An aging line especially designed for the aging of pollutant atmospheres is described. The line essentially is a nearly-square stainless steel duct with periodic 100-mesh stainless steel screens to promote plug flow. The line has been tested for uniformity of aging, aerosol losses, and ease of atmosphere control, Results show that a close approximation to plug flow can be achieved with excellent atmosphere control; aerosol losses in the particle size range 0.22-2.0 μm are less than 30% except at very long (96 min.) aging times, when significant sedimentation losses occur for larger particles. Copyright 1982, American Industrial Hygiene Association