Cosmogenic (7)Be and (22)Na in ground level air in Switzerland (1994-2011)

Airflow and teleconnection patterns driving the spatial and temporal variability of high 7Be air concentrations in Europe

The long-term monitoring of the cosmogenic ⁷Be activity concentrations has been used to better understand the influence of large-scale atmospheric circulation dynamics in the troposphere. With an aim, this study analyzes weekly ⁷Be data from 15 sampling stations in Europe over 2005-2014. We first define peak (or extremely high) events in each station as those activity concentrations above the 90th percentile, and then investigate their temporal and spatial variability. These events are most frequent in the spring and summer seasons, with a notable latitudinal variability in their number. Next, we use back-trajectory cluster analysis to identify the main advection pathways associated with these high concentrations. To achieve this, persistent periods, i.e., sampling periods over which at least 60% of the calculated backward trajectories arriving at a given site correspond to the same airflow pattern, are taken as reference. This method reveals large differences in the association between the airflow patterns observed at different stations in connection with the ⁷Be peaks. A comparison between stations shows no clear spatial pattern, which suggests a further influence of mesoscale/local physical processes on the surface ⁷Be activity concentrations. Finally, the main airflow pattern at each sampling site and the associated ⁷Be peaks, are related to the main teleconnection patterns of large scale and regional climate variability in Europe: North Atlantic Oscillation, Arctic Oscillation, East Atlantic, East Atlantic/Western Russia, Scandinavian pattern and Western Mediterranean Oscillation. The results point out the connection between the negative phases of NAO and We-MO, and the positive phase of EA with the spatial and temporal variability and occurrence of the ⁷Be peak activity concentrations. These results show a latitudinal division between northern and southern sites, with similar influence of teleconnection patterns, while those located in the central part of Europe present larger variability in the impact of teleconnection patterns.

Solar and meteorological influences on seasonal atmospheric 7Be in Europe for 1975 to 2018

Assessing the transport of natural radionuclides in the atmosphere provides a powerful tool to study air mass circulation. Here, we investigated the seasonal atmospheric distribution of the naturally produced ⁷Be in surface air over Europe between 40° N and 68° N during the period 1975-2018. The results suggest that the inter-annual variability of ⁷Be reflects production rates of the radionuclide induced by solar modulation of cosmic rays. Further analysis of the meteorological influences indicates that the meteorological influences on ⁷Be concentrations are geographically and seasonally dependent. We found that, in general, the tropopause pressure plays an important factor influencing ⁷Be activity for winter and spring seasons while the sea level pressure and temperature are more dominant during summer and autumn seasons. The combination of tropospheric production rates and meteorological parameters explains 24%-79% variances of the seasonal ⁷Be activity. We further applied a three-box model to study the influence of stratosphere-troposphere exchanges on ⁷Be concentrations. The simulation supports that the seasonal cycle of ⁷Be in Europe is controlled by two main factors: the changing height of the troposphere (seasonality of the tropopause height) and seasonal variations of the stratosphere-troposphere exchanges.

Origin and percolation times of Milandre Cave drip water determined by tritium time series and beryllium-7 data from Switzerland

Early observations of the tritium (³H) activity in precipitation from Switzerland started in 1967 in Bern and a monitoring program with improved analytical techniques was carried out between 1971 and 2009. Between 2012 and 2015, we performed tritium analyses on daily precipitation samples from north-western Switzerland to better constrain the tritium variability. We also collected waters dripping inside Milandre Cave (Jura Mountains) with a 4-6 months' resolution in order to estimate the age of the drip water, which is mandatory to interpret the high-resolution speleothem (cave carbonate deposits) records. Over the monitoring period, the mean tritium concentration in the daily precipitation samples was approximately 8.7 ± 4.7 TU with distinct seasonality showing higher values in spring and summer (from April-May to August-September). The similarity in trends with the other cosmogenic radionuclide beryllium-7 (⁷Be) suggests that, for the study site, tritium in precipitation essentially originates from stratospheric input in spring. In winter, precipitation dynamics with increasing moisture originating from the Atlantic and diluted during transport contribute to low values close to the Atlantic background. In cave drip water, the depleted activity level of tritium indicates a relatively long percolation time from the surface to the cave of several years. A residual superimposed short tritium variability provides evidence for the occurrence of fast flowing water paths. The contribution from both components were quantified based on a two-component lumped parameter-mixing model. Finally, we show that tritium concentrations in cave drip water are linked to the moisture origin and atmospheric transport pathways.

