Hygroscopicity of Organic Aerosols

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Hygroscopicity of Organic Aerosols

• Chak K. Chan
• Department of Chemical Engineering
• Hong Kong University of Science and Technology
• Clear Way Bay, Hong Kong
• IGAC - iLEAPS SOLAS Workshop
• on Organic Aerosols and Global Change, Finland,
  May 2004

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Hong Kong University of Science and Technology
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Introduction

• OC - two main fractions in term of their water
  affinity
• 1) Hydrophobic fraction, e.g., n-alkane, n-fatty
  acid, PAH
• 2) Hydrophilic fraction (Water soluble organic
  compounds, WSOC) e.g., dicarboxylic acids
• Surface active organic compounds

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Water-Soluble Organic Compounds (WSOC)

• Atmospheric Abundances
• 28-55 of total mass of aerosol carbon in urban
  sites (Sempéré and Kawamura, 1994 ) and 70 of
  total soluble mass of aerosol samples (Zappoli et
  al., 1999).
• WSOC play an important role in
• Formation of cloud condensation nuclei (Cruz et
  al., 1998).
• Acidity of atmospheric precipitation (Keene et
  al., 1984).
• Visibility degradation (Malm, 1997).
• Phase transition and hygroscopic growth of
  inorganic-organic mixed particles (Cruz and
  Pandis, 2000 Hameri et al., 2001 Gysel et al.,
  2004 ).

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Hygroscopicity Measurements of WSOC and their
mixtures with inorganic compounds

• Phase transition, evaporation and growth
  characteristics
• Dicarboxylic acids (Cruz and Pandis, 2000 Peng
  et al., 2001 Prenni et al., 2001, 2003 Hameri
  et al., 2002 Lightstone et al., 2000 Choi and
  Chan, 2002 Parson et al., 2004 Marcolli et al.,
  2004)
• Salts of organic acids (Peng and Chan, 2001)
• Amino acids (Na et al., 1995 Chan and Chan,
  2004)
• Multifunctional acids (Peng et al., 2001)

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Hygroscopicity Measurements of WSOC and their
mixtures with inorganic compounds

• Biomass burning derived organic species (Chan and
  Chan, 2004 Kawamura and co-workers, 2004)
• Naturally occurring fulvic and humic acids (Gysel
  et al., 2003 Chan and Chan, 2003 Brooks et al.,
  2004)
• Macromolecular organic species e.g., protein
  (Mikhailov et al., 2004)

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Hygroscopic Measurement Methods

• Tandem Differential Mobility Analyzer (TDMA)
• Aerosol Flow Tube Reactor
• Electrodynamic Balance (EDB)
• Aerosol Microscope-FTIR System
• Bulk Measurements

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Bulk Measurements vs. Aerosol Measurements

• Estimates of Deliquescence RH (DRH) from bulk
  measurements are
• Good for organic species that crystallize (e.g.,
  succinic acid and glutaric acid).
• Not good for organic species that do not
  crystallize (e.g., citric acid, malonic acid,
  glucose, and levoglucosan). No deliquescence.

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Low Water Solubility ? High DRH CRH?

• Organic species have low solubility are expected
  to crystallize and deliquesce at high RH.
• However, some organic particles of low
  solubility do not have phase transition. For
  example, sodium maleate (Peng and Chan, 2001),
  asparagine (Chan and Chan, 2004),
• aspartic acid, glutamic acid, and homophthalic
  acid (unpublished data).
• Phase transition properties of levitated
  particles can be different from those inferred
  from bulk measurements and their solubility in
  water.

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General Observations of Hygroscopic Growth of WSOC

• A lot of organic species do not exhibit
  efflorescence.
• Most organic particles have a smaller diameter
  growth ratio, (Gf 1.00 1.40) than pure
  inorganic particles after deliquescence (e.g.,
  NaCl (1.93) and (NH4)2SO4 (1.51)) at 85RH.
• Inorganic-organic mixed particles are less
  hygroscopic than pure inorganic particles.
• Inorganic species tend to dominate the water
  uptake of mixed particles at high RH while
  organic species dominate at low RH.
• However, some salts of organic acids (e.g.,
  sodium formate and sodium acetate) are as
  hygroscopic as NaCl (Peng et al., 2001).

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Effect of Organic Species onPhase Transition of
Mixed Particles
AS-Succinic Acid (mass ratio 11)

• AS-Succinic acid particles deliquesce at a RH
  close to that of pure AS particles.
• AS-malonic acid particles do not deliquesce at a
  low RH.
• The effect of organic species on the
  deliquescence characteristic of mixed particle
  is not well understood.

