Analysis of extreme beryllium-7 specific activities in surface air

Abstract

We present an analysis of the maxima in a large dataset of the beryllium-7 specific activities measured in surface air in Helsinki, Finland, over 25 years (1987-2011), which are stored in the online Radioactivity Environmental Monitoring (REM) database. The maxima are defined as events with the beryllium-7 specific activity above the 95th percentile, which, for the Helsinki data set, equals to 4.8210-3 Bqm-3. The beryllium-7 specific activity in Helsinki shows a seasonal pattern with the monthly means above 2.0010-3 Bqm-3 during the warm season (April– September), and below 2.0010-3 Bqm-3 during the cold season (October–March). The analysis of the extremes shows that 10 % occurred in the cold season, and these “cold extremes” are analysed in more detail. Amongst the cold extremes, three representative “episodes” are identified. The episodes, which occurred in March 1999, February 2003 and February 2005, show extremely high beryllium-7 specific activities measured over several consecutive days. Anomalies of potential vorticity, sea level pressure and surface temperature, as well as precipitation, over Europe and the Atlantic are investigated. A brief analysis of one cold extreme, classified as “burst” since it was an isolated event surrounded by measurements below the 95th percentile, is also presented in an attempt to find common mechanisms that contribute to both cold extreme episodes and bursts. Scandinavia teleconnection index seems to represent a good indicator of potentially preferential atmospheric conditions that could lead to cold extreme episode occurrences in the Scandinavian region.