A method for removing contaminants from a gas stream by feeding the gas stream into a scrubber that can use seawater as the scrubbing medium. The gas stream is first scrubbed with the seawater and then mixed with ozone to remove the contaminants. Nitrogen oxides that are present in the gas stream are converted to nitric acid/nitrates and these are recovered via condensate or coalesced mist for neutralization. The treated gas stream is then treated by a second scrubber whereby other contaminants such as sulfur oxides, particulates and the like are removed and the thus treated gas stream is exhausted to the atmosphere. Optionally a droplet separator may be present in the scrubber.