total sampling months into two parts as before the COVID-2019 pandemic and after the COVID-2019 pandemic, an obvious reduction was found after COVID-2019, which deserved a long-term observation.

4.3 Volume-weight activities of ⁷Be, ²¹⁰Pb, and ²¹⁰Po

We defined the volume-weighted activity of a specific nuclide in the atmosphere as the amount of the nuclide divided by the rainfall data (Equation (8)). We found that the activity (Bq L^-1) of ⁷Be varied from 0.05 ± 0.00 to 6.24 ± 0.37 with an average of 0.63 ± 1.07 (n=38) during our sampling period, which was much lower than the reported ⁷Be activity in rainwater of different cities, such as Shanghai, China (1.34 Bq L^-1, Zhong, 2020), Xiamen, China (1.48-2.10 Bq L^-1, Chen et al., 2016; Zhang et al., 2016), and Hakodate, Japan (4.07 Bq L^-1, Tokieda et al., 1996). During the sampling period, the volume-weighted activity (Bq L^-1) of ²¹⁰Pb ranged from 0.01 ± 0.00 to 4.86 ± 0.73 with a mean of 0.35 ± 0.80 (n=38), which was close to the observation in Xiamen, China during 2010-2012 (0.375 Bq L^-1, Wang et al., 2014), but lower than the reported data in Japan [from 0.49 Bq L^-1 in Hakodate during 1990-1991 (Tokieda et al., 1996) of 1.22 Bq L^-1 in Assabu during 1972 (Tsunogai and Fukuda, 1974)]. The variation of the volume-weighted activity (Bq L^-1) for ²¹⁰Po ranged from 0.00 ± 0.00 to 0.55 ± 0.05 with an average of 0.07 ± 0.13 (n=38), which was much higher than the reported ²¹⁰Po activity concentrations in rainwater of different areas, such as Harwell, UK (0.009 Bq L^-1, Burton and Stewart, 1960), Arkansas and Boulder, USA (0.0058 and 0.0185 Bq L^-1, Poet et al., 1972; Gavini et al., 1974), Hakodate and Assabu, Japan (0.035 and 0.0098 Bq L^-1, Tsunogai and Fukuda, 1974), and Shanghai and Xiamen, China (0.0048 and 0.034 Bq L^-1, Wang et al., 2014; Chen et al., 2016; Zhong, 2020). No significant difference (p>0.05) was observed among different sampling years for ⁷Be and ²¹⁰Pb. However, a significant difference (p<0.05) was found for ²¹⁰Po activity between different sampling years (June 2018-July 2019 and June 2020-July 2021). A much lower average ²¹⁰Po activity was obtained during June 2020-July 2021 than in June 2018-July 2019. A significant linear relationship was found between the activities of ²¹⁰Pb and ⁷Be (r=0.980, p<0.0001, n=38), but no significant relationships were found between the activities of ²¹⁰Po and ²¹⁰Pb, or between ²¹⁰Po and ⁷Be, indicating precipitation may be the main factor that influences the environmental behavior of ²¹⁰Pb and ⁷Be, but not ²¹⁰Po. Generally, the order of activities for these three radionuclides was ²¹⁰Po<²¹⁰Pb<⁷Be.

The temporal variations for activities of ⁷Be, ²¹⁰Pb, and ²¹⁰Po during the study period are shown in Figure 4A. As seen in Figure 4A, the maximum activity values of ²¹⁰Pb and ⁷Be were observed in January 2021 and the minimum value for ²¹⁰Pb was registered in August 2020, and for ⁷Be it was seen in May 2019. However, there was also a much lower activity in August 2020 for ⁷Be. The highest activity for ²¹⁰Po was observed in April 2019. Almost 85% of the deposition samples showed an activity of ²¹⁰Po of less than 0.1 Bq L^-1. Much similar temporal distribution patterns for the activities of all three radionuclides were found during our sampling period, that is, the activities started increasing in the fall of 2018, and reached the first peak in the spring of 2019, then suddenly decreased in the summer before another peak appeared in the fall of 2019. The variations repeated the same patterns in the following two years. In general, the variation of the activities for ⁷Be and ²¹⁰Pb within the years 2019 and 2021 was shaped like a positive parabola, showing high values at the end of the year and low values during the middle of the year. The monthly volume-weighted activities of ⁷Be and ²¹⁰Pb varied the most in January, with the maximum coefficient of variation of 125% and 128% for ⁷Be and ²¹⁰Pb, respectively. The minimum coefficient of variation was found in March for both ⁷Be and ²¹⁰Pb, with values of 8% and 21%, respectively. The maximum and minimum coefficients of variation for ²¹⁰Po were found in July and June with values of 142% and 39%, respectively.