Radioisotopes demonstrate changes in global atmospheric circulation possibly caused by global warming

In this paper, we present a new method to study global atmospheric processes and their changes during the last decade. A cosmogenic radionuclide measured at ground-level, beryllium-7, is utilized as a proxy to study atmospheric dynamics. Beryllium-7 has two advantages: First, this radionuclide, primarily created in the lower stratosphere, attaches to aerosols that are transported downwards to the troposphere and travel around the globe with the general atmospheric circulation. By monitoring these particles, we can provide a global, simple, and sustainable way to track processes such as multi-annual variation of the troposphere, tropopause heightening, position and speed of atmospheric interface zones, as well as the poleward movement and stalling patterns of jet streams. Second, beryllium-7 is a product of cosmic rays which are themselves directly linked to solar activity and the earth magnetic field. This study shows whether beryllium-7 observed concentration changes are correlated with such natural processes or independent of them. Our work confirms that major changes in the atmospheric circulation are currently ongoing, even though timeseries are too short to make climatological assessments. We provide solid evidence of significant and progressive changes of the global atmospheric circulation as well as modifications of tropopause heights over the past decade. As the last decade happened to be the warmest on record, this analysis also indicates that the observed changes are, at least to some extent, attributable to global warming.

Negative correlation between the number of sunspots and the occurrence of 7Be and 22Na in the surface air and their contribution to radiation doses

This article presents yearly mean concentrations of cosmogenic radionuclides 7Be and 22Na occurring in dry and wet depositions (fallout) and aerosols. Time dependencies negatively correlated with the yearly mean number of sunspots. Activity concentrations of 7Be and 22Na in aerosols in the surface air had a correlation of near-unity. 7Be in aerosols exhibited a smoother time dependence than 22Na, implying that the production of 22Na is more sensitive to the solar activity than the production of 7Be. The effect of the measured doses on the general population through internal and external exposure to radiation from cosmogenic radionuclides was small.

Deposition of naturally occurring 7Be and 210Pb in Northern Finland

This study presents observations of naturally occurring ⁷Be and ²¹⁰Pb in total deposition collected in three deposition sampling stations located in three towns in Northern Finland at Rovaniemi, Sodankylä and Ivalo. The deposition samples were measured in three-month intervals and the collected data covered the years from 1991 to 2017 in Rovaniemi and the years from 1999 to 2017 in Sodankylä and Ivalo. The ⁷Be deposition was observed to be dependent on the amount of precipitation where higher amount of precipitation meant higher ⁷Be deposition. The ⁷Be deposition showed a distinctive seasonal cycle where the highest ⁷Be deposition values were observed occur during July-September and the lowest values during January-March. From the annual total deposition 72-84% occurred during the months of April-September (2^nd and 3^rd quarters). The precipitation normalized enrichment factor (α) determined the excess or deficiency of ⁷Be relative to the amount of rain. The most excess of ⁷Be in the deposition was observed during April-June (2^nd quarter) while the most ⁷Be depleted precipitation occurred during January-March (1^st quarter). The large excess of ⁷Be in precipitation during April-June (2^nd quarter) was an indication of injections of ⁷Be rich stratospheric air into troposphere. By using simultaneously collected ⁷Be aerosol concentration data at Rovaniemi and Ivalo, the ⁷Be deposition velocities were determined. The deposition velocities (V_D) were also found to have a strong seasonal cycle where low velocities were observed during winter and higher ones during summer. The ⁷Be deposition time series were also compared with other time series that are known to affect ground level ⁷Be aerosol concentrations. These were precipitation, sunspot number (SSN), galactic cosmic ray flux (GCR), Arctic Oscillation (AO), North Atlantic Oscillation (NAO) and Scandinavian Pattern (SCAND) indices. The Pearson correlation and wavelet coherence (WTC) was used to analyze possible correlations between the time series. The analyses showed that the coherences were location dependent. Rovaniemi ⁷Be deposition data correlated strongly with rain and weakly with the SCAND index at the 6-8 year scale. The Sodankylä ⁷Be deposition data correlated strongly with the precipitation and SSN and GCR and weakly with AO and NAO indices. Ivalo ⁷Be deposition data correlated with strongly with ⁷Be aerosol concentration data, SSN, GCR and NAO index and weakly with the AO index. The WTC analysis revealed anti-phase coherences with the AO and NAO indices at 4-6 year scale. The ²¹⁰Pb deposition data was sparse and only available from 2011 onwards. ²¹⁰Pb deposition had a seasonal cycle where higher deposition values were observed during summer and lower ones deposition during winter. The annual ²¹⁰Pb deposition was determined at Rovaniemi and Sodankylä during 2014. The values of ⁷Be/²¹⁰Pb ratio was found to vary significantly seasonally, higher ⁷Be/²¹⁰Pb ratios were measured during summer and lower ones during winter.

