Introduction

The main source of 210Pb (half-life = 22.4 years) is the radioactive decay of 222Rn (half-life = 3.8 days) emitted to atmosphere from the earth’s crust; the other possible artificial sources in the air include, burning of fossil fuels (coal) [1, 2], use of phosphate fertilizers [3], iron and steel manufacture [4], biomass combustion [5], burning of leaded gasoline used for vehicle engines and so on [6, 7]. Since most of the 210Pb is originated from the 222Rn emanated from terrestrial surface, its concentration in air can be expected to be influenced by the local meteorological conditions, such as temperature, atmospheric pressure, relative humidity, precipitation or soil moisture, that affect the emanation rate of 222Rn from the land surface [7].

Once 210Pb have been produced, they are immediately attached to sub-micron-sized aerosol particles and can be transported with atmospheric aerosols [8, 9]. When people directly inhaled aerosols with 210Pb, which are collected by the respiratory tract of the body, and finally, it can be mainly accumulated in skeleton with a long-time radiation risk due to its long effective half-life. Haninger et al. [10] pointed out that in environments with enhanced radionuclides concentrations, direct inhalation of 210Pb is an important source for 210Pb accumulation in man. Therefore, its presence and activity levels in air is of the utmost concern in terms of radiation risk coupling with air pollutant which more and more seriously affecting public health via air inhalation.

With rapid economic development and urbanization for 40 years, China is experiencing severe haze pollution, especially in some important metropolis [11,12,13,14,15]. Shanghai is a mega-city with 24 million residents, ~ 4.3 million vehicles and nearly 60 million ton of standard coal per year, and there are many industrial facilities surrounding the city, including petrochemical factories, chemical plants, and solvent production facilities [15]. It is well known that Shanghai is still suffering from serious haze pollution though the number of haze day reduced from 124 in 2013 to 88 in 2017 [16]. The source of PM2.5 in Shanghai has been reported to include coal burning, vehicle exhaust emission, biomass burning and suspended mineral dust [17].

Ambient air quality in Chinese cities is monitored and reported daily using the air quality index (AQI) that was calculated on the basis of ground-based monitoring of a 24-h average atmospheric PM (PM2.5 and PM10 represent the particle aerodynamic diameter being equal or less than, 2.5 and 10 μm, respectively) [16, 18], carbon monoxide (CO), sulfur dioxide (SO2), nitrogen dioxide (NO2) and ozone (O3) concentration. The air quality was defined into some levels with the following scale: 0–50: excellent, 51–100: good, 101–150: light pollution, 151–200: moderate pollution, 201–300: severe pollution and > 300: very serious pollution [18]. Based on the AQI values, the weather condition was divided into two categories: 0–100, non-haze day; > 100, haze day. Many studies focused on the investigation of deposition fluxes of 210Pb and 7Be and used them as atmospheric tracers to characterize the sources of air masses, e.g. maritime versus continental air [19], but ignore the possible radiation risk in haze weather situation and the relationship between 210Pb level and air quality. And there is limited investigation on level of air 210Pb in China, especially in Shanghai [20]. Meanwhile, the variation of atmospheric 210Pb and its major control parameter are still unclear in Shanghai.

The aims of the study were (1) to measure temporal variations of 210Pb with changing air quality in a downtown area of Shanghai, (2) to analyze the correlation of 210Pb with meteorological data such as temperature and relative humidity, and air pollutants, (3) to estimate the possible radiation risk in haze and non-haze periods due to inhalation of 210Pb.

