Impact of soot on cloud formation
Rosario Guzman, Gustavo Sosa, and Abel Toribio
Why are we paying attention to the properties of soot that involve
water? This question sounds trivial: health and visibility problems
related to high concentration of soot are familiar enough. However,
there is a process associated with soot’s geometry that has
defied our intuition.
Soot is a black carbonaceous substance produced during incomplete
combustion of coal, wood, oil, etc., rising in fine particles that
adhere to and blacken surfaces on contact. It is hydrophobic (that
is, it does not absorb water). In fact, the soot-water contact angle
is around 80°. Contact angle measures “how much a substance
wets a surface,” thus a small contact angle reflects hydrophilic
(strong affinity for water) surfaces, while a large contact angle
indicates a hydrophobic surface.
![](../../../newsletter/nwsltr_2/jpgs/soot_1.jpg)
Figure 1. Soot aerosol particle.
Left, experimental photograph. Right, MIT-CAM team’s
simulation: fractal aggregate of 64 primary particles and
fractal dimension 1.7. |
Until recently, soot was modeled as a sphere having properties
equivalent to those of the real particle. This model con- firmed
our intuitive idea with respect to the amount of water able to condense
on soot’s surface: it was insignificant, and increasing this
amount required extremely supersaturated air. Let us not forget
though that these conclusions depend strongly on the model used,
and that soot is far from having a spherical contour.
In 1996, Xie and Marlow from Texas A&M University investigated
the effect of substrate surface curvature on water equilibrium vapor
pressure. Figuring out what they called “complex aerosols”
(two to four identical spheres stuck together, fixed in ten different
configurations), they determined the water volume content as a function
of relative humidity. In order to do this, they used “Surface
Evolver,” a free access program available in the Internet.
Evolver is a program that minimizes the energy of a system subject
to forces and restrictions specified by the user. In this way, Xie
and Marlow determined the equilibrium thermodynamic conditions for
an air-water-substrate system subject to waterair surface tension.
The results revealed strong sensitivity of vapor pressure with regards
to surface geometry; one of the more relevant results can be understood
as soot able to absorb limitless water amounts.
In 1979, Forrest and Wien noticed the fractal nature of several
aerosol particles such as metals, metal oxides, silicon dioxide
and soot, which are formed by aggregation of small, nearby spherical
particles (that we will call “primary particles”). A
fractal is a mathematical concept that “qualitatively can
be described as a rough object such that its roughness appears at
any scale” (Jullien and Botet, 1987). This quality can be
accounted for by a quantity called fractal dimension, “which
do not have any reason to be an integer” (contrary to the
dimension of a smooth curve or surface, Figures 1 and 2 respectively).
Although perfect fractals do not exist in nature, irregular fractal-like
shapes do. Such is the case of fractured surfaces, ecological systems,
cloud profiles, “the intricate arrangement of flying-bird-wing
feathers” (Stoyan and Stoyan, 1994), and fresh soot. This
last has a fractal dimension between 1.7 and 1.9.
In 1996, Thouy and Jullien proposed an algorithm to generate fractals
of fractal dimension between 1 and 2.55, and an arbitrary number
of primary particles. In nature, particles stick together in different
ways, such as one particle to another, a particle to an aggregate,
or an aggregate to another aggregate. Thouy and Jullien used the
aggregate-aggregate mechanism as it beer reflects soot’s aggregation
processes.
![](../../../newsletter/nwsltr_2/jpgs/soot_2.jpg)
Figure 2. Water uptake in
a four primary- particle a g g r e g a t e . Here we show
one of the 10 geometrical configurations studied by Xie. The
MIT-CAM team simulated water condensation over a substrate
of contact angle 60° using Surface Evolver. |
In 2002, Toribio and Guzman developed an aerosol simulation system
based on Thouy and Jullien’s algorithm. Once this simulation
system was available, Sosa (2002) and Guzman et al. (2002) investigated
if Xie’s aerosols appear in “real” fractal- like
soot aerosol particles and if they do, how frequent they are. The
answer was “yes, several of those 10 geometrical configurations
are present.” As some may know, cloud condensation nuclei
(CCN) must be available in the atmosphere for clouds to grow. Traditionally,
CCN were considered to be hydrophilic particles. Nevertheless, aer
their findings, Guzman et al. predict that soot is able to become
CCN, and they provide an estimated value for the relative humidity
necessary. Therefore, we come to important conclusions: thanks to
its roughness, soot absorbs more water than we thought; moreover,
it has the capability to form clouds under conditions commonly encountered
in the atmosphere. (Geometrical properties “against”
physical properties).
What properties do these “new” clouds possess? What
effect do they have on Earth’s energy budget? If soot absorbs
water, it probably absorbs other substances too. Does this property
affect people’s health? Does it influence particle chemistry?
These and other questions are being investigated as part of the
MIT-CAM Project.
Acknowledgement
This work is supported by the Integrated Program on Urban, Regional,
and Global Air Pollution with funds from the Comisión Ambiental
Metropolitana.
References
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Experimental Mathematics 1: 141-165. hp://www.geom.umn.edu/soware/download/evolver/evolver.html.
- Forrest, S.R., and Wien, T. A. (1979), “Long-range correlations
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- Guzmán, R., Sosa, G., Toribio, A., Molina, M., Molina,
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as cloud condensation nuclei”, to be submied.
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Aerosoles”, B.Sc. Thesis. Escuela Superior de Cómputo
del Instituto Politécnico Nacional. Mexico.
- Xie J. (1996) “Equilibrium vapor pressure of water over
multiple aerosol particles”. Ph. D. Thesis. Texas A&M
University. USA.
R. Guzmán, A. Toribio, Escuela Superior
de Ingeniería Química e Industrias Extractivas del
IPN; G. Sosa, Programa de Investigación en Medio Ambiente
y Seguridad, Instituto Mexicano del Petróleo.
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