How does radon affect ozone loss?

Question

How does radon affect ozone loss?

Answer 1

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!

Answer 2

It doesn't that I ever heard of.