Rain and radioactivity: rain-out or wash-out?

In this comment I will continue to comment some of the problems explaining what really happens (“radiationally” speaking) during short-lived precipitation events. Some very good papers will be shortly discussed, and I thank Marcel Severijnen for his really important comments.

1. A phenomenon known since long times ago

That atmospheric radiation peaks during precipitation events is known for many, many years. As an example, Thomas Thomson from the Swedish Meteorological and Hydrological Institute, Stockholm, published in 1962 a paper titled “Some observations of variations of the natural background radiation” (link) . He speaks of a 5% to 20% increase in background radiation which is correlated to precipitation. The following figure 2 from this paper (with my additions) documents this (note that 1 micro-roentgen per hour equals 10 nSv/h):

Thomas_fig2

From the duration of the gamma rays increase he concludes that the cause must be short-lived decay products from radon (with a half-live T1/2 less than 1 hour) and he suggests a specific activity in the rain-water between 10^-12 and 10^-10 curie/g, which corresponds to 37 to 1000 Bq/liter (Bq/kg). He also notes that this activity decreases with precipitation-rate, i.e. the higher the precipitation rate (in mm/h for instance), the lower the specific activity in the rain water. This is a sensible conclusion: when the rain falls through a slab of air loaded with radioactive elements, a fast fall-through will scavenge less particles (per rain drop for instance) than a slower one.

Let us conclude with figure 7, where Thomas calculates by linear regression the radioactive doses given by precipitation events: using SI units (yellow boxes) he finds, according to some months, a number between 50 and 100 nSv per mm of rainfall. Here in Luxembourg we are close to about 800 mm per year, which would correspond to an incredible high natural background dose of 40 to 80 mSv if this dose would be completely absorbed by a body (the usual assumptions of yearly natural doses are about 3 mSv).

Thomas_fig7

2. The Livesay paper

Marcel mentioned in his comment to the previous blog the Livesay paper by  R.J. Livesay et al. from the Oak Ridge National Laboratory, published in 2014 and titled “Rain-Induced Increase in Background Radiation Detected by Radiation portable Monitors” (link). This is a really interesting and very readable paper (with some not too difficult maths) that studies the increase in gamma counts given by portable radiation detectors installed at many places (for instance in Luxembourg at the entrances of trucks or train wagons delivering scrap metal to our steel founderies). Using gamma-ray spectroscopy they clearly show that the elements causing the radiation peaks during rain-fall are mainly the two radon daughters Pb214 and Bi214, with half-lives of 27 and 20 minutes (the Rn222 half-live is 3.8 days) and energies of approx. 352 and 609 keV, and which are deposited on the ground:

Livesay_fig5

The paper has a very nice record of a short rain pulse and the following decay of the gamma counts (fig.8 a&b, with my additions):

Livesay_fig8ab Clearly the time back to normal is about 3 hours, which corresponds to approx. 6 periods of the short-lived radon progeny.

The authors give an easy to understand mathematical model, which renders in a nearly perfect manner the observations:

Livesay_fig8dConcerning the specific activity in the rainwater, the authors do not give any number, but write that this parameters varies heavily with the rain event and the location, and needs further study.

3. The Fujinami paper: rain-out or wash-out?

I will conclude the scientific literature with a paper published by Naoto Fujinami in the Japanese Journal of Health Physics in 2009, titled “Study of Radon Progeny Distribution and Radiation Dose-Rate in the Atmosphere” (link). This paper answers the question given in the title of this blog: does the increase in gamma-activity come from rain-out or a wash-out? Rain-out means that the radon progeny is attached to the rain drops inside the rain delivering cloud, and wash-out means that the main process happens during the fall of the rain drops through the air volume below the clouds. The paper is a bit confusing, as a first (too)  rapid lecture gives the wrong impression that the authors concludes that gamma activity decreases with rain-fall. In fact this is not the case, as the author writes in chapter III (“Scavenging of radon progeny by precipitation in the atmosphere”) that “The radon progeny in precipitation produce an increase in adsorbed dose rate in air at ground level“; a remark that in my opinion should have been made much earlier in the paper. Fujinami reports that there exists an inverse relationship between the concentration of radon progeny (in rain water) and the precipitation rate, an observation reported by T. Thomson in his 1966 paper. What makes the Fujinami paper interesting, is that he tries to demonstrate that rain-out and not wash-out is the important cause of additional gamma activity.
In a first point of his study, the author shows a plot of the radon progeny concentration in surface air (implied by the measured gamma-activity) and precipitation, and he concludes that periods of precipitation lower the radon progeny concentration in surface air:

