A system, including apparatuses and methods, for removing sulfur dioxide (SO2) and nitrogen oxides (NOx) present in a stream of gases via a dual-stage irradiation process employing a pulsed electron accelerator in the first stage to remove a substantial majority of the sulfur dioxide (SO2) from the stream of gases before removing a substantial majority of nitrogen oxides (NOx) from the stream of gases in the second stage. In an exemplary embodiment, the system comprises a stack gas decontamination system having a reactor with a first portion in which a pulsed electron accelerator initiates a first series of ionmolecular reactions in an ammonia-rich environment to convert the sulfur dioxide (SO2) into sulfuric acid salts and a second portion in which at least one direct current electron accelerators initiates a second series of ion-molecular reactions in an ammonia-rich environment to convert the nitrogen oxides (NOx) into nitric acid salts.