I always have a hard time with the use of on and upon. I have looked at the general comments made in this question, but have the following specific use case in mind that does not seem covered to me: in the sentence we study the influence of anion size on/upon localisation, is one more common than the other?
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As pointed out in the answers to the question you posted, the difference lies in the usage, not the meaning. "On" and "Upon" are both correct and would both be understood in the context you present.
The key difference is whether you want your sentence to feel more old-fashioned (and by extension, more formal). In the context of writing a paper about negatively charged particles, one might want to use "on" to prevent said paper from feeling too outdated.
The choice between on and upon in the sentence
is not merely a matter of style. Two entirely different meanings are possible.