Temporal trends in the beryllium-7 activity concentrations in Serbia, Croatia, and Slovenia: 1991–2022

Beryllium-7, a cosmogenic radionuclide with a half-life of 53.28 days, can be used as a tracer of atmospheric transport as it travels attached to aerosols from the point of its production in the upper layers of the atmosphere to the surface. Beryllium-7 is a gamma emitter, and its activity concentration can be inexpensively measured using standard gamma spectrometry. Hence, it is closely followed in many national radioactivity monitoring programs. In our paper, records from the Ground Air Radioactivity Monitoring (GRAMON) database are used to investigate temporal trends of this radionuclide’s activity concentration. Specifically, the national air radioactivity monitoring programs of Serbia, Croatia, and Slovenia, all of which are contributors to the GRAMON database, offer the beryllium-7 time series since 1991. Four sampling sites are chosen: Belgrade (44.88333 °N; 20.583333 °E; 95 m a.s.l.) in Serbia; Zagreb (45.835361 °N; 15.982972 °E; 166 m a.s.l.) in Croatia; Krško, а town in the vicinity of the Krško Nuclear Power Plant (45.950414 °N; 15.512261 °E; 204 m a.s.l.), and Ljubljana (46.042356 °N; 14.487494 °E; 292 m a.s.l.) in Slovenia. Between 1991 and 2022, there are 384 mean monthly values of the beryllium-7 activity concentrations per site (with 22 missing values for Belgrade, and 6 for Ljubljana). Temporal trends are calculated as best-fit straight lines using the least-squares method. In addition to looking into the 1991–2022 period (32 years), we also calculate the trends over a shorter period of the latter 12 years, i.e. 2011–2022. The trends over 1991–2022 show an increase in the beryllium-7 activity concentration in all the sites, except Zagreb: 28 %/decade in Belgrade, -13.2 %/decade in Zagreb, 1.7 %/decade in Krško, and 12.4 %/decade in Ljubljana. The trends over 2011–2022 imply that this latter period differs from the overall period of 32 years. The trend in Krško still shows the least value (-2.8 %/decade) but is negative, indicating a slight decrease in the beryllium-7 activity concentration in recent years. In contrast, in Zagreb, the trend, negative over 1991–2022, is now positive (30 %/decade). In Ljubljana, the trends over the two periods are practically the same (12.4 %/decade vs 12.6 %/decade), while in Belgrade, the positive trend is even stronger in recent years (28 %/decade vs 33 %/decade). Our results bring out questions that might be fully answered only in a study that concurrently investigates the temporal trends of the meteorological parameters. One of the questions could focus on the Zagreb, Krško, and Ljubljana sites that are within 100 km, and yet, their trends, both long- and short-term, are notably different.