An exhaust gas stream from a combustion process is conveyed through a gas distributor at an inlet of a wet horizontal spray scrubber where it is partitioned into a plurality of spaced apart gas streams. The gas stream is partitioned in the scrubber without the need for any physical partitions. A preselected number of the partitioned gas streams are mixed with ozone to oxidize the contaminants. The oxidized gas streams are then sprayed with an aqueous medium to capture the oxidized contaminants and remove them from the treated gas streams. The treated gas streams are then recombined with the untreated gas streams. Excess ozone present in the treated gas streams is consumed by oxidation of contaminants in the untreated gas streams before the combined gas stream is released to the atmosphere.