A method and system for dynamically controlling a process chemistry above a substrate is described. The system for adjusting the process chemistry comprises a ring configured to surround a peripheral edge of a substrate in a vacuum processing system. The ring comprises one or more gas distribution passages formed within the ring and configured to supply an additive process gas through an upper surface of the ring to the peripheral region of the substrate, wherein the one or more gas distribution passages are configured to be coupled to one or more corresponding gas supply passages formed within the substrate holder upon which the ring rests.