Air is the most common atmospheric medium through which partial discharges propagate and, as such, it is worthy of consideration from a chemical viewpoint. Air is a complex mixture of gases, of which the major components are nitrogen and oxygen with minor concentrations of argon, water vapour and oxides of carbon. The molecular species, ions, etc., generated by discharges in air [10–14] are, therefore,most likely to be combinations of nitrogen, oxygen, carbon oxides and hydrogen (from breakdown in atmospheric moisture). The gaseous species produced will be, for an AC stress situation, of both positive and negative polarity e.g. O− and O−2 , N+and O+2 .
In addition, in a discharge atmosphere the polar nature of water molecules causes them to be attracted to charged species in the discharge.