Behavior of ambient concentrations of natural radionuclides (7)Be, (210)Pb, (40)K in the Mediterranean coastal city of Málaga (Spain)

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.

Transport and deposition of radionuclides from northern Africa to the southern Iberian Peninsula and the Canary Islands during the intense dust intrusions of March 2022

The present study focuses on the two consecutive and markedly intense Saharan dust intrusion episodes that greatly affected southern Spain (Málaga) and, to a lesser extent, the Canary Islands (Tenerife), in March 2022. These two episodes were the result of atypical meteorological conditions in the region and resulted in record levels of aerosols in the air at the Málaga location. The activity levels of various natural and artificial radionuclides (7Be, 210Pb, 40K, 137Cs, 239Pu, 240Pu, 239+240Pu) and radioactive indicators (gross alpha and gross beta) were impacted by these events and the results are described herein. These episodes caused, for example, the activities of 137Cs in aerosol samples at the Málaga monitoring station to reach the highest concentrations ever recorded since high-volume aerosol monitoring started at this site in 2009. A link between the activity levels of 137Cs, 40K and gross alpha in the atmospheric aerosols and daily PM10 concentrations during the episodes is also reported. In addition, isotopic ratios are discussed in the context of the source and destination of the various anthropogenic radionuclides measured. The atmospheric residence time of aerosols during these episodes is also evaluated because it concerns how intrusions to the Canary Islands should be analysed. Finally, for the first time, the concentrations of 137Cs deposition by rainwater during a Saharan dust intrusion are reported and the deposition rate of these radionuclides during these episodes is discussed.

Comparison of dimension reduction techniques applied to the analysis of airborne radionuclide activity concentration

Temporal variation of airborne radionuclide activity concentration is highly influenced by several meteorological parameters. A traditional key tool to perform a combined analysis on these data is Principal Component Analysis (PCA), a linear dimensionality reduction technique that prioritizes the conservation of the data set global structure. While it reveals important information regarding the correlation among the considered variables, the obtained visual representations do not usually allow to clearly discern different clusters of states with common properties. The main goal of this study is applying two recently introduced non linear dimensionality reduction techniques, t-distributed Stochastic Neighbor Embedding (t-SNE) and Uniform Manifold Approximation and Projection (UMAP) to a data set composed by ⁷Be and gross beta (A_β) activity concentration and other meteorological data gathered in Mallorca (Spain) between 2004 and 2014. Compared to PCA, both algorithms reveal more details on the local structure of the data set. UMAP allows to clearly identify data clusters with different characteristics that are not clearly identified with the alternative techniques.

Radionuclide and trace metal accumulation in a variety of mosses used as bioindicators for atmospheric deposition

Mosses can be used as biological monitors to study metal pollution and the depositional fluxes of radionuclides. In this study, we analysed the concentrations of radionuclides (²¹⁰Pb (²¹⁰Pb_ex), ⁷Be, ¹³⁷Cs, ⁴⁰K, ²³⁸U, ²²⁶Ra, ²²⁸Ra and ²²⁸Th) and metals (Fe, Zn, Cu, Al, Pb, Cd, Cr, Ni, V and Mn) in moss and soil samples from two different regions. The metal concentrations were higher in mainland China than in the Arctic region, and this is likely associated with the comparatively lower rates of industrial production and human activity in the Arctic region. Principal component analysis and correlation results revealed two radionuclides sources types in mosses, i.e., soil (⁴⁰K, ²³⁸U, ²²⁶Ra, ²²⁸Ra and ²²⁸Ra) and atmospheric (²¹⁰Pb (²¹⁰Pb_ex), ⁷Be ¹³⁷Cs). Clustering and correlation analyses showed that different sources such as traffic (suspended dust), fossil fuels, dry and wet deposition (atmosphere and rainfall), and soil contributed to metal accumulation in mosses. The correlation between radionuclides and metals supported these observations, confirmed the accuracy of our results, and suggests that radionuclides are useful for identifying the source of metals in moss samples. The concentration ratios (CR) values of the radionuclides and the bioaccumulation factor (BCF) and enrichment factor (EF) values of metals in mosses helped identify the most environmentally sensitive moss, i.e., BS (Bryum paradoxum), which can be used for screening and monitoring radionuclides and metal pollution in urban atmospheres. These results support the use of analysing radionuclides in mosses to identify metal sources, and the potential use of mosses can to determine the atmospheric deposition fluxes of radionuclides.

