The planetary hydrological cycle.
https://www.youtube.com/watch?v=BKorP55Aqvg
Why cant we have parallel lines that intersect. Geometry.
The sources cited includes https://doi.org/10.1016/j.watres.2020.116685, which is paywalled, but the snippet of the conclusion that is shown indicates that a possible major cause is industrial emissions:
> As local sources were determined to be significant, the results imply that local action can have an impact on PFAS contamination in precipitation. A three-way ANOVA model determined that functional group, chain length, and location were significant predictors of PFAS concentrations
If you can get the full text I'd be very interested in reading about it.
https://pubs.acs.org/doi/10.1021/acs.est.2c02765
> n Figure 1B, the levels of PFOS in rainwater are shown to often exceed the US EPA drinking water health advisory for PFOS, except for two studies conducted in remote regions (in Tibet and Antarctica).
I don't think there are a lot of industrial emissions in Antarctica.
https://pubs.acs.org/doi/10.1021/acs.est.2c02765
That paper alone has 61 references. There are plenty of papers to go read.