Characteristics of ⁷Be and ²¹⁰Pb behavior in the atmosphere over Osaka, Japan, as well as the size distributions of the chemical compositions of aerosols attached by ⁷Be and ²¹⁰Pb

Abstract

Air pollution, particularly particulate matter 2.5, is a significant environmental concern in Southeast Asia, with adverse effects on human health. Major sources include vehicular emissions, industrial activities, agricultural burning, and transboundary haze. ⁷Be and ²¹⁰Pb are tracers for studying aerosol behavior. Aerosol samples were collected using a high-volume air sampler, cascade impactor, and rainwater from the rooftop of Building No.16 (34.71° N, 135.64° E) at Osaka Sangyo University. Results showed that ⁷Be and ²¹⁰Pb-carrying aerosols were mainly attached to aerosols smaller than 2 μm, peaking at 0.33–0.55 μm. This study reports the world’s first simultaneous measurement of the activity size distribution of ⁷Be and ²¹⁰Pb-carrying aerosols across the entire aerosol particle size range, including the smallest sizes. A strong correlation between ⁷Be and SO₄²⁻ concentration and between ²¹⁰Pb and SO₄²⁻ concentration in summer 2020 suggests SO₄²⁻as their carrier. These findings improve understanding of atmospheric transport models and support air pollution mitigation strategies.

Introduction

Air pollution is a significant global environmental issue and impacts several countries in Southeast Asia including Thailand, Myanmar, Laos, and Indonesia. Fine particulate matter (PM2.5), recognized as a carcinogen, is linked not only to cancer but also to other health conditions such as heart and lung diseases. In metropolitan areas such as Bangkok, vehicular emissions are a primary source of PM2.5, with the industrial sector also contributing substantially. In northern Thailand, open agricultural burning, forest fires, and vehicle emissions are significant contributors to air pollution issues. The situation is further worsened by smog drifting from neighboring countries [1, 2]. Radionuclides such as beryllium-7 (⁷Be) and lead-210 (²¹⁰Pb) are tracers for understanding aerosol behavior. ⁷Be is a cosmogenic radionuclide produced in the lower stratosphere and upper troposphere by cosmic ray spallation, while ²¹⁰Pb originates from the decay of radon-222 (²²²Rn) emitted from the Earth's crust [3, 4]. ⁷Be and ²¹⁰Pb attach to aerosol particles after production, with their transport and deposition determined by the behavior of these carriers [5].

The size distribution of radionuclides in aerosols is influenced by atmospheric processes such as coagulation, condensation, washout, rainout, sedimentation, and contributions from dust storms and combustion [6]. However, numerous studies have explored the size distribution of ⁷Be or ²¹⁰Pb, most of which have been limited by an incomplete size range or a lack of simultaneous chemical composition analysis. These gaps remain in understanding their simultaneous size distribution and chemical composition.

This study addresses these gaps by investigating the simultaneous size distributions of aerosol behavior, contributing to the improvement of atmospheric transport models. These findings contribute not only to air pollution mitigation strategies but also to enhancing environmental radiation monitoring, dose assessment, and radiation risk management.

Material and methods

All samples were collected on the rooftop of building No.16 at Osaka Sangyo University, Daito, Osaka, Japan (34.71° N, 135.64° E), at a height of 31 m above ground level.

Weekly aerosol samples for evaluating monthly average ⁷Be and ²¹⁰Pb concentrations were collected from 8 January 2018 to 29 May 2023, using a high-volume air sampler (HV-1000R, SIBITA) operating at a constant flow rate of 1000 L/min, passing through a rectangular glass fiber filter (Advantec GB-100R, 203 × 254 mm², 0.3 μm pore size). Biweekly aerosol samples were collected from June to August 2020 using a cascade impactor (Tokyo Dylec Corp., Japan, model LP-20) operating at a constant flow rate of 20 L/min. The impactor classified aerosols into 12 size fractions (0.06–13 μm) on glass plates with a diameter of 80 mm. Collected samples were filtrated for ⁷Be and ²¹⁰Pb activity measurement using the co-precipitation method. Monthly rainwater deposition samples were collected from November 2019 to June 2022 using a 0.18 m² open surface bucket for total deposition and a 0.07 m² dust fall sampler for wet deposition. Rainwater samples were filtrated, evaporated to dryness on a qualitative paper filter (Advantec no. 1, with a diameter of 300 mm), and stored in U8 containers.