Seasonal evolution of 7,10Be and 22Na in the near surface atmosphere of Cáceres (Spain)

Cosmogenic radionuclides provide information about air masses exchanges between stratosphere and troposphere. The ⁷Be concentration in aerosols usually shows a seasonal variability, depending also of the climatic conditions. There are, however, fewer data available of the behavior of other cosmogenic radionuclides, such as ¹⁰Be and ²²Na. In this work about 7 years of aerosols collected in Cáceres (Spain) were analyzed. The ⁷Be concentration was higher than ²²Na and ¹⁰Be, being the ²²Na/⁷Be and ¹⁰Be/⁷Be ratios (1.16 ± 0.02)·10^-4 and (1.5 ± 0.3)·10^-4 respectively. For the ²²Na/⁷Be, a seasonal variation was observed, being higher in spring/summer. Seasonal variation of ⁷Be and ²²Na were explained using a model taking into account local values of the solar radiation, rainfall and dry deposition. The effective residence time for ⁷Be and ²²Na were (9.9 ± 1.0) and (11.3 ± 1.4) d respectively. Both ⁷Be and ²²Na seemed to decrease with increasing number of sunspots, although it was not statistically significant probably due to the low solar activity reported in the analyzed period.

The development of a digital gamma-gamma coincidence/anticoincidence spectrometer and its applications to monitor low-level atmospheric 22Na/7Be activity ratios in Resolute Bay, Canada

Using a previously developed digital gamma-gamma coincidence/anticoincidence spectrometer, daily aerosol samples collected at Resolute Bay, Canada (74.71°N, 94.97°W) from May 2016 to April 2017 were analysed for activity concentrations of ²²Na and ⁷Be. The spectrometer design allows a more selective measurement of ²²Na with a significant background reduction by gamma-gamma coincidence events processing. It has been demonstrated that the improved spectrometer provides a more sensitive and effective way to quantify trace amounts of ²²Na and ⁷Be with a critical limit of 3 mBq and 5 Bq respectively for a 20 h counting. The ⁷Be/²²Na ratio data set obtained in this study showed significant annual variation, which has a distinct spring (typically from February to May) maximum and winter (from September to February of next calendar year) minimum, which suggest that it could be used as a radiochronometer for studying the atmospheric processes. The ⁷Be/²²Na ratios are most likely connected to deep stratosphere-to-troposphere transport (STT) exchange events where air with a higher ⁷Be/²²Na ratio originates from downward flow from stratosphere to the troposphere. The aerosols with lower ⁷Be/²²Na ratios located between two oscillation peaks may have longer residence time. The correlations between ⁷Be and ²²Na activity concentration were the high during these time periods. Compared with other studies based on weekly collected aerosol samples, the techniques greatly improve the temporal resolution of ⁷Be/²²Na data set that will be able to provide more detailed information to study various atmospheric phenomena.

7Be spatial and temporal pattern in southwest of Europe (Spain): Evaluation of a predictive model

This study presents a comprehensive statistical analysis of the cosmogenic radioisotope ⁷Be measured in surface air in ten stations over Spain for a 9 years period (from January 2006 to December 2014). Besides the analysis of ⁷Be seasonal and inter-annual variability, ⁷Be frequency distributions and its correlations with meteorological variables observed in the 10 sampling sites were investigated. The second part of this paper focuses on a feasibility study for the application of the Artificial Neural Networks (ANNs) to predict monthly ⁷Be activity concentrations using meteorological variables, PM10 concentrations and the sunspot number as input parameters. Notwithstanding the low correlations found between ⁷Be and input parameter, the performance of the ANNs, as evaluated by the relevant statistical parameters, demonstrates their capability to correctly predict ⁷Be monthly activities in the 10 Spanish sampling sites.

