Ask a ScienceBlogger: What’s in the Air?

Image may be NSFW.
Clik here to view.There’s a new “Ask a ScienceBlogger” question out:

“A question from a friend’s 9-year old son:

What is in the air we breathe? What is it’s chemical composition?”

The short answer to this is “a little bit of everything.” Pretty much
any substance we have on Earth can be found in the atmosphere
somewhere. The atmosphere is a pretty big place– roughly
10⁴⁴ molecules worth of stuff (that’s
100,000,000,000,000,000,000,000,000,000,000,000,000,000,000, give or
take). In a collection that big, you’ll find just about anything you
want.

All we can really do when asked about the composition of the
atmosphere is make statistical statements. We know how much of various
gases there are in the atmosphere on average, but it can vary quite a
bit from place to place, and moment to moment. On a humid summer day
in New York, there’s a lot of water vapor in the air, but on that same
day in the middle of the Sahara desert, there’s hardly any water at
all.

The composition of the atmosphere is usually given in either
percentages, or “parts per million.” In either case, the answer comes
down to “if you grabbed a million molecules at random out of the air,
and counted how many of different things you had, what would you
find.” The vast majority of the atmosphere is nitrogen– about 78%, or
780,000 out of every million molecules. Oxygen is next, at about 21%,
and a good thing, because we need it to breathe. Then there’s about 1%
argon, and a whole bunch of other things.

Carbon dioxide, that you hear about all the time when people talk
about global warming, is somewhere around 400 parts per million– that
means that in a random sample of one million molecules, you expect to
get about 400 CO₂ molecules. There would be about five
helium molecules in that sample (unless you’ve been filling a lot of
balloons recently), and krypton is a one-in-a-million gas– roughly 1
krypton atom for every 1,000,000 molecules in the atmosphere (unless
you’re in my lab, where I use krypton in my research, and the
concentration is probably a little higher).

These are all really guesses, though. Nobody is ever going to count
all of the molecules in a room, let alone the entire atmosphere, and
tell you exactly what’s in the air you breathe– there’s just too much
of it. All we can do is put limits on what’s likely to be there, based
on lots of measurements at different places at different times. We
think that there will be about 10,000 molecules of water vapor in
every million molecules that you breathe, but it could be 40,000 on a
really humid day, or it could be 5,000 on a dry desert afternoon.

There’s no way to know exactly, but we know in a general way, and
that’s good enough for most practical purposes.