Measurement of ⁷Be and ²¹⁰Pb in the aerosol samples from the high-volume air sampler and cascade impactor, as well as rainwater samples, was performed using an HPGe detector (GX2018, CANBERRA) connected to a data acquisition device (APU8002 and DSA1000). Gamma-ray signals were analyzed at 46 keV (²¹⁰Pb) and 477 keV (⁷Be). HPGe detector (GX2018, CANBERRA) is a coaxial germanium detector. The detector is a cylinder of germanium with an n-type contact on the outer surface and a p-type contact of the axial well. The energy range of this detector ranges down to 3 keV to 3 MeV.

Detection efficiency calibration for aerosol samples collected using a high-volume air sampler and cascade impactor was performed using a standard point source of europium-152 (¹⁵²Eu). The cylindrical source, with a height similar to that of the aerosol samples, emitted X-ray energies at 39.91 keV and 40.12 keV and gamma-ray energies at 121.78, 244.7, 411.12, 443.96, 778.9, 867.37, 964.08, 1085.9 keV. For rainwater samples, the calibration was conducted using MX033U8PP No.6191, the standard source in a U8 container with a thickness of 5 cm. This standard source provided gamma-ray energies at 88.03, 122.06, 136.47, 165.86, 320.08, 514.00, 661.66, 834.85, 898.04, 1173.23, 1332.49, and 1836.06 keV. The detection efficiency curves constructed from these calibrations were used to determine the detection efficiencies of ⁷Be and ²¹⁰Pb.

Water-soluble inorganic ions, including sulfate ion (SO₄²⁻), ammonium ion (NH₄⁺), nitrate ion (NO₃⁻), sodium ion (Na⁺), and chloride ion (Cl⁻), in particulate matter [7] were collected using cascade impactor, were analyzed by ion chromatography (DIONEX, ICS-1100). Calibration was conducted using an Anion Mixture Standard Solution 1 (Multianion Standard Solution 1, 019-24011, FUJIFILM) for anions and a Multication Standard Solution III (FUJIFILM, 137-14611) for cations.

Results and discussion

Seasonal variations of ⁷Be and ²¹⁰Pb concentrations

The monthly average concentrations of ⁷Be and ²¹⁰Pb in aerosol samples collected in Daito exhibited a double-peak seasonal pattern, with maxima in spring and autumn and minima in summer (Fig. 1). A strong correlation between the concentration of ⁷Be and ²¹⁰Pb was observed, with a correlation coefficient (r) of 0.69. The concentrations of ⁷Be ranged from 2.39 ± 0.56 to 6.48 ± 0.71 mBq/m³, while those of ²¹⁰Pb ranged from 0.35 ± 0.06 to 1.09 ± 0.01 mBq/m³.

A similar double-peak seasonal pattern for ⁷Be was observed in Sakai during 1983–1997, for the average concentration was 6.06 ± 1.51 mBq/m³ [8]. In Tsukuba during 1988–1990, average concentrations ranged from 1 to 6 mBq/m³ for ⁷Be and 0.2 to 0.8 mBq/m³ for ²¹⁰Pb [9]. The higher concentrations in spring and autumn are likely due to air mass transport from the Asian continent or East China, while the lower concentrations in summer are possibly result from air masses transported over the Pacific Ocean.

Deposition of ⁷Be and ²¹⁰Pb

The average annual total depositions in Daito were 1454 ± 12 Bq/m² for ⁷Be and 182 ± 5 Bq/m² for ²¹⁰Pb. The annual deposition of ⁷Be observed in this study was within the range reported in Sakai [8] (967–1752 Bq/m²) and Fukuoka [10] (1250–1757 Bq/m²). Similarly, the annual deposition of ²¹⁰Pb was consistent with values also reported in Fukuoka [10] (215–289 Bq/m²).