7Be concentration in the near-surface layer of the air in Bialystok (north-eastern Poland) in the years 1992-2010

Weekly measurements of air ⁷Be concentrations (n = 769) were performed in the years 1992-2010 in Bialystok (north-eastern Poland) using gamma spectrometry. The arithmetic mean (AM) concentration of ⁷Be was 2.51 mBq m^-3, and the median (M) was 2.24 mBq m^-3 (range 0.47-7.81 mBq m^-3). The observed ⁷Be concentrations were within the range of levels recorded in Europe. Typical seasonal variability was observed. Concentrations of ⁷Be in the warm season (May, June, July) were almost twice as high as those in the cold season (November, December, January). A correlation was found between weekly ⁷Be concentrations and mean weekly values of relative humidity, temperature, and wind speed throughout the observation period. Pearson's correlation coefficients were -0.63, p < 0.001; 0.477, p < 0.001; -0.288, p < 0.001, respectively. The correlation coefficient between sunspot number and mean annual ⁷Be concentrations in the air in the years 1992-2010 was -0.609.

7Be, 210Pb and 40K depositions over 11 years in Málaga

The monthly bulk depositional fluxes of three natural radionuclides (⁷Be, ²¹⁰Pb and ⁴⁰K) were measured at a Mediterranean coastal station (Málaga) over an 11-year period from 2005 to 2015. The mean annual depositional fluxes of ⁷Be, ²¹⁰Pb and ⁴⁰K were 1215, 144 and 67 Bq m^-2 year^-1 respectively, showing a clear seasonal trend with minimum values recorded during summer and maximum values in winter. The rainfall regime with dry summers allows estimating the dry deposition. Assuming constant dry deposition through each year, ⁷Be, ²¹⁰Pb and ⁴⁰K would account for 12.5, 26.5 and 33% of the bulk fallout respectively which indicates that deposition for ²¹⁰Pb and ⁴⁰K are significantly higher than ⁷Be. The precipitation-normalized enrichment factor alpha used to explain seasonal variations in the depositional fluxes of radionuclides with respect the rainfall, indicates higher depositional fluxes during spring and summer than expected from the amount of rainfall. Despite their different origin, ²¹⁰Pb and ⁷Be monthly depositional fluxes have strong correlation. The atmospheric deposition fluxes of ⁷Be, ²¹⁰Pb and ⁴⁰K were controlled mainly by the amount of rainfall (r = 0.89, 0.91 and 0.66 respectively). Moreover, principal component analysis was applied to the datasets and deposition of radionuclides and rainfall in the same component highlighting the importance of the washout mechanism. The mean depositional velocity of aerosols evaluated using ⁷Be and ²¹⁰Pb are similar and are compared to other published values.

Analysis of 22Na using a spectral summation technique on high-volume aerosol samples

Measurement of cosmogenic ²²Na in daily aerosol samples is often difficult due to low atmospheric production rates. A new technique based upon spectral summation of sequential high-volume aerosol samples to measure ²²Na is described and validated. This summation technique has broad applications to any detection system that produces sequential representative sample measurements in which radioisotopes are just below the detection limit, provided the energy calibration is stable. It is anticipated that a global dataset of this radionuclide will have many important environmental science applications.

Gamma emitters in atmospheric precipitation in Krakow (Southern Poland) during the years 2005-2015

