A technique for stripping a certain molecule or class of molecules from an airstream. The molecules of interest diffuse to the walls of a flow-through reactor, and are deposited there, while particulate matter passes through the reactor unchanged. The reactors are usually cylindrical or annular tubes, to maximize the surface area for a given volume, and the inner walls can be coated with chemicals to remove the molecules of interest, for example, citric or oxalic acid for ammonia removal. At the end of a given time the chemicals of interest are removed from inside the denuder for quantitative chemical analysis.