i-54cced2edcf95f4db81bcbfc1c403d92-dice.jpgThere’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.

Comments

  1. #1 Uncle Al
    April 21, 2008

    Greenhouse gases are baaad! Water vapor doesn’t count. Carbon dioxide vapor only counts if the First World (China excepted) emits it. Chlorofluorocarbons are baaad because they chill beer and hole ozone. Hydrochorofluorocarbons are OK because – even though orders of magnetude stronger IR absorbers than HCFCs and in open spectral windows – their patents are in force and they corrode refigeration systems.

    SF6 is baaad because it wonderfully insulates high tension electrics and is remarkably harmless. SF5(CF3) is super baaad because at parts-per-trillion its dipole moment makes it a premier IR absorber. It is sooo baaad large poundages are injected as ocean tracer to replace SF6 with its nuisance background from heavy injection.

    What could be better? I(CF3)7 would have MW = 609.95, 21X as dense as air. IR absorber, ozone holer, and fluctional structure (spectrally model that, Pilgrim). If you care, if you want it in the air, if unknown hazards really ring your chimes… make it! IF7 + F3C-CF3(xs) in cold plasma discharge sounds plausible.

  2. #2 Clay B
    April 21, 2008

    A surprising thing I learned on a visit to the Savannah River National Lab is that they have a room made out of pre-1945 steel to do tests in. Because after that the atmosphere (and therefore any newly cast metals) had trace amounts of various radioisotopes that weren’t there before.

  3. #3 CCPhysicist
    April 21, 2008

    One of the trace materials in the atmosphere is the isotope Cs-137. With a half-life of 30 years, there is still plenty of it left over from atmospheric weapons tests even though the tests ended in 1962. So much, in fact, that the release from Chernobyl did not stand out by the time the radiation cloud got to the US. Only short-lived isotopes were obvious as a signature of the plume.

    The lead bricks used for shielding in the lab where I did my graduate work were all originally ballast from a submarine built during WW II. The lead was all refined before any atmospheric testing. There was even a big stack of corroded ingots (still looked fresh from the bilge) in a back hallway that were extras. They only squared up the ones that were actually needed at the time.

    The sensitivity of nuclear detectors makes this essential. You can count each single gamma-ray photon, so every contaminant matters.

  4. #4 Janne
    April 21, 2008

    How much does the composition of the major components vary, though? For instance, we say oxygen is about 21% of the air on average, but how high is it in a stuffy, unventilated lecture room during the third consecutive hour of a seminar, when half the audience is happily asleep? What’s the proportion in a greenhouse?

  5. #5 Neil
    May 8, 2008

    The vast majority of the atmosphere is nitrogen– about 78%, or 780,000 out of every million molecules.

    Aren’t these proportions (78% Nitrogen, 21% Oxygen) based on mass?

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