An eight-stage, low-pressure impactor is developed with 50% efficiency cutoffs of 4.0, 2.0, 1.0, 0.05, 0.26, 0.11, 0.075, and 0.05-µm aerodynamic diameter. Particles larger than 0.5 µm are sampled at near atmospheric pressure; four stages with stagnation pressures below the jet of 8-140 mm Hg absolute, size segregate the smaller aerosol. The 0.50, 0.26, and 0.11-µm cutoff stages are calibrated with monodisperse polystyrene latex spheres. The final two stages are compared with electrical aerosol analyzer measurements of a laboratory-generated sulfuric acid mist. Studies of impactor collection efficiency show that particle bounce and reentrainment are not important when the collection surfaces are Vaseline coated. The impactor has one jet per stage, and thus is especially suited for aerosol sulfur and nitrate determinations by the method of flash volatilization and subsequent gas-phase analysis.