Development of the Concept of the Bimodal Distribution of Particles: A Personal History.
William E. Wilson, U.S. Environmental Protection Agency, MD-52,
Research Triangle Park, NC 27711
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 1970s, 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 portable EAAs that provided an incentive for Thermo Systems
Incorporated 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
μm); others claimed that bimodality 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 bimodal 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 1970s, of 2.5 μm 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 μm as a 50% cut point, and the value of PM1 measurements.
This paper is dedicated to the memory of Kenneth T. Whitby, scientist, engineer and
educator.