Measurements of Particle Deposition Rates Inside Southern California Museums

Rates of deposition of submicrometer particles were measured inside five Southern California museums using an automated particle-counting technique on a scanning electron microscope equipped with an energy-dispersive X-ray detector combined with bulk chemical analysis. Deposition velocities to vertical and upward-facing horizontal surfaces were determined as a function of particle size for each site by analysis of indoor suspended particles sampled onto filters and particles deposited on vertically and horizontally oriented deposition plates. Measured deposition velocities to vertical indoor surfaces were in the range of 10 −6 to 10 −5 m/s at all sites but varied from site to site in terms of their dependence upon particle size. Deposition velocities to horizontal surfaces were in the range of 10 −6 to 10 −3 m/s and showed the expected increase with particle size due to gravitational settling. The deposition velocities measured by single-particle analysis are compared to deposition velocities determined by bulk chemical analysis of deposition plates and indoor ambient filter samples for several chemical species. Advantages and drawbacks of the automated particle counting technique are discussed.