This might be due to my location close to sea. This natural background comes from radon outgassing from the earth's crust, which might be filtered out by seawater as it bubbles up there. This way the air above sea is much less active as above land, and when the wind comes from the direction of the sea, this inactive air stretches over land a bit. This is just a loose theory of mine though, please comment below if I'm wrong.
Short story even shorter, calibrating using rain water as a source is not an option for me.