资源环境科技发展态势分析平台

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 τ_res 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 τ_res 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.