Observations of atmospheric variability and soil exhalation rate of radon-222 at a Russian forest site -: Technical approach and deployment for boundary layer studies

A monitor for continuous observations of the atmospheric Rn-222 daughter activity has been improved and successfully implemented in a field study in the European Taiga (Fyodorovskoye Forest Reserve). The alpha-activity of the short-lived Rn-222 and Rn-220 (Pb-212) decay products, which are attached to aerosols, is accumulated on a quartz aerosol filter and assayed on line by alpha-spectroscopy. The alpha-activity from the Pb-212 daughters is determined by spectroscopy and corrected for. This monitor is suitable to measure Rn-222 activities at hourly resolution down to 0.5 Bq m(-3) with an uncertainty well below +/-20%. The prototype of this monitor is run in Heidelberg on the roof of the Institute's building about 20 m above ground. For this site, the atmospheric radioactive disequilibrium was determined between the Rn-222 daughter Po-214 and Rn-222, which has to be known in order to derive the atmospheric Rn-222 activity with the static filter method. We derived a mean disequilibrium Po-214/Rn-222 = 0.704 +/- 0.081 for various meteorological conditions through parallel Rn-222 gas measurements with a slow pulse ionisation chamber. At the Russian field site, continuous activity observations were performed from July 1998 until July 2000 with half a year's interruption in summer/fall 1999. During intensive campaigns, a second monitor was installed at Fyodorovskoye at 15.6 m (July/August 1998), and at 1.8 m (July/August 1999 and October 1999) above ground. As expected, pronounced diurnal cycles of the Rn-222 daughter activity were observed at all sites, particularly during summer when the vertical mixing conditions in the atmospheric surface layer vary strongly between day and night. The lower envelope of the continuous measurements at Fyodorovskoye and at Heidelberg changes on synoptic timescales by a factor of 4-10 due to long-range transport changes between continental to more maritime situations. Generally, the Rn-222 activity at 26.3 m height at Fyodorovskoye is lower by a factor of 2-3 compared to Heidelberg at 20 m above ground. This unexpected result is due to considerably lower Rn-222 exhalation rates from the soils measured in the footprint of the Fyodorovskoye Forest tower compared to Heidelberg. With the inverted chamber technique Rn-222 exhalation rates in the range 3.3-7.9 Bq m(-2) h(-1) were determined at Fyodorovskoye for summer 1998 and autumn 1999 (wet conditions with water table depths between 5 and 70 cm). Only during the very dry summer of 1999 the mean Rn-222 exhalation rate increased by about a factor of five. All measured exhalation rates at the Fyodorovskoye Forest are considerably smaller by a factor of 2-10 compared to observations in the vicinity of Heidelberg (ca 50-60 Bq m(-2) h(-1)) and generally in Western Europe.