AN INTERNATIONAL INTERCOMPARISON OF MARINE ATMOSPHERIC RADON-222 MEASUREMENTS IN BERMUDA

As part of an international measurement intercomparison of instruments used to measure atmospheric Rn-222, four participating laboratories m ade nearly simultaneous measurements of Rn-222 activity concentration in commonly sampled, ambient air over approximately a 2-week period, a nd three of these four laboratories participated in the measurement co mparison of 14 introduced samples with known, but undisclosed (''blind '') Rn-222 activity concentrations that could be related to U.S, natio nal standards. The standardized sample additions were obtained with a calibrated Ra-226 source and a specially designed manifold used to obt ain well-known dilution factors from simultaneous flow rate measuremen ts. The exercise was conducted in Bermuda in October 1991. The Rn-222 activity concentrations in ambient Bermudian air over the course of th e intercomparison ranged from a few hundredths to about 2 Bq m(-3), wh ile the standardized sample additions covered a range from approximate ly 2.5 to 35 Bq m(-3). The overall uncertainty in the latter concentra tions was in the general range of 10% at a 3 standard deviation uncert ainty interval. The results of the intercomparison indicated that two of the laboratories were within very good agreement with the standard additions and almost within expected statistical variations. These sam e two laboratories, however, at lower ambient concentrations, exhibite d a systematic difference with an averaged offset of roughly 0.3 Bq m( -3). The third laboratory participating in the measurement of standard ized sample additions was systematically low by about 65-70%, which wa s also confirmed in their ambient air concentration measurements. The fourth laboratory, participating in only the ambient measurement part of the intercomparison, was also systematically low by at least 40% wi th respect to the first two aforementioned laboratories.