Effectiveness of airborne radon progeny assessment for atmospheric studies

In this paper, measurements of short- and long-lived Radon progeny attached to atmospheric fine aerosols are reported.Hourly measurements of ²²²Rn short-lived decay products (i.e. ²¹⁴Bi via on-line alpha spectrometry on ²¹⁴Po) in the atmosphere were carried out in Milan (Italy) from 1999 to 2016; ²¹⁴Bi 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 ²¹⁴Bi 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 ²¹⁰Pb concentrations were determined via off-line alpha spectrometry on ²¹⁰Po; atmospheric activity concentration values ranged between 0.13 and 3.05 mBq m⁻³. The seasonal behaviour of ²¹⁰Pb concentrations followed fairly well the ²¹⁴Bi temporal pattern, showing that mixing layer dynamics is paramount in determining short- and long-lived Radon progeny levels in the atmosphere.From ²¹⁰Pb/²¹⁴Bi activity ratio, the residence time τᵣₑₛ 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 τᵣₑₛ 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 ²¹⁰Pb/²¹⁴Bi (or equivalently ²¹⁰Pb/²²²Rn) couple provides reliable estimates.