Introduction to Aerosol Dynamics and Chemistry

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Introduction to Aerosol Dynamics and Chemistry

• Chao-Jung Chien
• Regional Modeling Center,
• Center for Environmental Research and Technology
  , UC-Riverside

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Outline

• Introduction
• Aerosol dynamics
• Physical processes leading to particle
  production/growth
• Visibility
• Aerosol chemistry
• Chemical processes transforming particles
• Secondary organic aerosol
• CMAQ vs. IMPROVE (aerosol model evaluation)

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Aerosol Definitions

• Aerosols relatively stable suspensions of solid
  or liquid particles.
• Properties
• Size, chemical composition, hygroscopiscity,
  density, and shape.
• 0.002-10 mm size range important with respect to
  atmospheric chemistry and physics.

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Impacts of Aerosol

• Direct and indirect radiative forcing (Global
  warming)
• Health effects
• Acid deposition
• Regional Visibility
• Chemical deposition budget for ecosystems
• All effects depend on Size, Mass and Composition.

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Reproduced from Chemistry of the Upper and Lower
Atmosphere, Finlayson-Pitts and Pitts, 2000
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Particle Size Distribution and National Ambient
Air Quality Standards (NAAQs)
Particular matter pollutant Primary and Secondary Primary and Secondary
Particular matter pollutant Conc. (mg/m3) Time
PM10 50 Annual arithmetic mean
PM10 150 24h
PM2.5 15 Annual arithmetic mean
PM2.5 65 24h
Lead 1.5 Quarterly average
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Types of Aerosols

• Marine Aerosol
• 100 300 /cc high coarse particle mass, but
  low number.
• Coarse marine aerosol are composed of salt from
  evaporated spray droplets.
• Fine aerosols formed from DMS reaction products.
• Remote Continental Aerosol
• 2000-10000 /cc produced naturally over land.
• Dust, pollen, or oxidation products from ammonia
  and sulfates.
• Background against anthropogenic emissions
• Urban Aerosol
• As high as 108 to 109 /cc
• Coarse particles contain crustal elements (Fe,
  Si, etc.)
• Fine particles mainly from combustion sources, or
  by gas to particle conversion involving reaction
  products of sulfates, nitrates, ammonium and
  organics.

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Fates of Atmospheric Aerosol

• Sources
• Direct emission (primary particles)
• Physical and Chemical processes (secondary
  particles)
• Sinks
• Wet (rain out),
• dry deposition
• Transformations
• Coagulation,
• Condensation,
• Cloud Processing

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Aerosol Dynamics

• Mathematical Representations of the Aerosol Size
  Distribution
• Discrete Distribution (Sectional Approach)
• uses discrete size bins
• very expensive for good size resolution
• used in CAMx
• Continuous Distribution (Modal Approach)
• uses moments of log normal distributions
• CMAQ uses 3 modes Aitken, Accumulation, Coarse
  modes.
• Three integral properties are included
• Total particle number concentration
• Total surface area concentration
• Total mass concentration of the individual
  chemical components

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Particle Production

• Primary Emissions
• In CMAQ
• Assumed PM2.5 mostly (0.999) in accumulation
  mode need to be evaluated.
• 90 of PM10 is estimated to be fugitive dust
  assigned to coarse mode (as ASOIL), among which
  70 is PM25.
• Emission rates (E) for aerosol number

D (mm) s
Aitken 0.03 1.7
Accum. 0.3 2
Coarse 6 2.2
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Particle Production/Growth

• Nucleation
• Process by which a gas interacts and combines
  with droplets.
• Can occur in the absence or presence of foreign
  material.
• Homogeneous nucleation
• Heterogeneous nucleation
• The production rate of new particle mass
  (mg/m3/s)

d3.5 diameter of the 3.5 nm particle r density
of the particle at ambient relative humidity
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Particle Growth (Mode Changing)

• Coagulation the process by which small particles
  collide with and adhere to one another to form
  larger particles.
• Condensation a process by which molecules in
  the atmosphere collide and adhere to small
  particles.
• Mode Merging by Renaming
• Growing particles reassigned to a larger mode and
  averaged with the new mode.

gas molecules
condens. nuclei
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Cloud Processing of Aerosols

• Aitken mode respond to in-cloud scavenging.
• Accumulation mode forms cloud condensation
  nuclei.
• Aqueous oxidation of SO2 by oxidants in cloud
  producing dissolved sulfate.
• New sulfate mass is added to accumulation mode.

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Aerosol Dry Deposition

• Deposition rate depends on particle size, surface
  resistance.
• Deposition velocity governed by
• Brownian particle diffusivity, and
• Gravitational settling velocity
• In CMAQ, species mass in each mode is deposited
  separately.

