Intercomparison study of atmospheric 222Rn and 222Rn progeny monitors

The use of the noble gas radon (Rn-222) as a tracer for different research studies, for example observation-based estimation of greenhouse gas (GHG) fluxes, has led to the need of high-quality Rn-222 activity concentration observations with high spatial and temporal resolution. So far a robust metrology chain for these measurements is not yet available. A portable direct atmospheric radon monitor (ARMON), based on electrostatic collection of Po-218, is now running at Spanish stations. This monitor has not yet been compared with other Rn-222 and Rn-222 progeny monitors commonly used at atmospheric stations. A 3-month intercomparison campaign of atmospheric Rn-222 and Rn-222 progeny monitors based on different measurement techniques was realized during the fall and winter of 2016-2017 to evaluate (i) calibration and correction factors between monitors necessary to harmonize the atmospheric radon observations and (ii) the dependence of each monitor's response in relation to the sampling height and meteorological and atmospheric aerosol conditions. Results of this study have shown the following. (i) All monitors were able to reproduce the atmospheric radon variability on a daily basis. (ii) Linear regression fits between the monitors exhibited slopes, representing the correction factors, between 0.62 and 1.17 and offsets ranging between 0.85 and 0.23 Bqm(-3) when sampling 2m above ground level (a.g.l.). Corresponding results at 100 m a.g.l. exhibited slopes of 0.94 and 1.03 with offsets of -0.13 and 0.01 Bqm(-3), respectively. (iii) No influence of atmospheric temperature and relative humidity on monitor responses was observed for unsaturated conditions at 100 m a.g.l., whereas slight influences (order of 10(-2)) of ambient temperature were observed at 2m a.g.l. (iv) Changes in the ratio between Rn-222 progeny and Rn-222 monitor responses were observed under very low atmospheric aerosol concentrations. Results also show that the new ARMON could be useful at atmospheric radon monitoring stations with space restrictions or as a mobile reference instrument to calibrate in situ Rn-222 progeny monitors and fixed radon monitors. In the near future a long-term comparison study between ARMON, HRM, and ANSTO monitors would be useful to better evaluate (i) the uncertainties of radon measurements in the range of a few hundred millibecquerels per cubic meter to a few becquerels per cubic meter and (ii) the response time correction of the ANSTO monitor for representing fast changes in the ambient radon concentrations.