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mode by the surface area concentration and the Aitken and nucleation modes by the
number concentration.
Aerosol formation arises from heterogeneous or homogeneous nucleation. The
former refers to condensation growth on existing nuclei, and the latter to the forma-
tion of new nuclei through condensation. Heterogeneous nucleation occurs prefer-
entially on existing nuclei. Condensation onto a host surface occurs at a critical su-
persaturation, which is substantially lower (< 1-2 %) than for homogeneous nucle-
ation in the absence of impurities (> 300 %). Examples of gas-to-particle conversion
are the combustion processes and the ambient formation of nuclei from gaseous
organic emissions. Particles in the Aitken/accumulation mode typically arise from
either: (i) the condensation of low volatility vapors; or (ii) coagulation. Particles
in the accumulation mode have a longer atmospheric lifetime than other modes, as
there is a minimum efficiency in sink processes. Particles in the coarse mode are
usually produced by weathering and wind erosion processes. Dry deposition (pri-
marily sedimentation) is the primary removal process. As the sources and sinks of
the coarse and fine modes are different, there is only a weak association of particles
in both modes (Hewitt and Jackson 2003 ). The aerosol chemistry data organized
first by Peter Mueller and subsequently analyzed by Friedlander and coworkers
showed that the fine and coarse mass modes were chemically distinctly different
(Husar 2005 ).
Husar ( 2005 ) has summarized the history of aerosol science as follows. The
modern science of atmospheric aerosols began with the pioneering work of Chris-
tian Junge who performed the first comprehensive measurements of the size distri-
bution and chemical composition of atmospheric aerosols (Junge 1952 , 1953 , 1955 ,
1963 ). Based on tedious and careful size distribution measurements performed over
many different parts of the world, Junge and coworkers have observed that there is
a remarkable similarity in the gathered size distributions (number concentration N
versus radius r a ): they follow a power law function over a wide range from 0.1 to
over 20 μm in particle radius.
dN
=
c.
r
dr
log
a
The inverse power law exponent α of the number distribution function ranged be-
tween 3 and 5 with a typical value of 4. This power law form of the size distribution
became known as the Junge distribution of atmospheric aerosols. In the 1960s, the
physical mechanisms that were responsible for producing these similarities in the
atmospheric aerosol size spectra were not known, although it was clear that homo-
geneous and heterogeneous nucleation, coagulation, sedimentation, and other re-
moval processes were all influential mechanisms. In particular, it was unclear which
combination of these mechanisms is responsible for maintaining the observed qua-
sistationary size distribution of the size spectra.
Whitby ( 1973 ) introduced the concept of the multimodal nature of atmospheric
aerosol and Jaenicke and Davies ( 1976 )added the mathematical formalism used
today. Around 1970-1971, Whitby et al. ( 1972 ) collected and analyzed several size
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