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Typical Time Scales for Various Aerosol Fates
Type of Air Mass Type of Air Mass Type of Air Mass
Fate Urban Remote Marine Non-urban Continental
Condensation 0.01 1 h 1 10 h 0.5 20 h
Coagulationof 0.03-um part. w/ larger particle 0.1 2 days 10 30 days 1 5 days
Deposition
0.3-um particles 0.5 10 days 0.5 10 days 1 month
0.03-um particles 1 month 1 month 1 month
Transport 2 5 days 1 2 weeks 1 2 weeks
Adapted from Pandis et al., 1995
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Aerosol Light Scattering and Absorption

• Visibility the furthest distance one can see
  and identify an object in the atmosphere.
• Light absorption
• Scattering
• Rayleigh scattering D ltlt l ( D 0.03 mm, 290 lt
  l lt 750 nm)
• Mie scattering D l ( 0.03 lt D lt 10 mm )
• Extinction coefficient
• Sum of the scattering and absorption
  coefficients.

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Visibility

• Deciview index, deciV
• Extinction coefficient,
• Mie theory extinction
• Reconstructed extinction

bext(1/m) 3fRH((NH4)2SO4NH4NO3) 4OC
10LAC SOIL 0.6CM
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Aerosol Chemistry

• Chemical Composition
• Reactions involved in particle formation and
  growth
• Thermodynamic of aerosols
• Aerosol liquid water content
• Chemical Equilibrium
• Chemical reactions
• Secondary organic aerosol

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Aerosol Composition

• Inorganics sulfate, nitrate, ammonium, crustal
  species, sea salt, hydrogen ions, and water
• Organics

Measured size distributions of aerosol sulfate,
nitrate, ammonium, chloride, sodium, and hydrogen
in Claremont, CA (Reproduced from Atmospheric
Chemistry and Physics, Seinfeld and Pandis,
1998).
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Organic Atmospheric Aerosols

• Components
• Elemental Carbon (EC)
• Black carbon or graphitic carbon
• Emitted during combustion processes
• Organic Carbon (OC)
• Primary OC, directly emitted by sources
• Secondary OC, formed in-situ by condensation of
  low-volatility products of the photooxidation of
  hydrocarbons

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Aerosol Liquid Water Content

• ZSR method
• Influenced by relative humidity and ionic ratio
• Chemical equilibrium sulfate, nitrate, ammonium
• NH4NO3(s) NH3(g) HNO3(g)

W liquid water content (kg/m3) Mn conc. of nth
species (moles/m3) mn0 molality (moles/kg3) aw
water activity
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Chemical Reactions involved in particle formation
and growth

• Reactions of gases to form low-vapor-pressure
  products followed by nucleation, condensation, or
  coagulation between particles.
• Reactions of gases on the surfaces to form
  condensed-phase products
• NaCl(s) HNO3(g) ? HCl(g) NaNO3(s)
• Reactions within the aqueous phases in fogs,
  cloud, or aerosol particles.
• SO2 oxidation to sulfate

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Formation of Secondary Organic Aerosol (SOA)

• Reactions of alkanes, alkenes, and aromatics with
  atmospheric oxidants (OH, O3, NO3 etc.) to form
  condensable multifunctional oxygenated products.
• Reactions of biogenic hydrocarbons, e.g.
  terpenes, to produce biogenic organic aerosol
  species.
• Production rates are passed from the
  photochemical component to aerosol component.
• Representations are limited and require
  improvement.

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Gas/Particle Partitioning

• Yield of SOA as a function of the amount of
  organic material already in the particle phase
  Pankow, 1994
• Adsorption to particle surfaces
• Mainly mineral particles
• Absorption into aerosol organic matter
• Important for SOCs

Kp (m3/mg) partitioning constant TSP (mg/m3)
total suspended particles conc. F, A (ng/m3)
particulate gaseous associated conc.
of compounds of interest
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Gas/Particle Partitioning

• Adsorption

pLo (torr) vapor pressure of the pure
compound Ns (sites/cm2) surface conc. of
sorption sites atsp (m2/g) particle surface
area Q1, Qv (kJ/mol) enthalpy of vaporization R
gas constant T (K) temperature
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Gas/Particle Partitioning

• Absorption

fom weight fraction of TSP in organic matter
(om) phase MWom mean molecular weight of the om
phase z activity coefficient
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Gas-Particle Partitioning

• Aerosol Yields

Odum et al. 1996
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Atmospheric Aerosol Chemistry
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References

• Binkowski, F. S., Science Algorithms of the EPA
  Models-3 Community Multiscale Air Quality (CMAQ)
  Modeling System, EPA/600/R-99/030, 1999.
• Seinfeld, J. H. and Pandis, S. N., Atmospheric
  Chemistry and Physics From Air Pollution to
  Climate Change, 1998.
• Finlayson-Pitts, B. J. and Pitts, J. N. Jr.,
  Chemistry of the Upper and Lower Atmosphere
  Theory, Experiments, and Applications, 2000.
• Lane, D. A., Gas and Particle Phase Measurements
  of Atmospheric Organic Compounds, 1999.
• dAlmeida, G. A., Koepke, P., and Shettle, E. P.,
  Atmospheric Aerosols Global Climatology and
  Radiative Characteristics, 1991.

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CMAQ vs. IMPROVE

• Evaluation of Model Performance