The results of the sum of dry and wet activity deposition for naturally occurring ⁷Be, ²¹⁰Pb, ⁴⁰K, ²²Na and anthropogenic ¹³⁷Cs radionuclides in Krakow (Southern Poland) for the samples collected over 10 years (from August 2005 to July 2015) are presented and discussed. The radionuclides were determined using low background gamma spectrometry with HPGe detectors. Additionally, in this paper there are shown the results of activity concentrations in water from air precipitation for ⁷Be, ²¹⁰Pb, ²²Na, ⁴⁰K and ¹³⁷Cs radioisotopes from the period of 7 years (from August 2008 to July 2015). For all these series the statistical analysis including Spearman correlations, effects of seasonal variation and multiple regression models were conducted. After excluding two months from 2011 affected by the Fukushima accident, high Spearman correlation factors (R > 0.5) for activity deposition were noticed for the pair of the cosmogenic radionuclides ⁷Be and ²²Na (R = 0.508) and between ²¹⁰Pb and ⁷Be (R = 0.570). High correlation was noted between activity deposition and amount of precipitation for ⁷Be (R = 0.677). The seasonal correlations between ⁷Be-²²Na, ⁴⁰K-¹³⁷Cs, ²¹⁰Pb-¹³⁷Cs and ⁷Be-²¹⁰Pb were investigated and the highest correlation coefficient R = 0.731 for the ⁴⁰K-¹³⁷Cs pair was in the spring season. High correlations were observed also between ²¹⁰Pb and ⁷Be for autumn (R = 0.594), ⁴⁰K-¹³⁷Cs in summer (R = 0.582), ⁷Be-²²Na in spring (R = 0.635) and ²¹⁰Pb-¹³⁷Cs in autumn (R = 0.672). The multiple regression approach showed the interesting difference in scavenging mechanisms of cosmogenic and terrestrial radionuclides. According to that model, the deposition of cosmogenic nuclides was noticeably related to the amount of precipitation, while the deposition of terrestrial radionuclides did not show such dependence.

7Be behaviour and meteorological conditions associated with 7Be peak events in Spain

This work regards a comprehensive analysis of the overall distribution of ⁷Be activity concentrations in Spain and the synoptic meteorological conditions associated with the highest ⁷Be peaks (>8 mBq/m³). The use of four sampling stations (Barcelona, Bilbao, Madrid, and Sevilla) included in REMdb, with different latitudinal location, as well as the relatively long time period used in this study (2001-2010), allowed to improve the understanding of ⁷Be spatio-temporal distribution in Spain. The comparison of the ⁷Be activity concentrations mean values indicated a north-south gradient (from 3.1 ± 1.1 mBq/m³ in Bilbao to 4.0 ± 1.8 mBq/m³ in Sevilla), even though not statistically significant (as indicated by the t-test). However, the analysis of frequency distributions and temporal evolutions of ⁷Be activity concentrations have suggested the presence of two main areas, namely northern (Bilbao and Barcelona) and southern (Sevilla) Spain. The identification and analysis of periods associated with the highest values of ⁷Be have allowed studying the different synoptic patterns associated with stratospheric-tropospheric transport (STT). In particular, three episodes (one in the north and two in the south) potentially associated with vigorous STT have been identified and analysed in detail. The results displayed that the omega block configuration, extending either over western Russia and Scandinavia or into the Atlantic Ocean, forced the prevailing jet stream to the northeast and south of Spain respectively with subsequent subsidence. In summer, this blocking configuration at high latitudes was combined with the presence of the Azores high pressure system to the west of Spain, affecting the ⁷Be activity concentration recorded in the south.

Variation in airborne 134Cs, 137Cs, particulate 131I and 7Be maximum activities at high-altitude European locations after the arrival of Fukushima-labeled air masses

The Fukushima-labeled air mass arrival, and later the cesium-134 (¹³⁴Cs), cesium-137 (¹³⁷Cs) and particulate iodine-131 (hereafter noted ¹³¹I_p) maximum levels were registered in Europe at different dates depending on the location. Most of those data were obtained at low-altitude sampling areas. Here, we compare the airborne levels registered at different high-altitude European locations (from 850 m to about 3500 m). The integrated ¹³⁷Cs activity concentration was not uniform with regard to the altitude even after a long travel time/distance from Japan. Moreover, the relation of integrated ¹³⁷Cs vs. altitude showed a linear decrease up to an altitude of about 3000 m. A similar trend was noticed for ¹³¹I_p (particulate fraction) while it increased above 3000 m. Comparison with ⁷Be activity concentration showed that, as far as the high altitude location is concerned, the ¹³⁷Cs and ¹³⁴Cs maximum concentrations corresponded to the ⁷Be maximum, suggesting downdraft movements from high tropospheric or stratospheric layers to be responsible for ^137,134Cs increase and peak values. This was also confirmed by high potential vorticity and low relative humidity registered during the peak values.

