Methods and apparatus for treating an exhaust gas in a foreline of a substrate processing system are provided herein. In some embodiments, a method for treating an exhaust gas in an exhaust conduit of a substrate processing system includes: flowing an exhaust gas from a process chamber into a plasma source via a foreline; injecting a reagent into the foreline; forming a plasma in the plasma source from the exhaust gas and the reagent; and injecting a cleaning gas into the foreline, wherein the cleaning gas and the reagent are different gases.