Aerodynamic Size Associations of 212Pb and 214Pb in Ambient Aerosols

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Abstract
The aerodynamic size distributions of short-lived Rn daughters (reported as 214Pb and 212Pb) in ambient aerosol particles were measured using low-pressure as well as conventional low-volume and high-volume impactors. The activity distribution of 214Pb and 212Pb, measured by .alpha. spectroscopy, was largely associated with submicron aerosols in the accumulation mode (0.08 to 2 .mu.m). The activity median aerodynamic diameter ranged from 0.09 to 0.37 .mu.m (mean 0.16 .mu.m) for 214Pb and from 0.07 to 0.25 .mu.m (mean 0.13 .mu.m) for 212Pb. The mean values of the geometric standard deviation (.sigma.g) were 2.97 and 2.86, respectively. By comparison, the median diameters of cosmogenic 7Be and ambient SO42- were about 0.24 .mu.m higher. In almost 70% of the low-pressure impactor measurements, the activity distribution of 214Pb showed a small shifts to larger particle sizes relative to 212Pb. This shift probably results from .alpha.-recoil detachment of parent 218Po, which preferentially depletes 214Pb from smaller particles. The subsequent recondensation of 214Pb causes an enrichment of larger aerosols. 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. Healthy physics implications of the results are also discussed.