A climatology of ⁷Be in surface air in European Union

This study presents a European-wide analysis of the spatial and temporal distribution of the cosmogenic isotope (7)Be in surface air. This is the first time that a long term database of 34 sampling sites that regularly provide data to the Radioactivity Environmental Monitoring (REM) network, managed by the Joint Research Centre (JRC) in Ispra, is used. While temporal coverage varies between stations, some of them have delivered data more or less continuously from 1984 to 2011. The station locations were considerably heterogeneous, both in terms of latitude and altitude, a range which should ensure a high degree of representativeness of the results. The mean values of (7)Be activity concentration presented a spatial distribution value ranging from 2.0 to 5.4 mBq/m(3) over the European Union. The results of the ANOVA analysis of all (7)Be data available indicated that its temporal and spatial distributions were mainly explained by the location and characteristic of the sampling sites rather than its temporal distribution (yearly, seasonal and monthly). Higher (7)Be concentrations were registered at the middle, compared to high-latitude, regions. However, there was no correlation with altitude, since all stations are sited within the atmospheric boundary layer. In addition, the total and yearly analyses of the data indicated a dynamic range of (7)Be activity for each solar cycle and phase (maximum or minimum), different impact on stations having been observed according to their location. Finally, the results indicated a significant seasonal and monthly variation for (7)Be activity concentration across the European Union, with maximum concentrations occurring in the summer and minimum in the winter, although with differences in the values reached. The knowledge of the horizontal and vertical distribution of this natural radionuclide in the atmosphere is a key parameter for modelling studies of atmospheric processes, which are important phenomena to be taken into account in the case of a nuclear accident.

Cosmogenic 22Na, 7Be and terrestrial 137Cs, 40K radionuclides in ground level air samples collected weekly in Kraków (Poland) over years 2003-2006

A low background gamma spectrometer with an Etruscan, 2500 years old lead shield and a muon veto detector were applied to study ²²Na and ⁷Be activity concentration in ground level air aerosol samples collected weekly over the years 2003–2006 in Kraków. Each sample was formed with ca 100 000 m³ of passed air, collected with two parallel ASS-500 high volume air samplers. The results for ⁴⁰K and ¹³⁷Cs are also presented for reference and comparison. Presented frequency distributions for activity concentration and correlation between the obtained results are discussed. The activity concentration results confirmed seasonal variation of activity to be different for all the investigated radionuclides. Moreover, the seasonal variation in nucleus activity ratio was also noticed for ²²Na and ⁷Be. Cosmogenic radionuclides being mainly of stratospheric origin, are subsequently attached to fine aerosols, via which they are transported to the ground level air. The mean aerosol transport time within the troposphere was estimated as equal to 7.5 days on average, reaching even 50 days in warm seasons. Limitations of the applied model were identified.

7Be, 210Pb and 137Cs concentrations in cloud water

Cloud water was sampled during 8 months, in 2008 and 2009, at the puy de Dome high-altitude atmospheric research station (France). The concentrations of (7)Be and (210)Pb, both naturally occurring radionuclides, and (137)Cs of anthropogenic origin, were determined. Those values are useful for a better knowledge of the aftermath cloud deposition and more generally for wet deposition assessment of radionuclides. This is of primary interest in case of a nuclear accident, especially considering (137)Cs deposition, both for high-altitude locations that are regularly embedded by clouds and also for lowlands where fog can occur. The (7)Be and (210)Pb average activity concentrations in cloud water found were 1.9±0.11 mBq m(-3) air and 140±10 μBq m(-3) air, respectively. For (137)Cs, the average concentration was 0.14±0.02 μBq m(-3) air. This very low-level is representative of the long term post-accidental background level. Indeed, for the studied period, the last accidental (137)Cs release was that of Chernobyl accident, in April 1986. To our knowledge this is the first data about (137)Cs reference level determination in cloud water. The comparison between cloud water and rain water concentrations showed a ratio cloud/rain ranging between 3.4 and 8.1, in agreement with previous studies performed on inorganic compounds. Scavenging efficiencies of aerosols by cloud droplets were also calculated with the additional aerosol concentrations routinely measured at the station and were quite low (0.13-0.40) compared to what has previously been observed for inorganic soluble ions.