FIGURE 4

Figure 4 The temporal variations for monthly volume-weighted activities (A), seasonal volume-weighted activities (B) of ⁷Be, ²¹⁰Pb, and ²¹⁰Po, activity ratios of ⁷Be/²¹⁰Pb and ²¹⁰Po/²¹⁰Pb (C) during the study period.

The seasonal volume-weighted activity was obtained by dividing the total rainfall of a specific season into the total amount of nuclides during that specific season. The seasonal volume-weighted activities of ⁷Be, ²¹⁰Pb, and ²¹⁰Po are shown in Figure 4B and indicate that the maximum seasonal volume-weighted activities were observed in the winter of 2018 for all three nuclides, and the minimum seasonal volume-weighted activities were observed in the fall of 2020 for ⁷Be, in the summer of 2020 for ²¹⁰Pb, and in the spring of 2021 for ²¹⁰Po. The seasonal variations during each year showed high values in winter, followed by spring, and low values in summer, which indicated the dilution of the precipitation in summer (Du et al., 2015). Maximum coefficients of variation of 45% were found in summer for ²¹⁰Pb, and 48% and 139% in winter for both ⁷Be and ²¹⁰Po, respectively. Minimum coefficients of variation of 0.8% and 3.7% were observed in spring for both ⁷Be and ²¹⁰Po, respectively, and 50% in summer for ²¹⁰Po.

4.4 ⁷Be/²¹⁰Pb and ²¹⁰Po/²¹⁰Pb activity ratios

The activity ratios of ⁷Be/²¹⁰Pb and ²¹⁰Po/²¹⁰Pb can provide information about the vertical movements of air masses, the residence time of the airborne particulate matter in the atmosphere, and the scavenging of aerosols (Leppänen, 2019). In the 38 monthly deposition samples, the activity ratio of ⁷Be/²¹⁰Pb varied from 1.17 to 4.35 with an average of 2.45 (n=38); the activity ratio of ²¹⁰Po/²¹⁰Pb varied from 0.00 to 1.31 with a mean value of 0.31 (n=38). The activity ratio of ²¹⁰Po/²¹⁰Pb was lower than that of ⁷Be/²¹⁰Pb, and less than 1 for almost all the samples except for two samples, showing a defect for ²¹⁰Po compared to its parent ²¹⁰Pb. The activity ratio of ⁷Be/²¹⁰Pb was greater than 1 in all samples. More than 50% of the deposition samples showed an activity ratio value of ⁷Be/²¹⁰Pb between 2 and 3, and more than 30% of the samples had a value between 1 and 2. There was no significant difference (p>0.05) for both activity ratios of ²¹⁰Po/²¹⁰Pb and ⁷Be/²¹⁰Pb among different sampling years. The activity ratio of ⁷Be/²¹⁰Pb on the coast of the Maowei Sea was much different from the reported data in other regions, ranging from 1.3 to 30 in Malaga, Spain (Dueñas et al., 2017), from 2.2 to 14 in the Arctic region (Baskaran and Shaw, 2001), from 1.2 to 10.1 on the coastal region of Niteroi, Brazil (Sanders et al., 2011), and from 1.1 to 13.3 in Shanghai, China (Du et al., 2015). The relatively stable ratios reported on the coast of the Maowei Sea may indicate that atmospheric processes did not change frequently.

