Radon Decay Product Aerosols in Ambient Air

Received: November 30, 2009
Revised: November 30, 2009
Accepted: November 30, 2009
Download Citation: RIS|BibTeX|https://doi.org/10.4209/aaqr.2009.02.0011  

Cite this article:
Papastefanou, C (2009). Radon Decay Product Aerosols in Ambient Air. Aerosol Air Qual. Res. 9: 385-393. https://doi.org/10.4209/aaqr.2009.02.0011

ABSTRACT

The aerodynamic size distributions of radon decay product aerosols, i.e. ²¹⁴Pb, ²¹²Pb, and ²¹⁰Pb were measured using low-pressure (LPI) as well as conventional low-volume 1-ACFM and high-volume (HVI) cascade impactors. The activity size distribution of ²¹⁴Pb and ²¹²Pb was largely associated with submicron aerosols in the accumulation mode (0.08 to 2.0 μm). The activity median aerodynamic diameter “AMAD” varied from 0.10 to 0.37 μm (average 0.16 μm) for ²¹⁴Pb-aerosols and from 0.07 to 0.25 μm (average 0.12 μm) for ²¹²Pb-aerosols. The geometric standard deviation, σg averaged 2.86 and 2.97, respectively. The AMAD of ²¹⁰Pb-aerosols varied from 0.28 to 0.49 μm (average 0.37 μm) and the geometric standard deviation, σg varied from 1.6 to 2.1 (average 1.9). The activity size distribution of ²¹⁴Pb-aerosols showed a small shift to larger particle sizes relative to ²¹²Pb-aerosols. The larger median size of ²¹⁴Pb-aerosols was attributed to α-recoil depletion of smaller aerosol particles following the decay of the aerosol-associated ²¹⁸Po. Subsequent ²¹⁴Pb condensation on all aerosol particles effectively enriches larger-sized aerosols. Pb-212 does not undergo this recoil-driven redistribution. Even considering recoil following ²¹⁴Po α-decay, the average ²¹⁰Pb-labeled aerosol grows by a factor of two during its atmospheric lifetime. Early morning and afternoon measurements indicated that similar size associations of ²¹⁴Pb occur, despite humidity differences and the potential for fresh particle production in the afternoon. In estimating lifetimes of radon decay product aerosols in ambient air, a mean residence time of about 8 days could be applied to aerosol particles in the lower atmosphere below precipitation cloud levels.

Keywords: Radon; Radioactive aerosols; 1-ACFM; AMAD; Residence time; Decay products; 214Pb; 212Pb; 210Pb; Cascade impactors