A method for the partial removal of contaminants such as nitrogen oxides from a process gas stream is described. The process gas stream is separated into at least two process gas streams by means of a partition, baffle, damper or other device. Ozone is fed into contact with at least one of the separated process gas streams to oxidize the contaminants therein and the at least one of the process gas streams contacted by ozone is fed to a scrubber for removal of the oxidized contaminants from the gas streams. The separation is proportional to the percentage removal of contaminants desired.