Investigations were carried out on a point-to-plane geometry reactor with variable gap length under a mean frequency (~60 kHz) AC voltage, with a dielectric barrier covering the plane electrode, in view of volatile organic compounds (VOC) removal. The physical and electrical properties of the discharge are correlated with its chemical reactivity on air polluted by vapours of isopropanol. In particular, for a given reactor geometry, it is shown that efficiency can be evaluated by global measurements of current, energy density or plasma temperature. Nevertheless, the different components of the active current must be taken into account to understand all the results obtained when the geometry is modified (gap length in our case). Such measurements being also of practical interest for the optimisation of the plasma processes according to the application in view.