Materials and method

Materials and sampling

The sampling station was installed on the roof of the State Key Laboratory of Estuarine and Coastal Research (SKLEC) building at the East China Normal University at ~ 20 m above ground level (31°13′39″N, 121°23′56″E), and ~ 50 km from the East China Sea coastline [21]. The aerosol samples were collected from January 2016 to February 2017 through a portable Staplex TF1A type (Clover Company, USA) high volume air sampler. The maximum sampling flow rate is 2 m3/min. Quartz microfiber filters of 0.2 mm pore (Whatman Company, UK) were used to collect particles in air. The collection efficiency of the membrane was larger than 99.995% for size of particles greater than 0.3 μm. Sampling usually lasted for 10–12 h, and in general the total sampled air volume was higher than 1000 m3. The air sampler was protected with a locked cover to avoid direct input of the rain. The location of sampling site is showed in Fig. 1.

Fig. 1
figure 1

Sampling location (black star) at site of Shanghai, eastern coast of China

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The meteorological parameters (temperature and RH) and air pollutants (PM, SO2, NO2, CO, O3) concentrations were obtained and recorded from the nearest official station (Putuo Station), which is within 1.5 km distance away from SKLEC building [22].

Measurement of 210Pb by using alpha spectrometry

The aerosol samples were stored for 1.5 years to ensure the radioactive equilibrium between 210Po and 210Pb. Therefore, the activity of 210Pb can be replaced by 210Po. The analysis of 210Po was referenced and modified from a previous method [2], which is described briefly here. For the determination of 210Po activity, a quarter of the sample filter was acid-digested with a mixture of HNO3, HF and H2O2 at a temperature of 150 °C in presence of 209Po spike (1–2 dpm) in a Teflon beaker. The certified reference material 209Po (1526-81-1) used in this work was purchased from the Eckert & Ziegler Isotope Products. The clear solution was taken to dryness followed by the addition of 2 ml of 2 N HCl acid and further diluted to ~ 0.2 N HCl with Milli-Q water (resistivity = 18.2 MΩ·cm). Approximately 100 mg ascorbic acid powder, 1 ml of 20% hydroxylamine hydrochloride and 1 ml of 25% sodium citrate solution were added into the solution. Polonium (209Po and 210Po) was automatically deposited on a silver disc while heating at 80–90 °C and stirring for 4 h. After that, the disc was taken out from the solution and rinsed with Milli-Q water and ethanol. Finally, the activities of 210Po (Eα = 5.33 MeV) and 209Po (Eα = 4.9 MeV) were assayed by alpha spectrometer (Canberra 7200). This ultra-low background alpha spectrometer (purchased from CANBERRA EURISYS Lit., France) was equipped with 12 PIPS detectors (active area of 600 mm2). The warranted alpha resolution is 23 keV, and no 210Po and 209Po peak overlapping was found for all the aerosol samples. The correction for decay of 210Po from the time of plating to mid-counting was done for obtaining accurate 210Pb activity. Blank filters were also analyzed for 210Po and was subtracted from the sample. The overall recoveries of Po ranged from 74.4 to 104.4% with an average of 85.1 ± 14.5% (n = 20). The error of the 210Po activity was estimated on the basis of the statistical counting only.

Results and discussion

Variation of atmospheric 210Pb activity concentration in Shanghai

The air quality parameters and concentrations of 210Pb for 20 aerosol samples are given in Table 1. The temporal variation of 210Pb activity concentration is shown in Fig. 2. 210Pb had a range from 0.29 to 6.10 mBq/m3 and an overall average of 2.07 ± 1.28 mBq/m3 (n = 20). The average of 210Pb in haze and non-haze day were 2.34 ± 1.43 mBq/m3 (n = 12) and 1.46 ± 0.76 (n = 8) mBq/m3, respectively.

