Some Impacts of Atmospheric Aerosols

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Some Impacts of Atmospheric Aerosols
HNO3
N2O5

• Effects on Gas-Phase Composition
• Surfaces for heterogeneous chemistry
• Multiphase reaction volumes S(IV)?S(VII)

• Direct and Indirect Effects on Climate
• directly scattering solar radiation
• altering number and size distribution of cloud
  drops

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Some Sources of Aerosol Particles
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Aerosol Particle Size Diameter vs. Effective
Diameters
For many particles, spherical geometry good
assumption. Diameter has physical meaning
Spherical?
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Some Effective Diameters
Relation to aerodynamic diameter and other
physical properties of particle not well
understood for fractal like soot particles.
rp
Aerodynamic Diameter
Electric Mobility Diameter
Same terminal falling speed in air as a particle
with density 1g/cm3 and radius rp
Same trajectory in calibrated electric field as a
spherical singly charged particle with radius rp
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Bean Counting Aerosol Size Distributions
0.005
0.05
0.3
1.0
0.001
0.02
0.1
0.6
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• Problems
• Information lost at small sizes due to large size
  range
• Comparing particle concentrations in different
  bins marred by varying bin size
• Area under curve is not proportional to total
  particle number concentration

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Visual Representation of Particle Size
Distributions
Ni/?Dpi vs. Dp
Ni/?log(Dpi) vs. Log(Dp)
Area under both curves yields Ntotal But
dN/dlogDp vs. logDp is more informative
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Questions

• The figure shows various representations of the
  same aerosol size distribution. Under which
  curve(s) is the area equal to the total particle
  number concentration?
• dN/dDp (blue)
• dN/dlogDp (green)
• dN/dlnDp (red)

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Area, Volume-Mass Distributions
Heterogeneous and multiphase reaction rates
depend on surface area or volume, respectively.
Gravitational settling rates depend on mass and
air quality standards are mass-based.
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Questions

• What are the units of Stot and Vtot?
• How is the mass distribution function calculated?
• What is the relevant property (area, volume,
  mass) for the following aerosol particle
  processes
• Scavenging of HNO3 by mineral dust
• Acidification of aerosols by gas-phase H2SO4
• Light scattering efficiency
• Amount of Fe deposited to ocean by dust

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Smoothed Vertical Profiles of Aerosol Number
Concentrations(highly variable)
Boundary layer 10 105 cm-3 range in number
concentration Free Troposphere 100-300 cm-3 on
average
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Common Modes of Atmospheric Aerosol Distributions
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Typical Number Distribution for Urban Aerosols
Solid line what would be observed, composed of 3
modes Dotted/Dashed lines Two common
parameterizations

• Junge Distribution (dashed line) is a power law.
  Has some useful properties but requires care.
• Log-Normal distribution (dotted line) is most
  often used

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Continental and Marine Number Distributions
Lower numbers in these regions relative to urban
aerosols, especially in the nucleation mode.
Dominant accumulation mode indicative of aged
particles.
Giant aerosols over ocean dominated by sea salt.
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The Log-Normal Distribution

• The Log-normal distribution
• Bell-curve shape in log space

• The familiar normal (Gaussian) distribution
• Bell-curve shape in linear space
• 68 of variance about mean ( ) captured by 2?
  (width)