Colourless gas which forms a layer within the Earth's atmosphere (5)

I believe the answer is:

ozone

(Other definitions for ozone that I've seen before include "By sea, you could take in this" , "Gas formed from oxygen" , "Invigorating air of the seaside" , "Toxic form of oxygen" , "Sea air (found in layer?)" .)

I've seen this clue in The Mirror.

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