A device for forming at an ambient atmospheric pressure a gaseous plasma comprising active species for treatment of a treatment region, the device comprising: at least one plasma cell for forming said gaseous plasma for treating the treatment region, the at least one plasma cell comprising: an inlet for receiving gas from a source and an outlet for discharging active species generated in the cell; a dielectric substrate enclosed around a flow path for gas conveyed from the inlet to the outlet; an electrode formed on or in the dielectric substrate for energising gas along the flow path to form active species; and a protective coating made of a dielectric formed on an inner surface of the dielectric substrate for protecting the dielectric substrate from reaction with the active species, the device further comprising: a nozzle having a nozzle outlet and a plasma chamber located upstream of the nozzle outlet; wherein the plasma cell (12, 112) is fixed within the nozzle, and wherein the outlet of the plasma cell is configured to discharge active species generated in the plasma cell (12, 112) to the plasma chamber; and an earth electrode comprising a dielectric substrate and an electrode formed on or in the dielectric substrate, wherein the earth electrode substantially surrounds and at least partially overlaps the at least one plasma cell, wherein the outermost end of the earth electrode is closer to the nozzle outlet than the outlet of the plasma cell. at least one plasma cell for forming said gaseous plasma for treating the treatment region, the at least one plasma cell comprising: an inlet for receiving gas from a source and an outlet for discharging active species generated in the cell; a dielectric substrate enclosed around a flow path for gas conveyed from the inlet to the outlet; an electrode formed on or in the dielectric substrate for energising gas along the flow path to form active species; and a protective coating made of a dielectric formed on an inner surface of the dielectric substrate for protecting the dielectric substrate from reaction with the active species, an inlet for receiving gas from a source and an outlet for discharging active species generated in the cell; a dielectric substrate enclosed around a flow path for gas conveyed from the inlet to the outlet; an electrode formed on or in the dielectric substrate for energising gas along the flow path to form active species; and a protective coating made of a dielectric formed on an inner surface of the dielectric substrate for protecting the dielectric substrate from reaction with the active species, a nozzle having a nozzle outlet and a plasma chamber located upstream of the nozzle outlet; wherein the plasma cell (12, 112) is fixed within the nozzle, and wherein the outlet of the plasma cell is configured to discharge active species generated in the plasma cell (12, 112) to the plasma chamber; and an earth electrode comprising a dielectric substrate and an electrode formed on or in the dielectric substrate, wherein the earth electrode substantially surrounds and at least partially overlaps the at least one plasma cell, wherein the outermost end of the earth electrode is closer to the nozzle outlet than the outlet of the plasma cell.