Wet Deposition of Radon-222 Progeny in Northern Finland Measured with an Automatic Precipitation Gamma Analyser

Abstract

An automatic wet deposition gamma analyser has been constructed. Precipitation is collected with a 1 m 2 funnel and its 214 Pb and 214 Bi contents measured in a Marinelli beaker with an NaI(Tl) detector and a multichannel analyser. The amount of water in the beaker is measured with an ultrasonic sensor. All the sensor and spectrum data are gathered, and the magnetic valves controlled, with a data logger and a microcomputer. The instrument was tested during a five-month field measurement campaign in northern Finland. A value of 750 Bq.l -1 for the geometric mean 222 Rn progeny concentration in precipitation, and a geometric mean washout ratio of 9.1 x 10 5 was obtained in this study. Their variations, however, were large: four orders of magnitude. The Rn progeny concentrations and washout ratios are inversely correlated with low precipitation intensities. With higher precipitation intensities this behaviour levels out. Higher washout ratios were observed during rainfall compared to snowfall. Low Rn progeny concentrations in precipitation are usually connected with cold or occluded fronts and surges of arctic air masses. On the other hand, high Rn progeny concentrations are usually related to warm fronts or sectors. They are also observed between two successive cyclones, when continental air masses from southern Russia move northwest. The results obtained could also be applied to wet-depositing chemical substances like heavy metals, which can be difficult to measure using on-line methods.