The article states that negative ions are "short-lived" compared to positively charged ions, probably because negatively charged atoms or molecules can get rid of an electron anytime (e.g. through absorption of light), while positively charged atoms or molecules need to get close enough to any other material in order to catch the missing electron. It could be just a matter of statistics: losing an electron is a process of 0. order (proportional to the density of charged matter), stealing an electron from another atom/molecule is a process of 1. order (proportional to the number of productive collisions between charged and uncharged particles, which is proportional to the product of the densities of charged and uncharged particles – and that density is very, very low once a particle has left the surface of the moon)
> Negative ions are short-lived and cannot make it to orbit. This is why the European scientists needed to operate their instrument close to the lunar surface – an unprecedented mission for a particle detector.
