A plasma source, comprising - an outer surface, interrupted by an aperture for delivering an atmospheric plasma from the outer surface; - a transport mechanism configured to transport a substrate and the plasma source relative to each other parallel to the outer surface, such that, in use, a surface to be processed of the substrate is in parallel with at least a part of the outer surface that contains the aperture; - a first electrode (16a,b) having a first and second surface extending from an edge of the first electrode that runs along the aperture, the first surface defining the outer surface on a first side of the aperture, the distance between the first and second surface increasing with distance from the edge; - a second electrode and a dielectric layer covering at least part of the second electrode, at least part of a surface of the dielectric layer facing the second surface of the first electrode (16a, b), substantially in parallel with the second surface of the first electrode (16a, b), 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 (16a,b); - a gas inlet (19a,b) arranged in communication with the plasma initiation space to provide gas flow from the gas inlet (19a,b) to the aperture through the plasma initiation space, characterized in that the plasma source comprises - dielectric and/or electrically conductive ridges between the second surface of the first electrode (16a,b) and the dielectric layer, running through the plasma initiation space and leaving openings between the ridges for the gas flow towards the aperture. - an outer surface, interrupted by an aperture for delivering an atmospheric plasma from the outer surface; - a transport mechanism configured to transport a substrate and the plasma source relative to each other parallel to the outer surface, such that, in use, a surface to be processed of the substrate is in parallel with at least a part of the outer surface that contains the aperture; - a first electrode (16a,b) having a first and second surface extending from an edge of the first electrode that runs along the aperture, the first surface defining the outer surface on a first side of the aperture, the distance between the first and second surface increasing with distance from the edge; - a second electrode and a dielectric layer covering at least part of the second electrode, at least part of a surface of the dielectric layer facing the second surface of the first electrode (16a, b), substantially in parallel with the second surface of the first electrode (16a, b), 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 (16a,b); - a gas inlet (19a,b) arranged in communication with the plasma initiation space to provide gas flow from the gas inlet (19a,b) to the aperture through the plasma initiation space, characterized in that the plasma source comprises - dielectric and/or electrically conductive ridges between the second surface of the first electrode (16a,b) and the dielectric layer, running through the plasma initiation space and leaving openings between the ridges for the gas flow towards the aperture.