This 2014 paper looked into possibility of forming perchlorates in the atmopshere by photochemical processes - but though they could make sulfates and nitrates they couldn't make anything like enough of the nitrates in that way.
The formation of sulfate, nitrate and perchlorate salts in the martian atmosphere
They suggest that maybe it's due to processes that happen on solid / air interfaces.
This is another 2014 paper, experimental work that suggests it could happen through UV radiation of chloride salts on the surface.
FORMATION OF PERCHLORATE FROM CHLORINE SPECIES UNDER SIMULATED MARS CONDITIONS.
So, I think that basically it is still work in progress to try to figure this out. But the research seems to suggest, probably not a purely atmospheric process.
The formation of sulfate, nitrate and perchlorate salts in the martian atmosphere
They suggest that maybe it's due to processes that happen on solid / air interfaces.
This is another 2014 paper, experimental work that suggests it could happen through UV radiation of chloride salts on the surface.
FORMATION OF PERCHLORATE FROM CHLORINE SPECIES UNDER SIMULATED MARS CONDITIONS.
So, I think that basically it is still work in progress to try to figure this out. But the research seems to suggest, probably not a purely atmospheric process.
About the Author
Robert Walker
Writer of articles on Mars and Space issues - Software Developer of Tune Smithy, Bounce Metronome etc.Studied at Wolfson College, Oxford
Lives in Isle of Mull
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