Particles and Clouds

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• Particles and Clouds

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Cloud albedo forcing Cooling
The shortwave rays from the Sun are scattered in
a cloud many of the rays return to space. The
resulting "cloud albedo forcing, taken by
itself, tends to cause a cooling of the Earth.
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Cloud Greenhouse Forcing Warming
Longwave radiation
When a cloud absorbs longwave radiation emitted
by the Earth's surface, the cloud re-emits a
portion of the energy to space and a portion back
toward the surface. This process is called "cloud
greenhouse forcing" and, taken by itself, tends
to cause a heating or "positive forcing" of the
Earth's climate.
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Overall effect of all clouds

• Cloud albedo forcing ? cooling
• Cloud greenhouse forcing ? warming
• The overall effect of all clouds together is that
  the Earth's surface is cooler than it would be if
  the atmosphere had no clouds
• The dominate factor in the global albedo is
  clouds.

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Aerosol Definition

• Aerosol is a suspension of solid or liquid
  particles in a gas.
• Atmospheric aerosols consist of small particles
  of liquid and solid material suspended in the
  air.
• Bioaerosol An aerosol of biological origin.
  (Examples viruses, bacteria, fungi, spores, and
  pollens.)
• Aerosol sizes are usually measured in the unit of
  micrometer (mm)
• 1 mm 106 m
• 1 mm 104 angstrom (Å)

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Typical Particle Diameters (mm)
Human hair 25-100 mm
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Example images of atmospheric particles
Source http//www.mpch-mainz.mpg.de/kosmo/
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Aerosol sources
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Aerosol sources

• Windblown dust from Desert
• Sulfate aerosol from volcano eruption, fuel
  combustion and microbial activities.
• Seasalt aerosol from seaspray and bubble bursting
• Soot from fuel combustion
• Secondary organic aerosol from volatile organic
  compounds

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Aerosol Radiative Forcing direct effect

• The effect of aerosol on the energy flux of the
  atmosphere depends on particle size and
  composition.
• Dark particles (sootcontaining) tend to absorb
  light, thus warming Earths atmosphere.
• Small particles tend to scatter light, thus
  increasing the albedo of the atmosphere.

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Aerosol and cloud condensation nuclei (CCN)

• Cloud Condensation Nuclei (CCN) are particles
  that can become activated to grow to fog or cloud
  droplets in the presence of a supersaturation of
  water vapor.
• If the Earths atmosphere were totally devoid of
  particles, clouds could not form.

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Aerosol radiative forcing indirect effect
An increase in the number of atmospheric
particles would increase the number of cloud
condensation nuclei, therefore more cloud cover
and higher albedo
Ship tracking phenomenon
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Volcanoes and climate change
Large volcano eruptions provide dramatic evidence
of the ability of aerosols to affect global
climate.
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Mt. Pinatubo eruption made 1992 the coolest year
since 1986
Red line modeled temperature changes Blue
line temperature changes observed from
meteorological ground stations
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Volcanic Cooling
Large volcanic eruptions can cool the Earth by
increasing the albedo. Calculate the expected
temperature change if the albedo increases from
30 to 30.5 percent. Compare this estimate with
the temperature record after the Mt. Pinatubo
eruption. (Textbook, pp150).
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1816-the year without a summer

• In 1815, Tambora in Indonesia exploded and the
  volcano dust blanketed the Northern Hemisphere.
• The following year, 1816, daily minimum
  temperatures were abnormally low in the northern
  hemisphere from late spring to early autumn.
• Famine was widespread because of crop failures.
  An estimated 82,000 were killed indirectly by the
  eruption by starvation, disease, and hunger.

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Sulfate aerosol formation from volcano eruptions

• Large amount of SO2 is injected into the
  atmosphere from the force of the volcano
  eruption.
• SO2 can be converted to sulfate in gas and
  aqueous phase.
• In gas-phase
• SO2 .OH M ? HOSO2. M
• HOSO2. O2 ? HO2. SO3
• SO3 H2O M ? H2SO4 M

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Sulfate aerosol formation from volcano eruptions
(Continued)

• In aqueous phase, dissolved SO2 is oxidized to
  sulfate by
• O3 (dominant pathway when pHgt5)
• H2O2 (dominant pathway when pHlt5)
• organic peroxides
• O2 catalyzed by iron and manganese
• The oxidation of SO2(aq) by H2O2 proceeds as
  follows
• HSO3- H2O2 ? SO2OOH- H2O
• SO2OOH- H ? H2SO4

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Sulfate formation from biogenic gases

• Dimethylsulfide (DMS) CH3SCH3
• Marine origin
• Produced in plankton by the enzymatic cleavage of
  dimethylsulfonopropionate, a compound that help
  plankton achieve osmotic balance in the salty
  ocean water.
• Oxidation of DMS? SO2 ? sulfate
• H2S
• Terrestrial origin
• Produced by sulfate-reducing bacteria.

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Example Kuwait Oil Fires

• Air temperatures below the plumes were reported
  to be about 7oC lower than in adjacent areas
  without smoke.
• Coldest May in 35 years.
• Average temperatures were about 4oC lower than
  normal.

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Aerosol Effects on Climate

• Indirect Effects
• Serve as Cloud condensation nuclei
• More CCN leads to more cloud cover
• Possible effects include changes in Earth's
  albedo and changes in hydrological cycle

• Direct Effects
• Aerosols scatter and absorb visible and infrared
  radiation
• Overall effect cooling
• Light scattering depends on size distribution and
  index of refraction of the particles