AS-Malonic Acid (mass ratio 11)
Source Prenni et al. (2003) Atmos. Environ., 37,
4243-4251
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Effect of Organic Species on Phase Transition of
Mixed Particles

• NaCl-citric acid particles have phase transition
  and retain 10wt of water at low RH.
• AS-citric acid particles do not exhibit phase
  transition and form anhydrous particle at low RH.
• Overall, role of organic species in phase
  transition is not well understood.

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Effect of Organic Species on Water Uptake of
Inorganic Particles

• To study the effect of organic species on the
  water uptake of inorganic particles, Cruz and
  Pandis (2000) define the enhancement factor as
• assuming that only the inorganic fraction absorbs
  water and the organic fraction is inert.
• Organic species can also absorb water. xw, is
  defined as (Chan and Chan, 2003)

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Role of Organic-Inorganic Interactions in the
Water Uptake of Mixed Particles

• ?w' 1 indicates that the the water uptake of
  the particles is close to the water uptake of
  mixtures predicted by the additivity rule
  (neutral effect).
• xw gt 1 (enhancement effect).
• xw lt 1 (reduction effect).

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Water Uptake of Mixed Particles

• The values of xw of most AS-organic
  particles gt 1 or 1 at 85RH.
• The values of xw of NaCl-organic particles lt 1
  at 85RH.
• The effect of the interactions between organic
  species and NaCl and between organic species and
  AS are different.
• Chemical interactions between organics and
  inorganics are important.

All mixtures are at molar ratio 11 except for
glutaric acid and pinonic acid which are at mass
ratio 11
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Mass Transfer Effects in water uptake 1) Organic
Coated Particles

• Organic surface layers have been detected in
  atmospheric aerosols, fog and rain droplets (Gill
  et al., 1983). They change the physico-chemical
  properties of atmospheric aerosols (Rudich, 2003
  Ellison et al., 1999 Moise and Rudich, 2000).
• Organic surface layers cause kinetic limitations
  of hygroscopic growth, phase transition, and
  microstructrual rearrangement processes of
  particles (Andrew and Larson et al., 1993 Xiong
  et al., 1998 Hansson et al., 1998 Mikhailov et
  al., 2004)
• It is important to understand the effect of
  organic surfaces layer on
• Water uptake and phase transition of atmospheric
  aerosols, and
• Other atmospheric processes such as cloud
  activation and aerosol chemistry

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Mass Transfer Effects2) WSOC Particles

• Peng et al. (2001) observed a larger Gf than did
  Cruz and Pandis (2000) after deliquescence.
• This difference is attributed to the mass
  transfer limitation.
• TDMA (Cruz and Pandis, 2000)
• Equilibrium time 30 sec.
• EDB (Peng et al. 2001)
• Equilibrium time 10 hours

Glutaric Acid
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Mass Transfer Effects in Hygroscopic Measurements

• The possibility of mass transfer effects on phase
  transition, and hygroscopic growth has been
  investigated using laboratory generated particles
  (Cruz and Pandis, 2000 Peng et al., 2001 Gysel
  et al., 2003 Chan and Chan, 2003 Mikhailov et
  al., 2004).
• Are aerosol hygroscopic measurements equilibrium
  measurements?
• Residence time of atmospheric particles
• gtgt Residence time in aerosol flow system

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Modeling hygroscopic growth

• Aerosol Inorganic - Organic Models
• Limitations
• Few laboratory data of single component and
  mixtures for model development and verification,
  especially at temperatures other than 25C.
• Performance in the supersaturated regime,
  including estimation of efflorescence
  characteristics, has not been fully tested.

Inorganic Water
Organic Water
Inorganic Organic


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Future Research

• Identification of Chemical Composition in
  Atmospheric Aerosols
• Functional Group Identification or Individual
  Compound Speciation?
• Humic-like substances (HULIS) - a standard
  isolation method?
• State and Morphology of Particles
• Solid (crystalline or amorphous) or liquid,
  internally or externally mixed, organic surface
  layer?

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Future Research

• Organic Water interactions
• Limited measurements, mostly at ambient
  temperatures
• Parameterization of UNIFAC or modified UNIFAC
  models for water uptake prediction using
  experimental data
• Chemical Interactions between Inorganic and
  Organic Species
• Composition and RH Dependent. Strong at low RH.
• Measurements for Model Verification
• Possibility of Mass Transfer Effects in
  Hygroscopic Measurements

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Future Research