The Emergence of the Bimodal Distribution
Concept
Rudolf B. Husar, CAPITA, Box 1124
Washington University, St. Louis, MO, 63130. rhusar@me.wustl.edu
Introduction
This historical account is focused on the 1969 ‘Pasadena Smog Caper’
The bimodal distribution of atmospheric aerosol mass, chemical composition and optical properties is
now accepted paradigm for the description of atmospheric aerosols. With the introduction of the fine
particle standard in 1997 bimodal distribution has gained full acceptance in air quality management as
well. Being a story of successful science-policy interaction, the history of the bimodal distribution
deserves recounting from several perspectives. In the first volume of the History of Aerosol Science, 2000
the scientific significance of the bimodal distribution and the associated paradigm shift has been discussed
by Sheldon Friedlander, George Hidy, Gil Sem, and Evan Whitby (Friedlander, 2000, Hidy 2000, Sem,
2000, and Whitby, 2000).
This report is a brief summary of the circumstances, developments and events during the “discovery” of
the bimodal distribution. This is mostly an account from a personal perspective from a point of view of a
graduate student that was at the right place at the right time. I had a distinct privilege being associated
with two of the pioneers of the atmospheric aerosol science, PhD student of Ken Whitby and postdoctoral
associate of Sheldon Friedlander, during the time when the bimodal distribution concept has emerged.
Wilson 2002, in this volume, describes the history from science and air quality management perspective
The history of the bimodal distribution
Background
The emergence of the bimodal distribution was facilitated by the confluence of three parallel
developments: 1) development of complete and continuous size spectrum measurement technologies; 2)
the introduction of aerosol dynamics or the explanation of atmospheric aerosol pattern; 3) engineering,
analysis collaboration….
The modern science of atmospheric aerosols began with the pioneering work of Christian Junge on
detailed size distribution and chemical composition of atmospheric aerosols. The nature of the particulate
matter is reviewed and summarized in a series of papers, Junge (1953, 1955, 1957, 1958, 1961) as well as
the book on “Air Chemistry and Radioactivity”, Junge, 1963. Based on tedious but careful size
distribution measurements conducted over many different parts of the world Junge has observed that there
is a remarkable similarity in the size spectrum: they follow a power law function over a wide range of
particle size (0.1 - 20m. The universal power law exponent of the number distribution function was
about -4. This was known as the Junge distribution, or Junge model for atmospheric aerosols. When
plotted as a volume distribution the Junge distribution is represented by a flat horizontal line between
about 0.1 and 20 m (Figure 1). Below and above this size range the distribution function dropped off.
The physical mechanisms that were responsible for producing these similarities in the atmospheric
aerosol size spectra were not known, although it was clear that homogeneous and heterogeneous
nucleation, coagulation, sedimentation and other removal processes are responsible. However, in the
1960s it was not clear which particular combination or mechanisms are responsible for maintaining the
similarity of the size spectra. In a series of articles based on dimensional analysis and similarity theory,
S. K. Friedlander (1960a, 1960b, 1961) has shown that IF coagulation and sedimentation were the
dominant physical mechanisms, then a significant part the aerosol spectrum would follow the power law
shape. In essence, the implication of this theory was that the observed quasi-stationary size distribution
could be the result of balancing aerosol production (nucleation) and removal (sedimentation).
It should be recalled that complete size distribution data covering the entire 0.01-10 m size range were
virtually non-existent. That has changed when Clark and Whitby (1967)reported an extensive set of
complete size distribution measurements for Minneapolis using the newly developed Minnesota Aerosol
Analyzing System (see next section). The new aerosol size distribution data were welcomed by the
atmospheric aerosol research community and sparked a vigorous debate regarding the Junge distribution
and its possible explanations. Clark and Whitby (1967) themselves spoke in favor Friedlander’s ideas but
Junge (1969) swiftly responded that the ‘self-preserving’ distritibution of a coagulating aerosol can be
ruled out as an explanation of the atmospheric power-law data with negative slope4: the slope of that
a self-preserving coagulation size distribution is much steeper. Also, Junge argued, that since thermal
coagulation is a weak mechanism for changing the size range >0.1 m, there must be other stronger
mechanisms that govern the atmospheric aerosol dynamics. He named homogeneous and heterogeneous
gas-particle conversion, and cloud scavenging as candidate processes. However, he has declined to
speculate the relative roles and magnitudes of these processes.