Table 1 Concentrations of 210Pb and meteorological and air quality parameters of Shanghai’s air during 2016–2017
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Fig. 2
figure 2

Temporal variation of 210Pb concentration in the Shanghai air

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And the 1-year average of 210Pb activity level was compared with other sampling sites and summarized in Table 2. It could be found that the 210Pb concentrations in most different cities of China were higher than those in districts of other countries. Most of these values have exceeded the world average 210Pb value (0.5 mBq/m3) that reported by UNSCEAR [31]. The 210Pb activity in ambient aerosols in Shanghai is similar to the reported values in Chinese metropolis (typically, 1–2 mBq/m3), such as Hangzhou [30]. From Fig. 2, the 210Pb activity was significantly high during September–February and relatively low during April–August. The maximum activity of 210Pb, 6.10 ± 0.53 mBq/m3 was observed in the sample collected in 23, December (the AQI value = 175), which may be attributed to enhancement of anthropogenic emission in Shanghai in the winter. The potential artificial sources include coal fly ash, building dusts, street dusts and industrial or agricultural emissions, because these materials always have very high level of 210Pb, for instance, coal fly ash (29.8–204 Bq/kg) [32], street dusts (high up to 344.7 Bq/kg) and industrial site top soils (66.4 Bq/kg) [33]. In addition, there are at least 33 units at 13 coal-fired power plants in Shanghai’s 6340 km2 area in 2017, and they consumed at least 27.6 × 106 kg coal per year [34]. Hence, the 210Pb contribution from coal combustion should be high. The 210Pb activity concentration in Shanghai exhibited a strong seasonal variability, nearly 3–5 times higher activity in autumn–winter season compared to that in spring–summer time (Fig. 2).

Table 2 Comparison of 210Pb activity concentrations in air with previous reports from different sampling sites in the world
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Factors controlling atmospheric concentration of 210Pb in Shanghai and East Asian regions

In general, the 210Pb concentrations in the urban air were controlled by its scavenging processes and by 222Rn production rate (source of 210Pb), and easily be influenced by local emission from some human activities and by meteorological conditions, such as temperature, atmospheric pressure, precipitation or soil moisture, that affect the scavenging strength of particle-reactive radionuclides and the emanation rate of 222Rn from ground [35].

From Fig. 2, the higher 210Pb activity level in dry season’s (September–February) atmosphere of Shanghai indicated that less clearing processes due to the low precipitation amount may partly contribute the higher 210Pb in the air. It is also necessary to mention the low values found in spring–summer period (March–September), which might be due to the high levels of rainfall. About 40% of annual rain concentrated in summer of Shanghai [21]. The decrease of washout of 210Pb from air by wet precipitation might promote the accumulation of 210Pb in the airborne particulate materials. The lack of precipitation during autumn–winter season caused an increase of 210Pb concentration due to both lack of scavenging of 210Pb-laden aerosols by rain and lack of resuspension of particles from the soil and dust, meanwhile a decrease of precipitation would have facilitated the 222Rn emanation from soil.

Similar to other East Asian regions, the weather in Shanghai is also influenced by the Asian monsoon system, with the northeast–northwest wind in autumn and winter (continental air mass) from inland area (Fig. 3a) and the east-southeast wind, southeast wind in spring and summer (marine air mass) from the East China Sea, South China Sea and Pacific Ocean (Fig. 3b). In spring–summer period (from March to September), Shanghai is always covered by marine air masses, these situations would carry low atmospheric 210Pb concentration due to a reduced supply of 222Rn associated with low 226Ra concentration in seawater, in contrast, the anticyclones would come to Shanghai from the continental Asia in which much higher atmospheric 210Pb levels were obtained by an increased supply of 222Rn from the Asian continent surface. Hence, the U-shaped distribution patterns of atmospheric 210Pb in many research sites of East Asian regions were observed (Fig. 3c). From the Fig. 3c, the results indicated that we can distinguish air masses from different sources by using 210Pb as a tracer and even study the possible atmospheric air mass mixing processes. In addition, weak solar heating in winter and the subsidence in a lower tropospheric air column associated with the Asian winter monsoon favor a more stable atmospheric environment, resulting in the accumulation of aerosol particles containing 210Pb [52].

