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

Abstract: 

The activity concentrations of beryllium-7, a natural radiotracer which has been known since long time as a tracer of stratospheric-tropospheric exchanges, shows a distinct temporal pattern in Northern Europe compared to the central and southern parts of the continent, generally related to its production source (i.e. solar activity). 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 16 %/decade to 35 %/decade. 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. In order to investigate the causes of the detected trends, we also calculate correlations of beryllium-7 with factors related to its production, transport and removal from the atmosphere: in particular, cross-correlations of the beryllium-7 residuals with residuals in the local temperature, precipitation and atmospheric pressure, as well as Arctic Oscillation index, North Atlantic Oscillation index, and sunspot number. 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.