Bimodal unattached 218Po Radon progeny and the influence of enhanced deposition effect at high-radon levels

Unattached fraction 218Po airborne particle size distributions from graded screen array diffusion batteries are analysed for bimodal constituents. The analysis is for the high 222Rn concentration range from 10 to 50 kBq m−3 where the recently reported enhanced deposition effect is in transition. We find that the single unattached fraction is distributed nominally at an arithmetic mean diameter (AMD) of 0.8–1.0 nm, and consists of two 218Po molecular species with AMD ≅ 0.756 and 1.101 nm, respectively. In unfiltered room air the 0.756 nm species is the least stable, decreases monotonically in concentration magnitude beginning at 12 kBq m−3 and disappears completely at ∼32 kBq m−3 (at a relative humidity of 54%). The AMD ≅ 1.101 nm mode does not become unstable until it reaches ∼32 kBq m−3 222Rn level and completely disappears at ∼50 kBq m−3. Similar behaviour is observed for filtered air but with some distinct differences. This work supports the bimodal findings for the 218Po unattached fraction of Porstendorfer and co-workers and provides further details of the high-radon enhanced deposition effect.