Fig. 3
figure 3

East Asian Monsoon in a summer and b winter and monthly variation of atmospheric 210Pb in East Asian regions (c). The study sites in this figure include Okinawa-jima, Minamidaito-jima, Kume-jima, 2008–2010 [36]; Xiamen, 2016 [37]; Lanzhou, 2009–2012 [38]; Guanfengshan, 2002–2005 [39]; Beijing, 2013–2016 [40]; Guangzhou, 2014–2015 [41]; Hangzhou, 2012 [42]; Shenzhen, 1994–2002 [43]; Qingdao, 2015–2016 [44]; Bombay, 1985–1987 [45]; Rokkasho, 2000–2006 [46]; Kumamoto, 2001–2003 [47]; Sarufutsu, 2001 [48]; Tsukuba, 1988–1990 [49]; Beijing, 1989–1991 [50]; Chengdu, 1990–1992 [50]; Seoul, 1989–1991 [50]; Mangalore, 2014–2017 [51]; Kaiga, 2015–2017 [51]; Shanghai, 2016–2017 (this study)

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Other anthropogenic sources, like coal burning and associated industrial emission (iron and steel factory) have become the predominant sources of 210Pb in Shanghai ambient air, because the biomass burning (agricultural waste and use of wood-fuel for domestic heating) and the use of leaded gasoline had been prohibited by the Shanghai government.

Correlation between 210Pb and meteorological parameters

Meteorological conditions, such as temperature, RH, atmospheric pressure and wind speed, are primary factors that can influence pollutant levels in the atmosphere [53]. In this study, Pearson correlation analysis was preferred after normality test for all the related parameters. The spring–summer season (with relatively high RH and temperature) of Shanghai always correspond to a fairly frequent precipitation period. An inverse relationship between air temperature and 210Pb level (R = − 0.40, P = 0.081) was observed in this study (Fig. 4). The explanation includes: (1) high temperature favors the dispersion of aerosol particles embedded with 210Pb; (2) the rainfall dominates the removal of the atmospheric 210Pb, although higher temperature facilitates the radon emanation. A weaker relationship between 210Pb concentration and RH was also found, which also demonstrated the significance of wet scavenging for 210Pb [35]. The relatively high RH increases the condensation processes as well as coagulation between the attached aerosol particles. The correlation may also indicate low emission of 222Rn from soil in humid conditions in summer-time, as the finding reported by Li et al. [35], in which a negative correlation between RH and Rn concentration was proved.

Fig. 4
figure 4

Linear-regression analysis between meteorological parameters (a temperature, b RH) and air pollution parameters (c AQI, d PM2.5, e PM10, f SO2, g NO2, h CO, i O3) and the atmospheric 210Pb concentrations in Shanghai (Pearson correlation analysis performed)

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The relations between 210Pb and air quality parameters

The correlation coefficients between 210Pb activity concentrations and AQI values, 210Pb activity concentrations and PM2.5 concentrations, 210Pb activity concentrations and PM10 concentrations were 0.54 (P < 0.05), 0.52 (P < 0.05) and 0.44 (P = 0.055), respectively, which indicated that there were significant relationship between 210Pb activity concentration and air quality parameters. The atmospheric 210Pb concentrations increased with the decrease in air quality, which were also reported by others [20, 35], showing that levels of 210Pb in air of Shanghai were higher in haze day (AQI value > 100) than levels in clean weather. One of the explanations was that particle-relative 210Pb could be strongly scavenged to the suspended both fine and coarse particulate matters. Figure 4 shows significant positive correlations between atmospheric 210Pb concentrations with concentrations of gaseous pollutant CO, which suggested that this pollutant CO might share the same source regions and transported pathway with 210Pb [28]. We boldly speculated that this relationship was caused by emission from burning, because high temperature can cause the discharge of 210Po, 210Pb, and other volatile radionuclides (the boiling point is 962 °C for 210Po, and 1749 °C for 210Pb [54]). Because of the good relation between 210Pb and air pollutant CO, 210Pb and these air pollutants could be used as indicators together to assess the health of the atmospheric environment. Interestingly, SO2 and NO2 showed weak positive correlations with 210Pb, but O3 showed a negative correlation with 210Pb.

