Why does pH of water change over time?

Here's my problem. I am doing a small experiment using salt water. Last week I filled some beakers with salt water and measured the pH. It was 7.5. I decided to let it sit till the next day and at that time the pH was 7.8. There was nothing else in these beakers: no animals, no aeration, nothing. Can anyone explain why the pH would go up? It seems like it would make more sense for the pH do go down.