Fig 1
In a significant statement, Junge also re-emphasized his earlier findings, that the slope of the power – law
distributions can be rather different, depending on the aerosol type as reproduced in Figure 1 (Junge
1969). For example, the volume spectra of upper tropospheric aerosols has a peak at around 0.1 0.1 m
radius, while marine aerosol has a broad peak at 1-20 m. Only the aerosols observed in the continental
boundary layer show the flat Junge distribution with 4 over the 0.1-100 m radius range. Finally, he
ventures to provide and alternative explanation for the broad Junge spectrum: Continental aerosol arise
from many different aerosol sources, each having a different size between 0.1-100 m. When these
aerosols are mixed in the atmosphere, they form a size spectrum that can be represented by a broad lognormal distribution that resembles the typical flat Junge spectrum.
This was the state of understanding in 1969 as the ………
Aerosol Measurement Technology
Until the late 1960s aerosol size distribution was obtained using tedious impactor and filter
measurements, electron microscopy and other time consuming techniques that precluded continuous
monitoring. The total number of measurements that covered the 0.1-10 m were very limited, probably
in the range of 50-100 spectra. Starting in the mid 1960s Ken Whitby and Ben Liu at the University of
Minnesota Particle Technology Laboratory have developed or adapted a suite of instruments for near
continuous in situ monitoring of atmospheric aerosol size spectra. The heart of the instrument package
was the electrical mobility analyzer covering the size range 0.008 – 0.5 m. Atmospheric particles were
passed through a diffusion charger a subsequently separated by the electric mobility. The current resulting
from deposition of particles in a given mobility range was used as a measure of aerosol concentration in a
given mobility range. The instrument relied on the fact that the electrical mobility of charged particles
was monotonically decreasing with increasing particle size. This allowed particle size segregation in the
size range 0.008 0.3 m. A history of the electrical mobility analyzer is given in Sem (2000)
In the mid 1960s optical particle counters were made available commercially. Ken Whitby and Ben Liu
have also made significant improvements in the size resolution of optical particle counters. In the
commercial counters a wide jet of atmospheric aerosols was passed through the illuminated volume and
the scattered light pulse from the individual particles was classified by a multi-channel pulse height
analyzer. Whitby and Liu have introduced the “sheath” air inlet such that the aerosol particles were
passed through a uniformly illuminated center of the light beam. The result was a significant
improvement in the size resolution.
Fig 2
The Minnesota Aerosol Analyzing System, MAAS, as prepared for the 1969 Pasadena experiment is
shown in Figure 2. The white cabinet houses one of the first TSI commercial mobility analyzers (Whitby
Aerosol Analyzer). The Royco optical counter, equipped with the sheath air inlet along with the multichannel pulse height analyzer is house in a separate cabinet. The suite of MAAS instruments was
completed by the General Electric condensation nuclei counter that was used to continuously monitor the
total nuclei count > 0.01 m.
The analog and digital data from these MAAS and other meteorological instruments were gathered by the
data acquisition system with the data acquisition frequency of 20 minutes. As a result a total of 350
complete size distributions were recorded during the three-week study. The digital data recorder was a
teletype machine equipped with the punch tape unit that recorded the digital data as holes in a continuous
paper tape. Ken Whitby was justifiably very proud of the modern data acquisition system. This data
acquisition and recording system allowed swift computer processing of the size distributions and other
monitoring data. In fact, the prompt availability of the size distribution data to the other collaborating
research groups allowed synergistic collaboration between the Minnesota and other groups. For example,
Dave Ensor and Bob Charlson could immediately work on calculating aerosol light scattering from the
size distribution data and compare those to their four wavelength nephelometer data.
In 1969 the editing of data was rather similar to the currently used cut-and-paste graphic user interfaces.
The difference was that in 1969 cut meant cutting the paper punch tape with scissors, replacing the bad
data with a new punched tape section and pasting the new tapes with the appropriately perforated sticky
tape.
Fig 3
A significant factor during the 1969 Pasadena study was the stimulating intellectual environment. Not
only the leaders of the project Ken Whitby, Sheldon Friedlander and Peter Mueller were providing the
stimulating environment, but the project has attracted several “junior” scientists e.g. Bob Charlson,
George Hidy who eagerly contributed their instruments and rich set of ideas while the monitoring was in
progress. In Figure 4, for example Bob Charlson is conducting an animated discussion regarding an
interesting shift in the four wavelength nephelometer signal.
