Long-term variations in the surface air 7Be concentration and climatic changes

Applying the Kolmogorov-Zurbenko filter followed by random forest models to 7Be observations in Spain (2006-2021)

In this study, we analysed 7Be weekly surface measurements from six Spanish laboratories from 2006 to 2021. The Kolmogorov-Zurbenko filter was applied to the six 7Be time series, and following an iterative process, the original data were divided into two fractions: one related to variations characterized by periods above 33 days (including, among others, the seasonal cycle) and the second noisier fraction related to mechanisms originating from variations with periods below 33 days. Both fractions were independent at the six locations. The second machine-based step using random forest models was applied with the aim of identifying the most influential inputs to the observed 7Be concentrations, and machine learning-inspired regression models were fitted. With respect to seasonal components, the results indicated that the memory of the system was the most influential input, as expected by the large fraction of variance explained by the seasonal cycle, followed by that of humidity and wind-related variables. For the fraction corresponding to periods below 33 d, precipitation-, humidity-, and radiation-related variables were the most influential. This methodology has made it possible to successfully describe the major mechanisms known to be involved in the generation of the surface 7Be concentrations observed in Spain.

Climate change as observed through the IMS radionuclide station in Spitzbergen

The International Monitoring System (IMS), installed and maintained by the Comprehensive Nuclear-Test-Ban Treaty Organization (CTBTO) with the support of States Signatories, is a global system of monitoring stations based on four complementary technologies: seismic, hydroacoustic, infrasound and radionuclide. One of the IMS radionuclide stations is located in Spitzbergen, the largest island of the Norwegian Svalbard Archipelago, which borders the Barents Sea and the Northern Atlantic Ocean. It has been demonstrated that signs of climate change are particularly noticeable in that region. As many other radionuclides observed in environmental measurements, 212Pb is always observed at IMS stations, in varying quantities. This is also the case for the IMS station RN49, Spitzbergen, where it can be demonstrated that the average concentration of the measured lead 212Pb increases. This is observable specifically October through December. This paper demonstrates the asset of IMS data to study climate change effects. Our conclusions are supported by global temperature anomaly data from NOAA's Global Surface Temperature Analysis, covering the period 1850 to 2023.

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.

Analysis of trends, periodicities, and correlations in the beryllium-7 time series in Northern Europe

The activity concentrations of beryllium-7, a natural radiotracer that is considered as a tracer of the stratospheric-tropospheric exchange, shows a distinct behaviour in Northern Europe compared to the central and southern parts of the continent. In this study, we use the measurements collected at four sampling stations in Scandinavia (Ivalo, Umea, Kista, Risoe) between 2001 and 2010 and investigate their trends, periodicities and residuals with the aim to further understand the common features in the beryllium-7 data records in northern sampling sites. The beryllium-7 activity concentrations exhibit statistically significant positive trends that range from an average value of 1.50%/year to an average value of 2.96%/year. We detect a one-year periodicity in all the sites, and in the southernmost site, Risoe in Denmark, additional higher-frequency harmonics. In the residual time series, we find outliers that represent occurrences of extremely high beryllium-7 activity concentration. Cross-correlations of the beryllium-7 residuals across the four sites decrease with increasing distance; similarly, as indicated by local Hurst exponents the records exhibit long-range correlations that weaken towards the end of the investigated period. To investigate the causes of the detected trends, we also calculate correlations between beryllium-7 and factors related to its production, transport and removal from the atmosphere: in particular, cross-correlations of the beryllium-7 residuals with residuals in sunspot number, local temperature, atmospheric pressure and precipitation, as well as Arctic Oscillation index and North Atlantic Oscillation index. Most of the obtained correlations, however, are not statistically significant, highlighting the need to analyse a longer time period in order to evaluate the impact of different factors on the airborne beryllium-7 activity concentration.

