A stark difference in the electrical, optical and thermal properties of an atmospheric pressure plasma jet source exploiting spark discharges in a gas stream was reported when argon is used instead of air in the flow. Electrical probe results are discussed together with optical ones in order to investigate the discharge properties and the temporal structure of the different pulses of electrical current. If air-argon mixtures are used as feeding gas for the discharge, a sharp transition from a hot plume to a cold plasma jet is observed when argon content exceeds 90 %. Characteristics of the source are examined from the point of view of their utility as a tool for plasma treatment of materials. In this respect, a 10 % air in argon mixture corresponds to a promising oxidizing condition, as revealed by the pronounced peak in atomic oxygen emission intensity.