Figure 4C is a plot of the monthly volume-weighted activity ratios for ⁷Be/²¹⁰Pb and ²¹⁰Po/²¹⁰Pb, from which we can see that there was no consistent monthly variation trend for ratios of ⁷Be/²¹⁰Pb and ²¹⁰Po/²¹⁰Pb. However, when a high activity ratio for ⁷Be/²¹⁰Pb appeared, the activity ratio for ²¹⁰Po/²¹⁰Pb also showed a high value, which may indicate that the months with higher levels of ⁷Be in the air were also the months with a longer residence time of atmospheric particulate matter. The highest value for ⁷Be/²¹⁰Pb was found in August 2020 and the lowest in July 2019. ⁷Be/²¹⁰Pb activity ratios did not show much variation in the same month of different years, with a maximum coefficient of variation of 34.4% in August, and a minimum of 5.6% in April. Seasonal ⁷Be/²¹⁰Pb activity ratios ranged from 1.77 to 3.24 with an average of 2.46 (coefficient of variation of 18.8%), which was observed to be higher during summer 2021 and lower during winter 2020. The highest value for ²¹⁰Po/²¹⁰Pb was found in February 2019. Except for January and February 2019, the activity ratios of ²¹⁰Po/²¹⁰Pb were less than 1.0, indicating a defect of ²¹⁰Po compared to its parent ²¹⁰Pb. Unlike ⁷Be/²¹⁰Pb activity ratios, ²¹⁰Po/²¹⁰Pb activity ratios showed many variations in the same month of different years, with a maximum coefficient of variation of 123% in July, and a minimum of 62% in November. Seasonal ²¹⁰Po/²¹⁰Pb activity ratios ranged from 0.01 to 0.99 with an average of 0.30 (coefficient of variation of 100%), which was observed to be higher during the winter of 2018 and lower during the winter of 2020. The relatively stable ratios of ⁷Be/²¹⁰Pb on the coast of the Maowei Sea may indicate atmospheric processes did not change frequently between different seasons.

4.5 Single rainfall

From the monthly precipitation shown in Figure 1, we can find that the distribution of rainfall throughout the year was uneven, with more than 80% of rainfall occurring in summer and only approximately 20% in winter. Therefore, it was necessary to study the specific rainfalls during the rainy season in order to study the influence of the precipitation on the nuclides’ deposition. From June to August 2019, the rainiest months in 2019, 18 samples for single rainfall were collected. Table 2 and Figure 5 show the details of the total precipitation, deposition fluxes, and activities of ⁷Be, ²¹⁰Pb, and ²¹⁰Po measured during these specific rainfalls on the coast of the Maowei Sea, Beibu Gulf, China. From this, we can see that the deposition fluxes and the activities varied with the precipitation. There was a positive relationship between the daily deposition fluxes and the precipitation, while a negative correlation was found between the activities and the precipitation. The activities (Bq L^-1) of ⁷Be, ²¹⁰Pb, and ²¹⁰Po in specific rainfalls ranged from 0.03 ± 0.00 to 4.86 ± 0.29, 0.01 ± 0.00 to 1.94 ± 0.29, and 0.00 ± 0.00 to 0.29 ± 0.01 with the mean (n=18) of 0.63, 0.20, and 0.03, respectively. By comparison with the reported data in rainwater of different cities in the world, the activities of ⁷Be and ²¹⁰Pb in rainwater on our study site were much lower than the average (2.25 Bq L^-1 for ⁷Be and 0.36 Bq L^-1 for ²¹⁰Pb, Zhong, 2020); but the activity of ²¹⁰Po was much higher than the average reported data (0.018 Bq L^-1, Zhong, 2020). The activity ratio of ⁷Be/²¹⁰Pb in these single rainfalls varied from 1.19 to 7.35 with an average of 3.22 (n=18). The activity ratio of ²¹⁰Po/²¹⁰Pb ranged from 0.07 to 0.90 with a mean of 0.18 (n=18) in these single rainfalls. The highest activity for all these three radionuclides was observed in sample R-17 collected on 26 August 2019, and the lowest for ⁷Be and ²¹⁰Pb was observed in sample R-7 collected on 3 July 2019. For ²¹⁰Po, it was observed in sample R-6 collected on 1 July 2019. The precipitation during sampling was found to have the highest value on 1 July 2019, and a lower value on 26 August 2019, which may verify the dilution effect of rainfall. In 2019, the single rainfall events in June, July, and August brought about 24.5% of the total annual precipitation, but deposited approximately 24.8%, 17.7%, and 5.43% of the total annual bulk deposition fluxes of ⁷Be, ²¹⁰Pb, and ²¹⁰Po, respectively, which may indicate that more ⁷Be could be removed from the atmosphere by a single rainfall. Otherwise, the average precipitation for single rainfall was higher in June and lower in August, and the deposition flux was higher in June and lower in August as well, except for ²¹⁰Po, which was higher in August and lower in July. The activity showed higher values in August and low values in July, which indicated a further dilution of the precipitation for these radionuclides. The cumulative deposition, cumulative activity, and activity ratio can be calculated from the view of the time series for the single rainfall in one month and are shown in Figure 6. We can see that in all three months, a positive correlation was observed between the cumulative deposition of radionuclides and the elapsed time from the first single rainfall in each month (Figures 6A, D, G). However a logarithmic correlation was shown in June, and an exponential correlation in July and August. The relationship between cumulative activities for the three radionuclides and the elapsed time showed a similar correlation as the cumulative relationship between deposition and elapsed time, except for ²¹⁰Po in August, which had a higher activity at first (Figure 6H). The activity ratio of ²¹⁰Po/²¹⁰Pb for the cumulative activities showed a general decline over time, while the activity ratio of ⁷Be/²¹⁰Pb increased with the elapsed time in June and August. It was much different than the activity ratio of ²¹⁰Po/²¹⁰Pb for the cumulative activities, which first rose then fell over time, and the activity ratio of ⁷Be/²¹⁰Pb, which went down and then up with time in July. Of all these single rainfall events, July registered the maximum rainfall intensity but the minimum cumulative deposition and activities for all three radionuclides despite their different origins and transport pathways in the atmosphere.

