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Secondary Organic Aerosols

Secondary Organic Aerosols. Formation and Characterization. Overview. Background Formation Modeling Theoretical investigations Chamber experiments. Particulate Matter in the Atmosphere. PM affects visibility, climate, health. Inorganic fractions well characterized.

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Secondary Organic Aerosols

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  1. Secondary Organic Aerosols Formation and Characterization

  2. Overview • Background • Formation • Modeling • Theoretical investigations • Chamber experiments

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

  4. 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”)

  5. Aerosols = liquid or solid particles suspended in a gas (e.g., the atmosphere) Physical state of compound largely dependent on pure-compound vapor pressure (p°L) 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 Formation of Atmospheric Organic Aerosols

  6. -COOH oxidation products -OH -C=O 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

  7. Fundamental Thermodynamics of SOA Formation by Accretion Reaction Ag + Bg Cg Cliq

  8. 2A C1 2A C2 + H2O A + B C3 A + B C4 + H2O 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

  9. 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

  10. Accretion Reactions of Dialdehydes, Methylglyoxal, Diketones

  11. 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

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

  13. 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

  14. 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 • Lot’s of work to be done!

  15. PTRMS Reaction Chamber Ozone Monitor Ozone Source SO2 Monitor 2 Cylinder Air CIMS Humidifier SMPS T, RH UFPC SO2 Scrubber HTDMA Filter Sampler TDCIMS Biomass Chamber Biogenic Aerosol Chamber

  16. 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

  17. GC x GC Spectrum alcohols polarity (← ret. time) aldehydes alkanes volatility (→ ret. time)

  18. 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

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