Exposure to Particle Beta Radiation in Greater Massachusetts and Factors Influencing Its Spatial and Temporal Variability

Particle radioactivity is a property of airborne particles caused by the presence of naturally occurring or anthropogenic radionuclides. Recent studies have found associations between particle radioactivity and adverse health outcomes, including changes in blood pressure and lung function. However, the spatiotemporal distribution of particle radioactivity and factors influencing its variability have not been extensively studied. We address these knowledge gaps using measurements of gross beta activity, collected at seven Environmental Protection Agency (EPA) RadNet monitors located in and around Massachusetts. We apply back-trajectory analysis to identify prevailing air mass trajectories and find that these trajectories strongly influence seasonal trends in beta activity. We also evaluate the effects of different meteorological predictors on daily beta activity concentrations using a mixed-effect model. Important predictors of beta activity include air mass trajectories, temperature, and relative humidity. Finally, we create a series of random forest models to impute missing beta activity concentrations at each RadNet monitor for use in future health studies. This is the first study to analyze spatiotemporal trends in particle radioactivity using measurements from the EPA RadNet system.

Accumulation of natural and anthropogenic radionuclides in body profiles of Bryidae, a subgroup of mosses

Mosses can be used as biomonitors to monitor radionuclide deposition and heavy metal pollution in cities, forests, and grasslands. The aims of this work were to determine the activity concentrations of natural (²¹⁰Po, ²¹⁰Pb or ²¹⁰Pb_ex (excess ²¹⁰Pb is defined as the activity of ²¹⁰Pb minus the activity of ²²⁶Ra), ⁷Be, ⁴⁰K, ²²⁶Ra, ²³⁸U, and ²³²Th) and anthropogenic radionuclides (¹³⁷Cs) in moss body profiles and in situ underlying soils of moss samples and to assess/determine the distribution features and accumulation of these radionuclides. Activity concentrations of radionuclides in the samples were measured using a low-background gamma spectrometer and a low-background alpha spectrometer. Consistent with their source, the studied radionuclides in the moss samples and underlying soils were divided according to the principal component analysis (PCA) results into an airborne group (²¹⁰Po, ²¹⁰Pb (²¹⁰Pb_ex), ⁷Be, and ¹³⁷Cs) and a terrestrial group (⁴⁰K, ²³⁸U, ²²⁶Ra, and ²³²Th). The activity concentrations of ²¹⁰Po and ²¹⁰Pb_ex in moss body profiles were mainly concentrated in the stems-rhizoid parts, in which we measured some of the highest ²¹⁰Po and ²¹⁰Pb_ex levels compared to the results in the literature. ⁷Be mainly accumulated in the leaves-stem parts. Different positive correlations were observed between ²¹⁰Po and ²¹⁰Pb and between ⁷Be and ²¹⁰Pb, which indicated that the uptake mechanisms of ²¹⁰Po, ²¹⁰Pb, and ⁷Be by moss plants were different, to some extent. ¹³⁷Cs was detected only in some moss samples, and the fraction of ¹³⁷Cs in the underlying soils was much lower than that in the moss, suggesting that mosses were protecting the underlying soils from further pollution. Except for ⁴⁰K, the terrestrial radionuclide (²³⁸U, ²²⁶Ra, and ²³²Th) content in mosses was predominantly at low levels, which indicated not only the inability of mosses to use those elements for metabolic purposes but also the rather poor capability of mosses to directly mobilize, absorb, and transport elements (U, Ra, or Th) not dissolved in water.

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.

Analysis of the evolution of gross alpha and gross beta activities in airborne samples in Valencia (Spain)

Gross alpha (A_α) and gross beta activities (A_β) were measured weekly in the airborne of the Universitat Politècnica de Valencia campus (in the east of Spain) during the period 2009-2015 (7 years). The geometric mean values of weekly A_α and A_β were 0.53·10^-4 Bq m^-3 and 5.77·10^-4 Bq m^-3, respectively; with an average ratio A_α/A_β of 0.097. This study highlights the heterogeneity of gross alpha and gross beta activities depending on the different periods of the year. Data show seasonal variations with the highest activity in summer months and the lowest one in winter months. Several atmospheric factors were considered in order to explain this intra-annual variation (wind speed, temperature, relative humidity, precipitations, dust content and prevailing wind directions). Multiple Linear Regression Analysis were performed in order to obtain information on significant atmospheric factors that affect gross α and gross β variability, which could be useful in identifying meteorological or atmospheric changes that could cause deviations in gross α and gross β activity depending on the seasons considered. Models obtained explain more than 60% of variability for global data, and also for winter and spring-autumn months. However, more research is needed to explain gross α and gross β variability in summer months, because the atmospheric factors considered in the MLR explain less than 35% of variability.

