Aerosols

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Aerosols and Climate S.K. Satheesh Centre for
Atmospheric and Oceanic Sciences Indian Institute
of Science Bangalore.
Aerosol Cooling or Global Warming?
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What are Aerosols?
Particles of solid or liquid phase suspended in
the atmosphere is called aerosols. The atmosphere
is the dispersion medium and particles are the
dispersed phase. They are capable to affecting
climate.
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Tiny Particles

• Aerosols are too small to be observed by the eye.
  

• We can see their presence as red sunsets and
  sunrises, or haze.

• Aerosols scatter the red color in the sunlight
  most efficiently.

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Why Aerosols are Important?
Aerosols
Radiative Effects
Remote Sensing
Agriculture
Radiation Budget
Health
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Aerosol Types
Soil dust Sea salt Volcanic dust Organic
particles.

• The aerosols are

• Natural

• Man-made

Industrial sulfates Soot (black carbon) Organic
particles.
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Radiative Effects Sea-salt
Scattering Industrial Sulphate
Scattering Soil Dust Partly
absorbing Organics Partly absorbing Soot
(Carbonaceous) Absorbing
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Common Aerosol Systems
Smoke
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Haze

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Fog
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Smog
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Mono-dispersed and Poly-dispersed
Number (cm-3)
Number (cm-3)
Radius (?m)
Radius (?m)
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Shape of Aerosols
Isometric particles are those for which all three
dimensions are roughly the same. One common
example is spherical particles. Platelets are
particles that have two long dimensions and one
small third dimension. Leaf fragments are
examples of this type of aerosols. Fibres are
particles with great length in one dimension and
much smaller dimensions in the other two.
Threads or mineral fibres are examples of this
type of aerosols.
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Size Range Aerosol size varies in the range
of about four orders of magnitude from about
0.01 ?m to 100 ?m size. Particles greater than
100 ?m do not normally remain suspended in the
air for long time.
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0.5 cm s-1
0.05 cm s-1
Typical Rain Drop R 1000 ?m or 1 mm V 650
cm s-1
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Aerosol Mixtures Externally Mixed Intern
ally Mixed
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Sources
Extra-terrestrial
Terrestrial
Natural
Anthropogenic or man-made
Mechanical Disintegration
Gas-to-Particle Conversion
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Estimate of Source Strengths
Sources Source Strength (Tg
Yr-1) Ocean 1000 to 2000 Soil
Dust 2000 Biosphere 450 Biomass
Burning Volcanoes 15 to 20 Gas-to-Particle
1300 Conversion
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Mechanical Disintegration Production of Soil
Dust Aerosols The fine soil and sand particles
became air- borne by the action of wind Wind
speeds above 0.5 m s-1 are capable of picking
up and keeping the airborne soil particles as
large as r2 ?m in size
Rajasthan Desert
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Production of Sea-salt AerosolsOceanic Whitecaps

• Waves break at high winds, and the ocean is
  covered with whitecaps.

• Whitecaps are the major source for sea-salt drops.

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Wave Breaking

• As waves break, air mixes into the water, and
  forms clouds of bubbles.

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Sea Spray Formation

• The large bubbles rise to the surface, and while
  floating on the water, burst.

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Film Droplets

• Upon bursting, bubble caps shatter into thousands
  of small droplets.

Film drops form.
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Jet Droplets

• As the bubble cavity collapses, a water jet
  rises,

• and several small drops are formed from the tip.

Jet drops form.
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Spray Droplets

• Under very high winds drops are torn from the
  wave crests and blown directly into the air.

Spume drops form.
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Sea-Salt Aerosols

• In the air, this sea spray

• evaporates,
• shrinks,
• and forms

sea-salt aerosols.

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Volcanic Eruption
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Industrial Emissions
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Biomass Burning
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Biomass Burning
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Aerosol Removal Processes
Gravity Settling
Rain
Clouds
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Aerosols and Climate

• Directly

• Aerosols can influence climate

- by scattering incoming sunlight back to
space. - by absorbing sunlight

• Indirectly

- by modifying the properties and lifetime of the
clouds.
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Effect of Aerosols on Albedo Mean temperature of
the Earth is given by the balance between the
absorbed solar energy and emitted energy and can
be written as, where H is the net energy input
to the climate system and S0 is the solar power
per unit area intercepted at the mean Sun-Earth
distance (1365 to 1372 W m-2). The quantity ?
is the albedo (reflectance) of the planet which
is the fraction of the solar radiation reflected
by the Earths surface and atmosphere. The value
of ? is 0.3. Aerosols have a significant
influence in determining the value of albedo (?),
and has consequent impact on the climate system.

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Causes of Climate Change The main processes
which determine the overall state of the climate
system are heating by incoming short-wave
radiation from the Sun and cooling by long-wave
(infrared) radiation from the Earth into
space. Any process which can disturb the overall
energy balance can cause climate change. A
process which alters the energy balance of the
climate system is known as radiative forcing
mechanism.
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Radiative Forcing Mechanisms Radiative forcing
mechanisms can be internal or external
mechanisms. External forcing mechanisms
operate from outside the Earth's climate system
and include orbital variations and changes in
incident solar flux. Internal forcing mechanisms
operate from inside the Earth's climate system
and include changes in atmospheric composition
(such as greenhouse gases and aerosols),
volcanic activity etc.
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Aerosol Radiative Forcing ?
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The Effect

• Both effects lead to cooling of the
  Earth-atmosphere system.

• Hypothesis Aerosol cooling might offset global
  warming.

• But this hypothesis is not yet proven.

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Aerosol Cooling

• The most important aerosols in climate cooling
  are

• Natural sulfates.

• Man-made sulfates

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Background Aerosols
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Sea-Salt Aerosols

• Sea-salt aerosols dominate in a clean marine
  atmosphere.

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Sea-Salt Aerosols and Climate

• Among all types of aerosols, sea-salt aerosols
  are the major contributor to sunlight reflection.

• Sea-salt aerosols facilitate the formation of
  drops in marine clouds.

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Sea Salt and Cloud Properties

• More aerosols more cloud droplets.

• Hence, clouds reflect more sunlight.

• Net result cooling.

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Sea Salt and Cloud Lifetime

• More aerosols inhibit the growth of cloud
  droplets.

• Hence, droplet condensation and rain are delayed.

• Net result prolonged cloud lifetime.

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Sea-Salt Aerosols
A link between the ocean and the atmosphere!
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ISRO Geosphere Biosphere Programme A project was
initiated to monitor the aerosol characteristics
over the Indian region at a few selected sites.
The programme, became operational in the
nineties has as part of Aerosol Climatology and
Effects (ACE) project of ISROs Geosphere
Biosphere Programme.
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INDOEX WAS A FOCUSED FIELD EXPERIMENT IN THE
INDIAN OCEAN WITH THE FOLLOWING MAJOR OBJECTIVES
ASSESS THE SIGNIFICANCE OF SULFATES AND OTHER
CONTINENTAL AEROSOLS FOR GLOBAL RADIATIVE
FORCING. ASSESS THE MAGNITUDE OF THE SOLAR
ABSORPTION AT THE SURFACE AND IN THE
TROPOSPHERE. THE INTERACTIONS BETWEEN NATURAL
AND MAN-MADE AEROSOLS TRANSPORTED BY CONTRASTING
AIR MASSES.
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Why the Indian Ocean?
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Sagar Kanya Department of Ocean Development
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Ronald Brown National Oceanic Atmospheric
Administration
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C-130 NASA
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Maldives Observatory