Fig 4
For us graduate students this was an intense learning experience on how “science” is conducted.
Throughout the experimental period our host Sheldon Friedlander has brought in many visiting
dignitaries, including Ari Hagen-Smit, the “discoverer” of the Los Angeles smog and chairman of the
California Air Resources Board. This high visibility provided the impetus to analyze and display the
collected data and to discuss the features of the size distribution with the participants and visitors.
Fig 5
Since the MAAS data acquisition and recording was completely automatic it freed much of our time for
special experiments. Discussions with the participants
RESULTS
In the 1950s, Christian Junge has proposed that atmospheric aerosols obey the monotonic power-law of
the size spectra, n (D) = D- , where the exponent was generally found to be about  = 4 over particle
diameter range 0.05 < D < 20 m. An alternative ‘model’ is the bimodal distribution, introduced around
1970, by K.T. Whitby according to which atmospheric aerosols can be divided into two distinct
aerosol classes: fine and coarse particles, each having a distinct mode in the aerosol volume distribution
function. The emergence of the bimodal concept was the result of simultaneous developments in highresolution in-situ size distribution measurement techniques and theoretical studies in aerosol size
distribution dynamics. In the 1960s K.T. Whitby and co-workers have developed the Electrical Aerosol
Analyzer (EAA) for in situ size spectrum measurements in the range 0.01 < D < 0.3 m. When combined
with modified commercial optical particle counters (OPC) and data acquisition electronics, the ‘Whitby
system’ was capable of near-continuous size spectrum measurements over the range 0.01<D<10 m. The
full system was first field-tested in 1968 in Colorado and operated routinely during the 1969 ‘Pasadena
Smog Caper’.
The Pasadena smog aerosol data have shown a remarkably persistent peak in the volume distribution
around D = 0.2–0.5 m followed by a minimum at about 1-2 m. The second (coarse particle) peak was
not fully covered due to the 10 m upper size of the optical counter. Both, the persistent volume peak and
the volume minimum were unexpected and evoked many doubts, particularly in the accuracy of the
instrumentation: Is it due to the poor size resolution of the EAA? Is it an artifact of OPC? Is it universal or
unique to the Los Angeles smog aerosol under specific conditions? The concept of the bimodal
distribution was formally presented by Whitby and co-workers in 1972.
The emergence of the bimodal distribution was also a boost to the theory of atmospheric aerosols. It
allowed a more rigorous treatment of atmospheric aerosols as a dynamic system governed by well-defined
physical and chemical processes. Following the ideas of S.K. Friedlander and co-workers, the early
theoretical work was focused on the understanding of the dynamics of the sub-micron (accumulation)
mode. Monte Carlo and other simulations by R.B. Husar have demonstrated that condensational growth
of gaseous precursors on existing nuclei is mainly responsible for the aerosol growth in the 0.1-1.0 m
accumulation mode. The simulated size distributions have confirmed the observations that the volume
growth (accumulation) occurs without much
change in characteristic particle size.
Coagulation was found to be the key mechanism
for the fast decay of sub-0.1 particles at night
and did not appear to influence the dynamics of
the ‘accumulation-mode’ particles in the LA
basin.
Subsequent confirmation the bimodality as a
robust concept came from (1) comparing size
spectra from numerous locations and times (2)
chemical composition data showing different
aerosol species in the fine and coarse modes (3)
theoretical calculations on the size spectrum
dynamics. Research over the past 30 years has
demonstrated that fine and coarse particles have
different sources and are also responsible for
different effects on human health and welfare. In
1998 EPA has adopted a separate air quality
standard for fine particles (PM2.5).
References
Clark and Whitby (1967)
Wilson 2002
Junge 1953,
Junge, 1955,
Junge 1957,
Junge, 1958,
Junge, 1961
Junge 1967
Junge, 1963Air Chemistry and Radioactivity,
S. K. Friedlander (1960a,
S. K. Friedlander (1960b)
S. K. Friedlander (1961)
(Friedlander, 2000, Hidy 2000, Sem, 2000, and Whitby, 2000)