Aerosol size distribution of the radon progeny in outdoor air

The activity size distribution of the short-lived radon decay products in ambient air was measured continuously over several weeks. Two different measurement techniques were used: A direct measurement with a low-pressure on-line alpha cascade impactor (OLACI) and an indirect determination based on measurement with a differential mobility analyser (5–200 nm) and a laser aerosol spectrometer (100–5000 nm). In parallel, the meteorological parameters like temperature, rainfall intensity and wind velocity were registered. The measured activity size distribution of the radon progeny can be approximated by a sum of three log-normal distributions. The greatest activity fraction was adsorbed on aerosol particles in the accumulation size range (100–1000 nm) with activity median diameters (AMDa) and geometric standard deviations (σga) values of 250–450 nm, and 1.5–3.0, respectively. The influence of the weather conditions on the activity of the accumulation particles was not significant. In contrast to the results of measurements in houses, a small but significant fraction of the radon progeny (average value: 2%) was attached to coarse particles (>1000 nm). This fraction varied between 0 and 10%. The direct measurements with the OLACI yielded a higher activity fraction (20–40%) in the nucleation size range (<100 nm) compared to the values evaluated from the number size distribution measurement (indirect measurement). This fraction depends on the number concentration of the atmospheric aerosol in this size range. A diurnal variation of the particle number concentration and of the activity fraction for the nucleation particles was found. In the case of lower particle concentrations (diameters <50 nm) in air during the night no nucleation mode was registered using the indirect measurement technique.