Horizontal gas-liquid scrubbing systems and associated gas scrubbing methodologies are provided. In one embodiment, a horizontal duct scrubbing system includes a horizontally disposed housing having a waste gas inlet and a treated gas outlet, a liquid inlet manifold disposed within the horizontally disposed housing, the liquid inlet manifold comprising a plurality of nozzles oriented to spray a scrubbing liquor co-current to the flow of a gas stream flowing through the horizontally disposed housing, and a demister located proximal the treated gas outlet, where the horizontally disposed housing is substantially free of flow deflection members between the liquid inlet manifold and the demister. The gas stream may include sulfur dioxide, and the system may be capable of removing at least 71 vol. % sulfur dioxide from the gas stream.