Chapter 6 Atmospheric Aerosols

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Chapter 6Atmospheric Aerosols

• CH350/EV350
• Spring 2008

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Atmospheric Aerosols

• Solids in gas
• Dust, smoke, fly ash, pollen
• Liquids in gas
• Cloud, mist, fog, smog

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• Particles can scatter light if they are the same
  size as the wavelength of light

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• Large particles settle quickly
• Small particles stay suspended
• Very small particles coagulate
• PM10 particles less than 10 mm
• PM2.5 particles less than 2.5 mm

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Breathing particles

• PM10 particles are not removed when breathing
  through the nose
• Particles can enter lungs
• PM2.5 particles pose more of a health threat
• Particles contribute to breathing problems

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Ways of measuring particles

• Coefficient of haze (COH)
• 300 linear meters of air are drawn through filter
  and the absorbance of the filter is measured
• COH 100 times absorbance
• COH gt 6 indicates air that will cause severe
  respiratory problems
• Total suspended particulate (TSP)
• Gravimetrically measure quantity of particulates
  by filtering air at a rate of 1.4 m3/hr
• 10-30 mg/m3 common, but 500 mg/m3 has been
  observed in Toronto
• Total dustfall (TSF)
• Passively weigh dust that settles into an open
  topped container over 30 day period
• gt 7.0 g/m2/month considered excessive and is
  commonly found in highly industrialized areas

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Ways of measuring particles

• Inhalable particulates (IP)
• Course (2.5 10 mm) to fine (lt 2.5 mm)
• Measure with an impactor
• Can make small models to be carried with people
  to measure their exposure to particles

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Sources of particles

• Natural and anthropogenic sources

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Sea spray aerosols

• Small droplets 5 25 mm contain 2 -200 pg salt
• Small enough to stay in air
• Large droplets 25 500 mm contain 200 pg to 2 mg
  salt
• Fall back to water
• Sea surface microlayer is often different
  composition than bulk water
• Polar-nonpolar interface
• Aerosols may be enriched in nonpolar molecules
  compared to that predicted by bulk water

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Dust

• Rural dust
• Soil components
• Desert dust storms
• Agricultural lands
• Urban dust
• Cement particles
• Tire particles
• Brake lining particles
• Solid aerosols from vehicle exhaust

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Combustion products

• Carbon fuel gt fuel rich conditions gt black
  smoke
• Common in diesel engines
• White smoke gt condensed water vapor
• Coal combustion
• Fly ash
• Bottom ash
• Bottom ash from university power plant is used
  for road traction in Cape County

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Polycyclic Aromatic Hydrocarbons (PAHs)

• Formed during incomplete combustion of wood,
  coal, liquid fuels, meat, charcoal
• 150 compounds 2-8 rings
• Hamilton Ontario 10 ng/m3 in summer and 30 ng/m3
  in winter
• 80 of PAHs associated with soot particles less
  than 3.3 mm.

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Condensation aerosols - sulfate

• Sulfur compounds in -2 oxidation state are
  oxidized to sulfuric acid (Chapter 5)
• COS, H2S, CS2 etc
• Ammonia is released during microbial breakdown of
  organic matter
• Ammonia and sulfuric acid react to form solid
  particles (0.1 1 mm)
• Water condenses on these particles

NH3 H2SO4 ? NH4HSO4 2NH3 H2SO4 ? (NH4)2SO4
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Condensation aerosols - organic

• Terpenes emitted by trees cause condensation
  aerosols similar to those formed in photochemical
  smog events
• a-pinene is produced by pine, spruce, and fir
  trees
• 0.1 50 ppbv in forest air
• Isoprene is produced by willow, oak, poplar, and
  aspen
• 1 10 ppbv in forest air

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Condensation aerosols - organic

• Terpenes are oxidized to aldehydes, ketones, and
  carboxylic acids which condense to form small
  particles (lt 0.3 mm)
• Photochemical haze over forests (Great Smokey
  Mountains, NC)
• Aerosol production from forests 20 Tg/yr

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Aerosol concentrations

• mg/m3 rather than mol/m3
• Lower concentration of aerosols than trace gases
• CH4 1200 mg/m3

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Aerosol lifetimes - Settling

• Settling velocity due to gravity (Stokes Law) is
  dependent on
• Density of particle (rp)
• Density of air (ra)
• Stokes-Cunningham slip correction (C)
• Acceleration due to gravity (g)
• Particle diameter (dp)
• Viscosity of air (?)

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Aerosol lifetimes Settling 10 mm particle

• Settling due to gravity is in competition with
  air currents lifting the particle

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Settling velocity

• gt10 mm considered settleable
• lt10 mm remain suspended due to upward movement of
  air

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Aerosol lifetimes - Coagulation

• Brownian diffusion of small particles to form
  larger particles dependent on
• Particle concentration (N)
• Diffusion coefficient of the particles in air (D)
• Stokes-Cunningham slip correction (C)
• Particle diameter (dp)

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Aerosol lifetimes Coagulation 0.01 mm particle

• Brownian diffusion of small particles to form
  larger particles dependent on
• Particle concentration (N)
• Diffusion coefficient of the particles in air (D)
• Stokes-Cunningham slip correction (C)
• Particle diameter (dp)

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Coagulation

• gt 0.1 mm coagulation not important
• lt 0.1 mm coagulation will occur removing this
  size of particle

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Residence time of particles in atmosphere

• Large particles have short residence times due to
  settling
• Small particles have short residence times due to
  coagulation
• Particles in the 0.1 100 mm range have longest
  residence times

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Control of anthropogenic point sources of
particles

• Cant control sources like volcanoes and sea
  spray
• Can prevent forest fires
• Settling chamber
• Baffles and open spaces designed to allow the
  particles time to settle before being released

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Control of anthropogenic point sources of
particles

• Cyclones
• Cone-shaped device causing waste stream to swirl
  rapidly
• Large particles collide with wall due to
  centrifugal force

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Control of anthropogenic point sources of
particles

• Filter
• Waste stream passes through a porous filter
  similar to a vacuum cleaner
• Small particles plug the pores

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Control of anthropogenic point sources of
particles

• Scrubbers
• Fine mist of water to add water to particles
• Larger particles are easier to remove by settling

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Control of anthropogenic point sources of
particles

• Electrostatic precipitator
• Particles in waste stream become charge when
  passing in areas of electron flow between two
  electrodes
• Charged particles move toward positive electrode
  and are removed

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Summary of control of anthropogenic particles