Why the Ozone layer is 15-30km above the earth surface despite the fact that O3 is heavier than O2?
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Because O3 is mostly created by UV rays hitting O2 in the high athmosphere. Closer to the surface most UV is already shielded off
Follow-up question people might be asking: Even if ozone forms only in the upper atmosphere from UV rays hitting oxygen, why doesn’t it sink right away, being heavier than the rest of the air?
Answer: Same reason our air contains a mix of oxygen and nitrogen and carbon dioxide despite these gases being of different densities: turbulent mixing. Gas molecules bounce off each other in a chaotic way, and this makes the slightly different gravitational pull almost irrelevant.
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Ozone is created in the upper atmosphere. Once created it lasts between 30 minutes and 4 hours. That’s not enough time for gravity or turbulence to bring it into the lower atmosphere.
If you’ve heard about ozone holes:
1. An “Ozone hole” isn’t really a hole. It’s just thinner (sometimes a lot thinner. The arctic ozone hole has only a third of the normal levels of ozone).
2. Ozone depletion is mainly due to gasses containing chlorine rising up into the stratosphere, where they too react with UV, and release chlorine. Chlorine then reacts with Ozone (O3) and breaks up lots and lots of Ozone molecules into Oxygen molecules (O2). These gasses containing chlorine can muck about in the stratosphere for a long time (decades). Once these gasses are there the average lifetime of an ozone molecule drops significantly, and so there is less ozone at any one time (which drops the chance of a UV ray hitting an ozone molecule).
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