Secondary Organic Aerosols

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Secondary Organic Aerosols

• Formation
• and
• Characterization

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Overview

• Background
• Formation
• Modeling
• Theoretical investigations
• Chamber experiments

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Particulate Matter in the Atmosphere
PM affects visibility, climate, health. Inorganic
fractions well characterized. Organic fractions
are poorly characterized, very complex.
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Modeling Atmospheric Aerosol Formation

• Model aerosol formation to understand its affects
  on air quality and climate change
• Accurately represent organic fraction
• Better characterization chemical composition of
  atmospheric organic aerosols
• Better understanding of secondary organic aerosol
  (SOA) formation, including the role of
  MW-building reactions (i.e., "accretion
  reactions)

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Formation of Atmospheric Organic Aerosols

• Aerosols liquid or solid particles suspended in
  a gas (e.g., the atmosphere)
• Physical state of compound largely dependent on
  pure-compound vapor pressure (pL)
• How can a compound have low/lower volatility?
• Inherent compounds emitted as PM
• Undergo oxidation VOCs NOx,O3, OH ? oxidation
  products
• Undergo MW-building reactions oxidation
  products/ atmospheric compounds? high-MW products
• Lowering volatility increases the tendency of a
  compound to condense, thereby forming PM

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Formation of Atmospheric Organic Aerosols
OA
gas/particle (G/P) partitioning
gas/particle (G/P) partitioning
high molecular-weight (MW)/ low-volatility
products
accretion reactions
Biogenic
Anthropogenic
oxidation
Emissions Volatile Organic Compounds
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Fundamental Thermodynamics of SOA Formation by
Accretion Reaction

• Ag Bg Cg
  Cliq

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Mathematical Solution Process
Multiple accretion reactions and products from
parent compound A
Mass balance leads to
A and C denote concentrations (µg m-3) N number
of accretion products from A
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Accretion Reactions of Aldehydes and Ketones

• Based on work of Jang and Kamens
• Reaction of 4 n-aldehydes and ketones (C4, C6,
  C8, C10)
• 5 Accretion products for each aldehyde/ketone
  (hydrate, dimer, trimer, hemiacetal, acetal,
  hydroxy carbonyl, unsaturated carbonyl)
• Considered same reactions for pinonaldehyde,
  inputs representative of ambient conditions

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Accretion Reactions of Dialdehydes,
Methylglyoxal, Diketones
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Accretion Reactions of Carboxylic and
Dicarboxylic Acids Ester and Amide Formation

• Accretion reactions of 5 acids
• Ester formation w/ MBO, amide formation w/DEA and
  NH3
• Inputs representative of ambient conditions

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Results for Carboxylic and Dicarboxylic Acids
Predicted OPM as a Function of A0

• MBO0 and DEA0 1 µg m-3
• NH3 0.1 µg m-3
• OPMna 10 µg m-3
• RH 20, T 298 K
• For malic, maleic, and pinic acids OPM formation
  is significant
• For acetic acid, accretion products do not
  condense into OPM phase
• Esters and at least 1 amide contribute to
  predicted level of additional OPM

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Implications for Observed OPM Formation in
Chamber Experiments

• MW 256-695 g mol-1 dominant accretion reactions
• MW 200-900 g mol-1, combination of monomers
  (Tolocka et al., 2004)
• MW 250-450 g mol-1 dimers, MW 450-950 g
  mol-1 trimers and higher oligomers (Gao et al.,
  2004a,b)

OPMna 0, RH 50, T 298 K
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Summary of Dissertation Research

• Accretion reactions appear to play a role in
  atmospheric SOA formation
• Currently, the dominant accretion
  reactions/products are not known
• Developed a first-cut approach to identifying
  favorable reactions and estimating their
  potential contribution to SOA
• Lots of work to be done!

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Biogenic Aerosol Chamber
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Filter Sample Analysis GC x GC

• Entire sample passed through two different
  columns
• First column usually separates based on
  volatility, second usually separates based on
  polarity

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GC x GC Spectrum
alcohols
polarity (? ret. time)
aldehydes
alkanes
volatility (? ret. time)
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Future Plans

• Look for accretion products in filter samples
  from chamber experiments and field experiments
• Use PTR-MS to track gas phase species
• Use GC x GC to analyze filter samples
• Compare data with thermodynamic model predictions
• Parameterize reactions to include in
  regional/global models