Apparatus for removing ammonia from a gas stream includes primary and secondary scrubbers each of which includes a gas inlet, a gas outlet, and a slurry atomizer. The gas inlet of the primary scrubber is operatively connected to a gas stream having ammonia contained therein. A primary slurry feed system operatively connected to the slurry atomizer of the primary scrubber feeds to the slurry atomizer a primary slurry composition of a molybdic acid at a pH level. The gas inlet of the secondary scrubber is operatively connected to the gas outlet of the primary scrubber. A secondary slurry feed system operatively connected to a slurry atomizer of the secondary scrubber feeds to the slurry atomizer a secondary slurry composition of a molybdic acid at pH level. The pH level of the secondary slurry composition is lower than the pH level of the primary slurry composition.