When a vacuum system containing particle-free, cleanroom air is pumped down from atmospheric pressure to produce a vacuum, the gas in the chamber expands and cools to cause water vapor condensation and droplet formation. Experiments have been performed to show that a stable aerosol of submicrometer residue particles on the order of 0.2μm are formed when the water droplets have re-evaporated. The residue particles are spherical in shape and appear to contain mainly sulfuric acid. The theoretical and experimental studies conducted strongly suggest that the main steps involved in the residue particle formation process are: (i) simultaneous absorption of SO2 and H2O2 in the ppb range from the air into the water droplets during pump down, (ii) concentration of SO2 and H2O2 in the liquid when the droplets re-evaporate, and (iii) oxidation of SO2 by H2O2 in the concentrated liquid solution during the final period of evaporation to produce stable, residue particles of H2SO4. The mathematical model developed gives results that are consistent with the experimental observations.