Jeff
When a body of water evaporates, it essentially changes form from a liquid into a vapour or gas. Water can change its form into vapour but absorbed minerals cannot and so these are left behind. This means that the water vapour that forms clouds is pure water.
When this water cools or condenses, it falls as rain. As the pure water falls it starts to absorb air-borne particles and as it does so, the water TDS reading rises. That is why the air seems cleaner and fresher after a really heavy shower. This is especially noticeable if we have had a few dry days.
You can test this theory if you test the quality of rainwater gathered in a puddle, you will find the TDS reading is lower than you may think.
David.