An apparatus for providing a local clean microenvironment near optical surfaces of an extreme ultraviolet optical assembly maintained in a vacuum process chamber comprising: one or more extreme ultraviolet optical assemblies (204a, 204b) including at least one optical element with an optical surface, wherein the one or more extreme ultraviolet optical assemblies (204a, 204b) comprise at least a portion of at least one of an extreme ultraviolet inspection tool, an extreme ultraviolet metrology tool and an extreme ultraviolet lithography tool; a supply of cleaning gas stored remotely from or delivered remotely to the one or more optical assemblies, wherein the gas supply is not incorporated into the vacuum process chamber; and a gas delivery unit (220a, 220b) comprising: at least one plenum chamber; one or more gas delivery lines (222a, 222b) connecting the supply of gas to the at least one plenum chamber; at least one delivery nozzle configured to control the local microenvironment by directing the cleaning gas from the plenum chamber to one or more portions of the one or more extreme ultraviolet optical assemblies (204a, 204b), wherein the delivery nozzle is suitable for causing the flow of cleaning gas from the plenum chamber to converge and then diverge, thereby directing the flow of gas uniformly along the optical surface of optical element; at least one collection nozzle for removing gas from the one or more extreme ultraviolet optical assemblies (204a, 204b) and the vacuum process chamber; and a guide skirt configured to contain and direct the flow of gas from the at least one delivery nozzle through the immediate vicinity of the optical surface toward the collection nozzle, wherein the collection nozzle is mounted across the optical surface from the delivery nozzle. one or more extreme ultraviolet optical assemblies (204a, 204b) including at least one optical element with an optical surface, wherein the one or more extreme ultraviolet optical assemblies (204a, 204b) comprise at least a portion of at least one of an extreme ultraviolet inspection tool, an extreme ultraviolet metrology tool and an extreme ultraviolet lithography tool; a supply of cleaning gas stored remotely from or delivered remotely to the one or more optical assemblies, wherein the gas supply is not incorporated into the vacuum process chamber; and a gas delivery unit (220a, 220b) comprising: at least one plenum chamber; one or more gas delivery lines (222a, 222b) connecting the supply of gas to the at least one plenum chamber; at least one delivery nozzle configured to control the local microenvironment by directing the cleaning gas from the plenum chamber to one or more portions of the one or more extreme ultraviolet optical assemblies (204a, 204b), wherein the delivery nozzle is suitable for causing the flow of cleaning gas from the plenum chamber to converge and then diverge, thereby directing the flow of gas uniformly along the optical surface of optical element; at least one collection nozzle for removing gas from the one or more extreme ultraviolet optical assemblies (204a, 204b) and the vacuum process chamber; and a guide skirt configured to contain and direct the flow of gas from the at least one delivery nozzle through the immediate vicinity of the optical surface toward the collection nozzle, wherein the collection nozzle is mounted across the optical surface from the delivery nozzle. at least one plenum chamber; one or more gas delivery lines (222a, 222b) connecting the supply of gas to the at least one plenum chamber; at least one delivery nozzle configured to control the local microenvironment by directing the cleaning gas from the plenum chamber to one or more portions of the one or more extreme ultraviolet optical assemblies (204a, 204b), wherein the delivery nozzle is suitable for causing the flow of cleaning gas from the plenum chamber to converge and then diverge, thereby directing the flow of gas uniformly along the optical surface of optical element; at least one collection nozzle for removing gas from the one or more extreme ultraviolet optical assemblies (204a, 204b) and the vacuum process chamber; and a guide skirt configured to contain and direct the flow of gas from the at least one delivery nozzle through the immediate vicinity of the optical surface toward the collection nozzle, wherein the collection nozzle is mounted across the optical surface from the delivery nozzle.