THE PHASES OF THE TROPOSPHERIC AEROSOL: THERMODYNAMICS vs' KINETICS

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THE PHASES OF THE TROPOSPHERIC AEROSOL
THERMODYNAMICS vs. KINETICS Claudia Marcolli and
Thomas Peter Institute for Atmospheric and
Climate Sciences, ETH Zurich, Switzerland
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IMPORTANCE OF THE AEROSOL PHASES

• GAS / PARTICLE PARTITIONING
• Adsorptive vs. Absorptive partitioning.
  Absorptive partitioning is dependent on activity
  of the solutes in the solution.
• WATER UPTAKE AND RELEASE
• Continuous water uptake / release of a liquid vs.
  deliquescence / efflorescence of crystalline
  solids.
• CCN ACTIVATION
• Critical supersaturation for CCN activation
  depends on the particle phase (e.g. Broekhuizen
  et al., GRL 2004 adipic acid crystalline vs.
  liquid).
• REACTION MEDIUM
• Adsorptive solid surface vs. absorptive liquid as
  a reaction medium.

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WHAT IS NEEDED TO MODEL THE PHASES?

• COMPOSITION AND MIXING STATE
• What substances are present within the same
  particle?
• ACTIVITY COEFFICIENTS
• What are the activities of all components in a
  given solution or liquid mixture?
• VAPOR PRESSURES
• What is the partitioning between the gas and the
  particulate phases?
• MINIMIZATION OF THE FREE ENERGY Gives the
  number and the composition of the gas phase and
  the thermodynamically stable particulate phases.

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CALCULATED PHASES OF PURELY ORGANIC SYSTEM

• Erdakos and Pankow, AE 2004
• Cyclohexene oxidation products Dicarboxylic
  acids, hydroxy- and oxo- carboxylic acids and
  aldehydes.
• Lipid mixture n-alkanes, n-acids, n-alcohols.
• Absorptive partitioning between gas and PM
  phases.
• Activity coefficients calculation of by UNIFAC.
• Free energy minimization using Ficks first law
  to model a pseudo-diffusion process.
• Gas-phase concentrations calculated for one and
  two phase cases.

Error in predicted PM assuming one phase -21.8
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LIQUIDS AS THE THERMODYNAMICALLY STALBE PHASES OF
THE ORGANIC PM
Marcolli et al., J. Phys. Chem. A., 108, 2216
2224, 2004.
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PHASES OF AMMONIUM SULFATE / POLYOL / WATER
MIXTURES AT RT
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MIXING STATE OF THE AEROSOL EXPERIMENTAL EVIDENCE
Single particle mass spectrometry - Continental
region internal mixture of organics / sulfate as
a general characteristic (Lee et al., JGR
2002). - Urban area organics / sulfate,
hydrocarbon / soot, mineral particles and more
complex mixtures (Lee et al., JGR 2002, 2003) -
marine area internal mixtures of sea-salt and
organic species are frequent (Middlebrook et al.,
1998).
H-TDMA Measurements
Weingartner et al., 2002.
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MIXING STATE OF THE AEROSOL THEORETICAL
CONSIDERATIONS

• MIXING OF NON-VOLATILE SUBSTANCES
• Substances from the same source will be
  internally mixed. Example fresh sea-salt aerosol
  (Cl-, Na, Mg2, SO42-, Ca2, K, Br- some
  organics).
• Further mixing by coagulation.
• MIXING OF SEMIVOLATILE SUBSTANCES
• Mixing by gas phase diffusion.
• Degree of mixing depends on equilibrium
  thermodynamics.
• Example Secondary organic aerosol.

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VAPOR PRESSURES AND EQUILIBRATION TIMES AT 25ºC
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VOLATILITY AND MOLECULAR WEIGHTS OF HUMIC LIKE
SUBSTANCES (HULIS)

• Samples from K-puszta (Gelencsér et al., AE 2000,
  Kiss et al., AE 2003)
• Overnight at 250ºC (in air) 30 42 of fine
  aerosol carbon remained.
• Overnight at 340ºC (in air) 1 3 of fine
  aerosol carbon remained.
• Ultrafiltration All the water-soluble organic
  matter passed through a membrane having a 500 Da
  nominal molecular weight cut-off.
• Mass spectrometry average molecular weight in
  the 200 300 Da range.
• Vapour pressure osmometry average molecular
  weight 215 345 Da.
• Sample from Jungfraujoch (Kriváscy et al., JAC
  2001)
• Ultrafiltration All the water-soluble organic
  matter passed through a membrane having a 500 Da
  nominal molecular weight cut-off.
• HPLC-MS mass-to-charge ratios from 53 303.

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THERMODYNAMICS vs. KINETICS
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THERMODYNAMIC MODELS WHAT IS NEEDED? WHAT IS
AVAILABLE?
NEEDED Model that can treat inorganic ions and
organic substances in concentrated aqueous and
organic solutions.
Pitzer ion-interaction model for aqueous
solutions up to fairly high concentrations. Zdanov
skii-Stokes-Robinson (ZSR) approach calculation
of water uptake / release. No calculation of
activity coefficients. UNIFAC model developed
for organics and water, based on functional
groups. LIFAC model consisting of Debye-Hückel
term, UNIFAC term, and the osmotic virial
equation (Yan et al., FPE 1999).
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EXPERIMENTAL DATA WHAT IS NEEDED? WHAT IS
AVAILABLE?

• INORGANIC SALTS
• Water activities for aqueous solutions of the
  relevant salts are known up to high
  supersaturations and also for low temperatures.
• ORGANICS
• Vapor-liquid-equilibrium data for small molecules
  are known. However, parameterization based on
  this data is often inaccurate for more complex
  molecules. Data for complex molecules and low
  temperatures are missing.
• ORGANIC / INORGANIC MIXTURES
• Data for mixtures of atmospheric importance is
  missing.

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CONCLUSIONS (I)
- A better knowledge of the aerosol phases is
desirable. - The phases of a system can be
predicted theoretically if the components and
their interactions are known. - To determine
the components, the aerosol composition and its
mixing state have to be known. - To determine
the interactions, the activity coefficients have
to be known.
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CONCLUSIONS (II)

• COMPOSITION
• A characterization with respect to functional
  groups could be sufficient.
• ACTIVITY COEFFICIENTS
• Development of a thermodynamic model to
  calculate the phases of mixed organic / inorganic
  systems.
• Data for the parameterization of the model.
• MIXING STATE
• Better characterization of organics with respect
  to volatility and/or molecular weights.
• Experimental data on the mixing state of
  different organic substance classes and inorganic
  constituents.

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CHARACTERISTIC TIMES FOR GAS PHASE DIFFUSION
a accommodation coefficient r particle
radius D molecular diffusivity in the gas
phase v mean molecular speed H dimensionless
Henrys law constant rRT/Mp0 n particle number
density
Gas/particle equilibration
Particle/particle equilibration
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TIME EVOLUTION OF EQUILIBRATION OF AN ORGANIC
SPECIES
For a dimensionless Henrys law constant of 3 x
1014 with a) 20 wt b) 2 wt of organic species
in distribution 1
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EFFECT OF ENTROPY OF MIXING ON MELTING TEMPERATURE
- Melting point of component i
- Entropy
- Melting point of pure component i in a mixed
solution
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EFFECT OF ENTROPY OF MIXING EXAMPLES
--- Succinic acid --- Adipic acid Malic
acid - - Glutaric acid -- Lactic acid