Figure 2 presents the average total deposition velocities of ⁷Be and ²¹⁰Pb as 1.07 ± 0.12 cm/s and 0.89 ± 0.46 cm/s, respectively. These values fall within the ranges reported in Rokkasho, Japan [11], where ⁷Be deposition velocities ranged from 0.17 to 12 cm/s and ²¹⁰Pb deposition velocities ranged from 0.25 to 13 cm/s. In Daito, the total deposition velocities of ⁷Be and ²¹⁰Pb exhibited seasonal variations, peaking in summer. This seasonal pattern for ²¹⁰Pb aligns with observations in Germany [12]. In contrast, Rokkasho [11] recorded elevated deposition velocities for ⁷Be and ²¹⁰Pb during winter and summer, while Taiwan reported a spring peak for ⁷Be [13]. Precipitation influenced the seasonal variation of deposition velocities in Daito, with strong correlations between precipitation and deposition velocities (r = 0.85 for ⁷Be and r = 0.69 for ²¹⁰Pb). In Taiwan, the deposition velocity of ⁷Be was also influenced by strong vertical air-mass mixing [13].

Dry deposition was calculated by subtracting wet deposition from total deposition. Wet deposition was the dominant removal process for ⁷Be, accounting for 85% of its total deposition, compared to 75% for ²¹⁰Pb. In contrast, the dry-to-total deposition ratio showed more effective removal of ²¹⁰Pb (25%) than ⁷Be (15%). Similarly, in Kumamoto, Japan, dry deposition contributed 24.9% to ²¹⁰Pb removal and 7.8% to ⁷Be removal [14]. The wet and dry deposition fractions for ⁷Be in this study were consistent with those reported in Hsinchu, Taiwan, where dry deposition contributed 3.7%–22.6%, and wet deposition accounted for 77.4%–99.2% [13]. Similarly, Laguionie et al. [15] reported a lower dry deposition contribution of 21% and 28% for ⁷Be and ²¹⁰Pb, respectively, in Nantes, France. Globally, the low fraction of dry deposition (12% for ⁷Be and 21% for ²¹⁰Pb) highlights the dominant role of wet deposition in the atmospheric removal of these isotopes [16].

Precipitation was particularly for the wet removal of ⁷Be in all seasons except summer (r = 0.74). Conversely, ²¹⁰Pb was more effectively removed through dry deposition (r = 0.35).

Aerosol particle size distribution

Over 80% of ⁷Be and ²¹⁰Pb were associated with aerosols smaller than 2.1 μm, with peak activity observed in the 0.33–0.55 μm diameter range, as shown in Fig. 3. These findings are compared with data from Dazaifu [17] and Finland [18, 19]. In Dazaifu, ⁷Be was associated with aerosols ranging from 0.43 to 1.1 μm, with 97% of ⁷Be linked to particles ˂1.1 μm [17]. Similarly, in Finland, most of ⁷Be and ²¹⁰Pb were associated with particles below 1.3 μm and 1 μm, respectively [18, 19]. The average Activity Median Aerodynamic Diameter (AMAD) values were determined to be 0.63 μm for ⁷Be and 0.65 μm for ²¹⁰Pb. These values are consistent with those reported in Nagano [20] (0.44–0.67 μm) but differ from those observed in Dazaifu [17] and Finland [18, 19].

In Fig. 4, the mass concentrations of NH₄⁺ and SO₄²⁻ in the particulate matter across size distribution (0.06–12.1 μm) compared with data from Santiago, Chile [21]. The average Mass Median Aerodynamic Diameter (MMAD) values of SO₄²⁻ and NH₄⁺ were 0.63 and 0.61 μm, respectively. The correlation coefficients between the size distributions of ⁷Be and ²¹⁰Pb in particulate matter with SO₄²⁻ were 0.93 and 0.87, and with NH₄⁺ were 0.90 and 0.85. The results differed from those observed in Santiago, Chile [21], where the particles ranged from 0.1 to 18 μm. In this study, ⁷Be, ²¹⁰Pb and ions in the particulate matter were measured across a broader size range, with simultaneous measurements conducted, as shown in Fig. 5.

Time variation in total activity concentration of ⁷Be [3, 4] and ²¹⁰Pb indicates that the total activity concentration ranged from 3.63–9.67 mBq/m³ for ⁷Be and 5.62–18.55 mBq/m³ for ²¹⁰Pb (Fig. 6). Correlation analysis revealed a strong positive relationship between ⁷Be and SO₄²⁻ (r = 0.98) and between ²¹⁰Pb and SO₄²⁻ (r = 0.72). Conversely, negative correlations were observed with NH₄⁺ (r = −0.40 for ⁷Be and r = −0.47 for ²¹⁰Pb), while moderate positive correlations were noted with NO₃⁻ (r = 0.69 for ⁷Be and r = 0.67 for ²¹⁰Pb).

