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California Air Resources Board, and as an Associate
Editor for Environmental Science & Technology. A member
of AAAR since 1986, Dr. Hildemann has served as an
Associate Editor of AS&T, as a member on the AAAR’s
Board of Directors and Editorial Board, and as Chair of the
Awards Committee and the Publications Committee.
Wednesday, October 5
8:00 am – 9:15 am
AeroSol, Cloud, And preCIpITATIon
InTerACTIonS: AnThropoGenIC And
nATurAl effeCTS
Bruce Albrecht, University of Miami, Miami, FL
Abstract: The role of aerosols in cloud and precipitation
formation and evolution was documented in some of
the earliest cloud studies based on aircraft observations
made in the 1950s. A key finding of these early studies
was that the microphysical characteristics of marine
and continental clouds exhibited marked differences
that were attributed to aerosol differences between
the two environments. But the possible importance of
aerosol-cloud-precipitation interactions in affecting
cloud properties critical to climate was not brought
to the forefront until some 20-30 years later when
mechanisms that that are now called the first and second
aerosol indirect effects were postulated. The first is
a brightening of clouds associated with increases in
aerosols that serve as cloud condensation nuclei (CCN)
and will distribute cloud water among smaller droplets
that more effectively reflect solar radiation. The second
is an increase in the cloud lifetime (or cloud amount)
as increased CCN reduces precipitation efficiency and
the removal of cloud water. Both effects would lead to
increased cloud albedo with increased aerosols. The first
indirect effect has a strong theoretical basis, since the
relationship between droplet size and cloud reflectivity
can be defined theoretically and the relationship
between CCN concentration and droplet number
(and size) is well established. But the second indirect
effect has a weaker foundation, since the precipitation
mechanisms as they relate to aerosols are not defined
easily in terms of basic physical principles. Further,
an additional element of the second indirect effect is
that in an environment of low CCN and precipitating
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