Abstract: The present invention provides an apparatus and method to improve the performance of a variety of particulate collection devices. Gaseous ammonia is injected by one or more injectors into a waste gas upstream from a particulate collection device. The amount of ammonia injected from each injector is controlled so that the local concentration of ammonia in the waste gas is approximately the same as the corresponding local concentration of sulfur trioxide in the waste gas. The flow of ammonia from each injector is controlled by measuring several local sulfur trioxide concentrations in the waste stream after the ammonia has been injected. The sulfur trioxide can occur naturally or result from injection. By keeping the concentration of ammonia approximately equal to the concentration of sulfur trioxide, ammonia and sulfur trioxide react to form a liquid product. This liquid reaction product coats the surfaces of particles entrained in the gas stream.