Plasma processing apparatus and associated methods for detecting air leak are provided. In one example implementation, the plasma processing apparatus can include a processing chamber to process a workpiece, a plasma chamber separated from the processing chamber by a separation grid, and an inductive coupling element to induce an oxygen plasma using a process gas in the plasma chamber. The plasma processing apparatus can detect afterglow emission strength from reaction between nitric oxide (NO) and oxygen radical(s) in a process space downstream to an oxygen plasma to measure nitrogen concentrations due to presence of air leak.