A plasma source has an outer surface (12), interrupted by an aperture (14) for delivering an atmospheric plasma from the outer surface. A transport mechanism (11) transports a substrate (10) in parallel with the outer surface, closely to the outer surface, so that gas from the atmospheric plasma may form a gas bearing between the outer surface and the substrate. A first electrode (16a,b) of the plasma source has a first and second surface extending from an edge of the first electrode that runs along the aperture. The first surface defines the outer surface on a first side of the aperture. The distance between the first and second surface increase with distance from the edge. A second electrode (17) covered at least partly by a dielectric layer (18) is provided with the dielectric layer facing the second surface of the first electrode, substantially in parallel with the second surface of the first electrode, leaving a plasma initiation space on said first side of the aperture, between the surface of the dielectric layer and the second surface of the first electrode. A gas inlet (19a,b) is in communication with the plasma initiation space to provide a gas flow from the gas inlet to the aperture through the plasma initiation space. Atmospheric plasma initiated in the plasma initiation space flows to the aperture, from which it leaves to react with the surface of the substrate.