FIGURE 5

Figure 5 The details of the total precipitation, deposition fluxes, and activities of ⁷Be, ²¹⁰Pb, and ²¹⁰Po measured during specific rainfalls in 2019 on the coast of the Maowei Sea, Beibu Gulf, China.

FIGURE 6

Figure 6 Cumulative depositions, cumulative activities, and activity ratios as time elapsed from the first single rainfall in June (A–C), July (D–F), and August (G–I) during 2019.

4.6 Sources and migration of the air masses for a single rainfall

In this study, the HYSPLIT model (Hybrid Single Particle Lagrangian Integrated Trajectory model) developed by the NOAA (National Oceanic and Atmospheric Administration) was used to analyze the sources of air masses during every single rainfall of our sampling period. The backward trajectory was calculated at heights 1.5 km, 5 km, and 12 km above mean sea level, starting from every specific sampling time, which was converted to the UTC (abbreviation for Universal Time Coordinated) time, ending at 72 h before the sampling time. The height of 1.5 km was assumed to represent the base of the precipitation cloud (Li, 2009) and the transportation of air masses was through the lower atmospheric layer. The altitude of 5 km was selected to analyze the migration of atmospheric air masses in the middle troposphere. The 12 km was set as the top layer of the model to analyze the movement of atmospheric masses between the tropopause and the lower stratosphere (Peng et al., 2011). Figure 7 shows the results of the backward trajectory analyses model, from which, we can see that the air masses were somewhat similar in the lower atmospheric layer (1.5 km, Figure 7A) and the middle troposphere (5 km, Figure 7B). Most of them were maritime air masses from the Beibu Gulf and the South China Sea, and some of them were mixed with continental air masses in the process of their transportation to the sampling site. However, the model results for the height of 12 km showed mainly continental air masses from the north and the west (Figure 7C). The ⁷Be/²¹⁰Pb ratios of the single rainfall ranged from 1.19 (on 10 July 2019) and 7.35 (on 20 August 2019) with an average of 3.22, which was higher than the monthly and seasonal activity ratios. This may be due to the changes in the air masses during a single rainfall. In addition, the air mass backward trajectories on both 10 July and 4 August (with an activity ratio of 1.25) showed the vertical air masses moved from the tropopause to the lower stratosphere, which led to a lower ⁷Be/²¹⁰Pb ratio in the sampling site. However, the backward trajectory on 20 August 2019 showed the sampling site was mainly influenced by the air mass in the lower atmosphere and mainly by the maritime air mass of Beibu Gulf in the south. Since the maritime air mass generally derived with a high ⁷Be/²¹⁰Pb ratio (Du et al., 2015; Du, 2019; Peng et al., 2019), the source of this sample was mainly from the maritime air mass of Beibu Gulf in the lower atmospheric layer.

