Effectiveness Of Airborne Radon Progeny Assessment For Atmospheric Studies

In this paper, measurements of shortand long-lived Radon progeny attached to atmospheric fine aerosols are reported.Hourly measurements of Rn-222 short-lived decay products (i.e. Bi-214 via on-line alpha spectrometry on Po-214) in the atmosphere were carried out in Milan (Italy) from 1999 to 2016; Bi-214 mean concentrations ranged from 0.2 to 38.1 Bq m(3). It is noteworthy that minima occurred in springtime although the strongest convective turbulence can be expected in summer, when the highest solar radiation is available; one order of magnitude higher values were observed in winter when the Po valley experienced poor atmospheric dilution. The Theil-Sen method was applied to investigate the long-time trend in de-seasonalised data series. Results showed that although inter/intra-annual variations in Bi-214 concentrations were observed in connection with differences in atmospheric dispersion conditions - no statistically significant trend over the investigated period was detected.On a sub-set of these samples, also weekly Pb-210 concentrations were determined via off-line alpha spectrometry on Po-210; atmospheric activity concentration values ranged between 0.13 and 3.05 mBq m(3). The seasonal behaviour of Pb-210 concentrations followed fairly well the Bi-214 temporal pattern, showing that mixing layer dynamics is paramount in determining shortand long-lived Radon progeny levels in the atmosphere.From Pb-210/Bi-214 activity ratio, the residence time Tres of fine aerosols in the atmosphere was estimated to be on average 1 day, ranging from 11.0 to 55.3 h without any evident temporal trend. By exploiting the availability of mixing layer height data at our site, an alternative approach to estimate aerosol residence time was tested; this was based on a simple relationship relying on deposition velocity (from literature data) and mixing layer height (available at our monitoring station). Mean experimental Tres resulted in 1.2 days which was comprised in the 0.6-2.0 days range estimated by the alternative method. This result brings a noteworthy contribution to the scientific debate about differences among aerosol residence time estimates obtained by different radioactive parent-daughter couple; our results show that the Pb-210/Bi-214 (or equivalently Pb-210/Rn-222) couple provides reliable estimates.