Have you noticed how rain is dirtier if it hasn't rained in awhile?
It could probably work both ways though, so depending on the environment some might start lower and then get higher as it rains depending on what's in the air.
I would assume the closer to the city the lower the pH would start out. And if you're out in the country I would assume it's not as low to start. Might even be higher than 7 depending on wind and dust in the air.
I am just guessing though. But it kinda makes sense to my stoner brain,
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