Atmospheric pressure plasma units do not require an earthed counter electrode during pre-treatment.
The spark discharge between two electrodes takes place at the end of the electrode head or internally, depending on the type of equipment. A stream of air transports the generated plasma to the substrate surface. The closed molecular chains of the surface are cracked open, and activated species, mainly oxygen radicals, can be deposited. On the surfaces that were non-polar before, polar molecules are now generated to which colour molecules are able to bind chemically. The function is shown below in a simplified manner:

The treatment width of an electrode head depends on the equipment type and can be Ø = 15 mm (Arcospot®) up to an area of 60 x 40 mm (Arcojet®). Larger working widths can be achieved with a battery bank.
If process gases are required, they can be mixed into the air gap. Special units are available for medical applications and clean-room applications.