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So the air pressure on earth remains relatively constant, right?

Is there air gained or lost through transitions of any sort? e.g. plumes going out into space, earth gaining new air particles with gravity from air in the solar system.

I was curious to know whether in general the particles that brushed up against the dinosaurs, and against Homo erectus's face, are the same as the wind today, ebbing and flowing as pressure variation directs it. Is this true? :)

I suppose air goes into trees, and into mammals, and recycles around. It seems like the recyclability rate must be very very high.

Is this logic correct? It's all very fascinating!

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Fun fact: Due to Quantum Physics, there is actually no way to distinguish identical particles, e.g. the oxygen molecules that touched dinosaurs and those that didn't. –  Tobias Kienzler Sep 10 '13 at 9:11
    
That's neat Tobias! Could you reference something? Would love a read –  Baconbeastnz Sep 10 '13 at 10:29
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(note that you should write @Username in your responses in general when it is not obvious whom you're replying to, otherwise they may not get notified of your comment and falsely appear to ignore you) Phew, it's a big field, but as a shameless plug here's a link to my answer on "How are forces “mediated”?", where I included a bunch of Wikipedia links for further reading. I don't know your background so I cannot tell whether that's too boring or too vague for you though... –  Tobias Kienzler Sep 10 '13 at 10:55
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3 Answers 3

up vote 28 down vote accepted

The atmosphere of the Earth is mainly composed of nitrogen (N2, 78%) and oxygen (O2, 21%) molecules, which together make up about 99% of its total volume. The remaining 1% contains all sorts of other stuff like argon, water and carbon dioxide, but let's ignore those for now.

As you probably know, the oxygen we breathe is produced by plants from water and carbon dioxide as a byproduct of photosynthesis. Conversely, animals (including humans) use the oxygen to burn organic compounds (like sugars, fats and proteins) back into water and carbon dioxide, obtaining energy in the process. So do many bacteria and fungi, too, and some oxygen also gets burned in abiotic processes like wildfires and the oxidization of minerals.

The result is that oxygen cycles pretty rapidly in and out of the atmosphere. According to the Wikipedia article I just linked to, the average time an oxygen molecule spends in the atmosphere before being burned or otherwise removed from the air is around 4,500 years. The most recent known Homo erectus fossil dates from about 143,000 years ago, so the probability that a particular oxygen molecule hitting your face today has been around since that time is roughly $\exp(- 143000 / 4500) = \exp(-31.78) \approx 1.58 \times 10^{-14}$, i.e. basically zero.

Of course, the oxygen atoms used for respiration don't disappear anywhere: they just become part of the water and carbon dioxide molecules. Those that end up in carbon dioxide usually get photosynthesized back into free oxygen pretty soon, unless they happen to get trapped in a carbonate sediment or something like that. The oxygen atoms that end up in water, on the other hand, may spend quite a long time in the oceans before being recycled back into the air; if I'm not mistaken, the total amount of oxygen in the hydrosphere is about 1000 times the amount in the atmosphere, so the mean cycle time should also be about 1000 times longer. Still, eventually, even the oxygen in the oceans gets cycled back into the atmosphere. Thus, while the oxygen molecules you breathe might not have been around for more than a few thousand years, the atoms they consist of have been around since long before the dinosaurs.


How about nitrogen, then? Perhaps a bit surprisingly, given how inert nitrogen generally is, it's also actively cycled by the biosphere. Unfortunately, the actual rate at which this cycling occurs seems to be still poorly understood, which makes estimating the mean cycle times difficult.

If I'm reading these tables correctly, the annual (natural) nitrogen flux in and out of the atmosphere is estimated to be somewhere between 40 and 400 teragrams per year, while the total atmospheric nitrogen content is about 4 zettagrams.

This would put the mean lifetime of a nitrogen molecule in the atmosphere somewhere between 10 million and 100 million years, well above the time since Homo erectus first appeared (about 1.8 million years ago). Thus, it seems that most of the air molecules around you have probably been around since the days of Homo erectus, and some of them might even have been present during the age of the dinosaurs, which ended about 66 million years ago.

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Excellent answer. Slightly off topic, I have read that on average, every other nitrogen atom in your body has been through the Haber process. –  Marty Green Sep 9 '13 at 12:10
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"so the probability that a particular oxygen molecule hitting your face is [..] $\approx 1.58 \times 10^{-14}$, ie. basically 0" - Except that the number of molocules that hit your face in a second is somewhere on the order of $10^{23}$, which means that the probability that one of these atoms existed that long ago is $1-(1-1.58 \times 10^{-14})^{10^{23}} \approx 99.99999999999...\%$ –  BlueRaja - Danny Pflughoeft Sep 9 '13 at 16:35
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@user80551: It was a guesstimate. But even using, say, 10^18 molecules, pari/gp tells me there are still around 6800 $9$'s after the decimal. –  BlueRaja - Danny Pflughoeft Sep 9 '13 at 18:29
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One question you didn't discuss in your answer is whether Earth has gained or lost significant amounts of air in space. Can you please add information about that? –  Kevin Sep 9 '13 at 19:22
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Actually, I'll ask that separately. –  Emilio Pisanty Sep 10 '13 at 2:38
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For some atmospheric gases, this is very unlikely, but for some it is possible to a limited extent. Each gas has its own residence time, which is the average time that

every substance released into the atmosphere is eventually removed, so that we have a cycle of elements.

Also known as biogeochemical cycles, from "Atmospheric Composition, Global Cycles and Lifetimes".

These can range from hours and days for air pollution species to 100's days for ozone and well over a million years for nitrogen, according to this table from Hobbs - so in this case, the same nitrogen gas we breath and brush against our face may have indeed made contact with our distant relative.

An old, yet still relevant article is "Residence time and spatial variability for gases in the atmosphere" (Hamrud, 1983).

I hope this helps.

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I think the other posters may have forgotten something.

The Earth’s atmosphere is formed and replenished from volcanic emissions of water vapor, etc.

And that’s good, because it’s constantly being depleted by leaking away into Space, from friction with the Solar Wind.

I believe the calculation was that if the atmosphere wasn’t being constantly replenished, Earth would be uninhabitable in about 20 Million years.

So, on your 2/3rds Billion timescale, I’d say the atmosphere has been replaced multiple times since then.

On the Human timescale, the air you breathe blew in Caesar’s face while Rome burned.

On the geological scale, only a trace of that original atmosphere still exists in gaseous form.

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No, the flow into the earth from the solar wind is very small. The cycles described in the other answers are between the solid/liquid earth and the atmosphere. The leakage into space is also very small. –  Ross Millikan May 22 at 17:18
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protected by Qmechanic May 22 at 17:58

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