FIGURE 7

Figure 7 Air masses backward trajectory analyses of the specific rainfall events from R-1 to R-18 in different heights (A) 1.5 km, (B) 5.0 km, and (C) 12 km. Using different line colors to distinguish different rainfall events.

4.7 Factors influencing atmospheric deposition of radionuclides

When we attempted to investigate the factors influencing the atmospheric depositions of ⁷Be, ²¹⁰Pb, and ²¹⁰Po, the levels of precipitation, AQI, the concentrations of particulate matters (PM, including PM_2.5 and PM₁₀), SO₂, NO₂, CO, and O₃ were assumed to explain the ambient air nuclides behavior in the present study. Figure 8 was a Pearson correlation matrix result of those possible environmental factors and the studied variables during our sampling period at the Maowei Sea coast. The color bar represents the correlation coefficients converted to percentages as shown in the correlation plot. Variables with significant correlations at a 95% confidence level (p< 0.05) are denoted by an asterisk (*)-marked. From Figure 8, we can see that the deposition flux and activity of ⁷Be showed a strong positive correlation with the deposition flux and activity of ²¹⁰Pb, respectively, indicating a similar scavenging behavior for ⁷Be and ²¹⁰Pb from the atmosphere. At the same time, the deposition flux and activity of ²¹⁰Po also showed positive correlations with the deposition fluxes and activities of ²¹⁰Pb and ⁷Be, respectively. However, the correlation between ²¹⁰Po and ²¹⁰Pb was not significant when compared with the correlation between ²¹⁰Po and ⁷Be. Since the ²¹⁰Po/²¹⁰Pb activity ratio can trace the residence time of PM in the air, this further confirmed that a longer residence time of PM made it able to adsorb the particle reactive radionuclides in the air and generated higher deposition results. From Figure 8, precipitation, AQI, and PM (both PM_2.5 and PM₁₀) were the major factors that may influence the deposition of the three radionuclides. Precipitation had significant positive correlations with the deposition fluxes of all three radionuclides, indicating that rainfall was the main scavenging way from the atmosphere for these radionuclides. Meanwhile, both AQI and PM had significant negative correlations with the deposition fluxes of all three radionuclides, indicating that particle settlement could also remove the radionuclides in the air. The particle settlement was strongly affected by the precipitation (a significant negative correlation between AQI, PM, and the precipitation in Figure 8). This suggests that the total nuclide deposition might be the result of the scavenging of PM by precipitation. To further confirm the aforementioned hypothesis, the products of PM_2.5 and precipitation, PM₁₀ and precipitation were used. The ⁷Be and ²¹⁰Pb deposition fluxes were found significantly dependent on the arithmetic product of the PM level and precipitation. This demonstrated that PM was representative of the atmospheric ⁷Be and ²¹⁰Pb levels and the scavenging of radionuclides was from atmospheric PM by precipitation. For ²¹⁰Po, although there was a positive correlation relationship between the deposition and the PM×precipitation, it was not significant, which indicates that it may depend on its production rate from ²¹⁰Pb and the residence time of the PM. Otherwise, the concentrations of SO₂, NO_2, and O₃ showed slightly weak negative correlations with the nuclide deposition, indicating, except for the decay of ²²²Rn, that other sources, such as the burning of fossil fuels, could contribute ²¹⁰Pb and ²¹⁰Po in the atmosphere. Meanwhile, the level of O₃ can prevent some short rays from reaching the ground, so it could impact the deposition of ⁷Be.

FIGURE 8

Figure 8 A Pearson correlation matrix result of the possible environmental factors and the studied variables during our sampling period at the Maowei Sea coast. The color bar represents the correlation coefficients converted to percentages as shown in the correlation plot. Variables with significant correlations at 95% confidence level (p< 0.05) are denoted by asterisk (*)-marked.

