Trajectory Analysis of 210Pb and 7Be in Ground-Level Air in Southern Finland

Abstract

The Finnish Meteorological Institute has been monitoring airborne natural radionuclides ²¹⁰Pb and ⁷Be at its geophysical observatory at Nurmijüarvi, southern Finland (60°30'N, 24°39'E, h = 105 m above see level). Daily aerosol samples (3500 m³/d) were collected onto glass-fiber filters with an aerosol sampler having an aerodynamic cutoff diameter of 10-15 μm depending on the wind speed. The filters were analysed for ⁷Be with an electrically cooled HPGe γ-ray spectrometer and for 210Pb with α-counting of the daughter ²¹⁰Po using an automatic α/β-analyzer. A trajectory analysis of the observed activity concentrations revealed that the highest ⁷Be and ²¹⁰Pb activity concentrations are associated with continental air masses coming from southeast. Meteorologically these situations often originate from high-pressure areas in central Russia. The associated air masses are often dry, which prevents the removal of radionuclides through precipitation scavenging. ²²²Rn, and thus ²¹⁰Pb too, originate practically entirely from land areas and not from oceans. The descending movement of air in high-pressure areas promotes the transfer of ⁷Be from upper air to the ground level. The production of ⁷Be increases toward the magnetic poles of the Earth. This is not reflected in the source areas of ⁷Be, because the concentration variations in the ground-level air are due to large-scale weather phenomena.