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

https://doi.org/10.1016/j.apradiso.2019.03.038Get rights and content

Highlights

  • 7Be records over 2001–2010 show statistically significant positive trends.

  • Only in Risoe, shorter 7Be periodicities are superimposed on the seasonal cycle.

  • 7Be cross-correlations across the sites decrease with the increasing distance.

  • 7Be long-range correlations evident early in the investigated years.

Abstract:

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.

Introduction

Natural radioactivity is the main contributing factor to the level of radioactive contamination of the atmosphere (e.g., Shahbazi-Gahrouei et al., 2013). Furthermore, around three quarters of the radiation in the environment comes from natural radiation sources (e.g., Bossew et al., 2017). It is, therefore, essential to assess the radiation doses in order to control possible health effects from such natural sources (Ahmad et al., 2015). More information on radiation in the environment and human exposure to radioactivity can be found in Eisenbud and Gesell (1997).

Article 36 of the EURATOM treaty (EU, 2010) requires the competent authorities of each Member State (MS) to provide regularly the environmental radioactivity monitoring data resulting from their Article 35 obligations to the European Commission (EC) in order to keep the EC informed on the levels of radioactivity in the environment (air, water, milk and mixed diet) which could affect population. Since 1988, the Radioactivity Environmental Monitoring Database (REMdb) (https://rem.jrc.ec.europa.eu/RemWeb/) has brought together and stored in a harmonised way environmental radioactivity data received from the European MS in the aftermath of the Chernobyl accident. Nowadays, the total number of data records contained in the REMdb exceeds 5 million.

Under Article 36 (EU, 2010), “control” of natural radioelements, such as cosmogenic isotope beryllium-7 (7Be), is required. Due to this legal requirement, 7Be is very closely monitored in the EU. As a confirmation of this, over 1984–2016, around 28000 results of 7Be measurements have been stored in the REMdb, and its long-term distribution has been characterised in surface air over Europe (Ajtić et al., 2017; Hernández-Ceballos et al., 2015).

Beryllium-7 (radioactive half-life 53.12 days) is created in the stratosphere and upper troposphere, in the interactions of high-energy cosmic particles with nuclei of the most abundant elements in the air, nitrogen and oxygen (Lal and Peters, 1967). After formation, 7Be rapidly attaches to submicron-sized aerosols, and follows all atmospheric paths of transport and deposition of the aerosol carriers. Hence, several meteorological factors of different scales, such as the synoptic horizontal and vertical transport in the troposphere, and regional-local meteorological conditions and their seasonal variations (e.g., rainfall, temperature and winds), affect the 7Be distribution (Feely et al., 1989), and introduce complexity in the characterisation of the 7Be activity concentration in the surface air. There is an extensive bibliography investigating the use of 7Be as a natural tracer of stratospheric influence and subsidence (e.g., Liu et al., 2016), and the impact of meteorological factors at different scales (synoptic, mesoscale and local) on the temporal and spatial variability of the 7Be activity concentrations in the surface air (e.g., Gordo et al., 2015; Ioannidou et al., 2014). Modelling studies of the 7Be production and transport (Usoskin and Kovaltsov, 2008; Usoskin et al., 2009) have shown that its surface concentration can be well reproduced over a timescale of a week, but small-scale patterns are not as readily discerned–the fact that may imply our understanding of all the influencing factors is still only partial.

Several authors have identified spatial patterns in the 7Be surface concentration in Europe (Ajtić et al., 2017; Hernández-Ceballos et al., 2016). These studies showed three distinguishable cluster groups (south, central and north of Europe) that clearly differ in terms of both intensity and temporal patterns of the 7Be concentration levels. The spatial distribution of the sampling stations agrees with the geographic (latitude, longitude) and seasonal variability of processes that affect the abundance of 7Be in the surface air (Brattich et al., 2017). Based on these groupings, for instance, extreme 7Be events have been analysed to understand the particular synoptic meteorological conditions that cause these anomalous surface concentrations (Ajtić et al., 2017; Hernández-Ceballos et al., 2017).

The aim of the present paper is a further understanding of the impact of local conditions on the temporal and spatial variability of the 7Be activity concentration measured in nearby sampling sites. We made an effort to discriminate local effects from signals possibly due to atmospheric patterns at synoptic and global scales. To achieve that, we applied a spectral analysis, previously used in Bianchi et al. (2018a), which highlights differences and similarities among time series in terms of harmonic components, random fluctuations behaviour, noise dynamic and similarities exponents. As a case study, we analysed the 7Be activity concentrations recorded at ground level in four Scandinavian sampling sites measured over 2001–2010. Previous studies (Ajtić et al., 2017; Hernández-Ceballos et al., 2016) grouped these stations in the Northern European cluster of 7Be, so an investigation of their local variations could provide more insight into the impact of geographic proximity on observing stations and, hence, on the spatial representativeness of the 7Be measurements.

Section snippets

Data records

Measurements of the 7Be activity concentration in the surface air (Be-7) at four monitoring sites in Northern Europe (Fig. 1) were obtained from the REMdb. This database is maintained by the REM-Emergency Preparedness and Response group of the DG Joint Research Centre (JRC). Records until 2006 are stored in the REMDdb and are publicly accessible through an unrestricted repository “REM data bank - Years 1984–2006” (De Cort et al., 2007). Access to data since 2007 has been granted only after

Methods

A time series Xiis a collection of data in chronological order, and can be considered as a sum of three terms (Chatfield, 1995):Xi=Ti+Pi+Riwhere i=1,...,N, and N is the length of the time series. Term Tiis called trend, and it represents the overall growth (towards higher or lower values) of Xi. Term Pi is a periodic term, and it is a sum of periodicities within the time series. Finally, Rirepresents the residuals, considered as a stochastic process underlying the other two processes. In other

Trends

Fig. 2 presents the time evolution of the 7Be activity concentrations in the four sites. In all four sites, a positive trend was detected, ranging from an average value of 1.50%/year for Ivalo, to an average value of 2.96%/year for Kista (Fig. 2). Please note that the Umea data set (Fig. 2b), with its missing data points during a part of 2007, was analysed as an unevenly sampled series in the regression. Hence, no data interpolation was performed and the output of the regression was based only

Conclusions

We looked into the 7Be records covering ten years of measurements in Northern Europe, a region of a distinct 7Be behaviour. We separated trend, periodicities and residuals from the 7Be time series, and analysed each term in an effort to delineate the similarities across this region.

Our analysis indeed showed common features in the 7Be surface concentrations across the investigated sites. First, we found statistically significant concentration increase over 2001–2010. The trends ranged between

Funding

The paper is a part of the research done within the project “Climate changes and their influence on the environment: impacts, adaptation and mitigation” (No. 43007) financed by the Ministry of Education, Science and Technological Development of the Republic of Serbia (2011–2019).

Acknowledgments:

The authors would like to thank the REM group for provision of the 7Be activity concentration measurements from the REM database (REMdb, REM group of the DG Joint Research Centre, Ispra site, European Commission) and the EU Member States for providing the data. The authors would also like to deeply acknowledge Prof. José Antonio Garcia Orza (Universidad Miguel Hernandez de Elche, Department of Physics, Spain) for his support and help in developing the codes for removing the influence of the

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