Ambient air radioactivity peaks due to radon washout?

The last 7 days from 01 to 07 August 2013 give a nice example of the increase of ambient air radioactivity with precipitation. During these days we had at Diekirch 3 very short precipitation events, which leave a visible fingerprint in the data series of the gamma Geiger counter:

We see that the short precipitation events (in blue on the top plot) correlate perfectly with activity peaks (in red on the bottom plot). The first event in the night of the 03 August has 3.4mm precipitation and an increase of radioactivity by about 19 nSv/h, to be compared to the “normal” increase for this period of the day of 3 nSv/h. So this rainfall event produced  approx. a 500% higher radioactivity level. The 3 events are too few to look for a relationship; but obviously the greatest precipitation event also produced the highest radioactivity rise.

This phenomenon is well-known and usually attributed to radon washout, with radon daughters like the gamma emitter Pb214 gathering on the instrument in the wet rainwater film. After evaporation has removed this wet cover, levels return to normal.

I found some discussions on this effect here:

http://onlinelibrary.wiley.com/doi/10.1111/j.2153-3490.1963.tb01393.x/pdf

http://www.irpa.net/irpa10/cdrom/00586.pdf

http://www.atmos-chem-phys.net/6/2865/2006/acp-6-2865-2006.pdf
If you have more to say on this, thank you for a comment.

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08 Aug 13:  corrected units typing error: activity is nanoSv/h (nSv/h), not microSv/h (uSv/h)

10 Aug 13:  Two readers sent me interesting comments (thank you!), and I will summarize these here with their tacit authorization:

Antoine KIES is a friend, emeritus professor of physics of the University of Luxembourg (Laboratoire Physique des Radiations), and a well known radon and ambient radioactivity  specialist. He says this (translated from Luxembourgish): “This is a normal and well-known situation, as a short rain fall deposits the daughter products (as polonium, lead, bismuth) of radon which are attached to aerosols. As the periods of these radioactive daughters are short, the increase in measured gamma activity is a short peak. Precipitation can block the out gassing of radon from the soil, so usually one observes a minimum of activity when the soil is wet. The diurnal variation of ambient air activity is mostly caused by radon.”

Marcel Severijnen is a former head of the Environmental Monitoring Department of the Province of Limburg in the Netherlands and also has a climate blog. He writes (translated from German): “Twenty years ago I had in my room a terminal from the BMNI (Binnenlandse Zaken Meetnet Nucleaire Incendenten) which is a network with over 300 measuring stations (now integrated into the LMR (Landelijk Meetnet Radioactiviteit, see http://www.rivm.nl/Onderwerpen/N/Nationaal_Meetnet_Radioactiviteit/Resultaten). The peaks during heavy rain showers were always very visible, and represented something like a precipitation radar. Today values below 200 nSv/h are considered normal. Above this limit the RVIM makes an analysis of the situation and when 2000 nSv/h is reached, the local fire brigades are set on alert…”

The link to the Dutch network is very interesting. Luxembourg also has many “official” measuring stations (see here) but no website with real-time data (except our “unofficial” meteoLCD). Nevertheless monthly reports are available here.