Radon decay products and 10–1100 nm aerosol particles in Postojna Cave

Abstract. At the lowest point along the tourist route in Postojna Cave, the activity concentration of radon (²²²Rn) decay products and the number concentration and size distribution of aerosol particles in the size range of 10–1100 nm were monitored, with the focus on the unattached fraction (f^un) of radon decay products (RnDPs), a key parameter in radon dosimetry. The total number concentration of aerosols during visits in summer was lower (700 cm⁻³) than in winter (2800 cm⁻³), and was dominated by <50 nm particles (related to unattached RnDPs) in summer and by >50 nm particles (related to the attached RnDPs) in winter. This explains the higher f^un values in summer (0.75) and the lower winter measurement (0.04) and, consequently, DCF_D values of 43.6 and 13.1 mSv WLM⁻¹ respectively for the calculated dose conversion factors. The difference is caused by an enhanced inflow of fresh outside air, driven in winter by the higher air temperature in the cave compared to outside, resulting in the introduction of outside aerosol particles into the cave.