Cloud multiphase processes and their impact on climate

1
Cloud multiphase processes and their impact on
climate

• Maria Cristina Facchini
• Istituto di Scienze dellAtmosfera e del Clima -
  C.N.R.
• Bologna, Italy

2
Acknowledgements

• M. Mircea, S. Fuzzi, S. Decesari, E. Matta
• ISAC-CNR, Bologna, Italy
• R.J. Charlson
• University of Washington, Seattle, USA
• A. Nenes, J.A. Seinfeld
• California Institute of Technology, Pasadena,
  USA
• S.L. Clegg
• University of East Anglia, Norwich, UK
• M. Kulmala
• University of Helsinki, Helsinki, Finland
• E. Tagliavini
• University of Bologna, Italy

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Clouds and climate

• Clouds are the most important factor controlling
  the Earth albedo and hence the temperature of
  our planet
• Cloud optical properties are controlled by
  size/number of droplets which in turn are
  governed by the availability of aerosol
  particles to serve as CCN

4
Clouds and climate 2

• Changes in cloud optical properties induced by
  mans activity are at the moment highly uncertain
  

5
Parameters influencing CDN

• 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

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CDN and aerosol number

• The number of CDN is not a linear function of
  aerosol number (Ramanathan et al., Science, 2001)
• The large degree of variation suggests that
  cloud properties are controlled by many different
  factors

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The issue

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

8
an intuitive picture of cloud chemistry
gas phase
R
R
Dry particle
wet aerosol
Cloud droplet
s
Absorbing material
Soluble fraction chemical composition
RH
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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

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Theory of cloud formation
aw water activity s surface tension nw
water molar volume
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Water activity
?
?
Only one paper Clegg et al., J. Aerosol
Sci., 2001
for inorganic aqueous electrolytic solutions
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Modified Köhler equation
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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)

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Aerosol chemical composition
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Organic aerosols andKöhler theory

• Organic aerosols influence equilibrium
  supersaturation by
• adding soluble material
• decreasing surface tension with respect to pure
  water or an inorganic salt solution

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Speciation of organic aerosol

• The traditional analytical approach has usually
  been individual compound speciation, but less
  than 10 of OC mass has been accounted for
• A new method using functional group analysis has
  been developed which accounts for up to 90 of OC
  mass (Decesari et al.,
  J. Geophys. Res., 2000)

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Organic solutes in clouds

• WSOC are a complex mixture of highly oxidised,
  multifunctional compounds with residual aromatic
  nuclei and aliphatic chains
• Neutral compounds mainly aliphatic polyols,
  polyethers, sugars
• Mono-/di-acids hydroxylated aliphatic acidic
  compounds
• Polyacids unsaturated polyacidic compounds both
  aliphatic and aromatic with a minor content of
  hydroxyl groups
• This information can be used to construct a set
  of model compounds

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Why model compounds?

• Too often the physical and chemical properties of
  atmospheric OC are simulated in models using
  compounds which are not representative of the
  physical reality
• Modellers need a synthetic information of a few
  model compounds which can be used to simulate in
  a quantitative way the whole OC of aerosol and
  clouds

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neutral fraction
mono-/di-acids
polyacids
(Fuzzi et al., GRL, 2001)
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Modified Köhler equation
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Surface tension measurements
s K - b T ln (1a C)
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Effect of organics on Sc
inorganic only
0.05 mm
inorganicorganic
inorganicorganics
0.1 mm
from Mircea et al., Tellus, 2001
0.3 mm
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Trace gas dissolution
Laaksonen et al., JAS, 1998
HNO3
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Modelling of chemical effects
0.1 m s-1
0.3 m s-1
1.0 m s-1
3.0 m s-1
insoluble
organic no Ds
organic with Ds
5 ppb HNO3
? 2 conc.
polluted case
marine case
(Nenes et al., GRL in press)
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Effect of size segregated chemical composition
Dotted line bulk composition Solid line
size-segr. compostion
See poster Mircea et al., Session B
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Water activity of multicomponent solutions
aw Clegg treatment
as above measured s
modified Koher theory
data from Clegg et al., J. Aerosol Sci., 2001
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Conclusions

• Dissolution of gases, dissolution of soluble and
  slightly soluble organics and the associated
  decrease of s influence droplet population
• There are many conditions in the atmosphere in
  which chemical factors influence/control cloud
  microphysics to the same extent as cloud dynamics
  and/or aerosol number concentration

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still needed

• data on physical and chemical properties of
  aerosol are needed for different areas and
  aerosol types
• thermodynamic data and models of aw for the
  complex cloud droplet solutions are needed

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Model compounds molar composition ()

• pinonaldehyde
  16
• levoglucosan
  9
• catechol
  2
• azelaic acid
  14
• hydroxy-benzoic acid 15
• b-hydroxy-butyric acid 3
• fulvic acid
  41

(Fuzzi et al., GRL, in press)
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Models neutral compounds
pinonaldehyde hydrated
cathecol
levoglucosan
31
Models mono-/di-acids
azelaic acid
b-hydroxy-butyric acid
hydroxy-benzoic acid
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Model polyacids
fulvic acid
33
Surface tension depends onchemical composition