QUESTION:
Why you are interested in Radon gas in the atmosphere?  We know it is
dangerous if it is trapped in our houses, but what effect does it have in
the atmosphere?
ANSWER from Mark Kritz on June 9:
Thank you for your questions, and for your interest in our work on NASA's
Kuiper Airborne Observatory (KAO).  Right now we are in the midst of a series
of flights on the KAO and this is a very busy time for my group, so I'll have
to answer your questions in several installments.
Your first question asks about why we are studying radon.  I will
answer this with a quick review of where radon comes from (which you already
know!) and what happens to it once it leaves the soil and gets into the
atmosphere.
The radon measurements we are making on the Kuiper Airborne
Observatory are being used for a brand new and very unusual purpose.
That purpose is the testing of large computer models which are to
predict acid rain, climate change (for example, the greenhouse
effect and global warming) and the loss of ozone.  To understand how
this is done you have to know something about radon, and something
about these models and how they work.  Let's start with radon.
You may know that radon comes from the ground where it is
constantly produced by the very small quantities of trace elements
found in all soils and rocks.  There are three things about radon that
make it very interesting for my work.  The first thing is that radon
is not very soluble in water, so that once it gets into the atmosphere
it is not removed by being washed out by rain or snow, or by being
absorbed into the ocean.  The second thing is that radon is an inert
gas, like helium or neon.  That means that radon is not removed from
the atmosphere by chemical reactions...such as the kind of chemical
reactions that remove pollutants such as sulfur dioxide.  Now these
two facts lead to an interesting question.  Can you answer this?  If
radon is constantly entering the atmosphere, but it is not removed
by being absorbed by rain or snow, or by chemical reaction, than why
doesn't the atmosphere fill up with radon? (Hint: it doesn't escape
out of the top either!)
The reason is that radon is removed by its own natural radioactive
decay.  Of course this radioactive decay can be a health problem if
the concentration gets too high.  This doesn't happen in the open air,
but it can happen in an unventilated cellar.  This is why people
sometimes put fans in their cellars, to move the radon that gets into
the cellar from the ground out into the open air.  But radioactive
decay is an interesting process.  While radon is a noble gas, and not
so subject to chemical reactions, it does have a sort of built-in
instability, so that after a few days a typical radon atom "self-
destructs", or decays.  The average length of time that this takes to
happen is called a "half-life", and the half-life of radon is just a
little less than 4 days (The exact figure is 3.825 days!)  What this
means is that if you put 1000 atoms of radon in a jar, and waited
3.825 days, very close to half of those 1000 atoms will have
decayed away and only about 500 will be left.  But then if you wait
another 3.825 days, half of those 500 atoms will have decayed away,
leaving about 250!  And so on.  This decay is a statistical process, so
we can't say that in advance which of the 1000 or 500 radon atoms
will decay away in the next 3.825 days---but we can say that half of
those present at the start will decay in that time!
So that is what happens to radon that gets into the air from the
ground.  Imagine 1000 atoms of radon entering the atmosphere right
now, from the soil across the street from school.  Four days from
now about half of those atoms will have decayed away, and so will
no longer be in the atmosphere.  And if we wait another four days,
only about half of that, or 250 atoms will be left.
But something does happen to the radon in the atmosphere while it is
"waiting" to decay.  It isn't removed by rain or snow, or by chemical
reaction, or being absorbed into the ocean, or by escaping out of the
top of the atmosphere. Can you tell me what does happen?
What happens is that the radon gets blown around by the wind.
Which is a good thing, because if it stayed near the surface its
concentration there, where we live, would be very high and this
would be very unhealthy!  But radon moves with the wind.  And the
wind can move very far and very fast.  For example, in our flights on
the Kuiper Observatory we sometimes find radon high in the
atmosphere, eight miles above California, which has blown there
So there are three things going on here: radon gets into the
atmosphere from the ground; it is blown around by the wind; and it
finally is removed by radioactive decay.
But what does this behavior of radon have to do with the models that
are used to predict climate change or ozone loss?  The answer is
that while those models are very, very complicated, at the bottom of
it all they do two things.  The first thing these models do is try to
predict how the wind blows around, or distributes, chemically
reactive pollutants, like sulfur dioxide.  The second thing the models
do is try and calculate the chemical reaction of those pollutants.
Both of these things are very hard to do, and the models that are
used to do this are very complicated.  But how can we be sure that
those models are giving us the right answer?
Well, here, at last, is where the radon comes in.  We know where the
radon comes from, and how fast it is getting into the atmosphere.
We also know that radon isn't removed from the atmosphere by rain
or snow or chemical reaction.  What does happen is that it gets
blown around by the wind in exactly the same way as the chemically
reactive pollutants that people are interested in.
These facts let us do an interesting and important experiment.
Suppose we run one of these big models, only with radon rather than
chemical pollutants.  All we have to do is put in how fast the radon
is getting in from the ground - which we know - and how fast it is
being removed by radioactive decay - which we also know.  These
things aren't hard to do, and so once we have put them into the model
we can turn the model on and look at the way it says the radon
"ought" to be moving around, and distributed in the atmosphere.  But
how do we know the model is right?
This is the next step in the process - we can test the model by
comparing the distribution of radon that it predicts with the actual
distribution of radon that I measure on the Kuiper Airborne
Observatory.  If we get agreement, then we can have some
confidence in how well the models can predict how the chemically
reactive gases - the pollutants - are moving around the atmosphere.
This is important because if we can be sure that the models are
handling this part of the problem correctly, then the modelers can
focus their attention on the second half of their problem - which is
making sure that they are treating the chemical reactions correctly.
But they can't really that until they are sure that the models are
doing a good job with the first part!
And that is why I make all those radon measurements on the NASA
Kuiper Airborne Observatory - to help improve the models used to
predict things like climate change, so that we can be more confident
that the predictions of these models are correct!
That is all I have time for before your vacation starts.  I hope this
wasn't too complicated, and that you have a wonderful summer.
Cordially,
Mark "Captain Radon" Kritz