By combining all reported data listed in Table 1 in China, Figure 9 was constructed, from which, we can see that the latitude effect was obvious but different for all three radionuclides. For ⁷Be, there was a positive correlation relationship between the annual deposition flux and the latitude, which may be due to the production of ⁷Be in the atmosphere increasing with latitude (Usoskin and Kvaltsov, 2008) and the interaction between stratosphere and troposphere diminishing with decreasing latitude (Du et al., 2008). Except for the latitude effect, precipitation and the concentrations of PM_2.5 and PM₁₀ can also influence the deposition flux of ⁷Be (Liu et al., 2001), while the impact of precipitation was not so strong as the PM_2.5 and PM₁₀ (Figure 9), indicating that the particle matter in the atmosphere was an important isotopic carrier. Under the influence of monsoons, atmospheric particle matters from inland/sea/high latitudes may have been transported to other areas, where they settled to the surface, leading to a high deposition of the radionuclides. In contrast, there were negative correlations between the annual deposition flux and the latitude for both ²¹⁰Pb and ²¹⁰Po, which had never been reported before since it was expected that their deposition could not show the latitude effect like ⁷Be (Yamamoto et al., 2006; Du et al., 2015). However, because of the small amount of data, the relationship should be studied further. Besides, there were strong positive correlations between precipitation and the deposition fluxes of ²¹⁰Pb and ²¹⁰Po (Figure 9), indicating rainfall was the major scavenging way for ²¹⁰Pb and ²¹⁰Po in the atmosphere. Since the concentrations for PM_2.5 and PM₁₀ in the early years were not available, the relationship was not discussed in this study.

FIGURE 9

Figure 9 The correlations among the annual deposition fluxes for ⁷Be, ²¹⁰Pb, and ²¹⁰Po with the latitude, precipitation, PM_2.5, and PM₁₀ in China.

5 Conclusion

Based on the observations of ⁷Be, ²¹⁰Pb, and ²¹⁰Po on the coast of the Maowei Sea from June 2018 to December 2021, the following conclusions can be made:

(1) The annual atmospheric depositional fluxes (Bq m^-2 yr^-1) of ⁷Be, ²¹⁰Pb, and ²¹⁰Po on the coast of the Maowei Sea were 496.80, 201.72, and 58.08, respectively. Both of the distributions for ⁷Be and ²¹⁰Pb depositional fluxes during a whole year (the years 2019 and 2021) followed a bimodal pattern, with one peak from February to April and another peak from August to October, while the distribution for ²¹⁰Po depositional flux showed only one peak during the years of 2019 and 2021.

(2) The activities of ⁷Be and ²¹⁰Pb were lower than the reported value in the rainwater of different cities, however, the activity of ²¹⁰Po was higher than the reported data in the rainwater of different cities.

(3) A Pearson correlation matrix and backward trajectory analysis for the specific single rainfall events showed that precipitation, AQI, PM (both PM_2.5 and PM₁₀), and the air masses movement during the rainfall were the major factors that may influence the deposition of ⁷Be, ²¹⁰Pb, and ²¹⁰Po. A similar scavenging behavior was found for ⁷Be, ²¹⁰Pb, and ²¹⁰Po from the atmosphere.

Data availability statement

The original contributions presented in the study are included in the article/supplementary material. Further inquiries can be directed to the corresponding author.

Author contributions

XW: Conceptualization, Methodology, Software, Data curation, Writing – original draft, Visualization, Investigation, Validation. JZ: Conceptualization, Methodology, Writing – review and editing, Validation, KS: Conceptualization, Methodology, Software, Data curation, Writing – review and editing, Visualization, Validation. JZD: Conceptualization, Methodology, Writing – review and editing, Validation, Supervision. LW: Conceptualization, Methodology, Investigation, Data curation, Writing – review and editing, Investigation, Validation. XL: Conceptualization, Methodology, Writing – review and editing, Investigation, Validation. DL: Conceptualization, Methodology, Writing – review and editing, Validation, Supervision. All authors contributed to the article and approved the submitted version.

Funding

This work was supported by the National Natural Science Foundation of China (Nos. 41906150, 42107251), Open Found Project of Key Laboratory of Global Change and Marine-Atmospheric Chemistry (GCMAC2208), the Natural Science Foundation of Guangxi (Nos. 2021GXNSFDA075004, 2018GXNSFBA281051), Science and Technology Plan Projects of Guangxi Province (No. Gui Science AD19245147), and First-class Discipline Construction Project of Beibu Gulf University (No. DRA003).

Conflict of interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Publisher’s note

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