 

fujinami_fig5 Now I have some problem with this plot (I added the colored lines and text box): the time-resolution seems very large, about 8 hours as shown by the blue lines. So this figure says nothing about the immediate changes in ambient radioactivity in surface air after a rain fall, but possibly more on the coarse general evolution. And even here I have problems of understanding: the two first events (and possibly the last)  clearly show a drop in radioactivity prior to the rain fall. So this point damps seriously my first enthusiasm for the the Fujinami paper.

Let me show you a similar plot made today (05 Aug 2016) at Diekirch and covering the last 7 days:

rain_radioactivity_Diekirch_Aug2016
First one should note the visible daily rhythm of gamma activity, with higher radioactivity in the morning hours when the usual inversion blocks the mixing of ground air with higher levels of the atmosphere; the same situation happens for instance with our CO2 data . The blue arrows point to the main rain events: the first two arrows show that the radiation peak follows the rain pulse; the next three rain events do not have any visible influence, as the ambient radioactivity has come down to the normal background, and the clouds and air above ground seem depleted of additional radon progeny to be scavenged. So in my opinion, lower levels following a rain event seem more a return to normal than an effect caused by precipitation.

All other hints to the Fujinami conclusion that the phenomenon observed is a rain-out happening in the clouds and not a wash-out during the free fall of the rain drops to the air below the clouds rely on the same Japanese data series  from Maizuru (1977-1985). I am absolutely not convinced by the author’s argumentation, and would appreciate stronger logic and more fine-grained data before accepting his conclusion.

4. Do the Diekirch data show the typical time evolution of Pb214 and Bi214 shown in the Livesay paper?

As the half-lives of the radon progeny Pb214 and Bi214 are 27 and 20 minutes, inspecting the decreasing radioactivity after a very short rain pulse should show a return to normal levels after 2 to 3 hours, as given in the Livesay paper. A problem with our Diekirch data is the rather coarse time step of 30 minutes: the measurements stored in the datalogger at time xx:00 and xx:30 are the average of the preceding 30 minutes (radioactivity) or the total precipitation during  this time interval. Our Davis Vantage Pro Plus backup weatherstation also registers the “rain rate”, which is the maximum precipitation that would have been collected during the measuring interval (30 minutes) following the last bucket tip. So this number gives a bit of information on the brusqueness of the rain pulse, compared to the integral over 30 minutes, but it does not help much more. I will use the rain event of the 23 July following the storm from the previous day. The reason is that this rain-pulse is not followed by a second one which muddles the picture. Here are the numbers:

23Jul2016There clearly is a time lag of about 2 hours between the start of the first rain event and the excess radiation peak; if we set the origin of time at the moment where the activity peaks, we obtain with a rather good R2 = 0.97 a half-live of about 43 minutes, applying a simple decay model. Note that the return to normal interval is about 3 hours.

gammaDK_23Jul2016

Let’s verify the corresponding evolution given in the Livesay paper at figure 4:

Livesay_fig4period

I put the time origin at the second count peak and wrote down the corresponding counts obtained by inspection. The decay model gives this result for the decrease of the excess counts:

Livesay_fig4_model

Here the half-live is approx. 19 minutes, about half of what we found at Diekirch. These 19 minutes are close to the half-life of Pb214, the first of the two relevant radon daughters. Now one should remember that we do observe the combined effect of two radioactive decays, where the first element Pb214 creates the second Bi214. Our simplistic curve fitting exercise has no solid physical basis, but should be taken as a visualization tool.

What remains is that in our data the general evolution of the excess gamma activity caused by a rain event follows a similar albeit slower evolution as that shown in the Lindsay paper. Return to normal background levels takes about 3 hours, in accordance to this paper.

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