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.
Highlights
The decay of radon, 222Rn and thoron, 220Rn in the atmosphere produces low vapor pressure decay products that coagulate with other nuclei or condense on existing aerosol particles
This paper summarizes results of an investigation designed to characterize the aerodynamic size distributions and the residence time of atmospheric aerosols in the context of 222Rn- and 220Rndecay products for better understanding of how radioactive aerosols behave in the atmosphere, and of the aerosol growth mechanism
Measurements on aerodynamic sizes of atmospheric aerosols associated with radioactive nuclides, such as radon decay products 214Pb, 212Pb and 210Pb were carried with Anderson type 1-ACFM ambient cascade impactors with or without the Andersen model 2000 low-pressure modification, as well as with Sierra model 236, six-stage high-volume cascade impactors, HVI
Summary
The decay of radon, 222Rn and thoron, 220Rn in the atmosphere produces low vapor pressure decay products that coagulate with other nuclei or condense on existing aerosol particles. These decay products include 218Po (3.0 min), 214Pb (26.8 min) and 212Pb (10.64 h). Several methods have been used for estimating the mean residence time of atmospheric aerosol particles These include measurements of the activities and ratios of radioactive decay products of radon, 222Rn which emanate from continental surfaces into the atmosphere, such as 210Pb (22.3 y), 210Bi (5.01 d) and 210Po (138.38 d) (Francis et al, 1970; Poet et al, 1972; Lambert et al, 1980; 1983; Marley et al, 2000, Baskaran and Shaw, 2001). There is disagreement between the Papastefanou, Aerosol and Air Quality Research, 9: 385-393, 2009 derived values of the residence times due to various processes, including the fact that they refer to different portions of the atmosphere
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