What are Aerosols

1
What are Aerosols?

• Solid or liquid particles suspended in the air
• Sources
• Natural volcanoes, sea spray, dust, smoke,
  wind erosion, water droplets, ice
  particles Human plowing, surface
  mining, combustion
• Effects
• too small to see individually (100s - 1000s nm)
• scatter visible sunlight
• sky looks hazy or dirty
• brilliant orange and red skies at sunrise and
  sunset
• large aureole around the sun or full moon

2
Why Measure Aerosols?

• Energy Cycle
• Affect the amount of sunlight reaching Earth's
  surface
• Water Cycle
• Stimulate formation of water droplets and ice
  particles
• Biogeochemical Cycles
• Help track the effects of dust storms, fires,
  volcanic eruptions and pollution

3
Aerosols and Light Absorption

• Sunlight includes a range of wavelengths
• Aerosols and gases absorb and scatter light best
  at wavelengths that match their size
• By measuring sunlight at 2 wavelengths we can
  learn about aerosol size distribution, which can
  help us identify aerosol sources

4
Aerosols and Light Absorption

• Both gases and aerosols absorb and scatter
  sunlight
• If we know (1) atmospheric pressure, and (2) the
  elevation angle of the sun, we know the amount of
  gas between us and the sun.
• Any light absorption not caused by atmospheric
  gas is from aerosols (of course, there must not
  be any clouds partly obscuring the sun!)

5
Where and When to Observe Aerosols

• Where you have a clear, unobstructed view of the
  sun (e.g., where you take cloud observations)
• Daily, ideally at a particular sun angle in the
  morning when the sun is visible.
• Multiple measurements per day are wonderful.
• Six measurements (3 with open aperture and 3 with
  aperture covered) at each wavelength (red and
  green) are required

6
Aerosols Instrument

• Aerosols are measured with a sun photometer.
• Sun photometers can be obtained from the PI for
  the measurement a link is available from the
  Scientific Instrument and Equipment Suppliers
  page on the GLOBE Web site.

7
Remember

• Sun photometers measure in units of volts.
• The GLOBE Data Server will convert volts to
  aerosol optical thickness using calibration
  values for each individual sun photometer.
  (Advanced students can do this as a Learning
  Activity).
• Aerosol optical thickness (AOT) can range from 0
  (100 light transmission) to gt5.0 (lt1 light
  transmission)
• The larger the optical thickness at a particular
  wavelength, the less light of that wavelength
  reaches Earth's surface
• A typical AOT value for visible light in clear
  air is roughly 0.1 (90 of light is transmitted).
  Very hazy skies have AOTs of 3.0 or greater (5
  light transmission).

8
Aerosol Questions We Would Like to Answer

• How do aerosol concentrations change over
  seasons?
• How are aerosol concentrations are related to
  the weather?
• How does smoke from large forest fires affect
  sunlight reaching Earth's surface?
• How long do volcanic aerosols stay in the
  atmosphere?
• How are the amounts and distribution of aerosols
  are changing?