Radiological hazards assessment for inhaling 210Pb in aerosols

The higher 210Pb levels in air in haze days implied a greater human exposure to outdoor atmospheric 210Pb in haze weather events. Due to long half-live and difficult to be removed, more attention should be paid to the long-term internal radiation to peoples who work in outdoor (such as traffic police, building worker) during air polluted situation in Shanghai once the 210Pb deposited in respiratory system. To evaluate corresponding annual effective dose of 210Pb because of inhalation for inhabitants, the committed effective dose (Ei, μSv/a) was calculated by the following formula:

$$E_{i} = e(g)_{{i,{\text{inh}}}} \times R \times C_{i} \times T_{\text{f}}$$

where, e(g)i,inh is the 210Pb dose conversion coefficient (Sv/Bq) [31]; R is the inhalation rate (m3/day), values of R for infants, children and adults were 4.5, 7.6, 10.9, 14.0 and 13.3 m3/day, respectively [55]; Ci is the atmospheric 210Pb concentration (Bq/m3); Tf is the exposure frequency, indicating the time spent outdoors by inhabitants of the study area, the value of Tf for the children was estimated at 0.21, whereas those for infants, teenagers and adults at 0.12 [55]. Different from oral exposure, not all 210Pb in air, especially those attached to the large particles, may be captured and deposited in respiratory tract, therefore, it has to be recognized that the Ei value calculated by equation is a much more conservative estimation of the actual human radiation dosimetry of 210Pb.

The estimated Ei values under two weather qualities (haze and non-haze) and different age groups are presented in Fig. 5. The results showed that internal dose through inhalation of the aerosol particles attached with 210Pb was assessed to be 0.76–2.74 μSv/a. Furthermore, due to smaller body size, toddlers and children in general have higher committed effective dose values than teenagers and adults. The order of Ei value among different age group was children > toddlers > teenagers > infants > adults. This means that when being exposed to the same levels of atmospheric 210Pb, kids may suffer from two times higher radiation dose than adults. However, the maximum annual effective dose is much lower than the worldwide average annual effective dose 2.4 mSv/a [31]. Considering the long half-life of 210Pb (22.3 years), the organs affected by inhalation of 210Pb are successively bone surface, lung tissue, kidney and external thoracic cavity. And once the 210Pb deposits in the respiratory systems in the form of particulate state, especially the lung, it requires a much long time to be cleared. Besides, the potential harmfulness to human body that may be caused by the coupling effect of air pollutants and radionuclides in air (like 210Po and 210Pb) has not yet been assessed, which asks more people to pay more attention on this risk in the future.

Fig. 5
figure 5

The radiation dose of 210Pb through inhalation to outdoor air, stratified by different age groups under two air pollution levels (non-haze and haze)

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Conclusion

Reports focused on 210Pb’s associations with different air quality weather events are relatively rare in Shanghai. Haze events that occurred frequently in the past decades are undoubtedly of great concern to people and government on economy, ecology, tourism and human health in China. Monitoring the pollutants and radionuclides (e.g. 210Pb) simultaneously, in different air quality weather events can help in assessing the potential harmfulness to public. In this study, seasonal variation of 210Pb was observed in the urban aerosols of Shanghai during 2016–2017. The U-shaped distribution pattern of atmospheric 210Pb in the East Asian regions was influenced by the East Asian Monsoon and local weather condition. The important results in this study showed that atmospheric 210Pb levels increase with decreasing air qualities and the good correlations between 210Pb in aerosol particles and air pollutants indicated that 210Pb cooperated with other air pollutants in atmosphere, which implied that 210Pb could also be used as an indicator to evaluate the health of the atmospheric environment.

Radiation dosimetry of daily inhalation of 210Pb through exposure to different air quality situations for different age groups were calculated to be much lower than the worldwide average annual effective dose. However, in the future, more research is needed to assess the coupling risks of air pollutants and radionuclides in air for human health.