Chemical effects on cloud activation with special emphasis on carbonaceous aerosol from biomass burning

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Nov 3, 2013 (3 years and 7 months ago)

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Atmospheric Chemistry

Chemical effects on cloud
activation with special emphasis
on carbonaceous aerosol from
biomass burning

M. C. Facchini, S. Decesari, M. Mircea,
F. Cavalli and S. Fuzzi


Istituto di Scienze dell’Atmosfera e del Clima
-

C.N.R.

Bologna, Italy

Atmospheric Chemistry

Acknowledgements


M. O. Andreae

Max Planck Institute for Chemistry


O. Majol
-
Bracero

University of Puerto Rico


R.J. Charlson

University of Washington, Seattle, USA


A. Nenes, J.A. Seinfeld


California Institute of Technology, Pasadena, USA


M. Kulmala

University of Helsinki, Helsinki, Finland



Atmospheric Chemistry


Atmospheric Chemistry

how does the chemistry of the
cloud multiphase system
influence formation and
evolution of the cloud droplet
population ?


Atmospheric Chemistry


Many years ago, Twomey suggested that the
most important parameter influencing cloud
droplet number (CDN) is aerosol number
concentration, while aerosol chemical
composition has a relatively minor effect


Recently, model and experimental results
have induced to revisit this assumption and
to re
-
examine the relative importance of the
different factors influencing CDN
distribution

Atmospheric Chemistry

RH

Dry particle

wet aerosol

Cloud droplet

gas phase

R

R

…an intuitive picture of aerosol
-
cloud interaction and chemistry

Insoluble /absorbing material


s

Soluble fraction chemical composition

Atmospheric Chemistry

Cloud formation


Atmospheric thermodynamic
parameters (moisture availability,
updraft velocity, temperature, etc.)


Aerosol properties:
classically, the
controlling chemical variables are
CCN size distribution and water
soluble mass

Atmospheric Chemistry

Chemical factors controlling cloud
formation


Not simply inorganic soluble salts
influence cloud formation


Soluble or slightly soluble organics
influence equilibrium water vapor
pressure and decrease surface tension
of the droplets


Soluble gases condensation


(
Charlson et al.,
Science 2001;
Nenes et al.,

GRL, 2002
)



Atmospheric Chemistry

Modified Köhler equation

Kelvin term

Raoult term

Atmospheric Chemistry

Biomass burning

aerosol chemical composition

Atmospheric Chemistry

Fine fraction organic and
inorganic speciation

Atmospheric Chemistry


Biomass carbonaceous aerosols can
influence equilibrium supersaturation




adding” organic soluble or slightly soluble
material (with different chemical structure
with respect to wet season) ;


decreasing surface tension (HULIS)


changing mass and properties of insoluble
fraction (WIOC)

Atmospheric Chemistry

Surface tension decrease

Po Valley

Tenerife ACE II

SMOCC
-
dry season

Atmospheric Chemistry

Modelling of chemical effects


By using a cloud parcel model, we have
shown that there are many conditions in the
atmosphere in which chemical factors
(soluble and slightly soluble organic and
inorganic compounds, surface tension,
dissolution of soluble gases)
influence/control cloud microphysics to the
same extent as cloud dynamics and/or
aerosol number concentration






(Nenes et al.,
GRL 2002
)



Atmospheric Chemistry

(Roberts et al., JGR 2003),


Recent results, using the same 1D cloud
model, suggest that changes in cloud
albedo are more influenced by chemical and
physical properties in case of hight aerosol
loading (as in dry season)


“The knowledge on CCN spectra alone is
not sufficient to fully capture the climatic
influence of biomass burning aerosol”

Atmospheric Chemistry

Effect of size segregated
chemical composition

Dotted line: bulk composition

Solid line: size
-
segr. compostion

Mircea et al.,

oral A08432


Atmospheric Chemistry

Water activity of multicomponent
solutions

data from

Clegg
et al
.,

J. Aerosol Sci
.,

2001

modified

Koher theory

a
w

Clegg

treatment

as above +

measured
s

Atmospheric Chemistry

Conclusions


The potential impact of “chemical factors” on
cloud activation in the case of biomass burning
aerosol can be high for several reasons:


Chemical composition: WSOC concentration;
WIOC; INORGANIC/WSOC ratio


s

effect

Atmospheric Chemistry

…still needed


Size segregated chemical composition


Speciation of molecular organic forms: wet vs.
dry season


A
w


: the effect of mixture of organics with a
few percent of inorganic salts can be described
with traditional osmotic coefficient as used in
KC ??