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Summary form only given. Stable and long-lived atmospheric-density air plasmas with high electron densities are interesting for many applications. One of the difficulties in creating such plasmas at low temperatures is that electrons very quickly attach to oxygen, thus limiting the electron density. Another important factor limiting electron density and gas pressure is the ionization-heat instability, resulting in arcing. In the present paper we discuss methods of suppressing attachment and controlling instabilities based on the excitation of metastable molecular states. Enhanced population of metastable states may stimulate ionization in both electron-molecule and molecule-molecule collisions, and accelerate electron detachment from negative ions. Metastable states could also define the path of arc formation, allowing external control of arc location and timing.