Why atmospheric air on earth contains approximately 79% N2 and 21% O2 by volume ?

Why atmospheric air on earth contains approximately 79% N2 and 21% O2 by volume ?

We understand that there exist O2 cycle , that is human beings consume O2 and emit CO2. Where as trees split CO2 to C and O2.

Similarly there exist N2 cycle in the nature.We eat proteins and discharge urine.Plantation absorb urine and produce proteins.

But how the ratio of N2 to O2 is maintained automatically ?

Does this ratio has changed over a period of time ?

When man started polluting air and started producing NOX , SOX etc,
there must be some canges in the above ratios.

Further does this ratio remains same beyond atmospheric layer ?

Is it that gravitational force and speed of the earth responsible for maintaining this ratio?

Please comment.

P.J.LAKHAPATE
plakhapate@rediffmail.com

 

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The O2 limit is probably set by the behavior of fire, ie forest fires and the like. If the O2 partial pressure were any higher, even wet green plant matter would burn out of control. The N2 is probably an artifact of the primordial NH3 that oxidized to N2 and H20 as algae created the O2 atmosphere.

 

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The ratio of gases has changed over time, as determined by organic processes, which is in turn controlled by evolution.

Although this is a subject of debate, it is believed that the early atmosphere had an abundance of ammonia and hydrogen. Oxygen was scarce until there was enough carbon dioxide to support photosynthesis.

 

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I thought the early atmosphere was a reducing atmosphere, with a very high CO[sub]2[/sub] content from volcanic activity. The early ocean was high in dissolved iron. The appearance of blue-green algae led to the production of oxygen. However, this influx of oxygen did not make its way to the atmosphere at first because of all that dissolved iron. Once that dissolved iron rusted, the oxygen level in the atmosphere began to rise.

 

Much better. I stand corrected.

 

Hey, anyone remember what the range is regarding oxygen that is non-lethal for humans? I seem to recall that it is a very narrow range. As in a percent or two up or down from normal is lethal.

 

Here's a link.

Neither particularly narrow nor particularly broad. They say .2 to .5 bars for indefinite exposure.

 

So 20-50%? That's interesting. 50% sounds like a lot to me.

 

What I seem to remember is that if a human breathes an oxygen mix that is below 19% oxygen or above 23% oxygen (or thereabouts), he will die. Anyone know the exact numbers?

 

I believe all that matters is the partial pressure of the oxygen. The actual % composition doesn't matter.

 

I hope the patients with oxygen mask are safe

 

That's a good thought, and I don't have a good answer for that.

 

Aren't the two interchangeable?

 

Partial pressure and percentage composition are not interchangeable unless you are talking about a constant total atmospheric pressure. 3 pounds oxygen plus 12 pounds nitrogen makes 15 pounds and 20 percent oxygen. 3 pounds oxygen plus 120 pounds nitrogen makes 123 pounds total pressure, almost 10 atmospheres, about 2.5 percent oxygen.

 

Anyone know the lethal partial pressure of oxygen? I know that astronauts in mercury, gemini, and apollo had 100% oxygen air at 5 psi, or 5 psi oxygen partial pressure, and earth around 3 psi oxygen partial pressure as metakron shows.

 

I believe the partial pressure range is quite large. Hyperbaric chambers are used for medical purposes. Mostly, I think that it is a more "chonic exposure" situation. You could probably tolerate an atm of pure O2 for some time, like hours or even days. But at some point, maybe days or weeks, you would be in trouble.