A comparison of the time variation in the total activity concentrations of ⁷Be and ²¹⁰Pb with mass concentrations of SO₄²⁻, NH₄⁺, and NO₃⁻ reveals that only SO₄²⁻ shows a pattern similar to the activity concentration of ⁷Be and ²¹⁰Pb during the summer of 2020.

The AMAD values and total activity concentration variations of ⁷Be and ²¹⁰Pb aerosol particles were similar to the MMAD values and mass concentration variations of SO₄²⁻. The strong correlations between ⁷Be and SO₄²⁻ and between ²¹⁰Pb and SO₄²⁻ underscore the role of sulfate aerosols in their atmospheric behavior, since these aerosols are formed through the oxidation of sulfur dioxide (SO₂) [3, 4]. Narazakai et al. [17] reported that ⁷Be aerosol particles were attached to SO₄²⁻, with ammonium sulfate identified as a main compound in Dazaifu. Similarly, Modibo et al. [22] reported a strong correlation between ²¹⁰Pb aerosol particles and SO₄²⁻ (r = 0.76) in Qingdao.

In contrast, Uematsu et al. [23]. observed that ⁷Be aerosol particles over the Pacific Ocean were associated with NO₃⁻ from January 1985 to January 1986. Prospero and Savoie [24] reported that nitrate concentrations in the North Pacific were co-seasonal with Asian dust transport and derived from continental sources during the monsoon (July–August 1995, 1996), the intermonsoon (April–May 1994), and the winter monsoon (February–March 1995 and February 1997). The behavior of ⁷Be and ²¹⁰Pb suggests that sulfate ion (SO₄²⁻) served as their carrier in the summer of 2020.

Dazaifu's measurements were limited in that they did not provide stepwise data for aerosol particles of ⁷Be ˂0.43 μm. Therefore, this study represents the first simultaneous measurement of the entire particle and ion size distribution of ⁷Be and ²¹⁰Pb.

The residence time of aerosol particles associated with ⁷Be and ²¹⁰Pb was evaluated using the method described by Papastefanou and Ioannidou [25]. For Daito city, Osaka Prefecture, Japan, the estimated residence times, based on an average AMAD of 0.63 μm for ⁷Be and 0.65 μm for ²¹⁰Pb, were 5.1–6.4 days and 5.3–6.6 days, respectively. These values differ from those reported in previous studies, which estimated residence times of 5–6 days for ⁷Be and 4–5 days for ²¹⁰Pb in Germany [5], and 3.5–4.3 days for ⁷Be in Dazaifu, Fukuoka Prefecture, Japan. The actual residence times of ⁷Be and ²¹⁰Pb in the atmosphere may vary depending on meteorological factors, such as descent acceleration.

Conclusion

The study conducted at Osaka Sangyo University observed significant seasonal trends in ⁷Be and ²¹⁰Pb concentrations in air dust samples, peaking in spring and autumn. The deposition velocities of ⁷Be and ²¹⁰Pb peaked in summer. ⁷Be deposition was effectively removed by wet removal processes, while ²¹⁰Pb deposition was effectively removed through dry deposition.

Most of the ⁷Be and ²¹⁰Pb-carrying aerosols were associated with particles smaller than 2.1 μm. The study is the first to measure the entire particle and ion size distribution of ⁷Be and ²¹⁰Pb. The AMAD values and concentration variations of ⁷Be and ²¹⁰Pb were similar to MMAD values and mass concentration variations of sulfate ions (SO₄²⁻). These findings suggest that ⁷Be and ²¹⁰Pb were mainly associated with sulfate ion (SO₄²⁻) during the summer of 2020. The residence times of ⁷Be and ²¹⁰Pb in the atmosphere were estimated to be 5.1–6.4 days and 5.3–6.6 days, respectively, based on the measured AMAD.

However, technical issues prevented data measurement by ion chromatography for winter and other seasons. Future measurements are essential to validate these seasonal patterns and enhance our understanding of the environmental behavior of these isotopes.

Conflict of interest

None declared.

Funding

This work was supported by JSPS Bilateral Joint Research Project with Thailand NRCT [Grant number JPJSB120209203].

References