Fukushima-derived radioactivity measurements in Pacific salmon and soil samples collected in British Columbia, Canada

Despite the many studies that have shown minimal health risks to individuals living outside of Japan following the Fukushima Nuclear Accident, there are persisting concerns regarding the consumption of Pacific seafood that may be contaminated with radioactive species from Fukushima. To address these concerns, the activity concentrations of anthropogenic 134Cs and 137Cs, as well as naturally occurring 40K, were measured in Pacific salmon collected from Kilby Provincial Park, British Columbia (BC), in 2013 and from the Quesnel River, BC, in 2014 using low-background gamma-ray spectroscopy. In addition, soil samples and a single roof-debris sample were collected and analysed to provide a record of Fukushima-derived contamination in BC. Cesium-134 was not detected in the salmon samples. Cesium-137 was not detected in any of the sockeye or chum samples, although it was detected in all of the Chinook samples. The weighted average (±1σ) 137Cs activity concentration in the Chinook salmon collected in 2013 and 2014 was 0.23 (3) and 0.20 (3) Bq/kg fresh weight, respectively. A conservative annual dose estimate for an adult who consumes the average Canadian quantity of seafood per year, contaminated with radiocesium at the maximum concentrations measured in this campaign, was calculated to be 0.054 μSv per year. Cesium-134 was detected in all but two of the soil samples. A weak positive correlation was observed between presence of 134Cs and of 7Be suggesting that the 134Cs arrived via atmospheric deposition. Cesium-137 was present in every soil sample, although the total radiocesium activity concentrations measured were significantly less than action levels set by Health Canada.

Long-term variations of radionuclides in the Bratislava air

Variations of aerosol radionuclides (2001-2015) in the ground-level air in Bratislava (Slovakia) showed ⁷Be maxima in spring/early summer and minima in winter, however, an inverse trend was observed for ²¹⁰Pb, ¹³⁷Cs and ⁴⁰K. A decreasing amplitude and splitting of summer maxima for ⁷Be in the last years has been found. A temporal behavior of the ⁷Be/²¹⁰Pb activity ratio showed higher levels during warm seasons due to vertical convection of air masses from higher altitudes. The ¹³⁷Cs activity concentration in the surface air between 2003 and 2010 was decreasing with an effective half-life of 1.9 ± 0.3 years. The yearly average ¹³⁷Cs concentrations during 2009-2014 were almost constant, disturbed only by the Fukushima accident in 2011. The increased atmospheric ¹³⁷Cs and ⁴⁰K levels observed during the autumn-winter season may be due to surface soil resuspension, biomass burning and radionuclide transport by winds. Seasonal variations of ²²²Rn activity concentrations were found with maxima at the end of autumn and in winter, and minima in spring. The variability of the average annual course of ²²²Rn has been larger than that of ²¹⁰Pb. The ²¹⁰Pb/²²²Rn activity ratio was highest at the end of winter and in the spring, while from June to December remained nearly constant. More intensive atmospheric mixing in spring months caused a decrease in the ²²²Rn activity concentration, while the aerosol component of the atmosphere has been affected mainly during the autumn and winter seasons. The mean residence time of aerosols in the atmosphere was calculated using the ²¹⁰Pb/²²²Rn method to be 4.5 ± 0.9 days.

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.

High activity concentrations of (210)Pb and (7)Be in sediments and their histories

High activity concentrations of (210)Pb and (7)Be, in addition to (137)Cs, were found in a rooftop deposit in Japan. This deposit had activities of about 5 Bq/g from (210)Pb and 3.6 Bq/g from (7)Be, an order of magnitude greater than typically found in field soils and lake and sea bottom sediments. It is clear that under certain conditions aerosol particles can accumulate in deposits with little or no mixing and dilution by silicate materials, and that subsequent processes and factors result in a heterogeneous distribution pattern of activity concentration on the Earth's surface. A simple model suggests that the history of an environmental sample can be estimated by using the activities of (210)Pb and (7)Be.