The present invention relates to a process for removing nitrogen oxides, sulfur oxides, mercury and mercuric oxide from gas streams such as furnace or utility boiler flue gas streams, particularly those derived from coal-fired utility boilers, or from a gas stream from another pollutant abatement process. Ozone will react with the impurities in the gas stream to form mercuric oxide and higher oxides of nitrogen which can be removed by scrubber means. Additionally, and alternatively, the present invention provides for the use of ozone and ultraviolet radiation to remove nitrogen oxides, sulfur oxides and mercury from gas streams.