Any idea of what mechanism could produce only negative ions?

Is this what you are asking, "Will a saturated salt solution produce negative ions in the air?"

Dangerous are indeed waves that can produce ions, like radioactivity.

If it produces ions, then the dissociation is also an ionization.

They do not necessarily fully dissociate and some can produce multiple H+ ions per molecule.

X-rays going through a gas will ionize it, producing positive ions and free electrons.

They sense pressure indirectly by measuring the electrical ions produced when the gas is bombarded with electrons.

Both techniques involve bombarding the sample with electrons, and measuring the negative ions produced.

The soil and its radioactivity might produce atmospheric ions, but so must some mysterious radiation present in the upper atmosphere.

The ions being produced at the filament are focussed into an ion beam and then passed through a magnetic field to separate them by mass.