A feed aerosol comprising detectable particles is injected into a mix-enhancing swirler. Diluting gas is also injected into the mix-enhancing swirler and mixed with the feed aerosol in a swirling motion to form an aerosol with a particle concentration no greater than 1,000 particles per liter. The aerosol is then injected into a mixing chamber, where the aerosol is mixed and dried; the inner diameter of the mixing chamber is at least twice as great as that of the swirl chamber. The aerosol is then emitted through a flow straightener that removes swirl from the flow of the aerosol and passed through a delivery conduit, where the particles are detected and counted; and the particle count is compared with a target count. The respective flows of feed aerosol and diluting gas can then be increased or decreased based on a comparison of the particle count with the target count.