Todays lecture objectives:

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ATMS 455 Physical Meteorology

• Todays lecture objectives
• Atmospheric Aerosol (WH 4.1)
• What besides water vapor do we need to make a
  cloud?

http//www.artcyclopedia.com/feature-2001-08.html
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ATMS 455 Physical Meteorology

• Todays lecture topics
• Atmospheric Aerosol (WH 4.1)
• Total concentrations
• Size spectra
• Sources and sinks
• Effects on some atmospheric phenomena

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Introduction

• Cloud physics the study of how raindrops or
  snowflakes are formed.
• Aerosol ? cloud particle ? hydrometeor
• Hydrometeor precipitation particle (e.g.
  raindrop, snowflake, hail, etc.)

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Introduction

• Atmospheric aerosol important in
• Cloud microphysics
• Air pollution and visibility

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Total concentrations

• Example of an aerosol collector

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Total concentrations

• Aitken nucleus counter
• Supersaturates air (wrt water) through rapid
  expansion
• Droplets are allowed to settle out onto a
  substrate
• Counted (microscope or using optical techniques)

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Total concentrations

• Typical values of aerosol
• 103 particles per cubic cm (cc) over oceans
• 104 particles per cc over rural land areas
• 105 particles per cc or higher for polluted air

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Total concentrations

• Aerosol concentrations decline with increasing
  altitude

The surface is an important source of aerosol!
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Size spectra

• Aerosols range in size from 10-4 micrometers
  (10-6 meters) 10-4 x 10-6 10-10 meters to
  tens of micrometers
• Aerosols range in concentrations from 107 to 10-6
  particles per cc
• Measurement technique depends on size of aerosol

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Size spectra
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Size spectra

• Concentrations of aerosol fall off very rapidly
  with increasing size
• Relationship is linear for some ranges
• Total concentrations of aerosol are, on average,
  greatest in urban polluted air and least in
  marine air
• Concentrations of giant aerosol (diameters
  greater than 2 micrometers) are similar in
  continental, marine, and urban polluted air

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Size spectra

• Giant and large (between 0.2 to 2 micrometer
  diameter) aerosols make similar contributions to
  the total mass of aerosol in continental air
• Aitken nuclei (diameters less than 0.2
  micrometers) are large in number, but contribute
  only 10-20 to the total mass of aerosol

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Size spectra

• Surface and volume distribution measurements

14
Size spectra

• More structure than would be expected based on
  dN/d(log D) v. log (D) plot
• Multiple maxima/minima are associated with
  sources and sinks of the aerosol

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Sources and sinks

• Aitken nuclei (smallest aerosols)
• originate primarily from combustion processes
• also found in marine air (gas-to-particle
  conversion)
• Sulfur dioxide ? sulfates
• Large and giant nuclei
• Windblown dust, pollens and spores from plants,
  bursting of air bubbles over water

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Sources and sinks
Accumulation mode growth of Aitken nuclei by
coagulation
Coarse particle mode mechanical processes
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Sources and sinks

• Coarse particle mode over water

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Sources and sinks

• Coarse particle mode over water
• Drops ejected into the air when air bubbles in
  breaking waves burst at the ocean surface
• Some drops are thrown 15 cm into the air
• Evaporation of these drops leave behind giant
  sea-salt particles
• Other bubble bursting processes leave behind
  smaller sea-salt particles

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Sources and sinks

• Differences in aerosol from land and ocean
• Principal aerosol elements are sodium and
  chlorine above the sea surface
• Dissolve at RH of 95 (see Fig. 4.5)
• Principal aerosol element are silicon, potassium,
  and iron above the land surface
• Do not dissolve at RH of 95 (see Fig. 4.6)

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Sources and sinks
By 2000, it was estimated that aerosol production
from human activities will have been twice that
in 1968
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Sources and sinks

• Aitken nuclei may be converted into larger
  aerosol by coagulation
• Concentration of aerosol of average diameter 0.01
  micrometers should decrease by a factor of two
  within 30 minutes due to coagulation (500 h for
  0.2 micrometer diameter aerosol to halve their
  concentration)
• Allows Aitken nuclei to grow to a size that
  they can be removed by other mechanisms

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Sources and sinks

• other mechanisms sinks of aerosols
• Removal by precipitation particles (accounts for
  80-90 of the aerosol mass removal)
• Diffusiophoretic force capture of aerosols by
  cloud particles
• Gravitational settling (dry fallout)

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Sources and sinks

• Summary

24
Effects on some atmospheric phenomena
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Effects on some atmospheric phenomena

• Atmospheric electricity
• Electrical conductivity of air is a minimum (and
  the atmospheric electric field a maximum) when
  the concentration of large ions and other Aitken
  nuclei is a maximum
• Fair weather atmospheric electric field over
  cities appears to be related to the level of
  aerosol pollution (peak at 7-10am and 7-9pm)

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Effects on some atmospheric phenomena

• Observed decrease of at least 20 in the
  electrical conductivity of the air over the North
  Atlantic is attributed to a doubling in the
  concentration of Aitken nuclei

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Effects on some atmospheric phenomena

• Radiative transfer
• Form haze under high RH conditions that reduces
  visibility
• Absorb and scatter a small fraction of the total
  solar radiation passing downward through the
  earths atmosphere
• Uncertain whether aerosol increase will result in
  a net increase or decrease in average global
  temperatures

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Effects on some atmospheric phenomena

• Chemistry
• Solid aerosol provide surfaces upon which trace
  gases can be absorbed and then react
• Liquid aerosols absorb gases which may then react
  together in solution
• e.g. sulfur dioxide ? sulfuric acid
• 4000 deaths in London 5-8 Dec 1952 due to dense
  smog causing respiratory ailments