Development of the Concept of the Bimodal Distribution of Particles: A Personal History
The author was hired by EPA in 1971 to head the new Aerosol Research Section (ARS), the first EPA unit devoted to research on atmospheric aerosols. At that time the existing paradigm was that the atmospheric aerosol distribution was monomodal , the mass mean diameter was one micrometer or larger, and that atmospheric particles were formed by nucleation of gases and grew by coagulation and condensation until they became large enough to fall out of the atmosphere. During the 1970's, a team of researchers, including Whitby, Charlson, Friedlander, Winchester, and Husar, supported by EPA organizations directed by the author, played a key role in changing this view into the current paradigm of fine and coarse particles with nuclei, accumulation, and coarse modes. The paper will include a discussion of: 1) Early studies including size distribution measurement such as ACHEX (the California Aerosol Characterization Experiment), Project MISTT (the Midwest Sulfur Transformation and Transport Study), Project VISTTA (Visibility Impairment due to Sulfur Transformation and Transport in the Atmosphere), and the Tennessee Plume Study. 2) The difficulties of maintaining support for the development of instruments to measure size distribution. 3) The EPA purchase order for three compact, transportable EAAs that provided an incentive for Thermal Systems, Inc. to continue development of aerosol instruments and led to the extensive research and commercial use of EAAs and subsequent improvements in mobility analyzers. 4) Difficulties in getting EPA scientists and managers to accept the bimodal concept. Some scientists lobbied for a definition of fine particles based on dosimetry (with a cut point at 4 or 6 um); others claimed that bi-modality was unreasonable but maintained that particle and particle components were distributed in many varying modes. 5) Early studies, in which size distribution measurements were used to demonstrate important new concepts, including formation and long range transport of particles in power plant and urban plumes, the chemical evolution of power plant plumes, removal of nuclei and coarse mode particles but not accumulation mode particles by rain, sulfate formation from SO2 by early automobile catalysts, size distribution of chemical components, and growth of particles due to chemical reactions of gases dissolved in liquid particles. 6) Ultimate success in acceptance of the bi-modal distribution as demonstrated by the 1997 promulgation of a fine particle standard by the EPA Administrator and a 1999 decision by the District of Columbia Court of Appeals that a PM10 standard could not be used to control coarse particles because PM10 contained fine particles as well as coarse particles. 7) The data and thinking that led to the choice, in the early 1970's, of 2.5 um as the 50% cut point to separate fine and coarse particles, efforts over the years to obtain measurements of PM1, some problems introduced by the use of 2.5 um as the 50% cut point to separate fine and coarse particles, efforts over the years to obtain measurements of PM1, some problems introduced by the use of 2.5 um as a 50% cut point, and the value of PM1 measurements.
Wilson, W E. Development of the Concept of the Bimodal Distribution of Particles: A Personal History. Presented at History of Aerosols, Second Symposium, Portland, OR, October 13-14, 2001.