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.

Long-range atmospheric transport Beryllium-7 to region the Sea of Japan

Concentrations of cosmogenic beryllium-7((7)Be) and atmospheric aerosols were measured in the atmosphere of the coastal zone of Vladivostok in 2013-2014. The (7)Be concentrations ranged from 0.5 to 4.1 mBq/m(3), with the lowest values in summer and the highest in spring and autumn; the mean value was 2.2 mBq/m(3). Analysis of meteorological data in the synoptic scale showed an inverse correlation with wet deposition rates R = -0.55 (p = 0.0001) and H2O mixing ratio R = -0.49 (p = 0.0001) and a positive with an average maximum height of 120-h backward trajectories of air masses R = 0.65 (p = 0.0001). Angular cluster analysis showed the (7)Be concentration to be dependent on the north-western (R = 0.53, p = 0.001) and eastern winds (R = -0.7, p = 0.0001 for 2013 and R = -0.49, p = 0.002 for 2014). The multiple regression analysis identified five factors in (7)Be concentration: altitudes (b = 0.44), air temperature (b = 0.36), a portion of trajectories in the pacific (North-East direction) cluster (b = -0.32), aerosol concentrations (b = 0.28) and wet precipitation rates (b = -0.24). The model has a good correlation with the data (adjusted R(2) = 0.55). It was found that the direction and height of the air masses trajectories in the lower troposphere strongly influence the concentration of (7)Be.

Monitoring of (7)Be and gross beta in particulate matter of surface air from Mallorca Island, Spain

Measurements of airbone radionuclides (7)Be and gross beta (Aβ) jointly with atmospheric aerosols, i.e. particulate matter (PM) were routinely carried out for 10 years (2004-2014) at the University of the Balearic Islands, Spain. A total of 728 filter (0.8 μm pore size) samples were collected, and in all of them (7)Be and Aβ specific activities were detected. The maximum and minimum specific activities monitored were 0.73 ± 0.41 - 15.8 ± 1.26 mBq m(-3) of (7)Be and 0.14 ± 0.02 - 2.55 ± 0.04 mBq m(-3) of Aβ. PM concentrations were also determined, showing seasonal behavior with the highest concentration in summer and the lowest one in winter. Several meteorological parameters have been considered to explain this intra-annual variation. Principal component analysis (PCA) was applied to the dataset indicating that it is well represented by two principal components that explain 76.6% of total variance. Additionally, a second study with preliminary results of the specific activities of two natural radiotracers ((7)Be and (210)Pb) in PM10 samples was carried out. They were monitored for two years (2013-2015) in air of Mallorca Island. (7)Be and (210)Pb were detected in most of the PM10 filters, in 100% and 93% of them, respectively. The relationship between activities of both radionuclides and several relevant meteorological parameters was established at 95% confidence level. As a common result to PM and PM10 samples, a strong positive correlation between the evaluated radionuclides and temperature was found.

Study of ⁷Be and ²¹⁰Pb as radiotracers of African intrusions in Malaga (Spain)

The relationship between the particulate matter in the atmosphere of Malaga and the origin of air masses with special attention to African intrusions was analyzed. The concentrations of PM10 and the activities of some radionuclides ((7)Be and (210)Pb) as tracers of these intrusions are discussed. The origin of these radiotracers is completely different. (210)Pb is a good tracer of air masses traveling close to the ground and come from inland areas. On the other hand, the production of (7)Be is very low in the vicinity of the Earth's surface, and increases with altitude, making it a great tracer of stratospheric air masses in the troposphere. Studies of radionuclide activities in aerosol particles provide a means for evaluating the integrated effects of transport and meteorology on the atmospheric loadings of substances with different sources. Measurements of aerosol mass concentration and specific activities of (7)Be and (210)Pb in aerosols at Malaga (36° 43' 40″ N; 4° 28' 8″ W) for the period 2009-2011 were used to obtain the relationships between radionuclide activities and airflow patterns by comparing the data grouped by air mass trajectory. The average concentration values of (7)Be and (210)Pb over the 3-year period have been found to be 5.05 and 0.55 mBq m(-3), respectively, with mean aerosol mass concentration of 53.6 μg m(-3). Taking into account the outbreaks of the daily values limits of PM10 in the study period (2009-2011), the origin of air masses is studied to explain three different situations of both radionuclides (7)Be and (210)Pb: 1) high concentrations of (7)Be and low concentrations of (210)Pb; 2) high concentrations of (210)Pb and low concentrations of (7)Be and 3) high concentrations of (7)Be and (210)Pb.