What exactly is a negative air ion (on the atomic level), and is there any evidence that negative ions are the sole causative agent of the mental health benefits observed? Can you propose a mechanism for the perceived effect?
On a molecular level, at least, the preponderance of negative air ions consist of oxygen [molecules, O2] with an added electron, which creates a force attracting them to ground [GND], where they are quickly neutralized. In other words, these ions are unstable, although they persist a bit longer in the air circulation when enveloped in moisture [microdroplets of H2O, high humidity]. The causative mechanism of the antidepressant effect is still unclear, but a current working hypothesis is that inhalation of ionized air enhances blood oxygenation. The research is underway.